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Neogene planktonic foraminifer biostratigraphy at Site 999, western Caribbean Sea
Abstract
Planktonic foraminifers were examined in at least three samples per core at Site 999 in the western Caribbean Sea (12°45'N, 78°44'W; 2829 m water depth) through sediments representing the last ~18 m.y. An age model for Hole 999A was constructed using the available magnetic reversal record (down to the top of the Gilbert Chron, 3.58 Ma) and selected plank-tonic foraminifer datum ages. Near 10 Ma an interval of extremely slow accumulation (5 m/m.y.) corresponds to the "carbonate crash" detected in other Leg 165 studies. Planktonic foraminifer datum ages, as calculated with the Hole 999A age model, are compared to the astrochronological ages assigned to datums at Ceara Rise (Leg 154) and to other published ages. Although there is general agreement, some significant differences are found that may be attributed to either regional paleoceanographic conditions or to shortcomings of the age model for this site. In the middle Miocene temperate latitude globoconellids (Glob-oconella praescitula, Gc. panda, and Gc. miozea) and in the upper Miocene Neogloboquadrina pachyderma (s) are found reg-ularly at Site 999, suggesting the existence of an influx of cool Pacific surface water and/or regional seasonal upwelling before the emergence of the Central American Isthmus. Menardellid species (Menardella miocenica, M. pertenuis, and M. exilis) endemic to the tropical Atlantic are all encountered at this Caribbean site, although not as regularly or in as large numbers as they were found in the western tropical Atlantic (Leg 154). Several species that were absent from the tropical Atlantic for much of the Pliocene were also found to be missing from the Caribbean record during similar intervals.
Supplementary resource (1)
Data
January 2000
... Changes to species abundance and extinctions of larger species are also important for size variation. Throughout the late Pliocene, the foraminiferal assemblage at Site 999 is affected by changes in species, either by extinction or fluctuations in occurrence (Berggren, 1969;Chaisson & Hondt, 2000). Dentoglobigerina altispira (3.1 Ma) and many of the Neogloboquadrina species go extinct (Chaisson & Hondt, 2000) in the late Pliocene. ...
... Throughout the late Pliocene, the foraminiferal assemblage at Site 999 is affected by changes in species, either by extinction or fluctuations in occurrence (Berggren, 1969;Chaisson & Hondt, 2000). Dentoglobigerina altispira (3.1 Ma) and many of the Neogloboquadrina species go extinct (Chaisson & Hondt, 2000) in the late Pliocene. Fluctuations in the abundance of menardellids, Neoglobquadrina dutertrei, Globigerinita glutinata, Gs. ruber, T. sacculifer, and the Globoturborotalia group are observed (Chaisson & Hondt, 2000), all of which will impact the sizes ranges within the assemblages. ...
... Dentoglobigerina altispira (3.1 Ma) and many of the Neogloboquadrina species go extinct (Chaisson & Hondt, 2000) in the late Pliocene. Fluctuations in the abundance of menardellids, Neoglobquadrina dutertrei, Globigerinita glutinata, Gs. ruber, T. sacculifer, and the Globoturborotalia group are observed (Chaisson & Hondt, 2000), all of which will impact the sizes ranges within the assemblages. Throughout the record, Site 999 is dominated by species of the Globigerinoides, Trilobatus, and Menardella genera, all are of the largest extant planktic foraminifers. ...
Atmospheric carbon dioxide (pCO2atm) is impacting the ocean and marine organisms directly via changes in carbonate chemistry and indirectly via a range of changes in physical parameters most dominantly temperature. To assess potential impacts of climate change on carbonate production in the open ocean, we measured size and weight of planktic foraminifers during the late Pliocene at pCO2atm concentrations comparable to today and global temperatures 2 to 3 °C warmer. Size of all foraminifers was measured at Atlantic Ocean Deep Sea Drilling Project (DSDP) Site 610, Ocean Drilling Program (ODP) Site 999, and Integrated Ocean Drilling Program (IODP) Site U1313. Test size was smaller during the Pliocene than in modern assemblages under the same environmental conditions. During the cold marine isotope stage (MIS) M2, size increased at Site 999, potentially linked to intensified stratification of the surface ocean in response to the closure of the Central American Seaway. At Site U1313, test size tracks the warming throughout the late Pliocene. Size‐normalized weight (SNW) of Globigerina bulloides at Site U1313 decreased during warmer temperature intervals. SNW of Globigerinoides ruber (white) at Site 999 displays high‐frequency variability not correlated to temperature. Yet during the glacial period within MIS M2, test weight was higher during higher temperatures. Our results support studies in the modern ocean, which challenge the view that carbonate chemistry is the primary driver for calcification. To better understand processes driving changes in SNW, computer tomography was used to quantify calcite to volume ratios. During interglacial periods, lower calcite volume but higher test volume suggests less suitable conditions for calcification. As this signal is not evident in SNW, subtle changes in calcification might not be observed by the weight‐based method.
... Shoaling of the Isthmus of Panama to 1000 m resulted in the flow of Pacific waters through the CAS into the Caribbean basin in the model presented by Nisancioglu et al. (2003). The flow of Pacific water into the Caribbean Basin agrees well with coccolith and planktonic foraminiferal assemblages (Chiasson and D'Hondt, 2000;Kameo and Sato, 2000) during this time. Site 999 (Caribbean Basin) and site 844 (eastern equatorial Pacific) recorded identical assemblages from 16.2-13.6 ...
... Foraminiferal assemblages identified from site 999 also suggest the flow of Pacific water into the Caribbean (Chiasson and D'Hondt, 2000). Chiasson and D'Hondt (2000) identified temperate-latitude foraminiferal assemblages (Globoconellids) at site 999 until ~10.7 Ma, and interpreted their presence to represent an influx of cool Pacific surface water, either the California or Peru Current system depending on the position of the ITCZ (Chiasson and D'Hondt, 2000). ...
... Foraminiferal assemblages identified from site 999 also suggest the flow of Pacific water into the Caribbean (Chiasson and D'Hondt, 2000). Chiasson and D'Hondt (2000) identified temperate-latitude foraminiferal assemblages (Globoconellids) at site 999 until ~10.7 Ma, and interpreted their presence to represent an influx of cool Pacific surface water, either the California or Peru Current system depending on the position of the ITCZ (Chiasson and D'Hondt, 2000). Flohn (1981) predicts a more northerly position of the ITCZ at this time (~10ºN) as a result of thermal asymmetry attributed to the differences in ice sheet development between the northern and southern hemispheres. ...
... The inhibited development of the peripheral keel suggests that at Site 926 the environmental conditions were not optimal for the G. fohsi lineage. Similarly, not well developed assemblages of the G. fohsi group were recorded at Site 999 (Caribbean Sea) (Chaisson and D'Hondt, 2000). These authors suggested that Caribbean Sea was a suboptimal environment for the full development of this lineage. ...
... The distribution of this species (as G. ruber) is also reported in other deep ocean successions (e.g. ODP Site 999, Chaisson and D'Hondt, 2000). The LCO of G. subquadratus is commonly used in the Mediterranean zonal schemes (Iaccarino and Salvatorini, 1982;Iaccarino, 1985;Foresi et al., 1998;Iaccarino et al., 2001;Foresi et al., 2002b). ...
... In addition, both at Site 926 and the Mediterranean, the L(C)O of G. subquadratus is slightly preceded by the LCO of D. kugleri (Table 3). The same succession of bioevents has also been recorded in the western Caribbean ODP Site 999 (Chaisson and D'Hondt, 2000;Kameo and Bralower, 2000) and possibly at the mid-latitude North Atlantic DSDP Site 608 (Jenkins, 1986;Gartner, 1992). ...
High-resolution calcareous plankton (planktonic foraminifera and calcareous nannofossils) biostratigraphy is presented from the Middle to early Late Miocene interval (from 14.45 to 8.86 Ma) at Site 926 (ODP Leg 154, equatorial Atlantic Ocean). The main bioevents used in the low-latitude zonal schemes, and also auxiliary events revealing potential biostratigraphic value have been recognised. The investigated succession ranges from N.10 to N.16 Zones based on planktonic foraminifera, and from NN5 (CN4) to NN10 (CN8) Zones based on calcareous nannofossils. The evolution of the planktonic foraminiferal Globorotalia fohsi lineage appears to be environmentally controlled. The main diagnostic features of the species of this lineage are not always evident, rendering problematic the definition of the N.9/N.10, N.10/N.11 and N.11/N.12 zonal boundaries. Calcareous plankton events have been calibrated on the basis of the Astronomical Time Scale of Shackleton and Crowhurst 〚Shackleton, N.J., Crowhurst, S., 1997. Sediment fluxes based on an orbitally tuned time scale 5 Ma to 14 Ma, Site 926. In: Curry, W.B., Shackleton, N.J., Richter, C., Bralower, T.J. (Eds.), Proceedings of the Ocean Drilling Program, Scientific Results, College Station, TX (Ocean Drilling Program) 154, pp. 69–82〛. The astrobiochronology obtained at Site 926 has been compared with that of the Mediterranean astronomically calibrated deep marine successions, allowing the evaluation of the degree of synchroneity and diachroneity of bioevents. Some bioevents, such as the last occurrence of Globigerinoides subquadratus dated at 11.55 Ma, the last occurrence of Sphenolithus heteromorphus dated at 13.51 Ma and the last common occurrence of Cyclicargolithus floridanus calibrated at 13.32 Ma, are near-synchronous events between the equatorial Atlantic and the Mediterranean area indicating their high biostratigratigraphic value in global correlation. The diachroneity of the last occurrence of Paragloborotalia siakensis, the first occurrence of Neogloboquadrina acostaensis and the last occurrence of Globorotalia peripheroronda between equatorial Atlantic and the Mediterranean reflect a different spatial and temporal distribution of these marker species probably due to a sharp definition of surface plankton provinces related to the latitudinal thermal gradient.
... Age Chaisson and Pearson (1997) and Berggren et al. (1995a,b). b Age coincides with Chaisson and D'Hondt (2000). c Age coincides with Stewart and Pearson (2000). ...
... d Age coincides with Wright and Kroon (2000). Chaisson and D'Hondt (2000). b Chaisson and Pearson (1997). ...
... estimates for datum levels are derived from Berggren et al. (1995a,b), Chaisson and Pearson (1997), Chaisson and D'Hondt (2000), Stewart and Pearson (2000), and Wright and Kroon (2000). A comparison of faunal event ages as determined by different authors is found in Table 2. Ages in these studies are based primarily on the time scale of Berggren et al. (1995a,b) with planktonic foraminifer and nannofossil data from Curry et al. (1995). ...
An assemblage of planktonic foraminifera is described from 125 samples taken from the Cercado, Gurabo, and Mao Formations in the Cibao Valley, northern Dominican Republic. The primary objectives of this study are to establish a biochronologic model for the late Neogene of the Dominican Republic and to examine sea surface conditions within the Cibao Basin during this interval. The Cercado Formation is loosely confined to Zones N17 and N18 (∼ 7.0–5.9 Ma). The Gurabo Formation spans Zones N18 and N19 (∼ 5.9–4.5 Ma). The Mao Formation is placed in Zone N19 (∼ 4.5–3.6 Ma). Changes in the relative abundances of indicator species are used to reconstruct sea surface conditions within the basin. Increasing relative abundances of Globigerinoides sacculifer and Globigerinoides ruber, in conjunction with a decreasing relative abundance of Globigerina bulloides, suggests the onset of increasing sea surface temperature and salinity in conjunction with diminishing primary productivity at ∼ 6.0 Ma. Abrupt increases in the relative abundances of G. sacculifer and G. ruber at ∼ 4.8 Ma suggest a major increase in sea surface temperature and salinity in the early Pliocene. The most likely mechanism for these changes is isolation of the Caribbean Ocean through progressive restriction of Pacific–Caribbean transfer via the Central American Seaway. Periods of high productivity associated with upwelling events are recorded in the upper Cercado Formation (∼ 6.1 Ma) and in the middle Mao Formation (∼ 4.2 Ma) by spikes in G. bulloides and Neogloboquadrina spp. respectively. The timing of major increases in sea surface salinity and temperature as well as decreasing productivity (∼ 4.8 Ma) and periods of upwelling (∼ 6.1and 4.2 Ma) in the Cibao Basin generally corroborate previously suggested Caribbean oceanographic changes related to the uplift of Panama. Changes in sea surface conditions depicted by paleobiogeographic distributions in the Cibao Basin suggest that shoaling along the Isthmus of Panama had implications in a shallow Caribbean basin as early as 6.0 Ma. Major paleobiologic changes between ∼ 4.8 and 4.2 Ma likely represent the period of final closure of the CAS and a nearly complete disconnection between Pacific and Caribbean water masses. This study illustrates the use of planktonic foraminifera in establishing some paleoceanographic conditions (salinity, temperature, productivity, and upwelling) within a shallow water basin, outlining the connection between regional and localized oceanographic changes.
