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Integral view of Holocene precipitation and vegetation changes in the Nile catchment area as inferred from its delta sediments
... The oldest terrace (T1; Fig. 2 and Table 1) is dated to 9.42 ± 0.75 ka and forms the oldest present-day exposed surface at the eastern valley margin at an elevation of ~78. 5 The changes observed here in the Egyptian Nile's Holocene riverine landscape near Luxor are in line with observations of their drivers ( Fig. 4b-g), evidence for which is found elsewhere in the Nile Basin. From ~11.5 ka onward, wet conditions existed over northern Africa (Fig. 4d,e,l) 6,7,34-36 , due to a northern position of the Intertropical Convergence Zone 4 , which resulted in increased Nile discharge enlarging its erosivity and transport capacity (Fig. 4f) [11][12][13]37 . Wetter conditions also led to a denser vegetative cover 6,7 , reducing upstream sediment input ( Fig. 4g) 11,12,38 . ...
... From ~11.5 ka onward, wet conditions existed over northern Africa (Fig. 4d,e,l) 6,7,34-36 , due to a northern position of the Intertropical Convergence Zone 4 , which resulted in increased Nile discharge enlarging its erosivity and transport capacity (Fig. 4f) [11][12][13]37 . Wetter conditions also led to a denser vegetative cover 6,7 , reducing upstream sediment input ( Fig. 4g) 11,12,38 . The observed erosion and subsequent uptake of sediment in the Nile Valley through channel incision was, therefore, probably a direct result of the wetter Early Holocene Nile Basin's hydroclimatic regime. ...
... Conversely, rapid aggradation and formation of CB1-3b in the Nile Valley from ~4 ka onward is thought to have been triggered by a diminishing discharge (and hence erosion capacity) and an increase in (fine) sediment supply 11,12,17,39 . The driving factor behind this increase was the progressive aridification of the Nile Basin, especially between 5 and 6 ka (Fig. 4c,l) 3,6,34,40 , which, potentially in combination with changing human impact on the hinterland 41-43 , made soils increasingly prone to erosion 44 . ...
Although the Nile is one of the largest rivers in the world and played a central role in ancient Egyptian life, little is known about its response to climatic change during the Holocene. Here we present a framework for the evolution of the Egyptian Nile, demonstrating how climatic and environmental changes have shaped the landscape of the Egyptian Nile Valley over the past 11,500 years, including the civilization of ancient Egypt (~5,000 to 2,000 years ago). Using data from over 80 sediment cores drilled in a transect spanning the Nile Valley near Luxor, pinned in time by 48 optically stimulated luminescence ages, we reconstruct the dynamics of the Nile River during the Holocene in the vicinity of UNESCO World Heritage sites such as Karnak and Luxor temples. According to our reconstruction, valley incision occurred from the start of the record until approximately 4,000 years ago and then rapidly shifted to massive floodplain aggradation. We argue that this relatively abrupt change in the riverine landscape near Luxor from the Middle to Late Holocene was linked to a shift towards a drier regional hydroclimate around this time. Such a dramatic change in river sediment dynamics could have had local agro-economic consequences.
... Ti is the least soluble element in water (Broecker and Peng, 1982) and is a good measure of detrital input (e.g., Whitlock et al., 2008). Higher content of Ti indicates intensive chemical weathering in the Ethiopian Highland (Malick and Ishiga, 2016) and less Ti suggests dense vegetation cover resulting in limited contribution to the Blue Nile sediments (Hennekam et al., 2015), combined with a larger discharge of the White Nile, in the drainage area over which the crystalline basement rocks prevail (Krom et al., 2002). Sediment input from the desert, mostly wind-blown but also contributed by wadi flux, is enriched in Ca derived from carbonate rock outcrops and depleted in Ti (Schilman et al., 2001;Woronko, 2012;Garzanti et al., 2015;Pennington et al., 2019). ...
... Organic matter could be derived either from older fluvial deposits by waves or brought by the Sebennitic. A lack of Ti in the sediments of phase 2 (Fig. 6) indi-cates that sediment input from the Blue Nile was limited, caused presumably by denser vegetation in the Ethiopian Highland (Hennekam et al., 2015). Rising values of CIA and CIW indicate a gradual increase of chemical alternation in the catchment, but they are generally low (<50), suggesting little or no input of highly weathered material to the Nile (McLennan, 1993;Krom et al., 2002). ...
... The lagoon persisted during this time (logs S43 and 1 in Fig. 8). Since 6.5 cal ka BP, vegetation cover in the Nile catchment became abruptly scarce (Pennington et al., 2019) and precipitation gradually decreased in response to lower insolation in the northern hemisphere (Hennekam et al., 2015). Reduced flow in the Nile branches in the delta affected marshland vegetation to the south of the Burullus Lagoon (Bernhardt et al., 2012). ...
The modern Nile delta developed in the Middle and Late Holocene, and at its most northern-central point is situated at the Burullus Lagoon, which is environmentally diverse, including salt marshes, mudflats, and sand plains, and separated from a sea by a sand barrier overtopped with high sand dunes. The lagoon has been fed since the Middle Holocene by the Sebennitic branch of the Nile and marine intrusions through the Bughaz inlet. A sediment core (BO-1) was collected at the northeastern shore of the lagoon and sampled at centennial scale resolution in order to reconstruct the development of the lagoon. The results show that an initial and limited lagoon had developed at the end of the Early Holocene, but after a dry period ca. 7.2 cal ka BP it has been progressively transformed into a marshy area, with occasional inflows of sea water. Lower water level and higher salinity of the Burullus Lagoon at 6.0–5.5 and 4.8–4.2 cal ka BP reflected droughts in the Nile catchment. Thereafter, the river reactivated in the Burullus Lagoon area, and since 2.8 cal ka BP was accompanied by occasional inflows of sea water. Since ca. 0.8 cal ka BP, increased fluvial activity occurred in this part of the Nile delta, which terminated after construction of the Aswan dams in the twentieth century.
... However, these ratios are lower in the eastern part of Nile Delta (e.g. Hennekam et al., 2015). Hennekam et al. (2015) interpreted depleted Ti/Al values in the eastern Delta during the AHP as indicating reduced erosion in the Ethiopian Highlands and an enhanced vegetation cover in the Ethiopian Highland. ...
... Hennekam et al., 2015). Hennekam et al. (2015) interpreted depleted Ti/Al values in the eastern Delta during the AHP as indicating reduced erosion in the Ethiopian Highlands and an enhanced vegetation cover in the Ethiopian Highland. ...
... Sapropel S1 is subdivided into two subunits S1A (10.5-8.2 ka cal BP) and S1B (8-6.5 ka cal BP) (Fig. 11). Increase redox conditions in early Holocene sediment of Faiyum core also correlate with sapropel S1A (Hennekam et al., 2015). Weldeab et al. (2014) also mentioned high Nile discharge from 12 to 8 ka, reaching a maximum discharge from 10 to 8.5 ka cal BP, and gradually decreasing discharge from 8 to 4 ka cal BP. 5-Palynomorphs in the early and mid Holocene sediments of Qaru-9 indicate changes in freshwater availability. ...
... By contrast, the more radiogenic ε Nd during and immediately prior to the S1-period point to intensified Nile runoff and reduced EMS circulation that may have initiated thousand(s) of years before S1 formation started (Fig. 4). This observation not only coincides with the onset of Nile flooding ∼2-5 kyr prior to the S1 onset (Hennekam et al., 2014(Hennekam et al., , 2015Revel et al., 2015;Tachikawa et al., 2015), but also with a 3-kyr precession lag of sapropels, related to the occurrence of North Atlantic cold events (Ziegler et al., 2010). This pre-S1 conditioning observed in our ε Nd records, is also consistent with reported oxygen-depleted conditions for bottom waters (500-2300 m depth) starting around 16-11 kyr BP in the Nile and Adriatic areas (Schmiedl et al., 2010;Grimm et al., 2015;Tachikawa et al., 2015;Tesi et al., 2017). ...
... This can be attributed to a direct and persistent control of the Nile discharge on the easternmost Levantine, as confirmed by the small ε Nd differences between the S1 data, core-top samples, and modern seawater for the areas east of ∼30 • E, which are all characterized by spatially variable ε Nd signatures (Fig. 3). Being spatially specified, the large ε Nd range for this small area may further reflect the variable influence of surfacewater currents (Hennekam et al., 2015). ...
... Most importantly, for S1 scenarios, the sediment-related flux is assumed to be proportional to the Nile water flux. This is probably not the case, as the S1 sediment flux may have been similar as today due to the monsoon-generated denser vegetation cover on the Nile-particle source areas (Hennekam et al., 2014(Hennekam et al., , 2015. In addition, the offshore Nile-sediment spreading must be limited to within the eastern Levantine, being mostly deposited on the Nile delta (Hennekam et al., 2015;Wu et al., 2016Wu et al., , 2017. ...
The recurrent deposition of organic-rich sapropel layers in the eastern Mediterranean Sea (EMS) has been attributed to deep-water stagnation and enhanced biological production. However, the underlying climatic interactions, paleoceanographic processes, and associated ventilation dynamics are still debated. Here, we present a basin-wide reconstruction of circulation systematics during sapropel S1 formation (∼10.8–6.1 kyr BP), using the Nd isotope composition (εNd) as paleo-seawater tracer. Our εNd data from fish debris and foraminifera tests are remarkably radiogenic compared to today, and spatially and temporally constant. These results predominantly reflect enhanced Nile versus Atlantic contributions, and indicate that EMS deep-water stagnation prevailed below ∼800 m water-depth during S1 formation. Additional εNd records obtained from bulk sediment leachates show that such stagnation was preconditioned thousand(s) of years prior to S1 initiation by intensified Nile flooding, whereas it terminated with a basin-wide, deep-water renewal. In addition, decoupling of deep waters between the EMS and western Mediterranean Sea (WMS) is evident for the S1-period. Using a box-model for Nd in the EMS, the observed εNd distribution can be most adequately explained by a 2-fold increase in Nile discharge, and a 50% decrease in the EMS–WMS exchange that was mostly limited to the surface waters alone. The corresponding circulation during S1 was more sluggish and shallow for the EMS, but largely unaffected for the WMS. This implies that deep-water stagnation is a prerequisite for sapropel formation.
... Several studies on varied marine sedimentary records show that they are key proxies for reconstructing Holocene climate variability (e.g., Rossignol-Strick and Planchais, 1989;Schilman et al., 2001a;Hennekam et al., 2014Hennekam et al., , 2015Mojtahid et al., 2015). In these studies, whatever methodological approaches are used, the reliable reconstructions depend on sedimentation rates at different sites, accurate and sufficient radiometric datings in addition to the application of high resolution analyses. ...
... The marine sedimentary records from the SE Mediterranean slope show that during the mid-and late Holocene, climate was variable, changing to relatively drier conditions, with alternating episodes between wetter and drier conditions (e.g., Schilman et al., 2001a;Hennekam et al., 2014Hennekam et al., , 2015. This trend is also documented in the central and eastern Mediterranean regions by various independent terrestrial proxies such as speleothem record and Dead Sea levels (e.g., Migowski et al., 2006;Bar-Matthews and Ayalon, 2011). ...
... The studied core V-4 is located at the middle shelf at 46.8 m water depth, 34.809042/32.434228 close to core PS009PC (32 07.7 0 N, 34 24.4 0 E; 552 m water depth) that was studied by Hennekam et al. (2014Hennekam et al. ( , 2015 and used as a reference in this study. Also shown by arrows is the northeast direction of sediment transport by the prevailing longshore current (LSC). ...
The River Nile catchment is considered the major source of nutrient-rich freshwater and sediment draining into the eastern Mediterranean Sea. Thus, exceptional high-resolution record from the Nile Littoral Cell likely traces changes in the Nile outflows related to climatic changes driven by the monsoonal system. This study used multi-proxy analyses combining sedimentological, geochemical and organic stable isotope data along with foraminiferal data in a southeastern Levantine inner shelf sedi-mentary sequence to understand Mid-to Late Holocene northeast African climate variability. The palynological record is used to reconstruct local scale changes in the regional vegetation related to the Mediterranean climate system. The analyzed records reveal multi-decadal to centennial pacing of paleoceanographic and paleoclimatic changes, with most prominent events recorded at 4.28, ~3.6 and at 2.9 kyr BP. These transitional periods characterized by simultaneous high d 15 N values and low total organic carbon (TOC) suggest drier climatic conditions, decrease in Nile discharge, leading to lower nutrient supply. A stable and arid climate during Mid-Late Holocene (~4.5e3.6 cal kyr BP) associated with a weaker monsoon system as well as with a weaker Mediterranean climate system is indicated based on high and constant sedimentation rates (~400 cm kyr À1), along with relatively stable values and only occasional weak fluctuations documented in all of the proxies. This climatic pattern is punctuated by a peak in maximum aridity at 4.28 kyr BP evidenced by low TOC value and low arboreal pollen (AP) and Artemisia/Chenopodiaceae (A/C) pollen ratios and the sharp decline in the abundance of benthic fora-minifera Cribroelphidium poeyanum and Valvulineria bradyana. This change corresponds with the well-studied 4.2 kyr BP drought event in the Middle East, Mesopotamia and south Asia. During the late Holocene (~3.6e2.8 cal. Kyr BP) the climate became unstable characterized by shifts between relatively wetter and drier conditions as evident in considerable fluctuations in all proxies. Maximum positive excursions of TOC values, C/N ratio along with high dominance of opportunistic foraminiferal species (e.g., Ammonia tepida) indicate northeast African wetter conditions and increased Nile discharge at 3.5, 3.3, 3.2 and 3.0 kyr BP. The distinct and abrupt transition from a stable to an unstable climate regime around 3.6 kyr BP may have been influenced by the Thera volcanic eruption. Thus, the sedimentary record investigated in this study may provide evidence for the impacts of this well-known volcanic eruption not just on the climate regime in the Levant but also on the ancient Near East and Egypt.
... The age model for the top~30 kyr in core MS21PC is constrained by 6 previously published radiocarbon dates (Hennekam et al., 2015a), complemented by 3 new radiocarbon dates (Table 1, Fig. 2a). Radiocarbon analyses were done on 10 mg of mixed planktic foraminifer material at the Pozna n Radiocarbon Laboratory in Poland. ...
... The d 18 O ruber were measured as described in Hennekam et al. (2015a). For this study the existing dataset (~13 kyr interval; n ¼ 277) was extended to the base of the core (adding 318 samples). ...
... (b) Age-depth model including the minimal-maximum error margins (blue shading) and sedimentation rate through time of core MS21PC, both calculated with the Bacon program of Blaauw and Christen (2011). The d 18 O ruber values for the first 13 kyr (n ¼ 277) were published in Hennekam et al. (2015a). (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.) ...
Eastern Mediterranean sapropels S1 (~10.5-6.1 kyr BP) and S3 (~85.8-80.8 kyr BP) formed respectively under full interglacial and glacial inception conditions. Consequently, the environmental factors pre-conditioning and leading to sapropel formation (e.g., global sea level and monsoonal forcing) were different. These factors must have differently influenced processes such as marine productivity, water column processes, and related seafloor anoxia. Here we investigate these differences through an inter-disciplinary approach using dinoflagellate cyst and pollen/spore assemblages and sedimentary (redox-sensitive) trace-metal concentrations from a core in the central Nile delta area. Comparing S1 to S3, we demonstrate that (1) Nile discharge appears to be stronger during S3 than S1, as shown by d18O-residuals, higher ratio of pollen and spores, and the higher abundance of coastal dinocysts and freshwater paly-nomorphs, (2) Ba/Al, Corg , and dinocyst accumulation rates indicate that marine productivity was similar at least during the first phase of their deposition and started prior to the onset of both sapropels, (3) bottom water conditions were more reducing during S3, resulting in higher Mo/Al, S, and Corg/Ptot values, but preservation was high and similar for both sapropels, and (4) Sedimentary Mo-U covariation indicates that the depth of water-column ventilation during deposition of S3 was shallower than during S1 (~1000 m versus ~1800m, respectively). We attribute the observed differences to slightly enhanced precessional-forced monsoon intensity and potentially lower global sea level, resulting not only in increased North-African runoff , but also in reduced ventilation during S3 compared to S1.
... deMenocal et al., 2000) not only had a major influence on the settlement of Neolithic communities (e.g. Kuper and Kr€ opelin, 2006;Drake et al., 2011;Manning and Timpson, 2014;Timmermann and Friedrich, 2016), but also resulted in enhanced fluvial discharge to the surrounding ocean margins, such as the equatorial Atlantic (Schefub et al., 2005;Weldeab et al., 2007), western Sahara (Zühlsdorff et al., 2007;Tjallingii et al., 2008;Niedermeyer et al., 2010;Skonieczny et al., 2015), and northeastern Africa (Almogi-Labin et al., 2009;Hennekam et al., 2014Hennekam et al., , 2015Weldeab et al., 2014). Such enhanced freshwater influx to the eastern Mediterranean Sea (EMS) stimulated density stratification of the water column and was associated with higher productivity in the surface water. ...
... Planktonic foraminiferal d 18 O data can indicate freshwater input (e.g. Kallel et al., 1997;Rohling et al., 2002;Hennekam et al., 2014Hennekam et al., , 2015. In comparison with published results from other core-sites ( Fig. 1), we not only constrain the variability of detrital supplies into the central Mediterranean Sea, but also give new insights in the fluvial discharge from the North-African margin, with implications for the mechanism controlling the paleodrainage delivery. ...
... Substantially enhanced sedimentation rates during sapropel S1 occurred only for cores taken within/near the Nile delta (Hennekam et al., 2015). This indicates that during the AHP the offshore spread of Nile sediments was not larger than before or after. ...
Using elemental geochemistry, clay mineralogy, grain size end-member modeling, and planktonic foraminiferal δ¹⁸O, we characterize the provenance of central-Mediterranean sediments over the past 18 ka. The provenance is dust-dominated before and after the African Humid Period (AHP). By contrast, during the AHP (∼11–5 ka), largely concurrent with organic-rich sapropel S1 formation, it is predominantly riverine from North-African sources. Such fluvial supply is suggested to come from paleodrainage networks that were reactivated by intensified monsoon precipitation during the AHP. The supply is characterized by high Mg/Al and smectite contents, and has been accompanied by considerable freshwater influx, as indicated by the enhanced grain size and lighter foraminiferal δ¹⁸O. The clay-mineral assemblages in our core and in nearby cores correspond with a provenance from the Libyan-Tunisian margin, mainly via the paleo-river Irharhar. The inferred fluvial discharge is strongest during the late-AHP (∼8–5.5 ka), coinciding with reported enhanced fluvial dynamics and wettest conditions over western Libya and Tunisia/Algeria. This period is not only synchronous with the largest extension of open-water bodies in North Africa and lowest Saharan dust inputs, but also consistent with precipitation records of the West-African monsoon. Moreover, our records show a remarkable correspondence with that of a paleodrainage system towards the Atlantic West-African margin, inferring a common headwater region in the central Saharan mountains, and a similar climate mechanism. Taken together, we suggest a dominant control of North-African humid surfaces on the paleodrainage delivery, modulated by groundwater level, in response to the insolation-driven West-African monsoon precipitation.
... This has allowed us to explore the varied sedimentary archives of the Nile basin from source to sink and over a range of timescales. We can now consider the sediments throughout the entire river basin (Garzanti et al., 2015), the longer-term flood history over glacialeinterglacial cycles (Williams et al., 2015a), the more recent history of abrupt climate change, Holocene flooding, and channel dynamics , as well as the longterm pattern of flow, sediment yield and palaeoclimate recorded offshore (Hennekam et al., 2015;Revel et al., 2015). Fig. 1 provides a summary of the key depositional settings in the Nile basin and offshore that have been examined in the papers of this Special Issue. ...
... We can then attempt to relate the changing pattern of human occupation (Honegger and Williams, 2015;Vermeersch and Van Neer, 2015;Woodward et al., 2015) to our reconstructions of past environmental changes in the Nile Valley, including shifting river channels, encroachment of desert dunes, fluctuations in floods and droughts in the Nile Valley, and changes in precipitation and runoff in the adjacent Sahara (Kuper and Kr€ opelin, 2006;Manning and Timpson, 2014), Sinai, and the Eastern Desert of Egypt (Hamdan and Brook, 2015). Finally, we can examine the Nile sediment record in the delta region (Stanley and Warne, 1993;Krom et al., 2002;Stanley et al., 2003Hennekam et al., 2015; and that preserved in marine cores off the coast of Egypt and use these records to reconstruct past changes in Nile sediment discharge and sediment sources (Box et al., 2011;Blanchet et al., 2015;Revel et al., 2010Revel et al., , 2015. The key contributions of the twelve papers published here are now briefly reviewed under six broad themes. ...
... This study of strategically selected sedimentary records retrieved on the Nile Rosetta upper continental slope, thus provides new insights for our understanding of environmental changes in the Nile basin. Hennekam et al. (2015) take a somewhat different approach and have looked to infer changes in Holocene precipitation and vegetation in the Nile catchment area using multi-proxy analyses of Nile delta sediments. Importantly, they underline that the focus point of the Nile plume varied considerably, as shown by the oxygen isotopic composition of the planktonic foraminifer Globigerinoides ruber, which serves as a tracer of differences in Nile discharge along an eastewest delta transect. ...
... This has allowed us to explore the varied sedimentary archives of the Nile basin from source to sink and over a range of timescales. We can now consider the sediments throughout the entire river basin (Garzanti et al., 2015), the longer-term flood history over glacialeinterglacial cycles (Williams et al., 2015a), the more recent history of abrupt climate change, Holocene flooding, and channel dynamics , as well as the longterm pattern of flow, sediment yield and palaeoclimate recorded offshore (Hennekam et al., 2015;Revel et al., 2015). Fig. 1 provides a summary of the key depositional settings in the Nile basin and offshore that have been examined in the papers of this Special Issue. ...
... We can then attempt to relate the changing pattern of human occupation (Honegger and Williams, 2015;Vermeersch and Van Neer, 2015;Woodward et al., 2015) to our reconstructions of past environmental changes in the Nile Valley, including shifting river channels, encroachment of desert dunes, fluctuations in floods and droughts in the Nile Valley, and changes in precipitation and runoff in the adjacent Sahara (Kuper and Kr€ opelin, 2006;Manning and Timpson, 2014), Sinai, and the Eastern Desert of Egypt (Hamdan and Brook, 2015). Finally, we can examine the Nile sediment record in the delta region (Stanley and Warne, 1993;Krom et al., 2002;Stanley et al., 2003Hennekam et al., 2015; and that preserved in marine cores off the coast of Egypt and use these records to reconstruct past changes in Nile sediment discharge and sediment sources (Box et al., 2011;Blanchet et al., 2015;Revel et al., 2010Revel et al., , 2015. The key contributions of the twelve papers published here are now briefly reviewed under six broad themes. ...
... This study of strategically selected sedimentary records retrieved on the Nile Rosetta upper continental slope, thus provides new insights for our understanding of environmental changes in the Nile basin. Hennekam et al. (2015) take a somewhat different approach and have looked to infer changes in Holocene precipitation and vegetation in the Nile catchment area using multi-proxy analyses of Nile delta sediments. Importantly, they underline that the focus point of the Nile plume varied considerably, as shown by the oxygen isotopic composition of the planktonic foraminifer Globigerinoides ruber, which serves as a tracer of differences in Nile discharge along an eastewest delta transect. ...
We trace compositional changes of Nile sediments for 7400 km, from their sources in equatorial rift
highlands of Burundi and Rwanda to their sink in the Mediterranean Sea. All chemical and physical
controls on sediment petrography, mineralogy and geochemistry, including weathering, grain-size, hydraulic sorting, mechanical breakdown, anthropic impact, mixing and recycling are investigated in detail.
The Nile course is controlled along its entire length by the East AfricaneRed Sea Rift. In this anorogenic
setting, detritus is derived in various proportions from volcanicfields associated with tectonic extension
(Anorogenic Volcanic provenance) and from igneous, metamorphic and sedimentary rocks uplifted on
the rift shoulders or exposed on the craton (Continental Block provenance). The entire spectrum of such
detrital signatures is displayed in the Nile catchment. Volcaniclastic Atbara sand is generated by focused
erosion of the Ethiopian basaltic plateau in semiarid climate, whereas quartzose White Nile sand reflects
low erosion rates, extensive weathering and sediment trapping in lakes and swamps at equatorial to
subequatorial latitudes. In the main Nile, as in its main tributary the Blue Nile, suspended load is volcaniclastic, whereas feldspatho-quartzose bedload is derived largely from basement sources, withfine to
medium-grained eolian sand added along the lower course. Mixing of detrital populations with different
provenance and grain size is reflected in diverse violations of settling-equivalence relationships influvial
and deltaic sediments. Sediment delivery from Sudan has been cut off after closure of the Aswan High
Dam and accelerated erosion of deltaic cusps is leading to local formation of placer lags dominated by
ultradense FeeTieCr oxides, but mineralogical changes caused by man's radical modification offluvial
regimes have been minor so far. In beaches of Sinai, Gaza and Israel, the Nile volcaniclastic trace gets
progressively diluted by quartzose sand recycled from eolian coastal deposits and carbonaticlastic
detritus eroded from the Levant rift shoulder. Studying the compositional variability of modern sediments in big-river systems allows us to appreciate the richness of natural processes occurring in the vast
drainage basin, and provides us with a key to understand the information stored in sedimentary archives
and to reconstruct the evolution of the Earth's surface from the recent to the less recent past.
©
... According to the age model, the upper 7.3 m of core MD04-2726 span the last 10 kyr, covering the Holocene period ( Fig. IV.2a). Both the range (between 0.2 and 3.5 mm/yr) and the timing of changes in sedimentation rates are coherent with previous reports from nearby archives (Blanchet et al., 2013;Revel et al., 2014Revel et al., , 2015Hennekam et al., 2015). A large variability in radiocarbon ages (including several reversals) is observed in sections III-IV-V, where the highest sedimentation rates are computed (>1 mm/yr). ...
... In Mologni et al. (2020) we used a laminated sequence from the NDFS for track at centennial resolution the hydro-climatic changes occurred in the Ethiopian Highlands during the Early to the Middle Holocene. Similarly, several studies have based their paleoclimatic reconstructions from NDSF continuous sequences: based on the sedimentary provenance sources from Nd/Sr isotopic compositions (Bastian et al., 2017;Blanchet et al., 2015Blanchet et al., , 2013Box et al., 2011;Ducassou et al., 2009;Krom et al., 2002;Ménot et al., 2020;Revel et al., 2015), on precipitation rate estimation from organic chemistry (Castañeda et al., 2016) and on terrigenous input estimation based on major elemental distribution (Hennekam et al., 2015;Revel et al., 2010;Zhao et al., 2011). Some studies advanced the hypothesis of the role of vegetation spread in the Nile catchments as a modulation factor between erosion and sediment transfer processes and as a non-linear response of the environment in river discharges (Blanchet et al., 2014;Foucault and Stanley, 1989;Krom et al., 2002;Lézine et al., 2017). ...
During the last fourteen thousand years, East Africa was the locus of significant climatic and cultural changes, which highly affected landscape evolution and prehistoric ways of life. Enhanced monsoonal precipitations over the tropics, mainly driven by Earth’s precessional and eccentricity orbital parameters, were the primary causes of the African Humid Period (AHP; ~14 - ~6 ka). This humid period led to the transgression of continental lacustrine waterbodies, the increase of fluvial discharge and the spread of vegetation cover over the present-day desertic regions. The onset and the end of this wetter phase were not homogeneous across the continent, and the AHP was punctuated by centennial-scale episodes of hyper-aridity which varied in time and in space. This highly variable environmental scenario was the theatre of one of the most important socio-economic changes in human history, that of the transition between the last hunters-gatherers and the first food production societies. The concomitance of such cultural transformations and climatic disruptions raises the question of the role of changing environmental conditions on the distribution patterns of new socio-economic practices, namely herding.The Ethiopian Highlands (EH) represent a major receiver of monsoonal precipitations in East Africa. During the AHP, substantial variations of the monsoon regime affected the functioning of the hydro-systems originating in the EH. This included the Blue Nile River and Lake Abhe, the endorheic receptacle of the Awash River waters, and located in the northern East African Rift System (EARS). Additionally, the Nile valley and the EARS, constituted the main corridors for prehistoric human trajectories across the Horn of Africa. With the aim to better understand human-climate coevolution during this crucial period a multiscale and multidisciplinary approach is proposed.First, I decipher Holocene climatic variations from the functioning of the two main EH-originated hydro-systems by: a) tracking exceptional Blue Nile paleo-flood frequencies using a high resolution study of a laminated sequence from the Nile Deep Sea Fan deposits, and b) reconstructing the hydrological fluctuations of paleo-Lake Abhe (Ethiopia, Djibouti), from the study of wave-dominated clastic paleo-shoreline sequences. Second, I characterize the impact of hydro-climatic changes on littoral paleolandscape evolution along the Lake Abhe basin. And finally, I estimate the interactions between climatic induced landform dynamics and human occupations with respect to the main socio-economic shifts that occurred over the same lacustrine basin.This study improves our comprehension of the spatial-temporal patterns of Holocene hydro-climate in East Africa: a strong and stable monsoonal regime has been evidenced between ~11 and ~9 ka BP, followed by a stepwise ardification until the end of the AHP. Moreover, belated Mid-Holocene humid conditions in the Northern Rift suggest that the Congo Air Boundary had a significant role on rainfall controls in this area. Additionally, evaluating the interplay between climatic-, geomorphic- and tectonic-induced sedimentary transfer mechanisms over the investigated basins allowed us to localise some hydro-sedimentary processes decoupled from climatic forcing and to propose a diachronic palaeolandscape reconstitution of the Lake Abhe basin for this period.Early Holocene Lake Abhe waters and environments were strongly exploited by hunter-fisher-gatherers until the last lake drop around ~4.5 ka BP, when herding practices were integrated in this region. Associations between prehistoric settlements and Holocene hydrological variations demonstrate how humans were extremely reactive to fluctuating lake environments and how new socio-economic practices were often adapted to specific local to regional environmental trajectories.
... Marine sediment records from offshore the Nile delta and the Levantine Sea indicate that fluvial transport and Nile discharge were relatively low during the YD and increased more moderately and gradually after the return to full humid conditions of the AHP (Hamann et al., 2008;Hennekam et al., 2014Hennekam et al., , 2015Weldeab et al., 2014;Castañeda et al., 2016;Fig. 8). ...
... All records from the Nile catchment indicate decreasing precipitation after ca. 8.7-8.4 cal kyr BP and a corresponding decrease in Blue Nile water and sediment deposition (Revel et al., 2015;Hennekam et al., 2015). At the same time the influence of the White Nile catchment increased with rapid shifts in erosion and vegetation until 6.0 cal kyr BP (Blanchet et al., 2014; see also Castañeda et al., 2016;Fig. ...
A 12 m long sediment sequence was recovered from the eastern Dendi Crater lake, located on the central Ethiopian Plateau and in the region of the Blue Nile headwaters. 24 AMS radiocarbon dates from bulk organic carbon samples indicate that the sediment sequence spans the last ca. 12 cal kyr BP. Sedimentological and geochemical data from the sediment sequence that were combined with initial diatom information show only moderate change in precipitation and catchment runoff during that period, probably due to the elevated location of the study region in the Ethiopian highlands. Less humid conditions prevailed during the Younger Dryas (YD). After the return to full humid conditions of the African Humid Period (AHP), a ~2 m thick tephra layer, probably originating from an eruption of the Wenchi crater 12 km to the west of the lake, was deposited at 10.2 cal kyr BP. Subsequently, single thin horizons of high clastic matter imply that short spells of dry conditions and significantly increased rainfall, respectively, superimpose the generally humid conditions. The end of the AHP is rather gradual and precedes relatively stable and less humid conditions around 3.9 cal kyr BP. Subsequently, slightly increasing catchment runoff led to sediment redeposition, increasing nutrient supply, and highest trophic states in the lake until 1.5 cal kyr BP. A highly variable increase in clastic matter indicates fluctuating and increasing catchment runoff over the last 1500 years. The data from Lake Dendi show, in concert with other records from the Nile catchment and the Eastern Mediterranean Sea (EMS), that the Blue Nile discharge was relatively high between ca. 10.0 and 8.7 cal kyr BP. Subsequent aridification peaked with some regional differences between ca. 4.0 and 2.6 cal kyr BP. Higher discharge in the Blue Nile hydraulic regime after 2.6 cal kyr BP is probably triggered by more local increase in rainfall, which is tentatively caused by a change in the influence of the Indian Ocean monsoon.
... In contrast, suboxic bottom water conditions were identified in S1 sediment from core MD04-2722 recovered at 1780 mbsl water depth from the continental shelf south of Cyprus ( Tachikawa et al., 2015; Fig. 1). Sapropels in the Nile Fan were deposited at even shallower water depths (~ 500 to 1000 mbsl), but nevertheless show the absence of benthic foraminifera, together with evidence for enhanced Nile suspended sediment flux, pollen and spore transport ( Almogi-Labin et al., 2009;Box et al., 2011;Langgut et al., 2011;van Helmond et al., 2015;Hennekam et al., 2015). Hennekam et al. (2014 inferred redox state fluctuations in overlying waters from authigenic V enrichments in sapropel S1 from core PC009BC within the Nile Fan ( Fig. 1). ...
... Thus, Fig. 1. Map of the Eastern Mediterranean Sea showing the location of core sites mentioned in this work: cores 9509 and 9501 (this study), ODP core 967D ( Azrieli-Tal et al., 2014), MD042722 ( Tachikawa et al., 2015) and PC009BC ( Hennekam et al., 2015). depending on redox conditions, both oxic uptake by Mn oxides and the anoxic generation of thiomolybdate species can lead to isotopically light Mo in sapropels. ...
Redox conditions and the mechanisms of redox development are a critical aspect of Eastern Mediterranean sapropels, whose formation in oxygen-depleted waters is closely related to water column stratification at times of global sea level rise and insolation maxima. Sapropels in the Nile Fan formed at relatively shallow water depths under the influence of the monsoon-driven freshwater output from the River Nile. This work evaluates the redox evolution of Holocene sapropel S1 in VALPAMED cruise core MD9509, recovered at 880 mbsl in the NE Nile Fan, using a combination of geochemical element proxies, Fe speciation, Fe and Mo isotopes studies. The productivity and redox proxies (Ba/Al, Mo/Al, U/Al, V/Al, Sb/Al) show well-defined enrichments in the sapropel, but with a marked minimum at ca 8.2 ka indicative of reventilation corresponding to a well known global cooling event. Peak productivity and reducing signals occur close to the initiation of sapropel formation. The proxy signals in sapropel 9509 are stronger and of longer duration than those of a second sapropel S1, recovered at the same depth, but 380 km to the north (MD9501), supporting the notion (suggested in previous studies) of more reduced conditions in the Nile Fan. The MoEF vs. UEF enrichment factor variations in core 9509 infer a transition from open marine suboxic conditions in the enclosing non-sapropel sediments to anoxic non-sulphidic water column conditions in the sapropel. Correspondingly, the highly reactive Fe pool (FeHR) measured in Fe speciation studies is dominated by Fe(oxyhydr) oxide minerals in the background sediments, whereas pyrite (Fepy) becomes the dominant component of the FeHR pool in the sapropel. Maximum Fepy values in the sapropel coincide with peak productivity and reducing conditions, implying a clear link between trace element uptake, diagenetic bacterial sulphate reduction in anoxic porewater and Fe mobilization in the sapropel. Iron isotope compositions (δ56Fe) in the sapropel do not show any departure from primary (marine and detrital) source sediment values, and the absence of an Fe/Al vs. δ56Fe trend strongly argues against an Fe shuttle. Molybdenum isotopes, however, show marked non-conservative fractionation patterns. Background sediment δ98/95Mo values (0.2 to 0.7‰) are compatible with fractionation upon absorptive uptake by Fe (oxyhydr)oxides and pyrite. In contrast, minimum δ98/95Mo values exhibited at peak sapropel (reducing and pyrite producing) conditions are most closely modeled by Mo isotope fractionation during kinetically controlled conversion of aqueous molybdate to thiomolybdate species. The conservative Fe isotope behavior/Mo isotope fractionation minima in the sapropel may be a characteristic of organic-rich sediment diagenesis below an anoxic non-sulphidic water body, without the operation of a benthic Fe shuttle.
... However, only a few high-resolution studies on the detrital composition of marine sediments exist, and most of these are focused on the Nile-dominated region (e.g. Hamann et al., 2009;Revel et al., 2010Revel et al., , 2015Box et al., 2011;Hennekam et al., 2014Hennekam et al., , 2015. Very little is known about the supplies of numerous small rivers from other sources through time. ...
... Krom et al., 1999b;Brumsack, 1999, 2000;Box et al., 2011;Hennekam et al., 2014). The higher Ti/Al in sapropels for the western Niledelta (Revel et al., 2010; see also Hennekam et al., 2015) has been interpreted by Box et al. (2011) as overbank flood deposits from turbidity flows, which mainly reflect the magnitude of water flow. Box et al. (2011) also suggested that during S1 time the Nile sediment reaching distal locations within the Levantine Basin, was mainly from the White Nile, since the Sudd swamps no longer acted as a giant sediment trap (Williams et al., 2000). ...
Hydroclimate variability has exerted a fundamental control on the alternating deposition of organic-lean marl and organic-rich sapropel sediments in the eastern Mediterranean Sea (EMS). However, the exact mechanisms regarding the freshwater sources and related changes are still debated. Here, Sr and Nd isotopes and high-resolution elemental data are used to constrain different riverine and eolian supplies to the central Mediterranean over the past 9.8 ka. The detrital sediments in core CP10BC, taken at the margin of the Libyan shelf in the southwestern Ionian Sea, can be described by a three-endmember mixing system based on Sr and Nd isotopic compositions. The same systematics can also be deduced from Ti and K compositional variability. The endmembers comprise: Saharan Dust, Aegean/Nile, and Libyan Soil, representing the eolian supply from North Africa, the riverine inputs from the Aegean/Nile areas, as well as the riverine and shelf-derived fluxes from the Libyan-Tunisian margin, respectively. For the sapropel S1 period in particular, we find important detrital supplies from fossil river/wadi systems along the Libyan-Tunisian margin, activated by intensified African monsoon precipitation. Combining the temporal profiles with the consistent variability observed in the ⁸⁷Sr/⁸⁶Sr–1000/Sr diagram, such Libyan contribution has been most prominent during the uppermost period of sapropel S1 in core CP10BC. This observation is in agreement with hydroclimate reconstructions of northwestern Libya. Comparison of the Sr-Nd isotope data between core CP10BC and four cores taken along a west–east transect throughout the EMS shows that this detrital supply originated mainly from western Libya/Tunisia, and was transported as far eastward as ∼25°E while being diluted by an increasing Nile contribution.