... Western Caribbean ODP Site 999 and East Equatorial Pacific DSDP Site 503 ( Fig. 1; Table 1) were studied because of their high-resolution biochronology (Site 503: Keigwin, 1982;Zenker et al., 1987;Keller et al., 1989;Site 999: Kameo and Bralower, 2000;Chaisson and D'Hondt, 2000). Their relatively complete stratigraphy enabled precise Atlantic-Pacific diversity comparisons (Keigwin, 1982;McDougall, 1996;Bornmalm, 1997;Bornmalm et al., 1999;Chaisson and D'Hondt, 2000;Bickert et al., 2004). ...
... Western Caribbean ODP Site 999 and East Equatorial Pacific DSDP Site 503 ( Fig. 1; Table 1) were studied because of their high-resolution biochronology (Site 503: Keigwin, 1982;Zenker et al., 1987;Keller et al., 1989;Site 999: Kameo and Bralower, 2000;Chaisson and D'Hondt, 2000). Their relatively complete stratigraphy enabled precise Atlantic-Pacific diversity comparisons (Keigwin, 1982;McDougall, 1996;Bornmalm, 1997;Bornmalm et al., 1999;Chaisson and D'Hondt, 2000;Bickert et al., 2004). We also compared paleobathymetries for all the sites under study, to rule out paleodepth as a potential cause for changes observed in benthic foraminiferal diversity analyses. ...
... This age model is after Bickert et al. (2004), and compares well with the shipboard biostratigraphic datums based on nannofossil events. The planktic foraminiferal datum levels are also in close agreement (Chaisson and D'Hondt, 2000; for details see Jain and Collins, 2007). ...
Diversity trends 8.3–2.5 Ma in Caribbean deep-sea foraminifera, as interpreted from the indices Fisher's α and the Information Index [H(S)] parallel paleoproductivity proxies (benthic foraminiferal infaunal/epifaunal species ratio, benthic foraminifer accumulation rates and flux of organic carbon to the seafloor). Paleoproductivity never reached a eutrophic threshold value above which we would predict opposite trends of high paleoproductivity and low diversity, consistent with stressful conditions. Instead, results are similar to those from other studies of oligotrophic settings that show a positive and statistically significant correlation between paleoproductivity and diversity. The correlations between current intensity and watermass oxygenation with diversity are negative and statistically significant. These results are also borne out by Principal Component Analysis (PCA). Intervals of increased relative abundance of Epistominella exigua, a proxy for seasonality of phytodetrital input to the seafloor, coincide with increased diversity, suggesting that pulsed paleoproductivity enhanced the diversity signal in the Caribbean.
... The study focused on two main sites, one from the western Caribbean, ODP Site 999, the other from the EEP DSDP Site 503 ( Fig. 1a; Table 2). These sites were chosen for their proximity to the former Central American Seaway (Fig. 1a) and available high-resolution biochronology (Site 503: Keigwin, 1982;Zenker et al., 1987;Keller et al., 1989;Site 999: Kameo and Bralower, 2000;Chaisson and d'Hondt, 2000). Both sites have relatively stratigraphically complete sections (Keigwin, 1982;McDougall, 1996;Bornmalm, 1997;Bornmalm et al., 1999;Bickert et al., 2004). ...
... 5. Haug and Tiedemann (1998), Bickert et al. (2004). 6. Chaisson and d'Hondt (2000). The Colombian Basin in the Caribbean Sea, where both ODP Site 999 and DSDP Site 502 are located (Fig. 1b), exceeds a water depth of 4000 m, and is separated from the Atlantic Ocean by a shallow sill at a water depth of 1645 m at the Windward Passage, between Haiti and Cuba (Fig 1c). ...
... Benthic foraminiferal analysis was carried out on 47 samples from ODP Site 999. Sediment samples of 10 cm 3 Kameo and Bralower, 2000) and orbitally tuned planktic foraminifer datum points (solid squares; after Chaisson and d'Hondt, 2000). ...
This study tests the hypothesis that the late Miocene to early Pliocene constriction and closure of the Central American Seaway (CAS), connecting the tropical Atlantic and East quatorial Pacific (EEP), caused a decrease in productivity in the Caribbean, due to decreased coastal upwelling and an end to the connection with high-productivity tropical Pacific waters. The present study compared paleoceanographic proxies for the interval between 8.3 and 2.5 Ma in 47 samples from south Caribbean ODP Site 999 with published data on EEP DSDP Site 503. Proxies for Site 999 include the relative abundance of benthic foraminiferal species representing bottom current velocity and the flux of organic matter to the sea floor, the ratio of infaunal/epifaunal benthic foraminiferal species and benthic foraminifer accumulation rates (BFARs). In addition, we calculated % resistant planktic foraminifers species and used the previously published % sand fraction and benthic carbon isotope values from Site 999.During early shoaling of the Isthmus (8.3–7.9 Ma) the Caribbean was under mesotrophic conditions, with little ventilation of bottom waters and low current velocity. The pre-closure interval (7.6–4.2 Ma) saw enhanced seasonal input of phytodetritus with even more reduced ventilation, and enhanced dissolution between 6.8 and 4.8 Ma. During the post-closure interval (4.2–2.5 Ma) in the Caribbean, paleoproductivity decreased, current velocity was reduced, and ventilation improved, while the seasonality of phytodetrital input was reduced dramatically, coinciding with the establishment of the Atlantic–Pacific salinity contrast at 4.2 Ma. Our data support the hypothesis that late Miocene constriction of the CAS at 7.9 Ma and its closure at 4.2 Ma caused a gradual decrease in paleoproductivity in the Caribbean, consistent with decreased current velocity and seasonality of the phytodetrital input.
... Previously published data on the paleontological and micropaleontological content from rocks of the Tubará Cycle was compiled after an extensive search in databases and online libraries (Appendix A). We updated the taxonomic nomenclature of reported planktonic foraminifera using the modern Pforams@mikrotax catalog (Young et al., 2017) to recognize key taxa with updated biostratigraphy (Chaisson and Pearson, 1997;Chaisson and D'Hondt, 2000;Bylinskaya, 2004;Wade et al., 2011;Raffi et al., 2020). Our results follow the formally established chronostratigraphic framework and nomenclature for the Neogene on the Geologic Time Scale 2020 (Raffi et al., 2020). ...
... crassula, synonym of Gr. aemiliana according to Kennett and Srinivasan (1983), to suggest an Early Pliocene age for the Tubará Formation in localities such as Puerto Colombia (Bordine, 1974;Appendix A). However, it is now known that the first occurrence of these taxa can be traced back to the late Messinian, in the planktonic foraminifera Zone N18 (Chaisson and Pearson, 1997;Chaisson and D'Hondt, 2000;Raffi et al., 2020). This new information emphasizes the importance of updating previous age determinations for stratigraphic sections in the northeastern SSJB (Bordine, 1974;Appendix A). ...
... The stratigraphic range of the nannofossils and palynomorphs reported in this study were integrated with the paleomagnetic polarity record for the upper 140 msbd of the section (Kent and Spariosu, 1982), and planktonic foraminiferal data (Keigwin, 1982b) to establish a temporal framework for DSDP Hole 502A (Fig. 4). Data from Keigwin (1982b) were compared with recent time calibrations of planktonic foraminifera in the Caribbean (Chaisson and d'Hondt, 2000;Fraass et al., 2015), in order to adjust the age of planktonic foraminifera useful events. Magneto-stratigraphic data of Hole 502A from Kent and Spariosu (1982), was re-calibrated according to the Astronomically Tuned Neogene Time Scale (Raffi et al., 2020). ...
... This change is coincident with a relatively low number of terrestrial palynomorphs, so it is probable that increased nutrient availability was not driven by continental run-off. The most probable process that could explain this increase in nutrient availability is due to enhancement of seasonal upwelling as proposed by Keigwin (1982b) and Chaisson and d'Hondt (2000). ...
Palynology of the Neogene marine stratigraphic sequences in tropical latitudes is poorly studied Most studies focus on lower to middle Miocene deposits in the Caribbean, emphasizing the biostratigraphic value of dinoflagellate cysts. The palynological and calcareous nannofossil analysis of 50 samples from DSDP Hole 502A and their integration with the paleomagnetic and planktonic foraminifera data reveal a detailed and reliable chronostratigraphic framework, and the interpretation of the paleobathymetric and paleoenvironmental evolution of the sequence. Fossil content in a continuous late Tortonian to late Gelasian
(~7.56-1.80 Ma) sequence of deep-marine sediments serves to calibrate ages of dinoflagellate cysts, and comparison with high latitude records reveals synchronous (Operculodinium janduchenei, Selenopemphix
dionaeacysta, Barssidinium taxandrianum, and Lejeunecysta interrupta) and asynchronous events (Quadrina? condita) with biostratigraphic potential for the southwestern Caribbean Sea. Our results highlight the importance of analyzing continental palynomorphs in deep-marine deposits to increase the temporal resolution of future palynostratigraphic schemes. Micropaleontogical data indicate a lower bathyal–abyssal environment and four paleoceanographic settings defined by changes in surface water productivity. These intervals fluctuate from mesotrophic-oligotrophic (~7.56–6.31 Ma) to eutrophic (~6.18–4.98 Ma), oligotrophic (~4.74–3.41 Ma), and mesotrophic surface water conditions (~3.26–1.80 Ma). Changes in trophic conditions are strongly related to the paleoceanographic reorganization associated with the constriction and complete closure of the Central American Seaway, seasonal upwelling episodes, and the input of terrigenous organic matter probably transported from southern Central America and northern South America by turbidity currents.
... These assemblages were initially reported as indicating a middle to Late Oligocene age (Renz 1960). However, identification of Globorotalia semivera (Hornibrook) (=Paragloborotalia semivera) by H. Duque Caro from the Jimol Formation (Zapata 2010) supports a Late Early Miocene age (Chaisson and D'Hondt 2000). ...
... A diverse foraminiferal assemblage from the lower part of the Castilletes Formation has been described (Burgl 1960), including the planktonic foraminifera Globigerina trilocularis, Globigerinoides rubra, Globigerinoita morugaensis, Globorotalia mayeri, and Globorotalia praemenardii. Of these, G. mayeri (=Paragloborotalia mayeri) and G. praemenardii (=Menardella praemenardii) are presently regarded as indicative of the early Late Miocene (Chaisson and D'Hondt 2000;Rincón et al. 2007). ...
The Cocinetas Basin of Colombia provides a valuable window into the geological and paleontological history of northern South America during the Neogene. Two major findings provide new insights into the Neogene history of this Cocinetas Basin: (1) a formal re-description of the Jimol and Castilletes formations, including a revised contact; and (2) the description of a new lithostratigraphic unit, the Ware Formation (Late Pliocene). We conducted extensive fieldwork to develop a basin-scale stratigraphy, made exhaustive paleontological collections, and performed 87Sr/86Sr geochronology to document the transition from the fully marine environment of the Jimol Formation (ca. 17.9–16.7 Ma) to the fluvio-deltaic environment of the Castilletes (ca. 16.7–14.2 Ma) and Ware (ca. 3.5–2.8 Ma) formations. We also describe evidence for short-term periodic changes in depositional environments in the Jimol and Castilletes formations. The marine invertebrate fauna of the Jimol and Castilletes formations are among the richest yet recorded from Colombia during the Neogene. The Castilletes and Ware formations have also yielded diverse and biogeographically significant fossil vertebrate assemblages. The revised lithostratigraphy and chronostratigraphy presented here provides the necessary background information to explore the complete evolutionary and biogeographic significance of the excellent fossil record of the Cocinetas Basin.
... Vincent and Toumarkine (1995) found identical orbitally-tuned ages for these two biostratigraphic datums at ODP Site 846 in the eastern equatorial Pacific. In the southern Caribbean ODP Site 999, Chaisson and D'Hondt (2000) reported the LO of G. (F.) fohsi in the same sample as the FO of G. menardii. ...
... Proto-warm pool development in the WEP coincided with the onset of the "carbonate crash" in the Caribbean based on the LO datum of Globorotalia (Fohsella) fohsi (11.68 Ma) at ODP Site 806 (Chaisson and Leckie, 1993) and ODP Site 999 in the Caribbean (Chaisson and D'Hondt, 2000). These two events also occurred when the Atlantic-Pacific carbon isotope gradient (i.e., NCW production) intensified in two steps approximately coincident with Miocene isotope events Mi5 and Mi6 at 11.7-11.5 ...
We document the waxing and waning of a 'proto-Western Pacific Warm Pool' and Equatorial Undercurrent (EUC) based on a study of multi-species planktic foraminiferal isotope ratios and census data spanning the 13.2-5.8 Ma interval at ODP Site 806 in the western equatorial Pacific (WEP). We hypothesize that the presence or absence of a proto-warm pool in the WEP, caused by the progressive tectonic constriction of the Indonesian Seaway and modulated by sea level fluctuations, created El Ni~{n}o/La Ni~{n}a-like alternations of hydrographic conditions across the equatorial Pacific during the late Miocene. This hypothesis is supported by the general antithetical relationship observed between carbonate productivity and preservation in the western and eastern equatorial Pacific, a phenomenon caused by these alternating ocean-climate states in the modern ocean. The two-step development and intensification of a proto-warm pool 11.6-9.6 Ma coincides with ice sheet expansion associated with Miocene isotope events Mi5 and Mi6, resulting in a cumulative sea level fall of 50 m and production of Northern Component Water. It also marks the initiation of a more modern equatorial current system as La Ni~{n}a-like conditions became established across the tropical Pacific. This situation sustained carbonate and silica productivity in the eastern equatorial Pacific (EEP) at a time when carbonate preservation sharply declined in the Caribbean. Proto-warm pool weakening after 10 Ma may have contributed to the nadir of a similar 'carbonate crash' in the EEP. Decay of the proto-warm pool and resultant El Ni~{n}o-like conditions brought higher productivity to the WEP, particularly 9.0-8.8 Ma coincident with a major perturbation in tropical nannofossil assemblages. This interval of increased productivity records the initial phase of the widespread 'biogenic bloom'. Resurgence of a later proto-warm pool in the WEP 6.5-6.1 Ma may have spurred renewed La Ni~{n}a-like conditions, which contributed to a strong late phase of the 'biogenic bloom' in the EEP.
... Vincent and Toumarkine (1995) found identical orbitally-tuned ages for these two biostratigraphic datums at ODP Site 846 in the eastern equatorial Pacific. In the southern Caribbean ODP Site 999, Chaisson and D'Hondt (2000) reported the LO of G. (F.) fohsi in the same sample as the FO of G. menardii. ...
... Proto-warm pool development in the WEP coincided with the onset of the "carbonate crash" in the Caribbean based on the LO datum of Globorotalia (Fohsella) fohsi (11.68 Ma) at ODP Site 806 (Chaisson and Leckie, 1993) and ODP Site 999 in the Caribbean (Chaisson and D'Hondt, 2000). These two events also occurred when the Atlantic-Pacific carbon isotope gradient (i.e., NCW production) intensified in two steps approximately coincident with Miocene isotope events Mi5 and Mi6 at 11.7-11.5 ...
We document the waxing and waning of a “proto-warm pool” in the western equatorial Pacific (WEP) based on a study of multi-species planktic foraminiferal isotope ratios and census data spanning the 13.2–5.8 Ma interval at ODP Site 806. We hypothesize that the presence or absence of a proto-warm pool in the WEP, caused by the progressive tectonic constriction of the Indonesian Seaway and modulated by sea level fluctuations, created El Niño/La Niña-like alternations of hydrographic conditions across the equatorial Pacific during the late Miocene. This hypothesis is supported by the general antithetical relationship observed between carbonate productivity and preservation in the western and eastern equatorial Pacific, which we propose is caused by these alternating ocean–climate states. Warming of thermocline and surface waters, as well as a major change in planktic foraminferal assemblages record a two-step phase of proto-warm pool development ~ 11.6–10 Ma, which coincides with Miocene isotope events Mi5 and Mi6, and sea-level low stands. We suggest that these changes in the biota and structure of the upper water column in the WEP mark the initiation of a more modern equatorial current system, including the development of the Equatorial Undercurrent (EUC), as La Niña-like conditions became established across the tropical Pacific. This situation sustained carbonate and silica productivity in the eastern equatorial Pacific (EEP) at a time when carbonate preservation sharply declined in the Caribbean. Proto-warm pool weakening after ~ 10 Ma may have contributed to the nadir of a similar “carbonate crash” in the EEP. Cooling of the thermocline and increased abundances of thermocline taxa herald the decay of the proto-warm pool and higher productivity in the WEP, particularly ~ 9.0–8.8 Ma coincident with a major perturbation in tropical nannofossil assemblages. We suggest that this interval of increased productivity records El Niño-like conditions across the tropical Pacific and the initial phase of the widespread “biogenic bloom”. Resurgence of a later proto-warm pool in the WEP ~ 6.5–6.1 Ma may have spurred renewed La Niña-like conditions, which contributed to a strong late phase of the “biogenic bloom” in the EEP.
... Gt. insueta) (Plate 1, Figures 7a-c), a conclusion also noted by Pearson (1995) in his analysis of the holotype and paratypes of Gt. insueta, as well as the illustrated "topotypes" from Stainforth et al. (1975;figure 125.5-6). Forms referable to Globigerinatella sp. have been illustrated within the Caribbean region from the Pozon Formation, albeit tentatively (Blow, 1959;plate 15, figure 95), and ODP Site 999 on the Kogi Rise, western Caribbean Sea (Chaisson and D'Hondt, 2000; plate 2, figure 11). Due to the historical application of Gt. insueta, a subdivision of Zone M3 into Subzone M3a (=Base Globigerinatella sp.) and Subzone M3b (=Base Gt. insueta) may be beneficial for correlative purposes despite the extremely condensed zone (~50 Ka) this would create (see Section 6). ...
Planktonic foraminifera are widely used in marine biostratigraphy thanks to their small size, limited stratigraphic range and abundance in oceanic sediments. The utility of planktonic foraminifera in biostratigraphy was first fully recognised within the Caribbean region during the middle of the 20th century. The area was critical for the subsequent development of the low latitude biostratigraphic schemes and remains fundamental for modern day biogeochronologies. This study presents a historical review of the Oligo-Miocene component of these biostratigraphic schemes, including the first proposed scheme of Cushman and Stainforth (1945) and the subsequent development. The work of Hans Bolli and Walter Blow is particularly highlighted due to their heavy influence on modern day biostratigraphy, including these authors initially recognising the biostratigraphic utility of a number of bioevents still applied today. These Caribbean-centric schemes are correlated to the modern-day low latitude biogeochronology of Wade et al. (2011), with this synthesis highlighting that a number of bioevents (e.g. Top Paragloborotalia kugleri and Top Catapsydrax dissimilis) have been applied consistently since their initial recognition. This in turn allows the recognisability of these bioevents to be deduced based on how consistently applied each datum has been. In addition, the range charts of six studies focusing heavily on the Caribbean have been reassessed to determine whether there is potential to apply a given bioevent, and the original author merely did not recognise the biostratigraphic utility of the species or favoured another bioevent.
In considering this historical review, a number of amendments to Wade et al. (2011) and future priorities to planktonic foraminifera biogeochronologies are suggested. Most notably, the re-introduction of Base Globigerinatella insueta as a primary bioevent due to the historical biostratigraphic importance of this species. This event now defines early Miocene Subzone M3b (Gt. insueta/Ct. dissimilis PRZ) dividing Zone M3 into an upper Subzone M3b (Base Gt. insueta) and lower Subzone M3a (Base Globigerinatella sp.). Finally, the Miocene to Recent timescale of Wade et al. (2011) has been recalibrated following more recent updates to the magnetostratigraphy (Kochhann et al., 2016; Ogg et al., 2016; Drury et al., 2017; Beddow et al., 2018) and cyclostratigraphy (Wilkens et al., 2017). The overall effect on the planktonic foraminifera biogeochronology is minor but our results become the suggested biostratigraphic framework for the low latitudes.
... All age models (Chaisson and D'Hondt, 2000;Kameo and Bralower, 2000) were adjusted to the 2004 timescale (Lourens et al., 2005) facilitating a direct comparison between the sites. The measurement of environmental conditions and morphology on the same sample ensures synchronicity of sample comparison and environmental interpretation. ...
The impact of global change on marine ecosystems is a major concern for the future. Examples from the geological past may provide insight into how ecosystems respond to major shifts in environment. Here we use the progressive closure of the Central American Seaway over the last 10 Myrs, and the resulting new environmental conditions and niches on either side of the Panama Isthmus, as a time series documenting the reaction of planktic foraminifers to environmental change and vicariance. Our main finding is that the size and shape evolution of both investigated species is strongly influenced by temperature, despite their different ecology. The surface dweller Trilobatus sacculifer conserved the same shape on both sides of the Isthmus for most of the studied interval, and diverged only recently when environment diverged on both sides of the Isthmus. The shape response is a combination of a change in mean shape and in percentage of morphotypes occurring within T. sacculifer. This suggests a minor role of vicariance and the potential to react to changes in the local environment through ecotypic or plastic variation. This interpretation is corroborated by extensive phenotypic variability in the absence of genetic differentiation today in this species. The shape of the deeper-living species Gt. tumida, in contrast, diverged on both sides of the Isthmus at a time that coincides with the cut-off of the connection of its habitat. This divergence combines a response to temperature and to location, suggesting local adaptation in response to vicariance. These different reactions highlight both a high potential for adaptation, but also sensitivity to temperature variations. The species-specific responses to environmental pressures indicate the difficulty in upscaling from one species to foraminifers in general.
... Test calcification in G. crassaformis is under seasonal thermocline (Ravelo and Fairbanks, 1992), which is why its absence from the lower strata implies an environmental effect similar to that for Globorotalia tumida, and its first occurrence in Miyakojima Island does not conform to the date of its evolutionary appearance. Berggren et al. (1995) dated the extinction level of Sphaeroidinellopsis spp., which defines the top of Pl3, at 3.12 Ma; this was accepted in subsequent papers (Saito, 1999;Chaisson and D'Hondt, 2000). Gradstein et al. (2004Gradstein et al. ( , 2012 preferred 3.59 Ma for the upper limit of the range of Sphaeroidinellopsis seminulina (Schwager). ...
This is the first comprehensive, fully illustrated systematic report of both plank- tonic and benthic foraminifera from the Pliocene Shimajiri Group in Miyakojima Island, southern Ryukyu Island Arc. A total of 47 planktonic species in 18 genera and 267 ben- thic taxa in 151 genera have been identified in 31 matrix samples covering almost all horizons in the island sections. All species, together with morphological varieties, are illustrated using focus-stacking software by assembling digital camera images taken in optical microscopy. These multi-focused illustrations may show natural color and tex- ture (reflection and transmission), and inner structures, including shape of septum, which are unavailable in scanning electron microscopy. Systematic notes comprise comments on synonyms of ubiquitous/well-established species described from other areas. The present comprehensive report with novel illustrations provides an effective guide for future research of foraminifera in surrounding regions. Planktonic foraminiferal biostratigraphy in Miyakojima Island has also been reexamined by considering sedimentary environment to explain the absence of some index species. The Early to Late Pliocene age of the Shimajiri Group in Miyakojima Island, correlative with the Pl1 Zone (Oura Formation), Pl1 to Pl2 zones (Onogoshi Formation), Pl2 to Pl4 zones (Yonahama Formation) and Pl5 zone (Minebari Formation), respectively, is confirmed.
... This diversification is consistent with paleoceanographic records indicating increased productivity in the Caribbean during this period. O'Dea et al. (2007) characterized marine faunas of Central America prior to 5 Ma as being influenced by upwelling of Pacific waters entering the Caribbean from the eastern Pacific (Keigwin 1982, Keller et al. 1989, Maier-Reimer 1990, Chaisson and d'Hondt 2000, Schneider and Schmittner 2006, Jain and Collins 2007, Lutz et al. 2008. FSFM corals would have been well suited to capitalize on increased heterotrophic food sources (Porter 1976). ...
Changes in the Caribbean coral fauna associated with Plio-Pleistocene extinctions are unique in that origination preceded extinction. This pattern is particularly evident in free-living solitary and flabello-meandroid (FSFM) corals. Here we use the stable isotopes of carbon and oxygen to assess the paleoautecology of pre-turnover FSFM corals from the Cibao Basin, northern Dominican Republic. We assess photosymbiosis and resource utilization within FSFM corals and how changing Neogene paleoclimatic and paleoceanographic conditions may have contributed to originations and extinctions within this unique ecological group. Measurements of δ13C and δ18O were compared between two known zooxanthellate coral species, two known azooxanthellate species, and six FSFM species of the Cibao Basin. Skeletal δ13C provided the best indicator of resource utilization by FSFM corals. For both modern reef corals and Mio-Pliocene reef corals from the Cibao Basin, values of δ13C fall between 0‰ and –2‰ whereas the δ13C values of azooxanthellate corals generally fall between –3‰ and –9‰. Both modern and fossil FSFM corals lie intermediate to these end members. FSFM values of δ13C fall between –1‰ and –4‰ and suggest reduced photosymbiosis in comparison to typical reef corals. Reduced photosymbiosis of FSFM corals in comparison to reef corals is also reflected in a stronger covariation between δ13C and δ18O. Though FSFM corals are clearly utilizing translocated carbon from associated algal symbionts, the more depleated values of δ13C and covariation with δ18O suggest a greater dependence on heterotrophic feeding. This ecological lifestyle likely has made FSFM corals sensitive to changes in primary productivity that characterize the Caribbean Neogene.