... Cardiff University Ray Zammit silica standards. The following ratios were calculated to act as palaeoproxies: Ti/Al and Zr/Al for aeolian flux (Krom et al., 1999;Govin et al., 2012;Hennekam et al., 2015;Martín-Puertas et al., 2010;Martinez-Ruiz et al., 2015) , Sr/Ca for sea-level controlled export of shelf carbonate material (Bialik et al., 2020), K/Al for chemical weathering (Clift et al., 2014), and Si/Ti for biogenic silica production (Brown et al., 2007;Kylander et al., 2011 (Passier & de Lange, 1998;Wei et al., 2009;Filippidi & Lange, 2019). All X-ray fluorescence data are semi-quantitative. ...
The Miocene represents a key interval in the transition to our modern bipolar icehouse world. The early Miocene climate (23.0 Ma to 19.2 Ma) was typified by a cool-arid climate with large swings in global benthic δ18O records. This climate regime changed to a gradually warming trend (19.2 Ma to 17.0 Ma) until it was replaced by the relative warmth of the Miocene Climatic Optimum from 17.0 Ma to 14.7 Ma. Climate subsequently cooled in a step-wise manner, with a major expansion of the Antarctic ice sheet occurring at 13.8 Ma. In addition to these climatic shifts, a major change in oceanic circulation also occurred during the Miocene. This was due to the restriction of water flow through the Mesopotamian Seaway that connected the Mediterranean with the Indian Ocean. This flow restriction occurred in two steps, one during the early Miocene (19.0 to 20.2 Ma) and the second and complete disconnection between 13.8 Ma and 13.6 Ma. The Miocene deposits of Malta are temporally and spatially ideally situated to investigate local and global paleoenvironmental changes during the Miocene.
Lithological and geochemical changes from the Early Miocene il-Blata section outcropping in Malta indicate a regional shift from a cold arid climate to a warm humid climate following the first Miocene restriction of the Mesopotamian Seaway. A marked shift in sedimentation rate at ~19.1 Ma coincident with a change to organic-rich clays and changes in bulk CaCO3, Sr/Ca, K/Al, Ti/Al, Zr/Al and Si/Ti support this interpretation. The closure of the Mesopotamian Seaway and consequent diversion of warm intermediate waters into the Atlantic Ocean is a plausible mechanism for the intensification of the hydrological cycle over North Africa. This transition to a humid regime and associated influence of terrigenous sediment delivery likely helped in the termination of the extensive Early Miocene phosphorites of Malta.
Planktic and benthic foraminiferal geochemical records from the Ras il-Pellegrin section (Malta) indicate changes in regional hydroclimate following the Middle Miocene expansion of the Antarctic ice sheet at 13.82 Ma. Foraminiferal Mg/Ca was used to generate temperature records, which enabled the generation of surface and bottom seawater δ18O, and in turn an estimate of sea surface salinity variability. These records show that a significant input of freshwater occurred following the Middle Miocene expansion of the Antarctic icesheet and the closure of the Mesopotamian Seaway. The input of freshwater is precession and obliquity driven and indicates that a strong seasonal, monsoonal type climate was emplaced over North Africa.
A new planktic foraminiferal B/Ca record was used with previously published δ13C records from the composite Marsalforn (Gozo Island) and Ras il-Pellegrin sections to investigate carbon cycle dynamics in the Central Mediterranean from 14.4 Ma to 12.8 Ma. The vertical seawater δ13C gradient coupled with B/Ca, Mn/Ca, and calculated [B(OH)]4^-]/[DIC], indicates a highly active biological pump following the Antarctic ice sheet expansion at 13.82 Ma and coinciding with the global CM-6 δ13C excursion. These records also cast doubt on the interpretation of the pCO2 records from Malta published in Badger et al. (2013), and suggests that low latitude shallow water zones are particularly important in carbon cycle dynamics as zones of enhanced carbon burial.
Overall, this thesis highlights the importance of the tectonic closure of the Mesopotamian Seaway on regional climate, with possible effects on global climate and cryospheric feedbacks. It confirms the Maltese Islands as a natural laboratory for investigating the Miocene climate. This thesis also underlines the importance of considering the Miocene as a geological time suitable for investigating global environments and climate feedbacks at near-future pCO2 levels.
https://orca.cardiff.ac.uk/id/eprint/160007/
... BP. Nevertheless, Hennekam et al. (2015) observed that the ratios exhibit reduced values in the east Nile Delta. In several African lakes, the African Humid Period (AHP) has been documented between 12 and 5 ka cal. ...
The geochemical and mineralogical investigations conducted on a core retrieved from a paleolake in Faiyum Depression, Egypt, reveal a variation in two precipitation patterns; these are the Ethiopian African Monsoon (EAM) at the Nile Headwaters and the Mediterranean winter rains. The heavy mineral assemblage, and the geochemical characteristics, exhibit significant shifts in clastic input to the paleolake. These variations suggest shifting in the sources of sediment, with an East African origin during the African Humid Period (AHP) and in the Late Holocene, and a White Nile source during the Late Pleistocene and Middle Holocene. Geochemical data is additionally utilized in the determination of variations in the properties of the paleo-water (paleo-salinity and paleo-productivity). The paleo-water of Faiyum Lake exhibited characteristics of a substantial freshwater body during the Early to Mid-Holocene. During the subsequent period, there was a sudden drop in the lake level, accompanied by a subsequent rise in salinity due to hydrological modifications.
... The isotopic composition of active dust sources in the ESPSA (ε Nd ∼ −1.1, 87 Sr / 86 Sr ∼ 0.7059; Kunkelova et al., 2022; Fig. 6) shows the typical signature of Blue Nile sediments (Palchan et al., 2013). Blue Nile sediments are also characterized by high smectite concentrations derived from weathering of the basaltic material (Hamann et al., 2009;Revel et al., 2014;Ehrmann et al., 2016) and by high Ti contents (Revel et al., 2010;Hennekam et al., 2015). Thus, sediments from the ESPSA can be identified in the central Red Sea by their radiogenic ε Nd and nonradiogenic 87 Sr / 86 Sr signatures, high smectite contents, and high Ti / terr ratios (Figs. ...
Climatic and associated hydrological changes controlled the transport processes and composition of the sediments in the central Red Sea during the last ca. 200 kyr. Three different source areas for mineral dust are identified. The dominant source is located in the eastern Sahara (Sudan and southernmost Egypt). We identify its imprint on Red Sea sediments by high smectite and Ti contents, low 87Sr / 86Sr, and high εNd. The availability of deflatable sediments was controlled by the intensity of tropical rainfall and vegetation cover over North Africa linked to the African monsoon. Intense dust input to the Red Sea occurred during arid phases, and low input occurred during humid phases. A second, less significant source indicated by palygorskite input is probably located on the eastern Arabian Peninsula and/or Mesopotamia, while the presence of kaolinite suggests an additional minor dust source in northern Egypt. Our grain size data reflect episodes of fluvial sediment discharge to the central Red Sea and document the variable strength in response to all of the precession-paced insolation maxima during our study interval including both those that were strong enough to trigger sapropel formation in the eastern Mediterranean Sea and those that were not. The African humid period most strongly expressed in our Red Sea record was the one during the Eemian last interglacial at ca. 125 ka (when the Baraka River was far more active than today), followed by those at 198, 108, 84, and 6 ka.
... Looking in more details at the patterns of three key calcareous nannofossil taxa, which are shown individually in Fig. 6, a visual correlation with insolation/precession is clearly observed. As expected, this is evident for the pattern of F. profunda, as a result of its well-known relation with stratified surface waters and DCM development (Rohling and Gieskes, 1989;Castradori, 1992;Kemp et al., 1999;Negri et al., 1999;Negri and Giunta, 2001;Corselli et al., 2002;Thomson et al., 2004;Triantaphyllou et al., 2010;Incarbona et al., 2011;Grelaud et al., 2012;Incarbona and Di Stefano, 2019;Maiorano et al., 2019), occurring during precession minima/insolation maxima (Hilgen, 1991a;Lourens et al., 1996) and due to intensified monsoon freshwater release especially via the Nile River (Rossignol-Strick et al., 1982;Rohling et al., 2002;Marino et al., 2009;Weldeab et al., 2014;Hennekam et al., 2015). Interestingly, new evidence is here observed for H. sellii and C. pelagicus ssp. ...
We present the first high-resolution results on planktonic foraminiferal stable oxygen isotopes and calcareous plankton assemblages at the Monte San Nicola GSSP section (near Gela, Sicily), the type-section for the Lower Pleistocene Gelasian Stage. The oxygen isotope profile is remarkably similar to the Eastern Mediterranean oxygen isotope record and indicates that the studied section, extending from marine isotope stage (MIS) G4 to MIS 103, clearly records glacial-interglacial variability. Cyclic changes of calcareous plankton assemblages (calcareous nannofossils and foraminifera) indicate that warmer and stratified surface waters occurred during interglacials, while cooler and unstable conditions developed during glacial phases. Signatures of further increase in surface water stratification are also captured by our surface water proxies and are coeval with enhanced monsoon run-off, developed during precession minima/insolation maxima. The surface water changes recorded at the Monte San Nicola section are in phase with North Atlantic climate variability, even at suborbital scale, and
reveal evidence of the first significant southward migration of the Subarctic Front in the mid-latitudes during MIS 104, slightly below the GSSP. The overall dataset provides precise alignment of the Gelasian GSSP within MIS 103 as well as new climatostratigraphic constraints, close to the GSSP, thus improving its correlation potential outside the type-area.
... The current era of accelerated change makes it essential that designers find ways of initiating cultural adaptation through actions that help humans live with the trouble. 1 Ribonucleic acid (RNA) carries short, coded messages into or within cells, while deoxyribonucleic acid (DNA) is much larger and acts as the primary genetic code stored in the nucleus and mitochondria of a cell. 2 The Epic of Gilgamesh is an example of this, and as the oldest written narrative known to archaeologists it's likely that the story was told as an oral narrative for centuries before it was written in clay tablets. 3 See for example : Hennekam, Donders, Zwiep, Lange, 2015, andSun, Wang, Zhang, Chen, Lu, Liu, 2021. 4 See for example: Rost ,2019. 5 For example: Wiggermann, 2011. ...
Haraway and others have suggested reciprocity with the non-human world is a pathway to un- derstanding our humanness. Two urgent trends accelerate our need for this reciprocity: the first is the COVID-19 pandemic as a harbinger of future pandemics, and the second is our changing planetary climate. Our present time is increasingly becoming a “present-future,” linked irreversibly by scientific models to specific future states of our planet and local regions. At the same time our bodies are co-evolving with a virus in a global reciprocal process with no end in sight, collapsing our sense of scale and separation among bodies. A long view of time in the past could act as a counterbalance to this experience. Bringing the longue durée model of time into our present requires reestablishing our knowledge of a long-term past in which humans adapted to major changes in climate earlier in the Holocene. Forms of future urban adaptation can embody reci- procity by emphasizing strategies that anticipate change rather than seeking to prevent it, leap- ing forward in time to embrace global changes we are no longer able to prevent.
... The instrument was standardised using in-house carbonate and silica standards. The following ratios were calculated to act as paleoproxies: Ti/Al and Zr/Al for aeolian flux (Krom et al., 1999;Govin et al., 2012;Hennekam et al., 2015;Martín-Puertas et al., 2010;Martinez-Ruiz et al., 2015) , Sr/Ca for sea-level controlled export of shelf carbonate material (Bialik et al., 2020), K/Al for chemical weathering (Clift et al., 2014), and Si/Ti for biogenic silica production (Brown et al., 2007;Kylander et al., 2011). The bulk [Ca] was used to indicate the marine carbonate component while the bulk [S] indicates redox conditions (Passier & de Lange, 1998;Wei et al., 2009;Filippidi & Lange, 2019). ...
During the Miocene (23.0–5.3 Ma) North Africa experienced both humid and arid intervals, but the underlying cause of these transitions is unknown. Earth's climate was characterized by a unipolar icehouse with a dynamic Antarctic ice sheet, which may have influenced regional hydrology through atmospheric teleconnections. However, the Miocene also witnessed the restriction of the Mesopotamian Seaway, which may have had significant climatic impacts. The Maltese il‐Blata section (Central Mediterranean) comprises Late Oligocene to Early Miocene marine deposits previously used to constrain the timing of the Mesopotamian Seaway restriction using the εNd tracer. The location of this section also makes it sensitive to climatic changes in the North African region, and biogeochemical changes in the central Mediterranean. Here, we present lithological and geochemical records of the il‐Blata section. We find a marked shift in lithology and an increase in sedimentation rate coeval with the Early Miocene (∼19–20 Ma) restriction of the Mesopotamian Seaway. Concomitant changes in bulk sediment CaCO3, Sr/Ca, K/Al, Ti/Al, Zr/Al, and Si/Ti support a major humid climate transition and associated intensification of river systems over western North Africa. We propose that these changes in North African hydroclimate reflect either a tipping point effect in a gradually warming global climate, or are the result of the initial restriction of the Mesopotamian Seaway, perhaps through consequent changes in Atlantic Meridional Overturning Circulation and the West African Monsoon. We also suggest the restriction of the Mesopotamian Seaway inhibited phosphorite deposition at low latitudes.
... The EBF and MF have complex vertical structures (mainly in the eastern part of the PRB), and EBF and MF have deeper roots and stronger water storage capacity (Wang et al., 2014;Tania and Irma, 2020;Kilic, 2021), so the nonlinear relationship was relatively larger in the eastern part of the PRB. In addition, some studies have proved that climate change (such as changes in precipitation and temperature) and human activities (such as irrigation) are also important factors affecting the relationship between meteorological drought and vegetation response (Fang et al., 2005;Hennekam et al., 2016;Papagiannopoulou et al., 2017;Ding et al., 2021), which can be further explored in future studies. From subsections 4.2, and 4.3, the VRTs that were determined based on the NDVI and SIF were different, and the VRT determined based on SIF was shorter than that based on the NDVI, which might be caused by the different monitoring principles of the NDVI and SIF. ...
The frequent occurrence of drought events in recent years has caused significant changes in plant biodiversity. Understanding vegetation dynamics and their responses to climate change is of great significance to reveal the behaviour mechanism of terrestrial ecosystems. In this study, NDVI and SIF were used to evaluate the dynamic changes of vegetation in the Pearl River Basin (PRB). The relationship between vegetation and meteorological drought in the PRB was evaluated from both linear and nonlinear perspectives, and the difference of vegetation response to meteorological drought in different land types was revealed. Cross wavelet analysis was used to explore the teleconnection factors (e.g., large-scale climate patterns and solar activity) that may affect the relationship between meteorological drought and vegetation dynamics. The results show that 1) from 2001 to 2019, the vegetation cover and photosynthetic capacity of the PRB both showed increasing trends, with changing rates of 0.055/10a and 0.036/10a, respectively; 2) compared with NDVI, the relationship between SIF and meteorological drought was closer; 3) the vegetation response time (VRT) obtained based on NDVI was mainly 4–5 months, which was slightly longer than that based on SIF (mainly 3–4 months); 4) the VRT of woody vegetation (mainly 3–4 months) was longer than that of herbaceous vegetation (mainly 4–5 months); and 5) vegetation had significant positive correlations with the El Niño Southern Oscillation (ENSO) and sunspots but a significant negative correlation with the Pacific Decadal Oscillation (PDO). Compared with sunspots, the ENSO and the PDO were more closely related to the response relationship between meteorological drought and vegetation. The outcomes of this study can help reveal the relationship between vegetation dynamics and climate change under the background of global warming and provide a new perspective for studying the relationship between drought and vegetation.
... Sapropel formation has been attributed to stagnating deep-water conditions and/or enhanced biological production, induced by increased rainfall and river runoff (e.g. De Lange et al., 2008;Hennekam et al., 2015;Filippidi et al., 2016;Wu et al., 2019;Benkovitz et al., 2020). ...
In marine sediments, the Sr content and isotope composition (⁸⁷Sr/⁸⁶Sr) of the terrigenous detrital component are widely used to track changes in provenance and related transport and weathering processes. Accurately separating detrital-Sr from other sedimentary Sr-phases is a prerequisite for such studies. Conventionally, it is assumed that Sr in the carbonate-free residue corresponds to detrital Sr alone. However, the decarbonated residue may contain barite with significant Sr content and a non-detrital ⁸⁷Sr/⁸⁶Sr composition; this may substantially affect the measured Sr signal. To examine this chronically overlooked phenomenon, the Mediterranean Sea is an ideal area because 1) detailed provenance studies have been done using Sr and ⁸⁷Sr/⁸⁶Sr of the residual fraction, and 2) enhanced levels of barite repeatedly occurred in association with distinct, organic-rich sapropel sediments.
Here, we use the most-recent sapropel S1 interval to evaluate the effect of barite-bound Sr in the residual fraction after decarbonation. A total of 130 samples were taken from 10 cores in the eastern Mediterranean Sea (EMS) and 1 core in the western Mediterranean Sea. This selection represents a geographic and bathymetric coverage of the EMS and permits the basin-wide comparison between organic-rich and -lean sediments. After decarbonation using 1 M HCl solution, the residual sediments were subject to NH4Cl extraction (2 M, pH 7), known to selectively dissolve barite. Our results demonstrate the presence of Sr-bearing barite after traditional carbonate removal and its effect on the derived “detrital” Sr signature. This barite-Sr effect is considerable for samples with barite-Ba >400 μg/g in bulk sediment. The impact of barite is prominent if accompanied by a detrital provenance background of high ⁸⁷Sr/⁸⁶Sr (>0.713) or low Sr/Al (<1.0 mg/g). In such cases, removal of remaining barite is required to obtain an unbiased detrital Sr signal. We recommend an improved procedure for detrital Sr separation in marine sediments, with an additional NH4Cl leaching step to eliminate any remaining barite after decarbonation. This approach is particularly important for areas/times of high biological productivity, where sediments are often characterized by abundant barite content.
... Sediment deposition in deltas is usually dominated by the export of terrigenous material delivered from flooded rivers, highly sensitive to changes in precipitation rates and land cover in corresponding drainage basins (Macklin et al., 2012). The sediment records preserved at the Nile Deep-Sea Fan (NDSF) provide suitable archives for reconstructing past climate variations at a high temporal resolution (100e1000 years) in north-East Africa (Almogi-Labin et al., 2009;Bastian et al., 2017;Blanchet et al., 2013;Costa et al., 2014;Hamann et al., 2009;Hennekam et al., 2015;Mologni et al., 2020;Revel et al., 2015;Weldeab et al., 2014). Currently, about 95% of the terrigenous material deposited at the Nile deepsea fan is derived from the Ethiopian Highlands (Garzanti et al., 2015;Padoan et al., 2011). ...
We have investigated provenance and weathering proxies of the clay-size sediment exported from the Nile River basin over the last 110,000 years. Using neodymium isotope composition of sediments from both the Nile Deep Sea-Fan and Lake Tana, we show that the Nile River branches draining the Ethiopian Highlands have remained the main contributors of clays to the Nile delta during the Late Quaternary. We demonstrate that fluctuations of clay-size particle contribution to the Nile Delta are mainly driven by orbital precession cycle, which controls summer insolation and consequently the African monsoon intensity changes. Our results indicate that - over the last 110,000 years – the proportion of clays coming from Ethiopian Traps fluctuates accordingly to the intensity of the last 5 precession cycles (MIS 5 to MIS 1). However, there is a threshold effect in the transport efficiency during the lowest insolation minima (arid periods), in particular during the MIS3. Several arid events corresponding to the Heinrich Stadial periods are associated with small or negligible clay source changes while chemical weathering proxies, such as δ7Li, Mg/Ti and K/Ti, vary significantly. This suggests a straightforward control of weathering by hydro-climate changes over centennial to millennial timescales. Our data also suggests a significant but more progressive influence of the temperature decrease between 110kyr and 20kyr. Taken altogether, the observed tight coupling between past climate variations and silicate weathering proxies leads us to conclude that precipitation changes in northeast Africa can impact soil development over a few hundred years only, while the influence of temperature appears more gradual.
... Thus, we invoke a dominant water column source for Fe enrichments, which could potentially arise in a variety of ways, including from dissolved Fe(II) released in deeper anoxic waters, followed by oxidation and sedimentation under dysoxic conditions. Alternatively, Fe enrichments may also occur directly following release of Fe(II) from sediments under dysoxic OMZ-like conditions (Scholz, 2018), or the source of Fe could have been from shallower settings, since Zirks et al. (2019) noted that anoxic conditions had already developed at a depth of $550 m (core PS009PC; Hennekam et al., 2015). Whatever the source of Fe, the increased sulfidation of Fe phases (Fig. 6) was likely due to diagenetic pyrite formation, since throughout the core, an increased prevalence of sulfidized Fe minerals (e.g., pyrite) occurs at the expense of crystalline Fe (oxyhydr)oxides (Fig. 5). ...
Oxygen Minimum Zones (OMZs) are expanding in modern oceans due to anthropogenically-driven climate and environmental change. In the Eastern Mediterranean Sea (EMS), OMZs developed in the early Holocene as a result of decreased intermediate water ventilation, increasing temperature, and increased Nile discharge and primary productivity. Here, we report benthic foraminiferal numbers (BFN) and species abundances, together with redox-sensitive trace metals (RSTM), and iron and phosphorus speciation from two sediment cores sampled at intermediate depths (1200 and 1430 m) from the SE Levantine shelf. The main aim of our study is to better understand the sequence of redox changes during sapropel S1 deposition caused by biogeochemical processes affecting the sapropel intermediate water mass. The use of benthic foraminifera indices (diversity and oxygen) together with iron speciation and RSTM (V, Mo and U) enables detailed description of the changing oxygen/redox status of the overlying water. Prior to sapropel S1 deposition at ∼10.2 ka BP, RSTM suggest that the overlying water was well oxygenated, but benthic foraminifera numbers (BFN) suggest that oxygen levels had already begun to decrease. There was then a pulse of increased export carbon from the enlarged Nile flood plume, as shown by increased BFN at the beginning of sapropel S1. Shortly after, RSTM and Fe-S systematics suggest that the water column transitioned from dysoxic to anoxic, non-sulfidic. Anoxic conditions then persisted at 1200 m depth, but RSTM and benthic foraminifera indices suggest that deeper waters at 1430 m were more likely dysoxic, until the 8.2 ka BP global cooling event. The benthic foraminifera and inorganic redox proxies then suggest a second period of anoxic, non-sulfidic conditions, with a gradual return to well ventilated waters at the end of sapropel deposition at ∼6 ka BP. There was enhanced burial of authigenic P throughout sapropel deposition, derived from the deposition and subsequent release of organic-P and iron bound-P during diagenesis. Phosphorus recycling from the sediment and in the overlying water column added reactive P to these mid-depth waters, a process which has the potential to result in a positive feedback in systems where such waters are upwelled into the photic zone. The past EMS thus represents a template which can be used to predict biogeochemical changes in settings that evolve towards anoxic, non sulfidic conditions, which may occur in some areas as modern climate and environment change causes the continued expansion of modern OMZs and hypoxic areas adjacent to modern major rivers.
... More detailed stratigraphic constrains were obtained from correlation of the titanium over calcium (Ti/Ca) records 139 of cores P73 and P99 with core P33 (Fig. 2). It was shown that changes in sedimentation rates are coherent on the 140 western Nile DSF (Hennekam et al., 2015) and Ti/Ca records show similar patterns (Fig. S1). Six tie-points (marked 141 T1-T6 on Fig. 2) mark changes in Ti/Ca records identified in the upper parts of the cores and were used to further 142 synchronise the records of P73 and P99 with core P33 (Table 2). ...
Ocean deoxygenation is a rising threat to marine ecosystems and food resources under present climate warming conditions. Organic-rich sapropel layers deposited in the Mediterranean Sea provide a natural laboratory to study the processes that have controlled the changes in seawater oxygen levels in the recent geological past. Our study is based on three sediment cores spanning the last 10 thousand years (10 kyr BP) and located on a bathymetric transect offshore the western distributaries of the Nile delta. These cores are partly to continuously laminated in the sections recording sapropel S1, which is indicative of bottom-water anoxia above the western Nile deep-sea fan. We used a combination of microfacies analyses and inorganic and organic geochemical measurements to reconstruct changes in oxygenation conditions at seasonal to millennial time-scales. The regular alternations of detrital, biogenic and chemogenic sublayers in the laminated sequences are interpreted in terms of seasonal changes. Our microfacies analyses reveal distinct summer floods and subsequent plankton blooms preceding the deposition of inorganic carbonates formed in the water-column during spring-early summer. The isotopic signature of these carbonates suggests year-round anoxic to euxinic bottom waters resulting in high levels of anaerobic remineralisation of organic matter and highlights their potential to reconstruct seawater chemistry at times when benthic fauna was absent. Synchronous changes in terrigenous input, primary productivity and past oxygenation dynamics on millennial time-scales obtained by our multi-proxy study show that runoff-driven eutrophication played a central role in driving rapid changes in oxygenation state of the entire Levantine Basin. Rapid fluctuations of oxygenation conditions in the upper 700 m water depth occurred above the Nile deep-sea fan between 10 and 6.5 ka BP while deeper cores recorded more stable anoxic conditions. These findings are further supported by other regional records and reveal time-transgressive changes in oxygenation state driven by rapid changes in primary productivity during a period of long-term deep-water stagnation.
... According to the age model, the upper 7.3 m of core MD04-2726 span the last 10 kyr, covering the Holocene period (Fig. 2a). Both the range (between 0.2 and 3.5 mm/yr) and the timing of changes in sedimentation rates are coherent with previous reports from nearby archives (Blanchet et al., 2013;Revel et al., 2014Revel et al., , 2015Hennekam et al., 2015). A large variability in radiocarbon ages (including several reversals) is observed in sections III-IV-V, where the highest sedimentation rates are computed (>1 mm/yr). ...
Climate conditions in Africa have varied substantially during the Late Quaternary with alternating humid and arid periods controlled mainly by the African monsoonal regime. However, the duration and termination of the last African Humid Period (14-6 ka BP) and its internal climatic variability are still debated. Using a laminated sequence from the Nile Deep-Sea Fan, we reconstruct for the first time the monsoon-induced frequency of exceptional Nile floods at centennial resolution during the African Humid Period. By combining sedimentological, geochemical and microscopic tools and comparing our record with two proximal piston cores and with regional paleoclimatic records, we show: a) the occurrence of recurrent high-energy floods between 10.1 and 9 ka BP, during the height of the African Humid Period; b) a shift in the hydro-climatic regime as early as 9 ka BP, with a progressive reduction in flood frequency and magnitude until 8.2 ka BP, likely related to a southward migration of the monsoon rainfall belt; c) a drastic reduction of flood activity between 8.2 and 7.8 ka BP; d) an unstable Ethiopian-Nile hydrological system from 7.8 ka BP, followed by a further decrease in river runoff until~4 ka BP. The occurrence of a stepwise hydro-climatic deterioration over the Ethiopian Highlands from~9 ka BP brings into question the climatic linkages and feedbacks between low and high latitudes during the Early to Mid-Holocene and in particular around the 8.2 ka BP North Atlantic cooling event. Our unique record of flood frequency at centennial-resolution therefore allows us to draw new insights on fluvial and geomorphic feedbacks of the Nile hydrologic system to monsoonal regimes during a period of major environmental shifts.
... Sub-oxic bottom water conditions were identified in S1 sediment recovered at 1780 mbsl water depth in the EMS eastern Levantine basin, (Tachikawa et al., 2015). S1 sapropels deposited in the Nile Fan at sub-oxic to anoxic conditions show absence of benthic foraminifera, together with evidence for enhanced Nile suspended sediment flux, pollen and spore transport (Almogi-Labin et al., 2009;Box et al., 2011;Langgut et al, 2011;Revel et al., 2014;Hennekam et al., 2015;van Helmond et al., 2015;Matthews et al., 2017). Euxinic (sulphidic) water column conditions are less commonly identified for the S1 event. ...
Sapropels S5 and S7 formed in the semi-enclosed Eastern Mediterranean Sea (EMS) during peak interglacial periods MIS5e and MIS7a, respectively, are considered among the most strongly developed Quaternary sapropels. This study investigates the redox dynamics of the water column during their formation, via Fe isotope and Fe speciation studies of cores taken at 2550 m depth at site ODP-967. Both sapropels show an inverse correlation between δ⁵⁶Fe and FeT/Al, with slopes mostly matching that found for the Black Sea, pointing to a benthic shelf to basin shuttle of Fe and subsequent precipitation of Fe sulphides in euxinic bottom waters. An exception to these Black Sea-type trends occurs during the later, peak, stages of S7, where the negative δ⁵⁶Fe - FeT/Al slope shallows. Fe speciation studies reveal that the dominant highly reactive Fe phase (FeHR) in the sapropels is pyrite, with Fe (oxyhydr)oxides forming the second major mineral component. Correspondingly, FeHR/FeT plots show increased strengthening of anoxic water conditions during the passage from pre-sapropel sediment into the sapropel. Nevertheless, despite the evidence for euxinic conditions from both Fe isotopes and high Mo concentrations in the sapropel, Fepy/FeHR ratios remain below values commonly used to identify water column euxinia. This apparent contradiction is ascribed to the sedimentary preservation of a high flux of crystalline Fe (oxyhydr)oxide minerals to the basin, which resulted in a relatively low degree of sulphidation, despite the presence of euxinic bottom waters. Thus, the operationally defined ferruginous/euxinic boundary for EMS sapropels is better placed at Fepy/FeHR = 0.6, which is somewhat below the usually ascribed lower limit of 0.7. Consistent with the significant presence of crystalline Fe (oxyhydr)oxides, the change in the δ⁵⁶Fe - FeT/Al slope during peak S7 is ascribed to an enhanced monsoon-driven flux of detrital Fe(III) oxides from the River Nile into the EMS basin and comcomitant diagenetic sulphidation. Euxinic water column conditions in sapropel S5 and S7 are interpreted here to reflect the positive balance between dissolved sulphide formation and rates of reductive dissolution of Fe (oxyhydr)oxide minerals. Both of these parameters in turn depend on the extent to which water overturn times are reduced and export productivity increased during sapropel formation.
... The fine-grained sediments composing the studied core (i.e., >90% of clay and silt; Hennekam & de Lange, 2012), before the commissioning of the Aswan Dam, are mainly sourced from the Nile River (Krom et al., 1999). The large influence of Nile River discharge results in high sedimentation rates in this region, particularly throughout the Holocene (Almogi-Labin et al., 2009;Box et al., 2011;Castañeda et al., 2010;Hamann et al., 2009;Hennekam et al., 2015;van Helmond et al., 2015). In our study core, the sedimentation rate is constant from 16 ka to the mid-Holocene (12 cm/ka) and increases during the Late Holocene from 12 to 100 cm/ka (Hennekam et al., 2014). ...
We present a high‐resolution study covering the past 19 ka from the southeastern Mediterranean Sea, based on benthic foraminiferal faunas and their stable oxygen and carbon isotopes. These data are integrated with previously published and newly acquired planktic foraminiferal data from the same sediment core in order to investigate the benthic‐planktic coupling and its response to past suborbital climate variability. On a millennial timescale, foraminiferal communities and their isotopic signatures vary following three main time periods (late glacial, sapropel S1 [~10.1–6.5 ka], and mid‐Holocene to Late Holocene). Within these intervals, we identified short‐timescale changes related to the carbon export and hydrological conditions. During the deglaciation, and except for the Younger Dryas, the coupled benthic‐planktic data indicate an overall poorly mixed water column with a low productivity. During S1 event, our data confirm the presence of a highly stratified water column, with enhanced primary productivity export to the deep sea, being associated to high Nile River activity. While the foraminiferal ecosystem is strongly driven by the combined influence of overturning circulation and the Nile River activity until the end of the African humid period, we suggest a more regional eastern Mediterranean climatic‐driven response of the foraminiferal community over the mid‐Holocene to Late Holocene. A strong multicentennial variability, probably associated to solar forcing, was found for both benthic and planktic records, and a supplementary 1,600‐year mode was found for the benthic data, suggesting a potential overturning circulation‐driven forcing for the latter.
... These characteristics reflect increased physical erosion and material transport mainly originating from the Ethiopian highland during higher rainfall regimes related to summer monsoon precessional variability. Thus, the sediment records preserved at the Nile deep-sea fan provide a suitable archive to study basin-wide environmental changes in vegetation cover, lithology, and soil erosion/weathering at a high time resolution mainly during humid periods such as the AHP (Blanchet et al., 2014;Castañeda et al., 2016;Hamann et al., 2009;Hennekam et al., 2015;Weldeab et al., 2014). ...
Large fluvial systems, such as the Nile River, allow a basin-scale integrated view of climatological and environmental changes. In this study, we reconstructed the Nile discharge history for the last 20 ka using molecular ratios of glycerol dialkyl glycerol tetraethers (GDGTs) and neodymium (Nd) radiogenic isotopes. By characterizing both the organic and inorganic fractions, we assessed the relevance of the GDGT-based proxies in deltaic environments as tracers of terrigenous origin. A large increase in Nile discharge is documented from 14.8 to 8.4 ka BP reflecting enhanced physical erosion and transport processes from the Ethiopian Traps. We confirmed the primary control of insolation on precipitation on North East Africa through the last 20 ka. The centennial time resolution reached on the sedimentary sequence revealed a step-wise onset and termination of the African Humid Period (AHP) starting at 14.8 ka and ending at 8.4 ka BP, respectively. Our centennial-millennial records allowed furthermore pinpointing the abrupt periods of arid conditions corresponding to the Younger Dryas. These data illustrate the linkage between low and high latitude hydrological variability.
... Fluctuations in the main sources of sediment carried by this river can inform on the efficacy of erosional processes operating in different regions of eastern and north-eastern Africa, andthrough thisclimatic variability across the region. Much of the focus of hydroclimatic research in this region thus far has rightly been on understanding climatic changes in the headwaters of the Nilea very important topic since it is precipitation in this region that controls the river flow (Krom et al., 2002;Revel et al., 2010Revel et al., , 2014Hennekam et al., 2015;Macklin et al., 2015;Castañeda et al., 2016). However, it is also important to recognise that the history of aeolian sedimentary input (dust) into the Nile system can further inform on the pace and timing of the development of hyper-aridity in the Sahara, further north. ...
Elemental XRF analysis carried out on an 8 m long core from the Nile Delta reveals a gradual increase in the Ca/ Ti ratio between 5000 and 4000 cal BP which is linked to the progressive development of hyper-aridity in this region. The increase results from elevated flux of aeolian material entering the Nile river system from calcareous source rock geologies in the dryer Egyptian Sahara. The most major increase in hyper-aridity occurs around 4000 cal BP. Such a perspective suggests a locally abrupt, regionally time-transgressive inception of hyper-aridity in this region at the end of the African Humid Period. After this time, reorganisation of wind circulation meant that less Saharan-derived aeolian material entered the Nile Valley, and the contribution of aeolian material in the Nile's sedimentary signal was also dwarfed by an increase in Blue Nile sedimentary flux. Chronological control is provided by two radiocarbon dates and the top and bottom of a well-constrained pottery horizon that dates from the period of occupation of two nearby archaeological sites: Kom al-Ahmer and Kom Wasit.
... Pollen records in marine sediments provide accurate information on paleovegetation, palaeohydrology and paleoclimate on a local and global level. Pollen is studied in order to reconstruct changes in temperature and precipitation occurred in the past ( Kotthoff et al., 2008;Langgut et al., 2011) but is also used to investigate processes such as productivity ( Hennekam et al., 2015). As is evident from Fig. 5.A, however very few data (only 6 cores) are available for the investigated interval. ...
Science advances and, with it, the storage of large amounts of data. The need to use this data efficiently, quickly and safely is possible thanks to the Big Data Analytics (BDA) that allows us to store and relate data in order to obtain new knowledge. In this paper we present and explain how we constructed the new database, “BEyOND”, that provides a wide variety of organized and standardized paleoproxies relative to the past 20.000 years of Mediterranean Sea history. BEyOND makes available to all researchers the possibility to extract and analyze data. We focused on a specific interval of time corresponding to the deposition of the most recent sapropel (S1) and the potential uses offered by the tool.
BEyOND contains 126 sediment cores data from 79 scientific papers and a total of 1.678 different proxy related data that have been categorized in: geochemistry, isotopes, pollen, sediment grain size, coccolithophore, dinoflagellate and foraminifera.
Our work highlights the development of a new methodology to correlate data, including the cases where data regarding the precise age control for each core was missing. It highlights as well the potential of using data analytics to extract hidden patterns and new knowledge also in the field of paleoceanography.
... Several studies have been carried out, already examining the patterns of abundance and species distribution of benthic foraminifera as well as studies on redoxsensitive trace metals and their isotopes. [19][20][21][22][23]69 Future studies should include examining the effect of changing redox distributions on redox-sensitive nutrient elements such as P and Fe cycling in the water column. Such studies are important to enable predictions concerning the modern anthropogenically driven expansion of oxygen minimum zones. ...
In the Eastern Mediterranean Sea (EMS), natural climate-driven changes that impacted its physical circulation and created deep-water anoxia have led to periods of distinctive deposits referred to as sapropels. The most recent sapropel event (S1) occurred between ~6-10 kyr BP. Use of a global climate model has suggested that there was a previously unrecognized mid-depth (500-1800m) water mass present at that time. Here, a broad review of cores previously analyzed from across the EMS was undertaken to determine if the field evidence was compatible with the presence of this intermediate water mass. The proxy records document a widespread oxygenated interruption centered around 8.2 ka BP in water depths between 500 and 1800m, but not present in shallower or deeper water. We attribute this pattern to the formation and spread of a water mass, which we refer to as Sapropel Intermediate Water (SIW). It is shown that this water mass was formed in the Aegean Sea and became more depleted in oxygen, to anoxicity but not euxinicity, as it flowed west to the Adriatic and east across the Levantine basin. The rate of formation and flow of this water mass was estimated to be slow but not zero. An evolving Oxygen Minimum Zone in the SE Levantine is posited where oxygen was depleted first in shallower water and expanded with time into deeper water. The presence of SIW obviates the need for the previously suggested thin anoxic blanket, since the 1300m thick SIW can be fully deoxygenated in 200-500 years.
... A link between climate and sedimentation rate has been proposed by Schiffelbein and Dorman (1986), who used a very similar criterion, "spectral simplicity." Still the pattern deviates from that found in the Nile delta for sapropel S1 (Hennekam et al., 2015). This difference may be due to the fact that the record used here is from a more distal location, whereas the Nile delta record is affected directly by the local sedimentary system, where magnitude response and phase response may be very different. ...
Reconstructing millennial‐ to centennial‐scale climate variability for the Eemian—an interval with estimated sea surface temperatures ~0.5 °C warmer than “preindustrial”—requires records with high temporal resolution. Sapropel S5 sediments, deposited under anoxic conditions in the Eastern Mediterranean Sea, offer the rare opportunity to assess multicentennial climate variability during this time. Here we present high‐resolution S5 piston core data from the Nile delta region. Specifically, we focus on Ba/Ti, Br/Ti, and Mo/Ti, as they are proxies for paleo‐productivity, marine organic carbon, and sediment anoxia, respectively. A high correlation between our Ba/Ti values in core 64PE‐406‐E1 and well‐dated Ba records of nearby cores (LC21 and ODP967) was found. We, therefore, tuned our data to these cores obtaining an initial age model. A time‐frequency analyses indicated significant frequency content in the multicentennial band, although the frequency components drifted over time. Assuming spectral simplicity, we corrected for sedimentation rate changes on a multicentennial time scale. This novel approach grants a higher‐resolution age model. The resulting variability in sedimentation rate is similar to records of monsoon variability, indicating a possible link between sedimentation at the core location and low‐latitude monsoon variability, linked via the River Nile. Moreover, the periodicities found in the sapropel time series are similar to the frequency content of total solar irradiance and sunspot records known for the Holocene, at least at high frequencies (~50–150 years). Hence, our data suggest cyclic intrasapropel variability, at least during the deposition of sapropel S5, may be linked to solar cycles.