... According to , the HO of G. venezuelana is younger than HO of G. obliqua, but our data suggest the opposite. Chaisson and D'Hondt (2000) also observed this sequence at ODP site 999, western Colombia Basin. ...
Biostratigraphy in northern Colombia has traditionally been done using foraminifera. Quantitative biostratigraphic techniques could improve the zonations that have been proposed for the region. We analyze the biostratigraphic information from 190 planktonic foraminifera species, and 1961 ditch-cutting samples from 26 oil wells in northern Colombia to produce a zonation for the region. A quantitative biostratigraphic technique known as Constrained Optimization was used to analyze the data.
The proposed zonation relies exclusively on last occurrences, which are readily applied to petroleum exploration. It has thirteen zones and eight subzones for the Eocene to Pliocene interval. Three zones and two subzones are defined for the Eocene, three zones for the Oligocene, six zones and six subzones for the Miocene, and one zone for the Pliocene. The zonation reveals three major unconformities: (1) a late Eocene - early Oligocene hiatus; (2) a late Oligocene - early Miocene hiatus; and (3) a late Miocene hiatus; the hiatuses are related to the collision of the Caribbean with the South American plate.
... From about 10.7 Ma on, a decreased inflow of Pacific waters into the Atlantic through the Central American Seaway is reconstructed from increasing carbonate mass accumulation rates in the Caribbean, from changes in the foraminifera assemblage as well as from deviation of Caribbean the calcareous nannofossils, from those in the equatorial Pacific (Chaisson and D'Hondt, 2000;Kameo and Sato, 2000;Newkirk and Martin, 2009). Such a decreased inflow would lead to a northward flow of the North Brazil Current and, thereby, transport the terrigenous material away from the study site. ...
... ''Ambient'' temperatures (18-$40°C) and also temperatures of around 60°C (i.e., the approximate lower temperature bound of the ''partial annealing zone'') result in track shortening of about 10%. Fig. 9 shows AFT ages plotted against the biostratigraphic ages of the same samples (Cepek and Wind, 1979;Mazzei et al., 1979;McNulty, 1979;Okada and Thierstein, 1979;Salvatorini and Cita, 1979;Pospichal et al., 1991;Erba and Covington, 1992;Chaisson and D'Hondt, 2000;Kameo and Bralower, 2000; Ca 9.740 9.746 9.909 9.844 9.823 9.798 9.961 9.986 9.833 9.523 9.565 9.705 9.848 9.826 9.624 9.747 9.860 9.720 9.690 9.807 9.790 9.812 9.684 P 6.049 5.991 5.935 5.955 5.958 5.961 5.894 5.874 6.023 6.138 6.107 6.076 6.028 6.036 6.112 6.054 6.019 6.081 6.095 6.044 6.054 6.048 6.097 and Sanfilippo, 2000). Within the ±1r-error limits, all AFT ages are identical with their deposition ages, indicating that track shortening of about 10% does not affect fission track ages. ...
Since studies on deep-sea cores were carried out in the early 1990s it has been known that ambient temperature may have a marked affect on apatite fission track annealing. Due to sluggish annealing kinetics, this effect cannot be quantified by laboratory annealing experiments. The unknown amount of low-temperature annealing remains one of the main uncertainties for extracting thermal histories from fission track data, particularly for samples which experienced slow cooling in shallow crustal levels. To further elucidate these uncertainties, we studied volcanogenic sediments from five deep-sea drill cores, that were exposed to maximum temperatures between ∼10° and 70 °C over geological time scales of ∼15–120 Ma. Mean track lengths (MTL) and etch pit diameters (Dpar) of all samples were measured, and the chemical composition of each grain analysed for age and track length measurements was determined by electron microprobe analysis. Thermal histories of the sampled sites were independently reconstructed, based on vitrinite reflectance measurements and/or 1D numerical modelling. These reconstructions were used to test the most widely used annealing models for their ability to predict low-temperature annealing. Our results show that long-term exposure to temperatures below the temperature range of the nominal apatite fission track partial annealing zone results in track shortening ranging between 4 and 11%. Both chlorine content and Dpar values explain the downhole annealing patterns equally well. Low chlorine apatite from one drill core revealed a systematic relation between Si-content and Dpar value. The question whether Si-substitution in apatite has direct and systematic effects on annealing properties however, cannot be addressed by our data. For samples, which remained at temperatures <30 °C, and which are low in chlorine, the Laslett et al. [Laslett G., Green P., Duddy I. and Gleadow A. (1987) Thermal annealing of fission tracks in apatite. Chem. Geol. 65, 1–13] annealing model predicts MTL up to 0.6 μm longer than those actually measured, whereas for apatites with intermediate to high chlorine content, which experienced temperatures >30 °C, the predictions of the Laslett et al. (1987) model agree with the measured MTL data within error levels. With few exceptions, predictions by the Ketcham et al. [Ketcham R., Donelick R. and Carlson W. (1999) Variability of apatite fission-track annealing kinetics. III: Extrapolation to geological time scales. Am. Mineral. 84/9, 1235–1255] annealing model are consistent with the measured data for samples which remained at temperatures below ∼30 °C. For samples which experienced maximum temperatures between ∼30 and 70 °C, and which are rich in chlorine, the Ketcham et al. (1999) model overestimates track annealing.
... A total of 195 samples, 5 cm 3 in volume, were collected every 10 cm from the uppermost 20 m representing the late Pleistocene (Chaisson and D'Hont, 2000) of ODP Core 999A (12°44.64′N, 78°44.36′W, ...
Faunal analyses of planktonic foraminifera and upper-water temperature reconstructions with the modern analog technique are studied and compared to the magnetic susceptibility and gamma ray logs of ODP Core 999A (western Caribbean) for the past 560 kyr in order to explore changes in paleoceanographic conditions in the western Caribbean Sea. Long-term trends in the percentage abundance of planktonic foraminifera in ODP Core 999A suggest two hydrographic scenarios: before and after 480 ka. High percentage abundances of Neogloboquadrina pachyderma and Globorotalia inflata, low abundances of Globorotalia menardii and Globorotalia truncatulinoides, low diversity, and sea-surface temperatures (SST) under 24 °C are typical characteristics occurring from 480 to 560 ka. These characteristics suggest a “shallow” well-oxygenated upper thermocline and the influx of nutrients by either seasonal upwelling plumes and/or eddy-mediated entrainment. The second scenario occurred after 480 ka, and it is characterized by high and fluctuating percentage abundances of Neogloboquadrina dutertrei, G. truncatulinoides, G. menardii, Globigerinita glutinata, Globigerinella siphonifera, and Globigerinoides ruber; a declining trend in diversity; and large SSTs. These characteristics suggest a steady change from conditions characterized by a “shallow” thermocline and chlorophyll maximum to conditions characterized by a “deep” thermocline (mainly during glacial stages) and by more oligotrophic conditions. The influence of the subtropical North Atlantic on the upper thermocline was apparently larger during glacial stages, thus favoring a deepening of the thermocline, an increase in sea-surface salinity, and a dramatic reduction of nutrients in the Guajira upwelling system. During interglacial stages, the influx of nutrients from the Magdalena River is stronger, thus resulting in a deep chlorophyll maximum and a fresher upper ocean. The eddy entrainment of nutrients is the probable mechanism responsible of transport from the Guajira upwelling and Magdalena River plumes into ODP 999A site.
... The sediments at Site 999 are dominated by clay-bearing foraminiferalnannofossil oozes (Shipboard Scientific Party, 1997). The age model used for this Site is a combination of a δ 18 O based orbital solution for 0 to 465 kyr (Schmidt et al., 2006) and 1.7 to 7.5 Ma (Haug and Tiedemann, 1998), and the biostratigraphic models of Chaisson and D'Hondt (2000) and Kameo and Bralower (2000a) for 0.465 to 1.7 Ma. The lack of an astronomically tuned age model, and hence larger error on the absolute age between 0.46 and 1.7 Ma, prohibited a more directed sampling strategy or an interpretation of glacial-interglacial variability. ...
Pulleniatina is an extant genus of planktonic foraminifera that evolved in the late Miocene. The bottom and top occurrences of its six constituent morphospecies (P. primalis, P. praespectablis, P. spectabilis, P. praecursor, P. obliquiloculata, P. finalis) provide a series of more or less useful constraints for correlating tropical and subtropical deep-sea deposits, as do some prominent changes in its dominant coiling direction and a substantial gap in its record in the Atlantic Ocean. Biostratigraphic information about these events has accumulated over many decades since the development of systematic deep-sea drilling in the 1960s, during which time the geochronological framework has evolved substantially, as have taxonomic concepts. Here we present new data on the biochronology of Pulleniatina from International Ocean Discovery Program Site U1488, which has a record of its entire evolutionary history from the centre of its geographic range in the Western Pacific Warm Pool. We then present and compare revised calibrations of 183 published Pulleniatina bioevents worldwide, with stated sampling errors as far as they are known, using a consistent methodology and in the context of an updated evolutionary model for the genus. We comment on the reliability of the various bioevents; their likely level of diachrony; and the processes of evolution, dispersal, and extinction that produced them.
The Indonesian Throughflow region connects the tropical Pacific Ocean with the eastern Indian Ocean, and variability of the Throughflow during the Plio-Pleistocene has been related to major climate shifts at the global level. Planktic foraminiferal biostratigraphy integrated with magnetochronology provides a robust time framework for assigning age control to major paleoceanographic events. Understanding of the coupled histories of the El Niño Southern Oscillation, Western Pacific Warm Pool, Indonesian Throughflow, and the eastern Indian Ocean has greatly benefitted from the chronological framework provided by planktic foraminiferal biostratigraphies from these regions. Ocean Drilling Program (ODP) Hole 762 lies under the influence of the Leeuwin Current, originating from the Indonesian Throughflow. Multiproxy data have been collected from Hole 762B and other nearby sites for paleoceanographic interpretation. However, a detailed planktic foraminiferal biostratigraphy integrated with magnetochronology is not available from this site. We provide here the sequential order of planktic foraminiferal First Occurrence (FO) and Last Occurrence (LO) events, which allowed us to divide the examined section into seven biostratigraphic zones. The biostratigraphy was integrated with magnetostratigraphy using revised ages of magnetochrons, which yielded biochronological age estimates for planktic foraminiferal events. A major planktonic faunal turnover between 3.4 and 2.7 Ma is probably related to the onset of the Northern Hemisphere glaciation and related changes in the Indonesian Throughflow. We have compared our numerical age estimates with published ages from other parts of the world. The diachronism observed probably is related to the local oceanographic setting of ODP Hole 762B, which has been alternatively influenced by changing strengths of the warm Leeuwin Current and the cold west Australian Current. The biochronology established here will be useful for correlating paleoceanographic events in the region.
The Colombia basin contains large volumes of sediment accumulated during the last 17 My. The use of isochore maps, exploratory wells, micropaleontological and geochronological dates has enabled us to estimate the volumes of sediment and accumulation rates in this basin. The analysis of source of sediments and exhumation data from the Northern Andes of South America led to the definition of areas and thicknesses of material eroded during the Neogene - Quaternary, to obtain volumes or material eroded from the continent that can be correlated with sediment volumes accumulated in the Colombia Basin. The analyzed sediment volumes suggest that during the last 17 My ~72.06x1015 Tons accumulated in the Colombia Basin, while ~ 7.16x1013 Tons accumulated in the continental catchment areas. The sedimentation in the Colombian Basin has occurred at variable rates, with values ranging from 55 MTons/My to 295 MTons/My, with a peak of 803 MTons/My in the early Pleistocene (between 2.4 and 2.2 Ma). The evaluation between the total volumes of sediment accumulated in the offshore and onshore, suggests that in the continental part of the basin less than 4% of
the total volume of eroded sediment is trapped and, therefore, the behavior of the accumulation rates calculated in the offshore directly reflect the relief evolution of South America’s Northern Andes. It seems, at large, that the lithospheric convergence rates
and subduction angle (South America vs Nazca and Meso Atlantic opening) have controlled the regional exhumation of the Northern Andes, with the exception of the Pleistocene high sedimentation event, which seems to coincide with local events such as the collision of the Panama Arch against Western Antioquia. It may be concluded that thanks to this collision, drainage systems such as those of the Magdalena and Cauca rivers were modified, which resulted in the formation of the Magdalena Submarine Fan.
Classification of modern and fossil planktic foraminifers is based on a morphological species concept (i.e. morphotypes) for practical reasons, i.e. a non-destructive enumeration from strew-mounted samples, and economical (i.e. time-saving) analyses. Detailed classification of each test, for example, using scanning electron microscopy or analysis of the molecular genetics in case of live specimens would be too costly.
SEM images of recently acquired specimens of Globigerina siakensis LeRoy, 1939, from near its type locality in Indonesia and other areas verify previous claims about the spinosity of the test wall and confirm assignment of the species to Paragloborotalia. Furthermore, they show that the species has straight intercameral sutures on the spiral side of the test. Paragloborotalia siakensis has previously been synonymized with Globorotalia mayeri Cushman and Ellisor, 1939. Backscatter electron, high-vacuum SEM images of Gl. mayeri show that the holotype has curved spiral-side intercameral sutures and apparently lacks spinosity. The absence of the latter feature, however, cannot be confirmed because the holotype is poorly preserved, the single paratype belongs to a different species, and topotypes are unavailable. The straight spiral-side intercameral sutures in P. siakensis and the curved ones in the Gl. mayeri holotype argue against the synonymy of both species, certainly for those using a typological species approach. For those applying an assemblage-based concept, synonymy is demonstrated if specimens with curved and straight spiral-side intercameral sutures occur in single homogeneous assemblages, preferably from the type localities of both species, and are connected by intermediate morphologies. Unfortunately, neither the type sample of Gl. mayeri nor its washed residue is preserved, and there is no possibility of resampling the relevant interval. However, assemblages of P. siakensis from its type area, Java, and Trinidad are clearly devoid of specimens with Gl. mayeri-like suture patterns, providing sufficient reason to retain the name P. siakensis. If the wall of Gl. mayeri is not spinose, the species most likely is closely related to, or a senior synonym of, Globorotalia partimlabiata Ruggieri and Sprovieri, 1970.
In 2010 and 2011 we undertook an initial exploratory survey of the late Miocene-early Pliocene Castilletes Formation (Eastern Guajira Peninsula, Colombia). The Castilletes is composed of deltaic and shallow marine deposits, with an abundant continental fauna in some intervals. These new findings will contribute to a better understanding of the neotropical vertebrate communities and paleoenvironments during a time of high tectonic activity and global climate change. The fauna includes sharks (Carchariniformes), rays (Myliobatiformes), catfish (Siluriformes), dogtooth tetras (Cynodontidae), fresh water turtles (Podocnemidae), and one of the oldest records of Crocodylus in the Americas. The mammal assemblage comprises five orders and ten families. Xenarthrans include megatheriid and nothrotheriid sloths and the cingulates Boreostemma pliocena (Glyptodontinae), Neoglyptatelus sp. (Glyptatelinae), cf. Holmesina and cf. Pampatherium (Pampatheriidae). Rodents are represented by Chapalmatherium (Hydrochoeridae), cf. Neoepiblema (Neoepiblemidae) and Paramyocastor sp. (Echymidae). Indigenous South American ungulates include horse-like protherotheriids (Litopterna), rhino-like toxodonts (Notoungulata), and astrapotheres (Astrapotheria). Associated fossil wood is found in some localities. Overall, the Castilletes fauna and flora indicates the presence of extensive water bodies, in a delta complex with moderate to high rates of rainfall during the late Miocene-early Pliocene. Considering that today the Guajira peninsula is characterized by a very dry climate with xerophitic vegetation, the inferred paleoenvironment for the Castilletes Formation indicates that the region has suffered a drastic climatic change over the last 4 million years.
The mega-breccia at Hornitos, northern Chile, was recently re-interpreted as a mass flow deposit caused by cliff failure, without any link to a tsunami backwash or the Eltanin meteorite impact at 2.5 Ma. While agreeing with the latter in the light of new microbiological data, it is here argued that mass flow can also be caused by tsunami backwash events and would be difficult to distinguish from those caused by gravity alone, especially as the Hornitos outcrop is of limited extent. However, a mechanism for downward dyke injection can be postulated for tsunami-related mass flows, but would not be applicable to normal debris flows generated by cliff collapse. The new age range of the Portada Formation coincides with similar deposits at Carrizalillo, Ranquil, Caldera and possibly Caleta Verde, suggesting that one or more mega-tsunamis might have struck the Chilean coastline during the Messinian. © 2015, Servicio Nacional de Geologia y Mineria. All rights reserved.
Detailed descriptions and stratigraphic analysis of 159 species of foraminifers recovered from six existing wells drilled
in the Tofino Basin, offshore Vancouver Island support the establishment of 10 zones, ranging from upper Eocene to Pleistocene-Recent.
The newly refined stratigraphic ages improve correlations between the offshore Tofino Basin and onshore outcrops of the Carmanah
Group on western Vancouver Island and to other areas in the Pacific North-west and Canadian Arctic.
Paleobathymetric estimates from interpreted biofacies changes indicate a shallowing upward trend throughout the Oligocene,
with deep biofacies encountered during the early Miocene and early Pliocene, reflecting a major transgression or basin subsidence.
Late Pliocene-Pleistocene inner to outer neritic biofacies, cool-water faunal assemblages and coarser-grained lithology indicate
a regressive phase and regional onset of glaciation.
The TB represents a complex tectono-stratigraphy, which has undergone episodic uplift, tectonic underthrusting, climatic cooling
and eustatic fluctuations resulting in complex facies changes, stratigraphic breaks and structural complexities. The new,
updated biostratigraphy has constrained many of these regional geological events, with improved/high resolution analysis of
correlations and paleoenvironmental interpretations. Proposed lifting of federal and provincial moratoriums on offshore exploration
and development has renewed interest in offshore energy resource potential, reassessment of existing data, and interpretations
of the Tofino Basin (TB) and adjacent shelf architecture. This study provides baseline data for future taxonomy, biostratigraphy,
basin modeling and energy resource assessments.
The late Neogene section of ODP Hole 763A, Exmouth Plateau, southeast Indian Ocean, has been biostratigraphically subdivided into eight planktic foraminiferal zones. The zones are similar to those of the tropical northern Indian and tropical western Pacific Oceans. The sequential order of planktic foraminiferal events has been determined, and 32 late Neogene planktic foraminiferal events have been identified. A major faunal turnover (22 events) occurs between 80 and 45 meters below seafloor (mbsf), spanning 1.6 million years between 3.6 Ma and 2 Ma and may be related to the stresses transmitted to the upper ocean due to major cooling associated with Northern Hemisphere glaciation. Based on integration with magnetostratigraphy, numerical age estimates of planktic foraminiferal events have been determined for the first time from the southeast Indian Ocean. A comparison with the published dates for these events from other parts of the world suggests that 14 events are reliable for correlation over a wide geographic range. The useful synchronous events are the Globorotalia tosaensis FO (2.84 Ma), the Globorotalia tosaensis LO (0.59 Ma), the Globigerinoides extremus LO (1.87 Ma), the Globigerinoides fistulosits FO (3.26 Ma), the Globigerinoides fistulosus LO (1.73 Ma), the Dentoglobigerina altispira LO (3.05 Ma), the Dentoglobigerina globosa LO (3.18), the Neogloboquadrina dutertrei FO (2.77 Ma), the Sphaeroidinellopsis LO (3.35), the Globorotalia margaritae LO (3.38 Ma), the Globorotalia crassaformis FO (4.41 Ma), the Globoturborotalita nepenthes LO (3.44 Ma), the Globorotalia tumida tumida FO (5.10 Ma) and the Pulleniatina primalis FO (5.70 Ma). In contrast, the other events are diachronous and considered unsuitable for inter-oceanic correlation. The Pliocene-Pleistocene boundary is marked at ODP Hole 763A by the LO of Globigerinoides fistulosus, which is a synchronous event over a wide latitudinal range and occurs close to the top of the Olduvai Event (Chron C2n; 1.77 Ma). The Miocene-Pliocene boundary is approximated by the first evolutionary appearance of Globorotalia tumida tumida from its ancestor Globorotalia plesiotumida and occurs close to base of Thvera magnetic event (C3n.4n; 5.23 Ma). Recognition of both of these epoch boundaries is in accord with the International Commission on Stratigraphy. The detailed planktic foraminiferal biochronology developed for Hole 763A will aid correlation and dating of various paleoceanographic events over the last 6 million years in the southeast Indian Ocean.
Changes in circulation associated with the shoaling of the Isthmus of Panama and the Caribbean carbonate crash in the Miocene were investigated using Nd isotopes from fossil fish teeth and debris from two sites in the Caribbean Basin (Ocean Drilling Program Sites 998 and 999) and two sites in the eastern equatorial Pacific (Sites 846 and 1241). The total range for εNd values measured from 18 to 4.5 Ma in the Caribbean is -7.3 to 0. These values are higher than Atlantic water masses (∼-11) and range up to values equivalent to contemporaneous Pacific water masses, confirming that flow into the Caribbean Basin was composed of a mixture of Pacific and Atlantic waters, with an upper limit of almost pure Pacific-sourced waters. Throughout the Caribbean record, particularly during the carbonate crash (10-12 Ma), low carbonate mass accumulation rates (MARs) correlate with more radiogenic εNd values, indicating increased flow of corrosive Pacific intermediate water into the Caribbean Basin during intervals of dissolution. This flow pattern agrees with results from general ocean circulation models designed to study the effect of the shoaling of the Central American Seaway. Low carbonate MARs and high εNd values also correlate with intervals of increased Northern Com ponent Water production and, therefore, enhanced conveyor circulation, suggesting that the conveyor may respond to changes in circulation associated with shoaling of the Central American Seaway. Reduced Pacific throughflow related to shoaling of the seaway led to a gradual increase in carbonate preservation and more Atlantic-like εNd values following the carbonate crash.
Biostratigraphic and isotopic studies of planktic foraminifera suggest
that early to middle Pliocene closure of the Central American Seaway
caused vicariant speciation among previously cosmopolitan menardellid
foraminifera. New morphospecies were restricted to the tropical
Atlantic. Isotopic data suggest that newly evolved Atlantic menardellids
were adapted to more oligotrophic conditions, living higher in the water
column than ancestral forms and perhaps harboring photosymbionts. In the
late Pliocene, all but one menardellid morphospecies became extinct.
These extinctions coincided with a progressive increase in the amplitude
of periodic climatic change and the end of isolated tropical Atlantic
surface circulation as Northern Hemisphere glaciation intensified.
The stable isotopic composition of two planktonic foraminifer species (Globigerinoides sacculifer and Neogloboquadrina dutertrei) and two benthic foraminifer species (Cibicidoides wuellerstorfi and Uvigerina peregrina) was measured at sub-orbital resolution through the marine isotope stages (MISs) 10, 11, and 12 (345–460 ka) at Site 1056 on the Blake-Bahama Outer Ridge. Planktonic foraminifers were counted for the interval 405–450 ka at 2–4-kyr resolution. Site 1056 (32°29′N, 76°20′W) is located on the continental slope (water depth: 2167 m) beneath the Gulf Stream. The average rate of sediment accumulation through the interval is 11.4 cm/kyr, but sediment accumulation is much more rapid during glacial intervals (15–17 cm/kyr). The decline in percent carbonate during glacial intervals, and its rise during interglacials, indicates that the increased sediment supply is of terrigenous origin. Low carbonate values and low benthic δ13C, which are both associated with a weak Western Boundary Undercurrent and low North Atlantic Deep Water production, persist for 6 kyr after the benthic δ18O record indicates that ice volume has begun to decrease. Recovery of carbonate and benthic δ13C values is abrupt and rapid. Millennial-scale variation (∼3–4 kyr) is apparent in the glacial intervals of the planktonic δ18O records and is more pronounced in a Δδ18O record, which represents the temperature range in the photic zone. Semi-precessional (10–12-kyr) cycles are apparent in the planktonic Δδ13C record. The millennial-scale cycles are largely caused by an increase in G. sacculifer δ18O and represent surface warming. They are interpreted as representing periodic increases in westward intensification of the gyre. The semi-precessional cycles are driven by changes in the N. dutertrei δ13C and represent fluctuations in the Gulf Stream itself and therefore likely have a tropical origin. Planktonic foraminifer census counts did not show an expected response to one of the largest glacial/interglacial transitions of the late Pleistocene. The most obvious change was an increase in faunal diversity during MIS 12.2, the interval of maximum δ18O values. This suggests that cool slope water and warm subtropical gyre water penetrated a more sluggish Gulf Stream with greater frequency at this time. The millennial-scale maxima in the Δδ18O record are accompanied by decreases in diversity, which is consistent with the interpretation of surface warming during these events.