... Fluctuations in the main sources of sediment carried by this river can inform on the efficacy of erosional processes operating in different regions of eastern and north-eastern Africa, andthrough thisclimatic variability across the region. Much of the focus of hydroclimatic research in this region thus far has rightly been on understanding climatic changes in the headwaters of the Nilea very important topic since it is precipitation in this region that controls the river flow (Krom et al., 2002;Revel et al., 2010Revel et al., , 2014Hennekam et al., 2015;Macklin et al., 2015;Castañeda et al., 2016). However, it is also important to recognise that the history of aeolian sedimentary input (dust) into the Nile system can further inform on the pace and timing of the development of hyper-aridity in the Sahara, further north. ...
Elemental XRF analysis carried out on an 8 m long core from the Nile Delta reveals a gradual increase in the Ca/Ti ratio between 5000 and 4000 cal BP which is linked to the progressive development of hyper-aridity in this region. The increase results from elevated flux of aeolian material entering the Nile river system from calcareous source rock geologies in the dryer Egyptian Sahara. The most major increase in hyper-aridity occurs around 4000 cal BP. Such a perspective suggests a locally abrupt, regionally time-transgressive inception of hyper-aridity in this region at the end of the African Humid Period. After this time, reorganisation of wind circulation meant that less Saharan-derived aeolian material entered the Nile Valley, and the contribution of aeolian material in the Nile's sedimentary signal was also dwarfed by an increase in Blue Nile sedimentary flux. Chronological control is provided by two radiocarbon dates and the top and bottom of a well-constrained pottery horizon that dates from the period of occupation of two nearby archaeological sites: Kom al-Ahmer and Kom Wasit.
... MS21PC This core originates from the Central Province of the Nile delta and was recovered from 1022 m water depth (Fig. 1). Hennekam et al. (2015) published the . Location map in Fig. 1 d 18 O values of G. ruber which show a negative peak during the period 10,000-5000 years BP, reflecting the African Humid Period (AHP) (Fig. 5). ...
Holocene climate in North Africa and on the Arabian Peninsula has undergone major changes. In this contribution, we review hydroclimate and temperature changes in the region over the past 15,000 years by correlating and integrating all available case studies. A pronounced wet period corresponding to the ‘Green Sahara’ and its equivalent in Arabia commenced between 15,000 and 9000 years BP and ended sometime between 6500 and 3500 years BP, followed by arid conditions throughout the late Holocene. Start and end dates vary between locations, depending on local factors, climate amplifiers and chosen climate proxies, leading to a spatially and temporally complex distribution. Some studies show gradual transitions between the hydroclimatic states while other locations evidenced abrupt changes. The humid phase was triggered by a northward migration of the Intertropical Convergence Zone (ITCZ) due to orbital precession. The northernmost parts of North Africa and the Arabian Peninsula were not reached by the ITCZ. Here, increased early Holocene humidity may have been due to an intensification of southern Mediterranean winter rain and its deeper penetration southwards into the present-day desert areas. The early Holocene ‘Green Sahara’ forms part of a long series of wet periods that have occurred over the past hundred thousand to million years in North Africa and Arabia. Notably, climate models are still unable to match the observed hydroclimatic changes in a quantitative way. Simulated rainfall during the African Humid Period over the Sahara is not sufficient to sustain vegetation at a level seen in the palaeo record, indicating that processes such as vegetation and dust feedbacks still need to be refined. Sea surface temperatures in North Africa and Arabia during the early Holocene were generally one to several degrees C warmer than during the late Holocene. Warming began around 12,000 years BP and ended around 5000 years BP. The warm period generally coincided with the early Holocene wet phase in the region and is linked to the Holocene Thermal Maximum, an early Holocene period during which temperatures were globally elevated. The review suggests that the Holocene climate history of North Africa and Arabia is closely linked to the global development and that significant temperature changes have also occurred in subtropical climate belts.
... In general, we did not find differences in component contributions among the different samples, implying that component analysis is not applicable to sediment tracing in the study area. Several studies have shown that large river delta deposits experience sediment source changes, including the Mississippi and Nile deltas (Montero-Serrano et al., 2010;Hennekam et al., 2015). Such a sediment source change is linked to the intensity and location of the precipitation regime and landcover within catchments with varied parent rocks, which is further influenced by Holocene climate change. ...
The optically stimulated luminescence (OSL) sensitivity and OSL signal components of quartz grains were used to investigate provenance changes of Holocene sediments from the Yangtze River delta. The variation of luminescence sensitivity was observed in multiple grain aliquots and single grains of quartz from different sedimentary units of the Yangtze River delta. Laboratory experiments suggest that repeated dosing/bleaching cycles increase the luminescence sensitivity of quartz from the studied sediments. High variable thermal activation curves were observed even for samples from the same sedimentary unit, implying highly diverse sources for the delta deposits of the Yangtze River. Different sedimentary units show quartz with similar OSL component contributions, and repeated dosing/bleaching cycles and heating treatment are unable to affect the relative contributions of the fast and medium components to the bulk OSL signal. The samples from unit 1 (U1, tidal river, 15–11 ka), unit 2 (U2, estuary, 11–9 ka) and unit 6 (U6, delta plain, ca. 1 ka to the present) show relatively higher luminescence sensitivity in comparison to unit 3 (U3, tidal sand ridge, 9–4 ka), unit 4 (U4, prodelta, 4–2.5 ka) and unit 5 (U5, delta front, 2.5–1 ka), implying changing sediment sources over time. Such a temporal variation of sediment source can be explained by the transgressive/regressive history of the Yangtze River delta as well as by Asian monsoon variability since the last deglaciation. It demonstrates that luminescence sensitivity of quartz has great potential for tracing sediment sources in the Yangtze River delta, but more work is needed to characterize specific sources to establish a source-to-sink linkage.
... During the Early to Mid-Holocene humid climate (or African Humid Period "AHP"), there was wide spread dense vegetation cover, which stabilized slopes and reduced the amount of soil erosion (e.g. Box et al., 2011;Hennekam et al., 2015). ...
We present results of mineralogical (light, heavy and clay minerals), geochemical and palynological investigations of floodplain sediments at Saqqara. Our investigations provide new insights into our understanding of the source of the Holocene Nile sediments, local palaeoclimate and palaeoenvironment of the floodplain at Saqqara as well as the Holocene palaeoclimatic variations at the Nile headwaters. We also investigate the relationship between Nile floods and global climate changes and the impact of changing flood conditions on episodes of societal and central governmental collapse at the end of the Egyptian Old Kingdom. The Holocene sequences recorded in two shallow drill cores (SAQA 21 and 22) were investigated and subdivided into six sedimentary units, from underlying Late Pleistocene sand and gravels (unit I) to overlying modern soil (unit VIII). Early Holocene sediments were essentially missing in the studied cores, probably due to intensive erosion during the well-known 8.2 kyr cal BP arid event. Middle Holocene sediments are represented by unit II, which reflect high Nile floods of Equatorial African origin with less contribution from Ethiopian sources. The Old Kingdom sediments (unit III), seems to be mainly of White Nile sources with frequent contributions from the Blue Nile. Late Holocene sediments (unit IV-VII) show an increased contribution from Blue Nile River sources. Low Nile flow episodes were recorded at 5.2 (between units II and III), 4.2 (between units III and IV), 3.5 (within unit IV) and 2.7 kyr cal BP (between units V and VI). The second event corresponds to the First Intermediate Period (FIP) and the third probably coincides with the Second Intermediate Period (2nd IP), periods of disorder in Egypt.
... A further study (Leroy, 1992) also in general found progressively decreasing amounts of Cyperaceae and other wetland plants, and increasing Poaceae, Amaranthaceae and Chenopodiaceae since c.4000 cal BP. More recent work (Bernhardt et al., 2012;Hennekam et al., 2015) has also found decreasing Cyperaceae c.5000 cal BP, which may reflect the replacement of local wetland landscapes of the Bilqas 2 environment by the Bilqas 1 facies. the genesis and subsequent disappearance of LSC landscapes, represented by deposits of the Bilqas 2 Member, and their replacement by more well-drained floodplains of the Bilqas 1 Member. ...
The evolution of the Nile Delta, the largest delta system in the Mediterranean Sea, has both high palaeoenvironmental and archaeological significance. A dynamic model of the landscape evolution of this delta system is presented for the period c.8000–4500 cal BP. Analysis of sedimentary data and chronostratigraphic information contained within 1640 borehole records has allowed for a redefinition of the internal stratigraphy of the Holocene delta, and the construction of a four-dimensional landscape model for the delta's evolution through time. The mid-Holocene environmental evolution is characterised by a transition from an earlier set of spatially varied landscapes dominated by swampy marshland, to better-drained, more uniform floodplain environments. Archaeologically important Pleistocene inliers in the form of sandy hills protruding above the delta plain surface (known as “turtlebacks”), also became smaller as the delta plain continued to aggrade, while the shoreline and coastal zone prograded north. These changes were forced by a decrease in the rate of relative sea-level rise under high rates of sediment-supply. This dynamic environmental evolution needs to be integrated within any discussion of the contemporary developments in the social sphere, which culminated in the emergence of the Ancient Egyptian State c.5050 cal BP.
... . ThoughHennekam et al. (2015) identifies a slow and general increase in fine-grained Nilotic-Average particle size distribution of the samples (n = 42). ...
The study focuses on the Holocene appearance, chronology and drivers of beach sand deposition and
inland aeolian sand transport around the Roman–Byzantine ruins of Caesarea, Israel. Beach sand, sand
sheets, nebkha, linear and transverse dunes as well as parabolic and transverse interdunes along two
transects were sampled in the current study down to their substrate.
Sixteen new optically stimulated luminescence ages cluster at �5.9–3.3 ka, �1.2–1.1 ka (800–900 AD)
and �190–120 years ago (1825–1895 AD) indicating times of middle and late Holocene sand sheet depositions
and historical dune stabilization. The first age cluster indicates that beach sand accumulated when
rates of global sea level rise declined around 6–5 ka. Until �4 ka sand sheets encroached up to 2.5 km
inland. Historical and archaeological evidence points to sand mobilization since the first century AD.
Sand sheets dating to 1.2–1.1 ka, coevally found throughout the dunefield represent sand stabilization
due to vegetation reestablishment attributed to gradual and fluctuating decline in human activity from
the middle Early Islamic period until the 10th century. Historical and chronological evidence of the
existence of transverse and coppice dunes from the 19th century suggest that dunes only formed in
the last few centuries.
The study illustrates the initial role of natural processes, in this case decline in global sea level rise and
the primary and later role of fluctuating human activity upon coastal sand mobility. The study distinguishes
between sand sheets and dunes and portrays them as sensors of environmental changes.
The eastern Mediterranean Sea (EMS) sedimentary record is periodically interspersed with organic-rich ‘sapropel’ layers. Sapropels are characteristic of basin-wide anoxic events, triggered by precession-forced insolation maxima. Relatively subdued insolation maxima, however, are not always expressed as distinct sapropel events. The EMS sedimentary record is thus useful to investigate feedbacks between marine anoxia and the nitrogen (N)
cycle and offers an analogue for modern deoxygenation and past oceanic anoxic events. To this end, we investigated a ~68 kyr sedimentary record from the EMS containing the well-established sapropel S1 (deposited in two phases: S1a [~10.5–8.5 ka BP] and S1b [~7.8–6.1 ka BP]) and sediments timed to the ambiguous S2
sapropel (~53 ka BP). We used lipid biomarkers of microorganisms to reconstruct key N-cycle components: (1) anaerobic ammonium oxidation (anammox) using ladderanes and a stereoisomer of bacteriohopanetetrol (BHTx), (2) dinitrogen gas (N2) fixation using heterocyte glycolipids, and (3) nitrification by Thaumarchaeota using
crenarchaeol. Additionally, benthic foraminifera and trace metals (U, Mo, Mn) were used to reconstruct redox conditions. During S1a, abundances of Thaumarchaeota increased, likely promoted by elevated high-nutrient freshwater discharge. At this time, a combination of phosphorus supply and intensified loss of bioavailable N via water column anammox, may have reinforced anoxia by favoring diatom-diazotroph associations. During
S1b, anammox is equally intense. Yet, no positive feedback on N2-fixation is observed, likely because diazotrophs were phosphorus limited. Instead, anammox may have provided negative feedback on anoxia by quenching primary production. Ladderanes suggest additional episodes of anammox between ~69 to 39 cal ka BP, corresponding to brief periods of water column deoxygenation. Anoxia likely occurred at the sediment–water interface
in S2-timed sediments (53–51 cal ka BP). During these episodes, ladderanes co-occur with the later eluting BHT-34R stereoisomer. δ13CBHT-34R indicate an anammox source, potentially synthesized by marine sedimentary anammox bacteria. No corresponding increase in diatom-diazotroph associations is observed, likely due to the oligotrophic conditions and the limited effect of sedimentary anammox on N-availability in the euphotic zone. Our results highlight various modes of operation of the N-cycle at different degrees of deoxygenation, which depend amongst others on nutrient-availability and the niche-segregation of N-loss and N2-fixating
microorganisms.
We present a global atlas of downcore foraminiferal oxygen and carbon isotope ratios available at https://doi.org/10.1594/PANGAEA.936747
(Mulitza et al., 2021a). The database contains 2106 published and previously unpublished stable isotope downcore records with 361 949 stable isotope
values of various planktic and benthic species of Foraminifera from 1265 sediment cores. Age constraints are provided by 6153 uncalibrated
radiocarbon ages from 598 (47 %) of the cores. Each stable isotope and radiocarbon series is provided in a separate netCDF file containing
fundamental metadata as attributes. The data set can be managed and explored with the free software tool PaleoDataView. The atlas will provide
important data for paleoceanographic analyses and compilations, site surveys, or for teaching marine stratigraphy. The database can be updated with
new records as they are generated, providing a live ongoing resource into the future.
The Nile Basin drains about one‐tenth of the African continent and contains the longest river channel system in the world. This chapter reviews the evolution of the river – with a particular focus on the Quaternary Period including the present‐day hydrology and fluvial geomorphology. Fluctuations in the climate and hydrology of tropical Africa during the Quaternary exerted an important influence on the behaviour of the Nile sediment system. During the cold stages of the Pleistocene, low lake levels reflect a decrease in precipitation as the expanded ice sheets cooled the global oceans and monsoon intensity fell. The marine sedimentary record in the Eastern Mediterranean provides a valuable record of the long‐term behaviour of the River Nile because long‐term changes in the flux of water and sediment from various parts of the catchment have exerted an important influence on oceanographic and sedimentation dynamics. The chapter examines the contemporary suspended sediment budget of the delta complex.
We present a global atlas of downcore foraminiferal oxygen and carbon isotope ratios available at https://doi.pangaea.de/10.1594/PANGAEA.936747 (Mulitza et al., 2021). The database contains 2,108 published and previously unpublished stable isotope downcore records with 362,067 stable isotope values of various planktonic and benthic species of foraminifera from 1,265 sediment cores. Age constraints are provided by 6,153 uncalibrated radiocarbon ages from 598 (47 %) of the cores. Each stable isotope and radiocarbon series is provided in a separate netCDF file containing fundamental meta data as attributes. The data set can be managed and explored with the free software tool PaleoDataView. The atlas will provide important data for paleoceanographic analyses and compilations, site surveys, or for teaching marine stratigraphy. The database can be updated with new records as they are generated, providing a live ongoing resource into the future.
High-resolution seismic surveys were conducted off the eastern coast of Hainan Island to study the distribution
and mass budget of Holocene subaqueous clinoform sediments. The Holocene subaqueous clinoform extends 45
km seaward across the shelf and more than 80 km northeastward along the shore. It covers an area of over 3,000
km2, which is almost equivalent to the expansion of large river systems. Overlying the transgressive surface, the
deltaic sequence can be divided into a transgressive unit and a highstand unit separated by a downlap maximum
flooding surface. The volume of sediments within the Holocene subaqueous clinoform is estimated to be
33.12×109 m3, equivalent to an annual accumulation of 3.61–3.31 Mt/yr, which is substantially higher than the
global average. The high sediment yield presumably resulted from strong tropical weathering and active tectonics.
The sediment flux during the transgression and sea-level highstand were 3.85–3.08 and 3.47 Mt/yr,
respectively, suggesting no substantial changes in sedimentation. Compared to river systems originating from
high mountain glaciers, the sedimentation variation of small tropical rivers throughout the Holocene is relatively
minor. However, the sediment discharge of the Wanquan River has drastically decreased in recent decades
because of increased human activity.
Ocean deoxygenation is a rising threat to marine ecosystems and food resources under present climate warming conditions. Organic-rich sapropel layers deposited in the Mediterranean Sea provide a natural laboratory to study the processes that have controlled changes in seawater oxygen levels in the recent geological past. Our study is based on three sediment cores spanning the last 10 000 years and located on a bathymetric transect offshore from the western distributaries of the Nile delta. These cores are partly to continuously laminated in the sections recording sapropel S1, which is indicative of bottom-water anoxia above the western Nile deep-sea fan. We used a combination of microfacies analyses and inorganic and organic geochemical measurements to reconstruct changes in oxygenation conditions at seasonal to millennial timescales. Millimetre-thick laminations are composed of detrital, biogenic and chemogenic sublayers reflecting seasonal successions of sedimentation. Dark layers reflect the deposition of summer floods and two types of light layers correspond to autumn plankton blooms and authigenic carbonates formed in the water column during spring–early summer, respectively. The isotopic signature of the authigenic carbonates suggests permanent anoxic to euxinic bottom waters resulting in high levels of anaerobic remineralization of organic matter and highlights their potential to reconstruct seawater chemistry at times when benthic fauna was absent. Ratios of major elements combined with biomarkers of terrestrial and marine organic matter and redox-sensitive compounds allow changes in terrigenous input, primary productivity and past deoxygenation dynamics on millennial timescales to be tracked. Rapid fluctuations of oxygenation conditions in the upper 700 m water depth occurred above the Nile deep-sea fan between 10 and 6.5 ka BP, while deeper cores recorded more stable anoxic conditions. Synchronous changes in terrigenous input, primary productivity and past oxygenation dynamics after 6.5 ka BP show that runoff-driven eutrophication played a central role in rapid oxygenation changes in the south-eastern Levantine Basin. These findings are further supported by other regional records and reveal time-transgressive changes in oxygenation state driven by rapid changes in primary productivity during a period of long-term deep-water stagnation.
The paper focuses on one of the most productive wheat-growing regions in the entire Roman Empire, the Arsinoite nome (modern Fayum) in Egypt. Towards the end of the third century CE, multiple formerly thriving farming villages at the edges of the district went into decline and were eventually abandoned. This paper presents a new perspective on causes of this abandonment by synthesizing existing research. The papyri as well as the archaeological record imply that irrigation problems arising simultaneously from the third century CE lay at the heart of the problem and led to the progressive desertification of formerly agricultural land. The surviving documentation allows us to trace what increasing water stress meant on the ground for the local population and what adaption strategies they undertook to deal with the degradation and desertification of their farmlands. While socio-economic factors certainly played a role in the decline of these settlements, a change in environmental conditions should be considered as well. In fact, natural proxies record a general shift in East African Monsoon patterns at the source areas of the Nile and consecutively lower Nile flood levels from the beginning of the third century on.
Cambridge Core - Geomorphology and Physical Geography - The Nile Basin - by Martin Williams
The most recent organic carbon-enriched layer (sapropel S1) deposited at the Eratosthenes Seamount has unique features, such as an early lithological interruption, fine light silt laminae and an exceptional vertical extent that is over 25 cm thick. Here we investigate calcareous nannofossil assemblages to reconstruct very high-resolution palaeoenvironmental and palaeoceanographic variations recorded before, during and after the perturbation episode that involved the eastern Mediterranean Sea, due to the massive freshwater discharge via Nile River. Our results show that the deep chlorophyll maximum development, observed in all micropalaeontological groups from previous studies, is a gradual process that started well before the base of sapropel S1. A high-frequency variability in the nutricline depth is evident at millennial- and/or centennial-scale throughout the sapropel deposition time interval. Also we highlight the poor-preservation of delicate tiny holococcolith crystals while anoxia was occurring at the seafloor and we suggest that such a phenomenon may be used to mark the original thickness of sapropel deposition where oxygen re-ventilation fronts were developed. Finally, calcareous nannofossil reworking peaks shed light on the nature of fine silt laminae within the sapropel S1 at the Eratosthenes Seamount, which may be ascribed to fine sediment plumes from the Nile River deposited during exceptional runoff events.
During the mid-Holocene, some of the world's first large-scale complex societies came into being within the lower and middle reaches of a number of large river systems. Around this time, as global sea-level stabilised, the hosting fluvial environments of Lower Mesopotamia, the Nile Delta and the North China Plain were evolving from spatially varied landscapes dominated by swampy marshland, to better-drained, more uniform floodplain environments. It is necessary to consider whether such environmental changes could have guided aspects of sociocultural evolution in these settings.
In the Nile Delta, the setting for which most data are available, these palaeolandscape changes are comprehensively mapped through the construction of a four-dimensional aggradation model of the Holocene alluvial plain. Development of this model takes place within the context of a full reinterpretation of the Upper Quaternary stratigraphy of the Nile Delta, which is itself further informed by substantial programmes of fieldwork in the western delta. The environmental changes were forced by a decrease in the rate of relative sea-level rise within the context of decreased discharge and sediment-supply due to regional climate change.
A geoarchaeological model links these changes in the landscape to sociocultural developments taking place in Egypt between 5500 and 2500 BC. Increased adoption of agricultural practices in the delta was stimulated by a decrease in the primary productivity of the landscape, which then led to population growth and shifts in settlement styles. The emergence of the first Egyptian capital of Memphis at the delta apex can also be seen as having been facilitated by changes in the palaeogeography of the fluvio-deltaic environment. Such linkages between the changing deltaic landscapes and social change are crucial in understanding the formation of the Ancient Egyptian State (c. 3100 BC), which involved increased involvement of regional elites using the delta as both an agricultural resource and trade route.
This study aims to trace changes in the River Nile flows over the Late Quaternary and is based on palynomorphs which were embedded in the sea floor of the Levantine Basin. The palynomorphs were extracted from two marine sediment cores, which cover the last 86 ka and are located at the two ends of the Levantine Basin: MD-9509, at the southern part of the Levantine Basin, and MD-9501, at its northern part. Core MD-9509 was taken from the Nile cone and is characterized by high sedimentation rates and a good state of palynomorph preservation. The assemblages included palynomorphs which were transported via the Nile headwaters and therefore enabled the reconstruction of the River Nile flows. The results demonstrate that the last glacial period (∼75–16 ka) was characterized by a decrease in Nile water discharge and an increase in sediment influx, while opposite trends were observed prior to the last glacial period as well as during the deglaciation and the Holocene. Based on the study of the spores, it is suggested that during the last glacial, the main contributors of freshwater and sediment load to the Eastern Mediterranean Sea were the Blue Nile and the Atbara and only during interglacials was there a more significant contribution of the White Nile. Within the northern core, MD-9501, pollen was preserved only during the formation of sapropels S3 and S1. The comparison of the sapropelic palynological spectra in both core sites clearly indicates that during sapropel deposition, climate conditions were more humid in the Northern Levant, reflecting the north-south regional Mediterranean climatic moisture gradient. Sapropel formation was a result of the intensification of the monsoonal climate system which was most probably related to the maximum insolation values at 65°N, while, currently, the Atlantic is the main influencing climate system in the region. One of the most interesting observations in this study is that during Heinrich Events H2-H6, which originated in the north Atlantic and were identified in MD-9509 based on minimum arboreal pollen percentages, pollen originating from tropical regions was not embedded in the Levantine Basin. These results lend support to the view that episodes of dryness in tropical/sub-tropical Eastern Africa were associated not only with low-latitude climate controls, but also with high-latitude glacial stress.
Pollen spectra from three eastern Mediterranean cores have been used to docmnent the palcoclimates of the Levantine Basin borderlands over the last 250 kyr to establish the relationship between this regional climate data set and the global climate as recorded by foraminiferal • 18 0 and to compare it with proximal land pollen records. Core MD 84 642 with eight sapropels covers the last two climatic cycles up to the early Holocene, MD 84 627 with four sapropels goes back to 125 kyr, and MD 84 629 with one sapropel covers the last 70 kyr. The sedimentation rate decreases from core 629, located at the shallowest depth beneath the Nile River plume, to cores 627 and 642. During the interglacials defined by a low 180/160 ratio, the abundance of tree pollen is maximum and points to an optimum Mediterranean climate with greatest humidity, including some summer rainfall. During glacial maxima, with highest 180/160 ratio, the pollen abundance is high for steppe and semidesert plants and low for trees, indicating a definitely more arid, more continental, and probably colder climate. The variations of pollen abundance occur in phase with those of the foraminifer • 180 record. This signifies that the regional climate of the Levantine Basin borderlands had the same temporal pattern as the global ice volume docmnented by the ice volume curve.
The Nile delta sedimentation constitutes a continuous high-resolution record of Ethiopian African monsoon (EAM) regime intensity. Multi-proxy analyses performed on hemipelagic sediments deposited on the Nile deep-sea fan allow the quantification of the Saharan aeolian dust and the Blue/White Nile River suspended matter frequency fluctuations during the last 21,000 years. The radiogenic strontium and neodymium isotopes, clay mineralogy, elemental composition and preliminary palynological analyses reveal large changes in source components, oscillating between a dominant aeolian Saharan contribution during the Last Glacial Maximum (LGM) and the late Holocene (similar to 4,000-2,000 years), a dominant Blue/Atbara Nile River contribution during the early Holocene (15,000-8,000 years) and a probable White Nile River contribution during the middle Holocene (8,000-4,000 years). The following main features are highlighted: (1) The rapid shift from the LGM arid conditions to the African Humid Period (AHP) started at about 15,000 years. The AHP extends until 8,000 years, and we suggest that the EAM maximum between 15,000 and 8,000 years is responsible for a larger Blue/Atbara Nile sediment load and freshwater input into the eastern Mediterranean Sea. (2) The transition between the AHP and the arid late Holocene is gradual and occurs in two main phases between 8,400-6,500 years and 6,500-3,200 years. We suggest that the main rain belt shifted southward from 8,000 to similar to 4,000 years and was responsible for progressively reduced sediment load and freshwater input into the eastern Mediterranean Sea. (3) The aridification along the Nile catchments occurred from similar to 4,000 to 2,000 years. This dry period, which culminates at 3,200 year, seems to coincide with a re-establishment of increased oceanic primary productivity in the western Mediterranean Sea. Such a pattern imposes a large and rapid northward shift of the rain belt over the Ethiopian highlands (5-15A degrees N) since 15,000 years. Precipitation over Ethiopia increased from 15,000 to 8,000 years. It was followed by a gradual southward shift of the rain belt over the equator from 8,000 to 4,000 years and finally a large shift of the rain belt south the equator between 4,000 and 2,000 years inducing North African aridification. We postulate that the decrease in thermohaline water Mediterranean circulation could be part of a response to huge volumes of freshwater delivered principally by the Nile River from 15,000 to 8,000 years in the eastern Mediterranean.
monsoon precipitation experienced a dramatic change in the course of the Holocene. The pace with which the African monsoon shifted from a strong early to middle to a weak late Holocene is critical for our understanding of climate dynamics, hydroclimate-vegetation interaction, and shifts of prehistoric human settlements, yet it is controversially debated. On the basis of planktonic foraminiferal Ba/Ca time series from the eastern Mediterranean Sea, here we present a proxy record of Nile River runoff that provides a spatially integrated measure of changes in East African monsoon (EAM) precipitation. The runoff record indicates a markedly gradual middle to late Holocene EAM transition that lasted over 3500 years. The timing and pace of runoff change parallels those of insolation and vegetation changes over the Nile basin, indicating orbitally forced variation of insolation as the main EAM forcing and the absence of a nonlinear precipitation-vegetation feedback. A tight correspondence between a threshold level of Nile River runoff and the timing of occupation/abandonment of settlements suggests that along with climate changes in the eastern Sahara, the level of Nile River and intensity of summer floods were likely critical for the habitability of the Nile Valley (Egypt).
We present high-resolution records for oxygen isotopes of the planktic foraminifer Globigerinoides ruber (δ18Oruber), and bulk sediment inorganic geochemistry for Holocene-age sediments from the southeast Mediterranean. Our δ18Oruber record appears to be dominated by Nile discharge rather than basin-scale salinity/temperature changes. Nile discharge was enhanced in the early- to mid-Holocene relative to today. The timing of the long-term maximum in Nile discharge during the early-Holocene corresponds to the timing of maximum intensity of the Indian Ocean-influenced Southwest Indian summer Monsoon (SIM). This coincidence suggests a major influence of an Indian Ocean moisture source on Nile discharge in the early- to mid-Holocene, while, presently, the Atlantic Ocean is the main moisture source. Nile discharge was highly variable on multi-centennial timescale during the early- to mid-Holocene, being strongly influenced by variable solar activity. This solar-driven variability is also recorded in contemporaneous SIM records, however not observed in an Atlantic Ocean-derived West African summer monsoon record from the Holocene. This supports the hypothesis that the Indian Ocean moisture source predominantly controlled Nile discharge at that time. Solar-driven variability in Nile discharge also influenced paleoenvironmental conditions in the eastern Mediterranean. Bulk sediment Ba/Al and V/Al, used as indicators for (export) productivity and redox conditions, respectively, varied both in response to solar forcing on multi-centennial timescales. We suggest that changes in Nile discharge on these timescales have been concordant with nutrient inputs to, and shallow ventilation of, the eastern Mediterranean.
A 7000 yr palynologic record from Burullus Lagoon, Nile Delta, Egypt, is assessed to
investigate changes in terrestrial vegetation in response to Nile fl ow. Previous studies in this
region have shown that sea-level rise in the early to mid-Holocene, and markedly increased
human land use during the past several centuries, altered vegetation in and around the
lagoon. The pollen record from this study documents changes in delta vegetation that likely
refl ect variations in Nile fl ow. We suggest that Cyperaceae pollen is a sensitive marker of
precipitation over the Nile headwaters and the resultant Nile fl ow. Decreases in Cyperaceae
pollen, interpreted as a marker for diminished Nile fl ow, as well as the increase in relative
abundance of microscopic charcoal, occurred at ca. 6000–5500, ca. 5000, ca. 4200, and
ca. 3000 cal. yr B.P. (calibrated years before present). These correspond to extreme regional
and global aridity events associated with a more southerly mean position of the Intertropical
Convergence Zone. These changes, also recorded by other proxy studies, indicate that
several marked regional drought events affected the Nile Delta region and impacted ancient
Egyptian and Middle Eastern civilizations.
The World Ocean Database (WOD) is the most comprehensive global ocean profile-plankton database available internationally without restriction. All data are in one well-documented format and are available both on DVDs for a minimal charge and on-line without charge. The latest DVD version of the WOD is the World Ocean Database 2009 (WOD09). All data in the WOD are associated with as much metadata as possible, and every ocean data value has a quality control flag associated with it. The WOD is a product of the U.S. National Oceanographic Data Center and its co-located World Data Center for Oceanography. However, the WOD exists because of the international oceanographic data exchange that has occurred under the auspices of the Intergovernmental Oceanographic Commission (IOC) and the International Council of Science (ICSU) World Data Center (WDC) system. World Data Centers are part of the ICSU World Data System.
X-ray fluorescence (XRF) core-scanning is a convenient non-destructive tool to rapidly assess elemental variations in unprocessed sediments. However, substantial analytical deviations may occur in such data due to physical sedimentary properties. The consequences of these artifacts on element intensities are important for paleoclimatic studies, but as yet have not been adequately studied for high resolution (≤ 1-cm interval) records. We have done a high resolution comparison between results from XRF scanning and from two more conventional methods: XRF with beads and inductively coupled plasma optical emission spectrometry (ICP-OES), both on discrete samples. Our XRF scan data concord reasonably well with those from the conventional methods for most elements. In several discrete intervals large deviations occur that could have been attributed to major paleoceanographic events. However, these deviations appear to be mainly related to water content and distribution. We show that the variability in the water film thickness underneath the plastic foil is highly variable, causing significant apparent variability in all elements from Al to Fe. The unverified use of these elements in ratios can thus lead to deviations in the paleoenvironmental interpretation. Erroneous element (ratio) variability can, however, be recognized and corrected using conventional analytical data of a few discrete samples. The antipathetic behavior of Ca to K or Ti, and their similar behavior to water absorption, leaves their ratios relatively unaffected at variable water film thicknesses. Hence, Ca/K and Ca/Ti ratios are recommended for a preliminary down-core overview of sedimentary variability and for stratigraphic correlations between unprocessed marine cores.
Cultured planktonic foraminifera, Orbulina universa (symbiotic) and Globigerina bulloides (nonsymbiotic), are used to reexamine temperature:δ18O relationships at 15°–25°C. Relationships for both species can be described by linear equations. Equations for O. universa grown under low light (LL) and high light (HL) share a slope of −4.80 (0.21‰ °C−1) with a HL-LL offset of −0.33‰ due to symbiont photosynthetic activity. The effect of [CO32−] on O. universa is −0.002‰ µmol−1 kg−1 and is insensitive to temperature. For G. bulloides, ontogenetic effects produce size-related trends in temperature:δ18O, whereby larger shells are enriched in 18O relative to smaller specimens. The O. universa temperature:δ18O relationships are more accurate than previously published equations for describing plankton tow data. Our equations do not explain planktonic core top data with the same precision but provide a good fit to benthic Cibicidoides data below 10°C. Temperature:δ18O relationships for G. bulloides provide good agreement with field data for this species from the northeast Pacific.
The provenance pattern of Nile River sediments can be used as a proxy for paleoclimatic changes in East Africa. The 87Sr/86Sr ratios are particularly appropriate for such provenance investigations, because the White Nile drains predominantly crystalline basement rocks, whereas the Blue Nile and Atbara flow off the Ethiopian Highlands, which consist of Tertiary volcanic rocks. A high-resolution profile of 87Sr/86Sr and Ti/Al ratios from a well-dated core in the Nile Delta shows a close correspondence with known changes in Nile flow over the past 7000 yr. At times of higher river flow there was markedly decreased input of Blue Nile derived and total sediment. This change was caused by northward movement of the Inter Tropical Convergence Zone, resulting in increased vegetative cover in the Ethiopian Highlands due to higher rainfall and a longer wet season. This inverse relationship between Nile River flow and sediment flux may have had important implications in the development of agricultural technology in ancient Egypt. The marked minimum in 87Sr/86Sr at 4200 4500 yr B.P. is coincident with the end of the Old Kingdom in Egypt and provides independent evidence that demise of the Old Kingdom might have been associated with an extended period of catastrophic low floods. During the Quaternary and late Neogene, there was periodic deposition of organic-rich sediments (sapropels) in the eastern Mediterranean that represent important indicators of major environmental change. Evidence from the Ti/Al ratio suggests that the pattern of erosion and sediment supply from the Nile catchment observed in this study also occurred throughout much of the Neogene and Quaternary. The reduced inputs of Blue Nile sediment during times of sapropel formation contributed to the increased primary productivity by reducing the amount of phosphate removed on particles and to the observed change to N limitation in the eastern Mediterranean, which are important characteristics of sapropel deposition.
In this study we utilize two organic geochemical proxies, the U37k′ index and TEX86, to examine past sea surface temperatures (SST) from a site located near the Nile River Delta in the eastern Mediterranean (EM) Sea. The U37k′ and TEX86 records generally are in agreement and indicate SST ranges of 14°C–26°C and 14°C–28°C, respectively, during the last 27 cal ka. During the Holocene, TEX86-based SST estimates are usually higher than U37k′-based SST estimates, which is likely due to seasonal differences between the timing of the haptophyte and crenarchaeota blooms in the EM and is related to the onset of the modern flow regime of the Nile River. Both records show that SST varied on centennial to millennial timescales in response to global climate events, i.e., cooling during the Last Glacial Maximum (LGM), Heinrich event 1 (H1), and the Younger Dryas (YD) and warming during the Bølling-Allerød and in the early Holocene during deposition of sapropel S1. The H1 cooling was particularly severe and is marked by a drop in SST of ∼4.5°C in comparison to pre-H1 SST, with temperatures >1°C cooler than during the LGM. In contrast to high-latitude and western Mediterranean records, which indicate both an abrupt onset and termination of the YD event, the transition from the YD to the Holocene was much more gradual in the EM.
A Holocene Gulf of Guinea record of riverine runoff, based on Ba/Ca in tests of a shallow-dwelling planktic foraminifer, and sea surface temperature (SST), based on Mg/Ca, reveals centennial-scale instabilities in West African monsoon (WAM) precipitation and eastern equatorial Atlantic (EEA) thermal conditions. The long-term Holocene climate trend is characterized by a warm and wet early-mid Holocene and gradual drying and cooling during the late Holocene. Superimposed on this trend are numerous centennial scale drops in precipitation during the early-mid Holocene. The greatest declines in early Holocene monsoon precipitation were accompanied by significant SST cooling in the EEA and correlate with drops in air temperature over Greenland and fresh water outbursts into the North Atlantic (NA). This observation suggests that early Holocene climate instabilities in the NA were closely linked to changes in the WAM. The strong imprint of NA events in summer monsoon precipitation suggests that these events were not confined to winter-time. The late Holocene does not show large amplitude changes in riverine runoff at the centennial level. The relatively stable late Holocene conditions likely reflect a weakening and stabilization of the monsoon system, probably due to diminished influence of the NA region due to a reduction in ice sheet.
A detailed (ca. 100yr resolution) and well-dated (18 AMS 14C dates to 23 cal. ka BP) record of latest Pleistocene–Holocene variations in terrigenous (eolian) sediment deposition at ODP Site 658C off Cap Blanc, Mauritania documents very abrupt, large-scale changes in subtropical North African climate. The terrigenous record exhibits a well-defined period of low influx between 14.8 and 5.5 cal. ka BP associated with the African Humid Period, when the Sahara was nearly completely vegetated and supported numerous perennial lakes; an arid interval corresponding to the Younger Dryas Chronozone punctuates this humid period. The African Humid Period has been attributed to a strengthening of the African monsoon due to gradual orbital increases in summer season insolation. However, the onset and termination of this humid period were very abrupt, occurring within decades to centuries. Both transitions occurred when summer season insolation crossed a nearly identical threshold value, which was 4.2% greater than present. These abrupt climate responses to gradual insolation forcing require strongly non-linear feedback processes, and current coupled climate model studies invoke vegetation and ocean temperature feedbacks as candidate mechanisms for the non-linear climate sensitivity. The African monsoon climate system is thus a low-latitude corollary to the bi-stable behavior of high-latitude deep ocean thermohaline circulation, which is similarly capable of rapid and large-amplitude climate transitions.