The reinterpretation of biostratigraphic information by new models and quantitative correlation techniques substantially improves its resolution and its correlative potential, thus minimizing oil exploration risks. With this aim, biostratigraphic information, i.e. first (FO) and last (LO) occurrence events of benthonic and planktonic foraminifera was analysed from sixteen wells from the Lower Magdalena Valley (LMV). The inconsistencies found in the biostratigraphic record, i.e. varying successions of first and last appearances of species from well to well as a result of several factors such incomplete sampling and preservation, true variation in the distribution of fosil taxa, etc., and the great amount of biostratigraphic data makes it practically impossible to accurately constrain basin history from biostratigraphic information by unaided visual inspection. This motivates the treatment of biostratigraphic information with new quantitative approaches, such as constrained optimization (CONOP9 software) and graphic correlation concepts (GraphCor 3,0) and the comparative method approach of Cooper et al. (2001). The succession of biostratigraphic events found through the application of each technique was statistically filtered and compared with Kendall tau coefficients whose values were 0,8. An optimal biostratigraphic succession of LOs was found and calibrated with the Berggren et al. (1995) global time scale by a LOESS regression model for the middle Eocene-Pliocene interval, thus revealing three major changes in sediment accumulation rates for the basin during this time interval: (1) middle Eocene to Oligocene, with low accumulation rates, (2) early Miocene to middle Miocene, with high accumulation rates and (3) late Miocene to Pliocene, with lower accumulation rates. The calibrated composite succession enabled the construction of age-well depth plots, which indicate periods of local deposition and accumulation rates, and periods of erosion, no deposition or very low accumulation rates (unconformities). The best plots were used to build a model for the correlation of unconformities, which shows that they are heterochronous, lasted at least 2,5 Ma, and are of limited extent. Finally, a correlation model was proposed that includes: (1) a time-calibrated succession of biostratigraphic events, and (2) a Haq curve that shows how each geologic period is recorded in each well
Pliocene and Pleistocene deposits from Grande-Terre (Guadeloupe archipelago, French Lesser Antilles) provide a remarkable example of an isolated carbonate system built in an active margin setting, with sedimentation controlled by both rapid sea-level changes and tectonic movements. Based on new field, sedimentological and palaeontological analyses, these deposits have been organized into four sedimentary sequences (S1 to S4) separated by three subaerial erosion surfaces (SB0, SB1 and SB2). Sequences S1 and S2 ('Calcaires inférieurs à rhodolithes') deposited during the Late Zanclean to Early Gelasian (planktonic foraminiferal Zones PL2 to PL5) in low subsidence conditions, on a distally steepened ramp dipping eastward. Red algal-rich deposits, which dominate the western part of Grande-Terre, change to planktonic foraminifer-rich deposits eastward. Vertical movements of tens of metres were responsible for the formation of SB0 and SB1. Sequence S3 ('Formation volcano-sédimentaire', 'Calcaires supérieurs à rhodolithes' and 'Calcaires à Agaricia') was deposited during the Late Piacenzian to Early Calabrian (Zones PL5 to PT1a) on a distally steepened, red algal-dominated ramp that changes upward into a homoclinal, coral-dominated ramp. Deposition of Sequence S3 occurred during a eustatic cycle in quiet tectonic conditions. Its uppermost boundary, the major erosion surface SB2, is related to the Cala1 eustatic sea-level fall. Finally, Sequence S4 ('Calcaires à Acropora') probably formed during the Calabrian, developing as a coral-dominated platform during a eustatic cycle in quiet tectonic conditions. The final emergence of the island could then have occurred in Late Calabrian times.
Detailed descriptions and stratigraphic analysis of 159 species of foraminifers recovered from six existing wells drilled in the Tofino Basin, offshore Vancouver Island support the establishment of 10 zones, ranging from upper Eocene to Pleistocene-Recent. The newly refined stratigraphic ages improve correlations between the offshore Tofino Basin and onshore outcrops of the Carmanah Group on western Vancouver Island and to other areas in the Pacific North-west and Canadian Arctic. Paleobathymetric estimates from interpreted biofacies changes indicate a shallowing upward trend throughout the Oligocene, with deep biofacies encountered during the early Miocene and early Pliocene, reflecting a major transgression or basin subsidence. Late Pliocene-Pleistocene inner to outer neritic biofacies, cool-water faunal assemblages and coarser-grained lithology indicate a regressive phase and regional onset of glaciation. The TB represents a complex tectono-stratigraphy, which has undergone episodic uplift, tectonic underthrusting, climatic cooling and eustatic fluctuations resulting in complex facies changes, stratigraphic breaks and structural complexities. The new, updated biostratigraphy has constrained many of these regional geological events, with improved/high resolution analysis of correlations and paleoenvironmental interpretations. Proposed lifting of federal and provincial moratoriums on offshore exploration and development has renewed interest in offshore energy resource potential, reassessment of existing data, and interpretations of the Tofino Basin (TB) and adjacent shelf architecture. This study provides baseline data for future taxonomy, biostratigraphy, basin modeling and energy re-source assessments.
Possible causes to explain platform drowning have been hotly debated by carbonate sedimentologists for more than a decade now. In this paper, we present multiple evidence to explain the drowning of a carbonate megabank that covered most of the modern Northern Nicaragua Rise (NNR) during an interval spanning from late Oligocene to early Miocene by the interaction of several environmental factors. The recovery during ODP Leg 165 of late Oligocene to middle Miocene sedimentary sequences in the sub-seafloor of the modern channels and basin, Pedro Channel and Walton Basin, respectively, that dissect the NNR (Site 1000) and south of the rise in the Colombian Basin (Site 999), combined with information from dredged rock samples, allows us to explore in more detail the timing and possible mechanisms responsible for the drowning of the megabank and its relationship to Miocene climate change. The modern system of isolated banks and shelves dissected by a series of intervening seaways and basins on the NNR has evolved from a continuous, shallow-water carbonate “megabank” that extended from the Honduras/Nicaraguan mainland to the modern island of Jamaica. Available information suggests that this megabank broke apart and partially drowned in the late part of the late Oligocene at around 27 Ma and finally foundered during the late early Miocene around 20 Ma, resulting in limited neritic coral growth in the areas where the modern isolated carbonate banks and shelves are occurring today. Available information also suggests that the southern and central parts of Pedro Channel were already a deep-water area before the major episode of platform drowning, and its formation predates the initiation of the Caribbean Current. However, after the partial drowning of the megabank, the channel has become a major pathway for the Caribbean Current. Stratigraphic units identified in deep-water carbonates sampled at ODP Sites 999 and 1000 help to constrain the environmental setting leading to the drowning of the banks. Changes in lithology and mass accumulation rates of both the carbonate and non-carbonate fraction parallel stable isotope shifts and likely indicate regional changes in climate and circulation during the late Oligocene–middle Miocene interval. Carbonate mass accumulation rates (MARs) at Site 999 suggest increased regional productivity during the early Miocene. Terrigenous MARs at both Sites 999 and 1000 show a general increase from the Burdigalian through the Serravallian. The temporal association among episodes of neritic platform deposition, followed by increased productivity as identified by higher carbonate MARs and positive excursion in carbon isotopes, suggests that oceanographic changes such as local upwelling and nutrification have led to the partial drowning of the NNR “megabank”.
The GSSP of the Tortonian Stage, which per definition marks the base of the Tortonian and, hence, the boundary between the Serravallian and Tortonian Stages of the Middle and Upper Miocene Subseries, has recently been defined and ratified by the IUGS. The boundary stratotype-section is Monte dei Corvi (Italy) where the Tortonian GSSP is now formally at the mid-point of the sapropel of small-scale sedimentary cycle no. 76, close to the last common occurrences (LCOs) of the calcareous nannofossil Discoaster kugleri and the planktonic foraminifer Globigerinoides subquadratus and associated with the short normal subchron C5r.2n. The GSSP level coincides closely with oxygen isotope event Mi-5 and the associated glacio-eustatic sea-level low-stand of supercycle T3.1 and concurrent deep-sea hiatus NH4, and is dated astronomically at 11.608 Ma. The Monte Gibliscemi section is accepted as an auxiliary boundary stratotype because the better preservation of the calcareous microfossils in this section enables quantitative analyses and the construction of a stable isotope record.
The GSSP of the Tortonian Stage, which per definition marks the base of the Tortonian and, hence, the boundary between the Serravallian and Tortonian Stages of the Middle and Upper Miocene Subseries, has recently been defined and ratified by the JUGS. The boundary stratotype-section is Monte dei Corvi (Italy) where the Tortonian GSSP is now formally at the mid-point of the sapropel of small-scale sedimentary cycle no. 76, close to the last common occurrences (LCOs) of the. calcareous nannofossil Discoaster kugleri and the planktonic foraminifer Globigerinoides subquadratus and associated with the short normal subchron C5r.2n. The GSSP level coincides closely with oxygen isotope event Mi-5 and the associated glacio-eustatic sea-level low-stand of supercycle T3.1 and concurrent deep-sea hiatus NH4, and is dated astronomically at 11.608 Ma. The Monte Gibliscemi section is accepted as an auxiliary boundary stratotype because the better preservation of the calcareous microfossils in this section enables quantitative analyses and the construction of a stable isotope record.
High-resolution planktonic foraminiferal and oxygen isotope records are
presented from a Mediterranean deep marine succession, dated
astronomically between 12.12 and 9.78 Ma. Planktonic and benthic oxygen
isotope records are punctuated by two episodes of δ18O
increase, which have astronomical ages of 11.4 and 10.4 Ma and
correspond to the Mi5 and Mi6 events of Miller et al. [1991a]. These ice
growth events coincide with low-amplitude variations in the 1.2 Myr
obliquity cycle and are accompanied by significant faunal changes in the
Mediterranean, such as the arrival of neogloboquadrinids, the increase
in abundance of the G. apertura-G. obliquus group, and the areal
differentiation between N. atlantica and N. acostaensis. Short-term
variations in the planktonic foraminiferal and oxygen isotope records
correspond to dominantly precession-controlled sedimentary cycles.
Features of the sapropel/gray marl layers indicate that the short-term
astronomically controlled circum-Mediterranean climate changes remained
basically the same over the last 12 Myr.
Four sites were drilled during Ocean Drilling Program Leg 159 on the continental slope off the Cote d'Ivoire and Ghana in the eastern equatorial Atlantic. Samples rich in planktonic foraminifers range in age from the Holocene to early Miocene, whereas Cenozoic samples older than this contained either poorly preserved foraminifer assemblages or were completely barren. Preservation of foraminifers is best at the two shallowest sites (Site 959, 2090 m water depth, and Site 960, 2048 m water depth) and degrades with increasing water depth. Foraminifers are almost completely absent at Site 962 at 4648 m. Even in the shallow water sites, preservation is moderate in the Pleistocene, the earliest Pliocene, and the early to middle Miocene where there is extensive fragmentation of the <150-μm fraction and often dissolution of the interiors of large, robust specimens. Preservation is good through much of the upper Pliocene and the upper Miocene. Planktonic foraminifer assemblages are typical of the tropical oceans, although temperate species such as Neogloboquadrina pachyderma (sinistral), Globorotalia panda, and Globorotalia miozea are found sporadically. Globorotaloides hexagona is absent from Leg 159 sites throughout upper Pliocene Zones P14 and P15 and may prove to have a pattern of disappearance from upper Pliocene Atlantic sediments similar to Globorotalia tumida and species of Pulleniatina. The recovered section at Site 959 is almost biostratigraphically complete between lower Miocene Zone N7 and the Holocene, although there is probably a hiatus in the middle Miocene that has cut out, or greatly thinned, Zone N9. The other Leg 159 sites are less complete. The middle Miocene section is missing at both Sites 960 and 961, and Site 961 also has a hiatus across the Miocene/Pliocene boundary.
At Sites 925-929, an orbitally tuned time scale was generated for the Pliocene from 5 to 2.5 Ma by correlating precessional magnetic susceptibility cycles to the 65°N summer insolation record that was based on the astronomical solution of Laskar et al. (1993).
Two Pliocene time intervals were investigated. The time interval from 3.3 to 2.6 Ma includes the intensification of northern hemisphere glaciation. The second interval from 5 to 4.5 Ma monitors the warmer Pliocene and is believed to reflect a major event in the closure history of the Panama Isthmus. A bathymetric comparison between sand content and carbonate accumulation records suggests that the early Pliocene was marked by stronger carbonate dissolution and a shallower lysocline than the middle Pliocene. A drastic increase in carbonate preservation occurred at about 4.6 Ma that was probably associated with the emergence of the Panama Isthmus. Variations in the lysocline depth were dominated by the 41-k.y. tilt cycle, indicating a strong coupling to high-latitude climate forcing. In contrast, the dominance of precession cycles in less undersaturated water masses well above the lysocline may indicate changes in carbonate production and carbonate flux. Middle Pliocene maxima in carbonate dissolution at 4356 m water depth lag minima of ice volume by 12 k.y. at the obliquity band, which is typical for Pleistocene Pacific records. This phase lag decreased toward shallower water depths returning to the “Atlantic type” of carbonate preservation.
Cyclic fluctuations in the supply of Amazon sediments responded with nearly equal concentration of variance to the precession and obliquity periods. The 41-k.y. fluctuations in the supply of Amazon siliciclastics may result from sea-level changes as inferred from an in-phase relationship with benthic . Benthic oxygen isotopes show no response to orbital precession. This suggests that climatological changes in South America and/or changes in the intensity of the North Brazilian Coastal Current are strong candidates in controlling the precessional flux of terrigenous material to Ceara Rise.
The Neogene biostratigraphy presented here is based on the study of 230 samples through 737 m of pelagic sediment in Hole 806B. Sediment accumulation is interrupted only once in the uppermost lower Miocene, apparently coincident with a widespread deep-sea hiatus. Preservation of planktonic foraminifers through the section ranges from good to moderately poor. One hundred and ten species of planktonic foraminifers were identified; taxonomic notes on most species are included. All of the standard low-latitude Neogene foraminiferal zones are delineated, with the exceptions of Zones N8 and N9 because of a high first occurrence of Orbulina, and Zones N18 and N19 because of a high first occurrence of Sphaeroidinella dehiscens. The global pattern of change in tropical/transitional species richness is paralleled in Hole 806B, with departures caused by either ecological conditions peculiar to the western equatorial Pacific or by inexactness in the estimation of million-year intervals in Hole 806B. -from Authors
Five guyots were drilled on Ocean Drilling Program Leg 144, three of which possess thick caps of pelagic sediment. These guyots (Limalok, Site 871; Lo-En, Site 872; and Wodejebato, Site 873) belong to the Marshall Islands group of seamounts. Pelagic sediments of late Oligocene to Holocene age were recovered from them. In each case, the sediment was found to be unconsolidated on recovery and contain very abundant planktonic foraminifers, particularly in the >150-µm size range. Preservation of tests is generally good, with most showing only minor signs of dissolution or recrystallization, although many samples have a high proportion of fragmented material in the fine fraction. Planktonic foraminifer faunas are diverse and consist predominantly of warm-water species. A typically western Pacific fauna occurs throughout the Miocene. Biostratigraphic assignment was generally straightforward except for the bottommost interval of the pelagic caps where severe reworking (sediment mixing) is a common feature. A significant hiatus was found at each site between drowning of the carbonate platform and the onset of pelagic sediment accumulation. Thus, the platforms were apparently swept clean of sediments, with the exception of isolated ponds, until subsidence took the guyots sufficiently deep for sediment to accumulate in large quantities. Backtracking of subsidence paths for each guyot suggests that pelagic cap formation began at depths of between 700 and 1000 mbsl. The first phase of pelagic cap formation was accompanied by significant transport and mixing of sediment. Eventually, with further subsidence, more continuous and less disturbed sequences were deposited. Several hiatuses occur at various levels within the pelagic caps, the durations of which range from more than 5 m.y. down to the limit of biostratigraphic resolution. These probably relate to changes in the local hydrographic regimes because there is little similarity in the timing of hiatuses between the guyots.
During Ocean Drilling Program Leg 154, Þve sites (Sites 925, 926, 927, 928, and 929) were drilled as a depth transect on the Ceara Rise in the western Atlantic Ocean. Each site consists mainly of pelagic carbonate sediments. Planktonic foraminifers are abundant throughout the sequences except in intervals of intense dissolution at the bathymetrically deeper sites. Faunas are typical of the low latitudes, and they are diverse and reasonably well preserved for much of the record. Recrystallization is mostly moderate, but it is severe at the greater burial depths. A biostratigraphic framework is presented for the lower part of the stratigraphic interval, from Subzone P4c in the upper Paleocene to Zone N12 immediately below the most intensely dissolved level in the middle Miocene. This study replaces the previously published shipboard planktonic foraminifer biostratigraphy. It contains many additional datums and some revisions. Complete range charts of all the species recognized are also presented for the entire stratigraphic interval on the Ceara Rise. Sedimentation on the plateau was almost continuous from the Paleocene to the Holocene. Although several small hiatuses are identiÞed at individual sites, none extends across the whole plateau, so a remarkably complete geological history can be constructed. Much of this record is characterized by sedimentary cyclicity on Milankovitch time scales. This suggests that the Ceara Rise will be a key area for future advances in orbital-based chronology in which planktonic foraminifer biostratigraphy will be an important component.
Planktonic foraminifers were examined from 27 holes situated at 12 separate sites in the tropical Atlantic. The sites are located in various environments, including areas of upwelling, areas affected by cool currents, areas of strong dissolution, and areas that show little dissolution in warm-water settings. Paleomagnetic results were variable at these sites, but accumulation rate curves have been produced by combining the existing paleomagnetic data with the available microfaunal data. Determinations of the ages of the planktonic foraminifer datums from these accumulation rate curves show some species to be strongly diachronous, while others provide good stratigraphic markers. The warmest water sites with the least dissolution show the most complete ranges of species.
The sediments recovered during Leg 138 provide a remarkable opportunity to improve the geological time scale of the late Neogene. We have developed new time scales in the following steps. First, we constructed age models on the basis of shipboard magnetostratigraphy and biostratigraphy, using the time scale of Berggren, Kent, and Flynn (1985). Second, we refined these age models using shipboard GRAPE density measurements to provide more accurate correlation points. Third, we calibrated a time scale for the past 6 m.y. by matching the high-frequency GRAPE density variations to the orbital insolation record of Berger and Loutre (1991); we also took into account δ 18 θ records, where they were available. Fourth, we generated a new seafloor anomaly time scale using our astronomical calibration of C3A.n (t) at 5.875 Ma and an age of 9.639 Ma for C5n.ln (t) that is based on a new radiometric calibration (Baksi, 1992). Fifth, we recalibrated the records older than 6 Ma to this new scale. Finally, we recon-sidered the 6-to 10-Ma interval and found that this could also be partially tuned astronomically.
With its thick, glassy cortex and wide open, coalescing primary and supplementary apertures Sphaeroidinella dehiscens is a conspicuous component of low-latitude planktonic foraminifer faunas. Sphaeroidinella evolved from Sphaeroidinellopsis through the development of a supplementary aperture on the spiral side of th test. The Sphaeroidenellopsis-Sphaeroidinella lineage is important in tropical late Neogene stratigraphy. Changes in the proportion of specimens equipped with a supplementary aperture and the size of this aperture have been analyzed in terminal Miocene through basal Pleistocene sections from DSDP sites 502A from the Caribbean Sea, 503 from the eastern equatorial Pacific Ocean and 214 from the equatorial Indian Ocean.
A preliminary report is presented documenting the evolution of the Globorotalia crassaformis (Galloway and Wissler) lineage from the Early Pliocene G. cibaoensis (Bermudez) in Hydraulic Piston Core 72/516 on the Rio Grande Rise (western South Atlantic). A new technique, eigenshape analysis, is used to describe the gross changes in morphology, and the use of “Raupian” analysis of expansion rates, angular displacement, position relative to coiling axis, and translation rates of chambers is suggested as a useful vehicle for inferring the geometric causes of observed evolutionary changes. These changes include size increase, decrease in the number of chambers per whorl, inflation, reduction of relative height of aperture, and increase in the peripheral angle. The sequence is judged to be an example of phyletic gradualism.
1. From Baffin’s Bay, between 76° 30' and 74° 45' North Latitude. These specimens are derived from seven deep-sea soundings made during one of the Arctic Expeditions under Sir Edward Parry. These soundings were confided to us by Professor Huxley, of the Museum of Practical Geology, Jermyn Street, to which Institution they had been given in April 1853 by Mr. J. W. Lowry, who received them of Mr. Fisher, Assistant-Surgeon in the Expedition alluded to. The Foraminifera obtained by us from these soundings are tabulated in Tables I., IV., and VII. This material from the “Arctic Province” of Naturalists is but scanty. None of the Foraminifera here obtained are numerous, except Polystomella striatopunctata , Nonionina Scwpha , Truncatulina lobatu , and Cassidulina lœvigata ; the first two of which are at home in Arctic waters : and none have attained here a large size except lituolœ . The material from 150 fathoms yielded these relatively large and numerous specimens.
The Neogene deep water carbonate section exposed at Buff Bay, north coast of Jamaica, is about 300 m thick and spans the interval from early middle Miocene to mid-Pliocene (ca. 16 to 3 Ma). It has come to serve as a standard reference section for upper Neogene biostratigraphy because several of the N-zones of Blow (1969) are typified here. A diverse tropical planktonic foraminiferal fauna characterizes this section and enables a detailed biostratigraphic subdivision. One obvious hiatus in the upper part of the Bowden Formation spans about 4 m.y. of late Miocene-early Pliocene time; a shorter more subtle, ca. 1-m.y. intra-late Miocene hiatus is thought to exist separating beds of Zone N17 from Zone N16/15. Faunal analysis reveals general agreement with mid-late Neogene biostratigraphy of Bolli, Blow, and Robinson. A distinct stratigraphic separation between the last occurrence of Paragloborotalia mayeri and the first occurrence of Neogloboquadrina acostaensis suggests that "restoration" of Zone N15 to the planktonic foraminiferal hagiography may be required and that its apparent absence at North Atlantic Deep Sea Drilling Project (DSDP) Sites 558 and 563 may be due to a widespread early late Miocene hiatus. A new type level in the Buff Bay Formation is chosen for the Globorotalia menardii Zone (Bolli, 1966) = Globorotalia (Turborotalia) continuosa Consecutive Range-Zone (Blow, 1969) because the type sample in Trinidad for these zones (which is the same for both zones) has been found to be correlative with a level within the Glborotalia (Turborotalia) acostaensis-G. (T.) merotumida (N16) Zone of Blow (1969). Analysis of type level samples used to denote some classic late Neogene zones has shown that the type level of Blow's (1969) Zone N17 is in calcareous nannoplankton Zone NN10 (rather than NN11 as previously thought), the type level of Zone N18 is biostratigraphically equivalent to Zone PL 2 (-3.7 Ma, lower Pliocene; i.e., within Zone N19), and the type level of Zone N19 is equivalent to Zone PL3-PL4 (ca. 3+ Ma). Use of Blow's Pliocene N18 to N21 zones is discouraged. The taxonomy and stratigraphy of over 40 taxa are discussed and most are illustrated by scanning electron microscope (SEM) micrographs.
Pliocene and Pleistocene planktonic foraminiferal biogeography and paleoceanography have been examined in Deep Sea Drilling Project (DSDP) sites of the Panama Basin (Pacific Ocean) and Colombian and Venezuelan Basins (Atlantic Ocean) to determine the timing of the isolation of Atlantic and Pacific tropical planktonic faunas resulting from the development of the Central American isthmus. Previous studies have suggested a late Miocene to middle Pliocene occurrence of this event. The Panama Basin (DSDP site 157) and the Colombian Basin (DSDP site 154A) share two early Pliocene biogeographic events: (1) great abundance of sinistral coiling Neogloboquadrina pachyderma at 4.3 m.y. ago at site 157 and 0.7 m.y. later at site 154A, and (2) a sinistral-to-dextral change in the coiling-direction preference in Pulleniatina 3.5 m.y. ago at both locations. Identification of these events farther to the east in the Venezuelan Basin (DSDP site 148) is complicated by insufficient lower Pliocene core recovery, but abundant sinistral N. pachydcrma appear to have extended far to the east in the Caribbean 3.6 m.y. ago; perhaps the early Pliocene abundance of this form is not indicative of cool water. The coiling-direction history and stratigraphic ranges of Pulleniatina became different in the Atlantic and Pacific Oceans during the early Pliocene; this is inferred to result from geographic isolation of the assemblages. Saito (1976) used the temporary disappearance of this genus from Atlantic waters at 3.5 m.y. ago to mark the closure of the Isthmus of Panama, but I show that in the Colombian Basin (site 154A) its disappearance was closer to 3.1 m.y. ago. This suggests the possibility of surface-water communication between the Atlantic and Pacific until that time.