Radiocarbon measurements of nine known age shells from the Mediterranean and the Aegean Seas combined with previous measurements provide an updated value for ∆ R, the local variation in the reservoir correction for marine sam- ples. Comparison of pre-1950s samples from the Algerian coast, with one collected in 1954, indicates early incorporation of nuclear weapons testing 14C into the shallow surface waters of the Mediterranean. Comparisons between different basins indi- cate the surface waters of the Mediterranean are relatively homogenous. The recommended ∆ R for calibration of the Medi- terranean marine samples with the 1998 marine calibration dataset is 58 ± 85 14C yr, but variations in the reservoir age beyond 6000 cal BP should be considered.
a b s t r a c t The influence of the northern Atlantic and tropical monsoonal systems, as recorded by the River Nile, on the climate variability of the southeastern Mediterranean was studied in two cores taken by the R/V Marion Dufresne: one core taken SE of Cyprus representing the northern Levantine Basin (core 9501, 980 m water depth) and the other locate 380 km further south, represents the southeastern Levantine Basin in an area influenced by the River Nile plume (core 9509, 884 m water depth). The study was performed at relatively high resolution using several proxies: d 18 O of Globigerinoides ruber, sediment characteristics and index colour parameters in core sections representing the last 86 ka. A low-resolution alkenone sea surface temperature record was also measured. The time frame in both cores was mostly constrained by 'wiggle' matching with the nearby well-dated d 18 O and d 13 C record of the Soreq Cave, which is mainly influenced by the eastern Mediterranean water vapor. The sedimentary record of the southern core is strongly influenced by the River Nile contribution throughout the last 86 ka, as evi-denced by the higher sedimentation rates compared with the northern core (20 cm/ka vs. 5 cm/ka), continuously darker sediment colour, and higher TOC values (0.6–0.9 vs. 0.25 wt% not including sap-ropels). During sapropels S1 and S3, present in both cores, the influence of the River Nile became more widespread, reaching as far as Cyprus. Yet, the influence of the River Nile remained stronger in the south, as evident by the higher TOC values in the southern core throughout the entire 90 ka period and the longer duration of S1 in the southern core. An anomalous low d 18 O interval that is not recorded in western Mediterranean occurred between 58 and 49 ka in the Levantine Basin and is more developed in the northern core. This period correlates with D-O interstadial 14 and maximum northern hemisphere insolation during the lastglacial cycle, suggesting that the warming mainly impacted the northern Levant. The Eastern Mediterranean Sea and land area was considerably warmer than the western Mediter-ranean throughout the LGM – Holocene transition, and the d 18 O G. ruber drop of 4.5& is significantly greater than the 3& shift found for the western Mediterranean d 18 O G. bulloides , both differences reflecting an increased continental effect from the western to eastern Mediterranean. Comparison between the marine and the land d 18 O records suggests that the origin of rain over the land is composed of mixed signal from the southern and northern Levantine Basin. The study of Dd 18 O sea–land variations demon-strates that various factors have influenced the sea–land relationship during the last 90 ka. The 'amount effect' has an important influence on rainfall d 18 O during interglacial periods (particularly sapropel periods), whereas during glacial periods, increased land distances and elevation differences arising from decrease in sea level may have brought about decrease in d 18 O of rainfall due to Rayleigh distillation processes. These influences were superimposed on those of sea surface water d 18 O changes brought about by continental ice melting, and the strong effects felt in the southern Levantine Basin of the high River Nile input during periods of enhanced monsoonal activity. (A. Almogi-Labin), matthews@gsi.gov.il (M. Bar-Matthews), danch@pob.huji.ac.il (D. Shriki), martine.paterne@lsce.cnrs-gif.fr (M. Paterne), bschilman@gsi.gov.il (B. Schilman), ayalon@gsi.gov.il (A. Ayalon), zeev@vms.huji.ac.il (Z. Aizenshtat), alan@vms.huji.ac.il (A. Matthews).
1] The large (8%) changes in the past seasonal insolation have a well-documented influence on the Indian summer monsoon. However, the effect of the small (<1%) decade to century scale solar variability is less certain. Evidence is emerging that Earth's climate is sensitive to small changes in solar output on centennial time scale during the Holocene. Comparison of a recently published proxy record for sunspot activity with our newly-revised higher-resolution record of the Indian summer monsoon winds reveals multiple intervals of weak summer monsoon during the Holocene at multidecadal to centennial scales. Weak summer monsoon winds correlate with reduced solar output. Our results suggest that small changes in solar irradiance can bring pronounced changes in the tropical monsoon. The multidecade to century scale variations in the monsoon winds were much larger in the early Holocene coincident with increased sunspot numbers.
If radiocarbon measurements are to be used at all for chronological purposes, we have to use statistical meth-ods for calibration. The most widely used method of calibration can be seen as a simple application of Bayesian statistics, which uses both the information from the new measurement and information from the 14 C calibration curve. In most dating applications, however, we have larger numbers of 14 C measurements and we wish to relate those to events in the past. Baye-sian statistics provides a coherent framework in which such analysis can be performed and is becoming a core element in many 14 C dating projects. This article gives an overview of the main model components used in chronological analysis, their mathematical formulation, and examples of how such analyses can be performed using the latest version of the OxCal soft-ware (v4). Many such models can be put together, in a modular fashion, from simple elements, with defined constraints and groupings. In other cases, the commonly used "uniform phase" models might not be appropriate, and ramped, exponential, or normal distributions of events might be more useful. When considering analyses of these kinds, it is useful to be able run sim-ulations on synthetic data. Methods for performing such tests are discussed here along with other methods of diagnosing pos-sible problems with statistical models of this kind.
This paper presents an overview of the evolution of the Nile deep-sea turbidite system during the last 200 kyr, over a series of glacial to interglacial cycles. Six individual deep-sea fans were identified from an extensive field data set. Each fan comprises a canyon, channel system and terminal lobes. Two of these fan systems were possibly active at the same time, at least during some periods. Large-scale slope failures destroyed channel segments and caused the formation of new submarine fan systems. These slope failures thus played an important role in the overall evolution of the turbidite system. During the last glacial maximum (ca 25 to 14·8 ka) the central and eastern parts of the Nile deep-sea turbidite system were relatively inactive. This inactivity corresponds to a lowstand in sea-level, and a period of arid climate and relatively low sediment discharge from the Nile fluvial system. Rapid accumulation of fluvial flood-derived deposits occurred across the shallower part of the submarine delta during sea-level rise between ca 14·8 and 5 ka. The most recent deep-sea channel–lobe system was very active during this period of rising sea-level, which is also associated with a wetter continental climate and increased sediment and water discharge from the Nile. Increased sediment deposition in shallower water areas led to occasional large-scale slope failure. The Nile deep-sea turbidite system was largely inactive after ca 5 ka. This widespread inactivity is due to retreat of the coastline away from the continental shelf break, and to a more arid continental climate and reduced discharge of sediment from the Nile. The Nile deep-sea turbidite system may be more active during periods of rising and high sea-level associated with wetter climates, than during lowstands, and may rapidly become largely inactive during highstands in sea-level coupled with arid periods. These acute responses to climate change have produced sedimentary/stratigraphic features that diverge from traditional sequence models in their nature and timing. This large-scale sedimentary system responded to monsoon-driven climate change and sea-level change in a system-wide and contemporaneous manner.
If radiocarbon measurements are to be used at all for chronological purposes, we have to use statistical methods for calibration. The most widely used method of calibration can be seen as a simple application of Bayesian statistics, which uses both the information from the new measurement and information from the 14 C calibration curve. In most dating applications, however, we have larger numbers of 14 C measurements and we wish to relate those to events in the past. Bayesian statistics provides a coherent framework in which such analysis can be performed and is becoming a core element in many 14 C dating projects. This article gives an overview of the main model components used in chronological analysis, their mathematical formulation, and examples of how such analyses can be performed using the latest version of the OxCal software (v4). Many such models can be put together, in a modular fashion, from simple elements, with defined constraints and groupings. In other cases, the commonly used “uniform phase” models might not be appropriate, and ramped, exponential, or normal distributions of events might be more useful. When considering analyses of these kinds, it is useful to be able run simulations on synthetic data. Methods for performing such tests are discussed here along with other methods of diagnosing possible problems with statistical models of this kind.
A reconstruction of the foraminiferal record of the past 3500 years in the SE Levantine Basin is presented. The planktic assemblages are of low diversity and dominated by spinose epi-pelagic and shallow meso-pelagic species, most of them symbiont-bearing, accompanied by lesser amounts of deeper dwelling non-spinose and symbiontbarren species. Benthic assemblages are dominated by endobenthic or potentially endobenthic, food-dependent opportunistic species able to withstand hypoxia in muddy sediments. The foraminiferal record shows two partially overlapping sequences of events. The first can be discerned by cycles in the frequency of Globigerina sacculifer and reflects ecological conditions of the upper waters above the pycnocline. The second sequence comprises changes in the deep planktic species and endobenthic foraminifera. These two groups are relatively rare before 1700 y BP and increase in abundance after this date.
Over the past 13 ka, the hydrology for the southeastern Mediterranean wasmainly regulated by Nile River runoff, which in turn was controlled by climate forcing. Being affected by orbital forcing, and the position of the Intertropical Convergence Zone (ITCZ), planktic foraminiferal data (assemblages, stable isotopes, and size properties) indicate three major periods. (1) From 13.0 to 11.5 ka, the upper water column was well-mixed, cold, and productive. (2) From 11.5 to 6.4 ka, hydrology and foraminifers were affected by intensified monsoonal circulation. The enhanced size of Globigerinoides ruber is interpreted as a response to environmental stress caused by low-saline waters. (3) After 6.4 ka, the southward retreat of the ITCZ caused a decrease in freshwater discharge and hence a return to ecological equilibrium. A drop in foraminifer diversity from 2.9 to 1.1 ka was related to more arid conditions, and limited supply of nutrients from the Nile River. We suggest a link to a negative North Atlantic Oscillation (NAO)marking the Roman Humid Period in the westernMediterranean, and in anti-phase with the southeastern Mediterranean aridity. Because Nile River runoff exerted major control on surface hydrology, a connection to Indian and Pacific climate systems partially controlling precipitation over the Nile catchment area is hypothesized. From 1.1 to 0.54 ka, high foraminifer diversity indicates humid conditions synchronous to the Medieval Climate Anomaly under a positive NAO state. Over the past 0.54 ka encompassing the Little Ice Age, another arid period is indicated by a drop in foraminifer diversity.
The Age Calibration Program, CALIB, published in 1986 and amended in 1987 is here amended anew. The program is available on a floppy disk in this publication. The new calibration data set covers nearly 22 000 Cal yr (approx 18 400 14C yr) and represents a 6 yr timescale calibration effort by several laboratories. The data are described and the program outlined. -K.Clayton
Magnetic and geochemical core data spanning the last 17,000 years are correlated with new seismic stratigraphy from Lake Tana, Ethiopia, to infer past lake-level change and hence effective precipitation. The data confirm that low lake-level coincides with Heinrich Event 1 (H1) in the North Atlantic, as previously shown from diatom and pollen evidence (Lamb et al., 2007). The lake deepened at 15.3 cal kyr BP and abruptly returned to freshwater conditions, when the lake overflowed into the Blue Nile. Low runoff and lake levels and therefore rainfall are inferred between 13.0 and 12.5 cal kyr BP and may represent southerly suppression of the ITCZ and the associated monsoon front at the time of the Younger Dryas. Two drought episodes occurred at 8.4 and 7.5 cal kyr BP, and are also interpreted as a southward shift in the monsoon front. The first of these events appears to have preceded and been more significant than the 8.2 cal kyr BP. Precipitation declined after 6.8 cal kyr BP, although we do not see an abrupt end to the African Humid Period. This period culminated in a dry episode at ~ 4.2 cal kyr BP, supporting the view that reduced Nile flow was a contributing factor to the demise of the Egyptian Old Kingdom.
Both the channel of the lower White Nile and the soils adjacent to the river have a number of special characteristics, which are a direct reflection of its unique late Quaternary history. These attributes include a straight channel pattern (despite a flood gradient of 1 in 100 000 and a very fine suspension load) and localised concentrations of buried evaporites, carbonates, and highly saline subsoils at intervals alongside the river. Late Quaternary aeolian, fluviatile and lacustrine deposits in and near the lower White Nile valley reveal a strong contrast between the often dry, cold and windy late Pleistocene climates characteristic of the Last Glacial Maximum (18 000±3000 BP) and the wetter and warmer climates prevalent in those regions towards 11 500–11 000 BP, 9500 BP and 8500–7000 BP. Buried shell-beds and recessional strandlines indicate a sudden and rapid regression of the vastly expanded terminal Pleistocene White Nile in possible Younger Dryas times (11 000–10 000 BP) coinciding with probable temporary closure of Lake Victoria in the Ugandan upper reaches of the White Nile. The stable carbon and oxygen isotopic composition of freshwater gastropod shells in the lower White Nile region is consistent with a stronger summer monsoon towards 11 500–11 000 BP and again towards 8500–7000 BP. At intervals between ca. 18 000 and 12 000 BP, the lower White Nile was a strongly seasonal river, which shifted course frequently and carried large quantities of medium and coarse sand. The present Sudd swamps in the southern Sudan cannot have been in existence during those times, but came into existence (or became reestablished) during the Holocene.
Pollen and charcoal analysis of sediment cores from two lakes in the highlands of northern Ethiopia provide evidence that the vegetation has changed in response to human impact during the last 3000 years. The natural, pre-disturbance vegetation of the area was Podocarpus-Juniperus forest. At about 500 bc, following Semitic immigration to northern Ethiopia, the forests were cleared and replaced by a secondary vegetation of Dodonaea scrub and grassland that persisted for 1800 years. Grasslands were dominant from about ad 1200 to 1400, probably as a result of further intensification of grazing, perhaps exacerbated by drought. Juniperus forest, with Olea and Celtis, then expanded from ad 1400 to 1700, possibly because of drought-induceddepopu lation followed by increased rainfall. Deforestation and soil erosion has again intensified during the last three centuries. Since forest regrowth was possible after 1800 years of human impact, northern Ethiopia should again be capable of supporting forest under appropriate land management.
Interpretation of carbon isotope records from late Quaternary planktonic
foraminifers are confounded due to the presence of a significant
physiological component in the carbon isotopic signal. A quantitative
carbon isotope (QC) model is presented which relates the carbon isotopic
composition of a foraminiferal shell to the physiological processes of
respiration and symbiont photosynthesis and to the δ13C
value of seawater ΣCO2. The QC model is calibrated with
physiological and stable isotopic data from laboratory experiments with
living planktonic foraminifers. Model simulations of chamber and shell
δ13C values with the symbiont-bearing foraminifers,
Orbulina universa and Globigerinoides sacculifer, suggest (1) variations
in symbiont density and photosynthetic rate (light level or habitat
depth) are the primary physiological parameters controlling
intraspecific carbon isotopic variability in these species, (2)
respiration has little effect on the δ13C of O.
universa, and (3) each chamber in a multichambered foraminiferal test
will have a distinct δ13C value depending on its
position in the test whorl. Size:δ13C value
relationships reported for G. sacculifer from fossil assemblages can be
explained as a function of increasing symbiont density during
ontogenetic development.
Local sea level curves are obtained by using various methods, from land as well as from sea records and are based on different indicators. The most updated Holocene sea level curve for the coast of Israel, Eastern Mediterranean, from about 9500calyears ago up to about 700 hundred years ago (the end of the Crusaders period) is based mainly on coastal archaeological remains. For the last Millennia an attempt was made to use bio-constructions along the present-day abrasion platforms as a tool for reconstructing past sea levels. Here we present radiocarbon dating of pre-bomb mollusks from the intertidal zone in order to determine the reservoir age of the bio-construction.
Pollen spectra from three eastern Mediterranean cores have been used to document the paleoclimates of the Levantine Basin borderlands over the last 25O kyr to establish the relationship between this regional climate data set and the global climates as recorded by foraminiferal delta18O and to compare it with proximal land pollen records. Core MD 84642 with eight sapropels covers the last two climatic cycles up to the early Holocene, MD 84627 with four sapropels goes back to 125 kyr, and MD 84629 with one sapropel covers the last 70 kyr. The sedimentation rate decreases from core 629, located at the shallowest depth beneath the Nile River plume, to cores 627 and 642. During the interglacials defined by a low 18O/16O ratio, the abundance of tree pollen is maximum and points to an optimum Mediterranean climate with greatest humidity, including some summer rainfall. During glacial maxima, with highest 18O/16O ratio, the pollen abundance is high for steppe and semidesert plants and low for trees, indicating a definitely more arid, more continental, and probably colder climate. The variations of pollen abundance occur in phase with those of the foraminifer delta18O record. This signifies that the regional climate of the Levantine Basin borderlands has the same temporal pattern as the global ice volume documented by the ice volume curve.
In an East Mediterranean marine core, the upper sapropel begins soon after the start of a global event—a very heavy precipitation which occurred in the equatorial latitudes during the late Glacial–early Holocene. This heavy precipitation in Africa, channelled by the Nile River across 35° of latitude, produced a low-salinity surface layer in the East Mediterranean. In this confined basin, with high bottom salinity, the steep salinity gradient stratified the water column. The stagnant bottom waters triggered the sapropel formation. Cretaceous sapropels in the tropical oceans may result from the same chain of events in warm, humid climates, with contrasting wet-and-dry seasonal rhythm.
The organic carbon isotope record of the sapropels (S1 and S3-S10) and intercalated marl oozes has been determined in a 12m piston core from the eastern Mediterranean. The observed differences in δ13Corganic between the two lithologies are probably produced by changes in the isotopic composition and the concentration of dissolved CO2. First, freshwater flooding during the formation of the sapropels caused the isotopic composition of the dissolved inorganic carbon in the surface waters of the Mediterranean to become lighter because of the 13C deficiency in fresh waters. Second, changes in atmospheric pCO2 between glacial and interglacial periods, as shown by the Vostok ice core, caused marked changes in the concentration of free dissolved CO2 in the mixed layer. Concentrations of dissolved CO2 could also have been much higher during the deposition of the sapropels because of the supply of regenerated CO2 to the mixed layer by upwelling, and this could have further lightened the δ13Corganic values in the sapropels themselves. Carbon isotope records may provide an alternative method for estimating atmospheric pCO2 levels over longer time periods than can be obtained from ice cores. -from Authors
Both the carbon isotopic composition of seawater and physiological "vital effects" such as respiration and symbiont photosynthesis influence the deltaC-13 value of foraminiferal calcite. The symbiont-bearing planktic foraminifera, Orbulina universa, was grown in a series of laboratory experiments to determine whether vital effects differentially obscure the deltaC-13 signal such that changes in the deltaC-13 of seawater SIGMACO2 cannot be quantitatively evaluated. Foraminifera were cultured under three different irradiance levels to elicit a full range of symbiont photosynthetic effects and in seawater with deltaC-13 values ranging between +1.77 parts per thousand (ambient) and +5.76 parts per thousand. Power regression analysis of isotope data from individual shells plotted against culture irradiance yields three curves which are nearly parallel but are offset by the difference between the carbon isotopic composition of the culture waters. When irradiance groups are combined and O. universa deltaC-13 values are plotted against seawater deltaC-13, three parallel linear regressions are obtained with slopes ranging between 0.93 and 0.96. Because the slopes are near unity, it is concluded that the deltaC-13 of O. universa accurately tracks changes in the carbon isotopic composition of seawater regardless of the magnitude of its physiological vital effect. O. universa oxygen isotope data show that individuals maintained in the low and medium irradiance groups secrete their shells in isotopic equilibrium according to established paleotemperature equations. In contrast, individuals grown under high irradiance levels (maximum symbiont photosynthetic rates) are on average 0.35 parts per thousand depleted in O-18 relative to equilibrium. Symbiont-enhanced calcification rates in the high light group may be responsible for the observed deltaO-18 difference.