Prior work with marine sedimentary sections exposed on Jamaica had been interpreted as evidence for a curious pattern of diachrony between the tropics and subtropics in the ages of certain biostratigraphic zonal boundaries of middle to late Miocene age. We offer an alternative to the Jamaican low-latitude nannofossil calibration, using a combination of paleomagnetic, biostratigraphic, and lithostratigraphic data derived from tropical sections recovered by the Ocean Drilling Program (ODP) in the eastern Pacific and western Atlantic Oceans. Our nannofossil calibration is based on magnetic polarity stratigraphy obtained from the Pacific Ocean and is consistent with the notion that Milankovitch periodicities governed cyclic changes in the lithology of sediments from the Ceara Rise, a sea-floor high in the Atlantic Ocean located offshore of the Amazon delta. Our results thus appear to be representative of the tropics and so call into question the ages that had been assigned to nannofossil zonal boundaries based on results from Jamaican sections.
The primary types of the foraminiferal species Globorotalia barisanensis LeRoy, 1939, Globorotalia fohsi Cushman and Ellisor, 1939, and Globorotalia lobata Bermúdez, 1949, are refigured and redescribed; their morphology and taxonomy are discussed and reapplied to biostratigraphy. The new species Globorotalia (Turborotalia) peripheroronda, G. (T.) peripheroacuta and Globorotalia (G.) praefohsi are proposed and discussed in order to define taxonomically forms which have previously been incorrectly assigned to G. barisanensis and/or G. fohsi. All the taxa discussed have been biostratigraphically related, and their phylogeny is briefly outlined. The evidence from the Kassikan section of Sumatra (LeRoy, 1952) has been reviewed, compared and contrasted with the biostratigraphy proposed by Bolli (1950, 1957) and Blow (1959) for the relevant parts of the Neogene successions of planktonic foraminifera.
Studies of many geographically widely separated stratigraphical sections covering the Late Miocene to Recent interval have allowed the origin and evolution of species referable to the genus Pulleniatina to be traced. The evolutionary sequence Pulleniatina primalis, n. sp., P. obliquiloculata praecursor, n. subsp., P. obliquiloculata obliquiloculata and P. obliquiloculata finalis, n. subsp., is shown to be useful in biostratigraphy. Pulleniatina spectabilis is recognised as forming a separate evolutionary line from P. primalis and as having a short range in the Pliocene of the Indo-Pacific province. Pulleniatina seems to originate from Globorotalia (Turborotalia) acostaensis Blow, 1959, and the holotype of this form is re-illustrated. Globigerina inflata d'Orbigny, 1839, is shown to be unrelated to Pulleniatina obliquiloculata (s. l.), and a neotype for the former taxon is now proposed. The meaning of streptospiral coiling is examined. The new monotypic genus Rotaliatinopsis is proposed to accommodate "?Pulleniatina semi-involuta" Germeraad, 1946, which is unrelated to the Globigerinacea.
Some 48 species and subspecies of planktonic foraminifera, eight of which are new, have been identified and their stratigraphic ranges established in a closely sampled sequence of Miocene rocks from southeast Australia. Zoning based on the ranges of these foraminifera is discussed and the use of initial appearances of evolving species is strongly advocated. Intercontinental correlation of the foraminiferal zones is feasible and comparison with the Caribbean is reasonably close.
We have developed a detailed time scale for the late Miocene and later middle Miocene by correlating lithologic cyclicity observed at Site 926 to the astronomically determined variations in northern hemisphere summer insolation. Most of the equiv-alent section at Site 925 can be correlated in the same manner. The validity of the phase relationship that we have assumed, whereby maximum terrigenous content is correlated with peak insolation, is supported by examining the benthic δ 18 O data from the same site. The time scale is probably accurate to the level of the individual precession cycle back to about 7 Ma and in many intervals back to 13 Ma; elsewhere, there are intervals where further work may require adjustments of one or more pre-cession cycles. At least in the upper part of the section investigated, we conclude that the lithologic signal is present because the input of terrigenous material to the Ceara Rise varied inversely with northern hemisphere summer insolation. At the same time, dissolu-tion of foraminifers in the sediment as well as δ 13 C varied, chiefly in response to obliquity, with a significant phase lag that probably reflects the long residence time of carbon and nutrients in the ocean.
The evolutionary lineage of "Globorotalia' kugleri (planktonic foraminifer) spanning the interval from Zone P22 (late Oligocene) to late Zone N4 (early Miocene) begins with the ancestral form, "Globorotalia' pseudokugleri Blow, and ends with "Globorotalia' kugleri Bolli. On the basis of the spiny honeycomb wall texture, the entire lineage is placed in the genus Paragloborotalia Cifelli. The wall texture of "Globorotalia' kulgeri group is in fact similar to that described by Cifelli for the Paragloborotalia opima group. Paragloborotalia pseudokugleri differs from Paragloborotalia kugleri in having less arched sutures, a rounded profile and peripheral margin, and a lesser maximum number of chambers in the last whorl. -Author
Study of planktonic foraminifers from deep-water sediment cores in the northern Gulf of Mexico indicates the presence of alternating cold-- and warm--water faunas which are interpreted to represent glacial and interglacial stages of the Pleistocene. Globorotalia menardii (warm) and Globorotalia inflata (cold) are the most sensitive indicators for delineating severe changes in Pleistocene temperature in the Gulf of Mexico. On the basis of planktonic datums the cold-warm cycles are correlated with water-depth changes in the shallower shelf environment which are interpreted to represent eustatic sea-level changes. The cold intervals represent times of lowered sea level and are correlated with the glacial stages of North America. Age relationships between the glacially related events recognized in the marine Pleistocene section and the continental glacial stages are well documented to about 35,000 years ago on the basis of carbon 14 dates. Age relationships between the older part of the marine Pleistocene and the continental glacial stages are not so well documented but can be inferred from correlation between the glacial-interglacial cycles and paleomagnetic reversal events, which are well dated by potassium-argon. Onset of severe climatic deterioration and beginning of the Quaternary (Nebraskan Stage) in the Gulf of Mexico correspond with the extinction horizon of Globoquadrina altispira. Correlation of this horizon with the paleomagnetic reversal events gives an age of about 2.8 m.y. ago for the Pliocene-Pleistocene boundary. Placement of the Pliocene-Pleistocene boundary at 2.8 m.y. ago allows recognition of five major glacially related events in the marine Pleistocene based on climatic and eustatic criteria. Faunal data indicate that the Nebraskan, Illinoian, and Wisconsinan were severely cold periods, whereas the Aftonian, Kansan, Yarmouthian and Sangamonian were warmer, with the Aftonian the warmest. Carbon 13/12 isotope data support these conclusions. Foraminifers and discoasters from the marine Calabrian Stage of southern Italy at Santa Maria di Catanzaro indicate a correlation with the marine Nebraskan Stage, as here defined, in the Gulf of Mexico. Paleontological evidence indicates that the Pliocene-Pleistocene boundary of Glass et al. (1967), Berggren et al. (1967), and Ericson and Wollin (1968) falls near the base of the Kansan. The cold and warm intervals shown below the Pliocene-Pleistocene boundary of Glass et al. (1967) are interpreted to represent the Nebraskan and Aftonian Stages. Paleomagnetic evidence suggests that the Olduvai magnetic event of these authors probably represents the Gilsa event at about 1.6 to 1.8 m.y. ago. Advances in paleomagnetic stratigraphy indicate that the magnetic reversals facilitate dating and correlating marine stages at latitudes where differing paleontologic criteria must be used to delineate the glacial and interglacial episodes. Seemingly, the major paleomagnetic epochs are reliable for stratigraphic correlation; however, the minor events of relatively short duration appear to be less reliable as unique stratigraphic markers. Globorotalia pertenuis, n. sp., a right-coiling member of the Globorotalia menardii group, is described.
We present a revised Neogene geochronology based upon a best fit to selected high temperature radiometric dates on a number of identified magnetic polarity chrons (within the late Cretaceous, Paleogene, and Neogene) which minimizes apparent accelerations in sea-floor spreading. An assessment of first order correlations of calcareous plankton biostratigraphic datum events to magnetic polarity stratigraphy yields the following estimated magnetobiochronology of major chron- ostratigraphic boundaries: Oligocene/Miocene (Chron C6CN): 23.7 Ma; Miocene/Pliocene (slightly younger than Gilbert/Chron 5 boundary): 5.3 Ma; Pliocene/Pleistocene (slightly younger than Olduvai Subchron): 1.6 Ma.
Changes to the marine time-scale are relatively minor in terms of recent and current usage except in the interval of the middle Miocene where new DSDP data reveal that previous correlations of magnetic anomalies 5 and 5A to magnetic polarity Chrons 9 and 11, respectively, are incorrect. Our revized magnetobiostratigraphic correlations result in a 1.5-2 m.y. shift towards younger magneto- biochronologic age estimate in the middle Miocene. Radiometric dates correlated to bio- and magnet- ostratigraphy in continental section generally support the revized marine magnetobiochronology presented here. Major changes, however, are made in marine-non-marine correlations in the Miocene in Eurasia which indicate African-Eurasian migrations through the Persian Gulf as early as 20 Ma. The 12.5 Ma estimate of the Hipparion datum is supported by recent taxonomic revisions of the hipparions and magnetobiostratigraphic correlations which show that primitive hipparions first arrived in Eurasia and North Africa at c.12.5 Ma and a second wave in the tropics (i.e. Indian and central Africa) at c. 10 Ma.
Seismic-stratigraphic analysis of multichannel seismic-reflection profiles indicates that the sedimentary section in the western Colombian Basin comprises five principal units. The deposition of these units was strongly influenced by the basement relief of an oceanic plateau. The five units define three depositional episodes, the middle three units defining a single episode. The depositional history of the western Colombian Basin supports plate-tectonic models of this area based on the insertion of Pacific lithosphere. -from Author
We present an integrated geochronology for late Neogene time (Pliocene, Pleisto- cene, and Holocene Epochs) based on an analysis of data from stable isotopes, mag- netostratigraphy, radiochronology, and cal- careous plankton biostratigraphy. Dis- crepancies between recently formulated astronomical chronologies and magneto- chronologies for the past 6 m.y. have been resolved on the basis of new, high-precision Ar/Ar ages in the younger part of this in- terval, the so-called Brunhes, Matuyama, and Gauss Epochs ( Chrons C1n–C2An; 0–3.58 Ma), and revised analysis of sea floor anomalies in the Pacific Ocean in the older part, the so-called Gilbert Epoch ( Chron C2Ar–C3r; 3.58 –5.89 Ma). The magneto- and astrochronologies are now concordant back to the Chron C3r/C3An boundary at 5.89 Ma.
The Neogene (Miocene, Pliocene, Pleisto- cene, and Holocene) and Paleogene are treated here as period/system subdivisions of the Cenozoic Era/Erathem, replacements for the antiquated terms Tertiary and Qua- ternary. The boundary between the Miocene and Pliocene Series (Messinian/Zanclean Stages), whose global stratotype section and point (GSSP) is currently proposed to be in Sicily, is located within the reversed interval just below the Thvera (C3n.4n) Magnetic Polarity Subchronozone with an estimated age of 5.32 Ma. The Pliocene/Pleistocene boundary, whose GSSP is located at Vrica (Calabria, Italy), is located near the top of the Olduvai (C2n) Magnetic Polarity Sub- chronozone with an estimated age of 1.81 Ma. The 13 calcareous nannoplankton and 48 planktonic foraminiferal datum events for the Pliocene, and 12 calcareous nannoplankton and 10 planktonic foram- iniferal datum events for the Pleistocene, are calibrated to the newly revised late Neogene astronomical/geomagnetic polar- ity time scale.
A detailed analysis of planktonic foraminifera assemblages and benthic oxygen isotope values from the Late Pliocence of DSDP Site 548 in the Northeast Atlantic shows that the initiation of Northern Hemisphere glaciation developed gradually, through a series of cycles. These results contrast with those of other DSDP Sites which indicate constant paleoclimatic conditions preceding the sudden, step-wise, development of glaciations at 2.4–2.5 m.y. The difference between these sites and Site 548 might be explained by broad sampling interval and breaks in the sedimentary sequence which obscure the onset of Late Pliocence glaciation at those locations where it appears to occur in a step-wise fashion. The results from Site 548 indicate that the development of Northern Hemisphere glaciation could be regarded as part of a progressive climatic deterioration rather than a geologically sudden paleoclimatic event.