The provenance of sediments within the Levantine basin of the eastern Mediterranean was studied using 87Sr/86Sr isotopic ratios, together with major elements. Measurements were made on the detrital fraction of surface sediments, and the two most important sources of detrital matter to the region. Saharan dust was characterised by an 87Sr/86Sr isotopic ratio in the range 0.7160–0.7192. There was a small systematic decrease in 87Sr/86Sr from west to east which is interpreted as due to a change in the balance of aeolian source material. The Nile particulate matter had a 87Sr/86Sr isotopic ratio in the range 0.7057–0.7071. 87Sr/86Sr isotopic signatures together with Sr concentration were used to calculate the contributions made by Saharan dust and Nile particulate matter to the surface sediments of the Levantine basin. It was shown that Nile-derived sediment was dispersed widely across the Levantine basin enabling the area affected by retention of sediment behind the Aswan dam to be determined.
1] The result of paleoclimate studies on the relationship between the Indian monsoon and solar activity, inferred from the analysis of stalagmites in Oman, is confirmed by using a modern meteorological dataset from 1958 –1999. The present result suggests that the solar influence on monsoon activity is not due to a change in radiative heating in the troposphere but, rather, originates from the stratosphere through modulation of the upwelling in the equatorial troposphere, which produces a north-south seesaw of convective activity over the Indian Ocean sector during summer. Higher precipitation over Arabia and India, thus, occurs during high solar activity.
Two large-scale investigations of the eastern Mediterranean: POEM — an international cooperative effort, and GOIN — an initiative of the former USSR, were carried out between 1985 and 1995. The two projects were not coordinated and therefore the cruises of the two sometime overlap and sometime complement each other in time and in space. Statistical comparison between adjacent stations and comparisons between parameter maps indicated that there is no need to adjust any of the data and that the two data bases could be combined into a single POEM – GOIN data base (PGDB). The PGDB was employed for the description of winter and summer distributions of temperature and salinity in the eastern Mediterranean Sea. These appear to differ in some respects from the climatologies extracted from the MODB or from those extracted from the Naval Oceanographic Office gridded hydrographic data base. Since the MODB contains a large number of coastal cruises which were intended to monitor the Nile floods, the MODB salinity climatology reflects the conspicuous effects of those floods. However, this feature disappeared with the closure of the Aswan dam in 1965. On the other hand, some well-known mesoscale features of the eastern Mediterranean circulation do not appear in the climatological fields. We conclude that the differences are due to indiscriminate averaging, which does not produce a useful representative climatology. The PGDB also provided useful additional information in the southern region of the Ionian Sea, which was not covered by POEM cruises. Thus, details of the southern branch of the MAW, the branch that flows directly from the Sicily straits to the Cretan Passage, could be described. The PGDB allowed us to follow the history of the Irapetra Gyre and to show that, indeed, the gyre appeared only in the summer of 1987 and, apparently, lasted only until 1995. We describe the changes throughout this period, in intensity, size, and position of the Irapetra Gyre. The examination of previous cruises as well as a thorough search through the data of Med Atlas I indicated some occurrences of the Irapetra Gyre before its description by the POEM investigation, and therefore this gyre should be defined as a recurrent feature.
The distribution of living planktic foraminifers and their relation to the hydrography of the Caribbean Sea was investigated in plankton net tows and surface sediment samples taken along the Antilles island arc during April/May 1996. The planktic foraminiferal community was strongly influenced by spatial variations in salinity that were largely due to the influx of Orinoco River water into the southeastern Caribbean Sea and inflowing Sargasso Sea water in the north. Along the Antilles island arc, Globigerinoides ruber was the dominant species in the surface waters throughout. In the southeastern Caribbean Sea, where Orinoco River outflow influences the planktic community, standing stocks of planktic foraminifers (>100 μm) between 4 and 50 specimens m−3 were medium to low. The southeastern faunas between Tobago and Guadeloupe were characterized by increased proportions of Neogloboquadrina dutertrei. Highest standing stocks of 159 specimens m−3 in the upper 20 m of the water column were recorded in the northeastern Caribbean Sea and the assemblages were characterized by high proportions of Globigerinita glutinata, associated with cyclonic eddies. In the Anegada Passage, where Sargasso Sea water flows into the Caribbean Sea, low standing stocks of 18 specimens m−3 indicate oligotrophic conditions. Together with the oligotrophic surface waters, the Subtropical Underwater enters the Caribbean Sea through the Anegada Passage in water depths between 100 and 300 m. These waters are characterized by higher concentrations of Globorotalia truncatulinoides relative to the adjacent water masses. Comparison of the living planktic foraminiferal fauna with empty test assemblages from the water column and from surface sediments shows that differences in the faunal composition mostly correspond to the distribution of water masses and to the differential dissolution of species. In the vicinity of islands Globigerinoides ruber reaches higher relative frequencies than in the open ocean, pointing towards a higher tolerance of this species towards neritic conditions than in other species.
Our results show that the late Pleistocene Nile in northern Sudan was shifting position and actively aggrading at 145 ± 20 kyr, 83 ± 24 kyr, 32 ± 8 kyr and 20.7 ± 0.2 kyr and indicate, for the first time, a phase of high-energy flow in the White Nile at 27.8 ± 3.2 kyr, with still high but somewhat reduced flow in that river at 13.3 kyr, 10 kyr and 4.8–4.0 kyr. Beach ridges associated with a 386 m strandline of the White Nile have OSL ages of 27.5 ± 2.7 kyr and 14.5 ± 1.6 kyr. The Holocene terraces and former channels of the main Nile have ages of 11 kyr, 6.5–5.0 kyr and 4.8–4.0 kyr, after which there was a general decline in flood discharge. The now arid main Nile valley in northern Sudan was significantly wetter during the early to middle Holocene, with a lake up to 450 km2 in area, fed by an overflow channel from the early Holocene Nile between 9.5 kyr and 7.5 kyr. Previously stable late Pleistocene dunes were reactivated at intervals during the Holocene, with five samples from the White Nile valley indicating brief phases of Holocene dune activity at 9.9 ± 2.0 kyr, 9.0 ± 2.8 kyr, 6.6 ± 0.9 kyr, 4.8 ± 0.9 kyr and 2.9 ± 0.5 kyr, the earliest of which occurred within periods of generally wetter climate and higher Nile flow. The youngest freshwater shells on the Khor Abu Habl alluvial fan west of the White Nile correspond to a time of regionally wetter climate between 1.7 and 1.0 kyr. Our results suggest that millennial scale climatic instability may have been characteristic of Holocene climates in this region.
Changes in 87Sr/86Sr and major element geochemistry, from two sediment cores (9509 and 9501) in the Eastern Mediterranean (EM), were used to resolve changes in sediment provenance and, hence, determine climate changes in the Nile catchment and Eastern Sahara desert over the past 25 ka. The sediment was described by a three end-member system comprising Blue Nile (BN; 87Sr/86Sr = 0.7506; Sr = 210 ppm), White Nile (WN; 87Sr/86Sr = 0.7094; Sr = 72.5 ppm) and Saharan dust (SD; 87Sr/86Sr = 0.7183; Sr = 99 ppm). The sedimentary record of these cores represents the suspended load carried down the Nile river and discharged into the S.E. Levantine basin and thus records palaeoclimatically controlled changes in erosion and transport in the catchment. During arid periods (0–5 ka BP) and prior to 11 ka BP, fluxes of BN sediment at 9509 (∼6 g/cm2/yr & 10–12 g/cm2/yr, respectively) were greater than during the peak of the African Humid Period (AHP) from 5 to 11 ka BP (<2 g/cm2/yr); this latter period witnessed the deposition of the youngest organic-rich sediment, termed sapropel (S-1), in the EM basin. By contrast the flux of WN increased during the AHP from ∼5 g/cm2/yr at ∼13 ka BP to >15 g/cm2/yr. In the Ethiopian Highlands (BN catchment) increases in the amount and duration of the monsoon during the AHP caused more vegetation to grow resulting in less soil erosion. In the WN catchment increased rainfall caused more catchment erosion and higher sediment flux through the Sudd marshes. The sedimentation rate in core 9509 increased during the AHP because of the greater importance of the WN sediment flux relative to the BN sediment flux. Saharan dust flux also decreased during the AHP reaching a minimum at ∼6 ka BP (core 9509) due to ‘greening’ of the Sahara desert. At the onset of S-1, the changes in Nile flow as determined by 87Sr/86Sr and climatic changes in the EM basin determined by δ18O of planktonic foraminifera were simultaneous, confirming that such isotopic tracers cannot be used directly to determine the cause of the circulation changes in the EM at this time. The increase in the proportion of BN sediment at 9509 with a somewhat higher grain size during the H-1 period (15–17 ka BP) was caused by erosion and redistribution of sediment from the Nile delta and/or the Israeli coast as sea-level rose.
Distribution patterns of live planktic foraminifers are examined at two times of the year, late summer and late winter, through horizontal plankton towing along a NW-SE transect across the West and East Mediterranean Basins and in the Alboran Sea (French oceanographic cruises Vicomed I, II and III, respectively).Foraminiferal production is significantly different in the two sampling periods and across the West and East Mediterranean. Foraminiferal density is higher in winter than summer (e.g. in the western basin: 3700 specimens/1000 m3 in winter against 200–600 specimens/1000 m3 in summer). The production, averaged for the two sampling sets, is higher in the Western Basin than in the Eastern one. However, in late summer, peaks of maximal production are observed in the Central Mediterranean (Strait of Sicily and Ionian Basin) (1000 to 2000 specimens/1000 m3), where the assemblages are dominated by Globigerinoides ruber rosea and G. ruber alba.In the Mediterranean Sea, frontal regions and semipermanent eddies, which are essentially controlled by regional hydrography, represent a significant portion of the regional production. They also elicit a higher diversity of species. Their influence is observed in summer as well as in winter.At the end of summer, the rather shallow mixed layer hosts spinose symbiont-bearing species, such as G. ruber alba, G. ruber rosea, O. universa and G. trilobus. At that season, in the Western Basin, the nutrient content of the mixed layer is generally low, and the surface layer is rapidly nutrient depleted. Summer foraminiferal production remains low at the surface, with respect to the winter production. In the Eastern Basin, late summer and autumn correspond to the “biological spring”. Summer foraminiferal production in surface waters is higher than in winter. But standing crops are lower than in the Western Basin for the same species, at the same period. In summer however, frontal structures and eddies are generally better developed than in winter. In some particular cases (frontal boundaries in the Western Basin; eddies south of Cyprus), nutrients from intermediate layers can be upwelled, through isopycnal or diapycnal mixing, to the photic zone, and allow for secondary blooms. In the Western Basin these blooms favour the development of grazers at depth ( Globorotalia truncatulinoides, G. inflata). South of Cyprus, in the Eastern Basin, oligotrophic conditions promote the development of heterotrophic organisms and predatory foraminifers have higher standing stocks ( G. ruber alba, G. ruber rosea).In winter, the break down of the thermocline combined with deep turbulent mixing in the Northwestern Basin, and the deepening of the mixed layer in the Alboran Sea and along the North African coast, result in increased phytoplankton biomass and favour reproduction and proliferation of species such as G. truncatulinoides or G. inflata.Our observations indicate that the geographic and depth distribution of live foraminiferal species cannot be entirely explained by the general temperature and salinity differences among the different Mediterranean Basins but that they are strongly related to regional hydrographic patterns. Hydrography controls the seasonality and depth of the turbulent mixing (which differ between the Western and the Eastern Basins), the depth of the mixed layer and its seasonal stratification, and the strength of the pycnocline. Nutrient supply and primary production are mostly regulated by these dynamics. One may therefore expect that the species assemblages, on the sea floor and in deep-sea cores, will also correlate to the various hydrographic parameters of the overlying water masses.
High-resolution seismic data from Lake Tana, the source of the Blue Nile in northern Ethiopia, reveal a deep sedimentary sequence divided by four strong reflectors. Data from nearshore cores show that the uppermost strong reflector represents a stiff silt unit, interpreted as a desiccation surface. Channel cuts in this surface, bordered by levee-like structures, are apparent in the seismic data from near the lake margin, suggesting fluvial downcutting and over-bank deposition during seasonal flood events. Periphytic diatoms and peat at the base of a core from the deepest part of the lake overlie compacted sediments, indicating that desiccation was followed by development of shallow-water environments and papyrus swamp in the central basin between 16,700 and 15,100 cal BP. As the lake level rose, open-water evaporation from the closed lake caused it to become slightly saline, as indicated by halophytic diatoms. An abrupt return to freshwater conditions occurred at 14,750 cal BP, when the lake overflowed into the Blue Nile. Further reflection surfaces with downcut structures are identifiable in seismic images of the overlying sediments, suggesting at least two lesser lake-level falls, tentatively dated to about 12,000 and 8000 cal BP. Since Lake Victoria, the source of the White Nile, was also dry until 15,000 cal BP, and did not reach overflow until 14,500 cal BP, the entire Nile system must have been reduced to intermittent seasonal flow until about 14,500 cal BP, when baseflow was re-established with almost simultaneous overflow of the headwater lakes of both the White and Blue Nile rivers. Desiccation of the Nile sources coincides with Heinrich event 1, when cessation of northward heat transport from the tropical Atlantic disrupted the Atlantic monsoon, causing drought in north tropical Africa. The strong reflectors at deeper levels in the seismic sequence of Lake Tana may represent earlier desiccation events, possibly contemporaneous with previous Late Pleistocene Heinrich events.
In this paper, it is proposed that the formation of eastern Mediterranean sapropels occurred in an anti-estuarine type of circulation, which was to some degree weakened relative to the present in response to reduction of the eastern Mediterranean excess of evaporation over freshwater input. This reduction of excess evaporation would have been imposed by intensifications of (1) the Indian Ocean summer (SW) monsoon, influencing the eastern Mediterranean via increased Nile discharge, and (2) the system of Mediterranean depressions (an element of the westerly Atlantic system) causing increased precipitation and decreased evaporation. Both the Indian Ocean summer monsoon, and the westerly Atlantic system, would be intensified in response to the occurrence of distinct minima in the cycle of precession.
The Holocene climate evolution in Northern Africa is studied in a 9000-yr-long transient simulation with a coupled atmosphere–ocean–vegetation model forced by changes in insolation and atmospheric greenhouse gas concentrations. The model simulates in the monsoonal domains a significant decrease in precipitation under influence of the orbitally forced reduction in summer insolation. In the Western Sahara region, the simulated mid-Holocene transition from humid to arid conditions (the termination of the African Humid Period) is highly non-linear with the occurrence of centennial-scale climate fluctuations due to the biogeophysical feedback between precipitation and vegetation cover. This result is in agreement with proxy data from the Western Sahara region. The other monsoonal regions experience a more gradual climate evolution that linearly follows the insolation forcing, which appears in disagreement with available lake level records.
Multiproxy analyses were performed on core MS27PT recovered in hemipelagic sediments deposited on the Nile margin in order to reconstruct Nile River palaeohydrological fluctuations during the last 100,000 years. The strontium and neodymium isotope composition of the terrigenous fraction and the major element distribution reveal large and abrupt changes in source, oscillating between a dominant aeolian Saharan contribution during arid periods and a dominant Nile River contribution during pluvial periods. Iron content shows a strong correlation with strontium and neodymium isotopes. This allows the use of a high-resolution continuous Fe record as a proxy of Blue Nile sediment input over the last 100,000 years. The detailed Fe record, with approximately 10 years resolution during pluvial periods, is consistent with subtropical African records of well-dated lake level fluctuations and thus constitutes a first continuous high resolution record of the East African monsoon regime intensity over Ethiopia.
Paleohydrological data from the African tropics and subtropics, including lake, groundwater and speleothem records, are reviewed to show how environments and climates from both hemispheres are inter-related. Although orbitally induced changes in the monsoon strength account for a large part of long-term climatic changes in tropical Africa, the Late Pleistocene–Holocene hydrological fluctuations rather appear to have been a series of abrupt events that reflect complex interactions between orbital forcing, atmosphere, ocean and land surface conditions. During the Last Glacial Maximum (23–18 ka BP), most records indicate that generally dry conditions have prevailed in both hemispheres, associated with lower tropical land- and sea-surface temperatures. This agrees with simulations using coupled ocean–atmosphere models, which predict cooling and reduced summer precipitation in tropical Africa; the global hydrological cycle was weaker than today when the extent of large polar ice-sheets and sea-ice was a prominent forcing factor of the Earth's climate. Glacial-interglacial climatic changes started early: a first wetting/warming phase at ca. 17–16 ka BP took place during a period of rapid temperature increase in Antarctica. Next, two drastic arid-humid transitions in equatorial and northern Africa occurred around 15–14.5 ka BP and 11.5–11 ka BP. Both are thought to match the major Greenland warming events, in concert with the switching of the oceanic thermohaline circulation to modern mode. However, part of the climatic signal after 15 ka BP also seems related to the Antarctica climate. During the Holocene, Africa has also experienced rapid hydrological fluctuations of dramatic magnitude compared to the climatic changes at high latitudes. In particular, major dry spells occurred around 8.4–8 ka and 4.2–4 ka BP in the northern monsoon domain. Comparison with other parts of the world indicates that these events have a worldwide distribution but different regional expressions. In the absence of large polar ice sheets, changes in the continental hydrological cycles in the tropics may have a significant impact on the global climate system. Climate information gathered here allows to identify geographical and methodological gaps, and raise some scientific questions that remain to be solved to better understand how the tropics respond to changes in major climate-forcing factors, and how they influence climate globally.
We show that robust regressions can be established between relative sea-level (RSL) data and benthic foraminifera oxygen isotopic ratios from the North Atlantic and Equatorial Pacific Ocean over the last climatic cycle. We then apply these regressions to long benthic isotopic records retrieved at one North Atlantic and one Equatorial Pacific site to build a composite RSL curve, as well as the associated confidence interval, over the last four climatic cycles. Our proposed reconstruction of RSL is in good agreement with the sparse RSL data available prior to the last climatic cycle. We compute bottom water temperature changes at the two sites and at one Southern Indian Ocean site, taking into account potential variations in North Atlantic local deep water δ18O. Our results indicate that a Last Glacial Maximum (LGM) enrichment of the ocean mean oxygen isotopic ratio of 0.95‰ is the lowest value compatible with unfrozen deep waters in the Southern Indian Ocean if local deep water δ18O did not increase during glacials with respect to present. Such a value of the LGM mean ocean isotopic enrichment would impose a maximum decrease in local bottom water δ18O at the North Atlantic site of 0.30‰ during glacials.
The amount and type of organic matter in the sediments of lakes and oceans contribute to their paleoenvironmental and paleoclimatological records. Only a small fraction of the initial aquatic organic matter survives destruction and alteration during sinking and sedimentation. Selective degradation modifies the character of the surviving small fraction of organic matter which becomes incorporated in bottom sediments. Organic matter alterations can continue to sub-bottom depths of hundreds of meters, corresponding to millions of years. Source and paleoenvironmental information nonetheless remains preserved in the molecular, elemental and isotopic compositions of organic matter. C/N- and δ13C-values of total organic matter, in particular, appear to retain paleoenvironmental information for multi-Myr time periods.