No full-text available
To read the full-text of this research,
you can request a copy directly from the authors.
A reassessment of the sources and importance of land-derived organic matter in surface sediments from the Gulf of Mexico
Abstract
Organic matter in surface sediments from two onshore-offshore transects in the northwestern Gulf of Mexico was characterized by a variety of techniques, including elemental, stable carbon, radiocarbon, and molecular-level analyses. In spite of the importance of the Mississippi River as a sediment source, there is little evidence for a significant terrigenous input based on the low carbon:nitrogen ratios (8–5) and the enriched δ13C values of bulk sedimentary organic carbon (−19.7‰ to −21.7‰). Radiocarbon analyses, on the other hand, yield depleted Δ14C values (−277‰ to −572‰) which indicate that a significant fraction of the sedimentary organic carbon (OC) in all these surface sediments must be relatively old and most likely of allochthonous origin. CuO oxidations yield relatively low quantities of lignin products (0.4–1.4 mg/100 mg OC) along with compounds derived from proteins, polysaccharides, and lipids. Syringyl:vanillyl and cinnamyl:vanillyl ratios (averaging 1.6 and 0.5, respectively) and acid:aldehyde ratios for both vanillyl and syringyl phenols (averaging 0.8 and 1.2, respectively) indicate that the lignin present in sediments originates from nonwoody angiosperm sources and is highly degraded. The δ13C values of lignin phenols in shelf sediments are relatively depleted in 13C (averaging −26.3‰) but are increasingly enriched in 13C at the slope sites (averaging −17.5‰ for the two deepest stations). We interpret these molecular and isotopic compositions to indicate that a significant fraction (≥50%) of the lignin and, by inference, the land-derived organic carbon in northwestern Gulf of Mexico sediments ultimately originated from C4 plants. The source of this material is likely to be soil organic matter eroded from the extensive grasslands of the Mississippi River drainage basin. Notably, the mixed C4 and C3 source and the highly degraded state of this material hampers its recognition and quantification in shelf and slope sediments. Our data are consistent with higher than previously estimated inputs of land-derived organic carbon to regions of the ocean, such as the Gulf of Mexico, with significant sources of terrigenous C4-derived organic matter.
... There have been many studies focusing on the carbon sink function of sediments, and the carbon burial flux and its long-term changes in marine sediments have been studied (Thunell et al., 1992;St-Onge and Hillaire-Marcel, 2001;Justićet al., 2002;Ruiz-Fernańdez et al., 2007;Hayes et al., 2021;Zhao et al., 2021). The sources of organic carbon have also been discussed by isotopes or C/ N, etc. Andrews et al., 1998;Goñi et al., 1998;Huang et al., 2001;Bouchez et al., 2014;Xing et al., 2014;Sanderman et al., 2015;Geraldi et al., 2019;Zinkann et al., 2022). However, from a BC burial perspective, the burial behavior of BC, including patterns, burial flux, and its relationship with environmental changes, remains unclear. ...
... The term "organic blue carbon" (OBC) in this study refers to marine-source organic carbon that is buried in sediment. And C/N ratio was used to identify the source of organic matter Andrews et al., 1998;Goñi et al., 1998;Kaushal and Binford, 1999;Venkatesh, 2020). The end-member values of C/N for marine and terrestrial organic matter were set at 5 and 20, respectively. ...
... Various methods have been established to estimate organic carbon in sediments, including isotopes, C/N ratios, and biomarkers Andrews et al., 1998;Goñi et al., 1998;Huang et al., 2001;Bouchez et al., 2014;Xing et al., 2014;Sanderman et al., 2015;Geraldi et al., 2019;Zinkann et al., 2022); however, there are still challenges in identifying the origins of inorganic carbon in sediment samples. In this study, the source resolution of inorganic carbon was initially determined based on the morphological characteristics of inorganic carbon from various sources. ...
Marine primary production and terrestrial input are the main sources of buried carbon in sediments of marginal seas. Only marine-source carbon buried in sediments, fixed and stored by marine ecosystems, belongs to “blue carbon” and reflects marine ecosystems’ carbon sink function. The pattern of buried blue carbon in sediments, its flux, and its relationship with environmental changes remain unclear. The study aimed to investigate the composition of blue carbon in the sediments of Sanggou Bay, a special type of marginal sea. The analysis of sediment carbon sources was conducted through the C/N ratio and microscopic examination. The study also examined the long-term changes in the blue carbon burial fluxes. Results showed Blue carbon, which is sea-sourced carbon, accounted for about 23% of the total carbon content and its concentration ranged from 0.17% to 0.51%, with an average of about 0.25% ± 0.10%. The content of organic blue carbon in this sea area ranges from 0.09% to 0.26%, with an average of around 0.18% ± 0.04%. It constitutes approximately 72% of the buried blue carbon in the sediment, making it the primary component of buried blue carbon. Meanwhile, the content of inorganic blue carbon ranges from 0.01% to 0.32%. Over the past 70 years, the burial fluxes of sedimentary blue carbon, organic blue carbon and inorganic blue carbon in the Sanggou Bay are about 0.54 ± 0.22 mmol/(cm²a), 0.38 ± 0.07 mmol/(cm²a) and 0.17 ± 0.22 mmol/(cm²a), respectively; their long-term changes have been significantly affected by human aquaculture activities. Large-scale raft-rack aquaculture activities have caused a reduction in water flow velocity and an increase in the deposition of particulate organic matter, which in turn has led to the burial of organic blue carbon in the sediment. Additionally, the competition between aquaculture products and small calcareous organisms, such as mussels, foraminifera, may have inhibited the growth of small calcareous organisms. We suggest this has resulted in reduced burial fluxes of inorganic blue carbon and a decrease in its proportion among total blue carbon in the sea area. Our findings imply that aquaculture activities in Sanggou Bay had a negative impact on the burial of blue carbon in the sediments.
... The C/V ratios had an average of 1.7 in the sediment of Storsjön. Such high C/V ratios are not common in sediments, soils or most fresh plant tissues (Goñi et al., 1998;Hedges & Mann, 1979;Moingt et al., 2016). However, the pollen of Picea is rich in p-coumaric acid and has C/V ratios ranging between 13 and 35 and p-coumaric acid/ ferulic acid (CAD/FAD) ratios above 1.5 (Hu et al., 1999;Ishiwatari et al., 2006;Keil et al., 1998). ...
... (a) Relation between Λ 8 and C/N ratio with the boundary according to Meyers and Ishiwatari (1993). (b) Relation between Λ 8 and (Ad/Al) V with the boundary according to Goñi et al. (1998). (c) Relation between C/V and S/V ratios with the boundaries according to Goñi et al. (1998). ...
... (b) Relation between Λ 8 and (Ad/Al) V with the boundary according to Goñi et al. (1998). (c) Relation between C/V and S/V ratios with the boundaries according to Goñi et al. (1998). Λ 8 : sum of vanillyl, syringyl, and cinnamyl phenols normalized to 100 mg organic carbon; (Ad/Al) V : ratios of vanillic acid to vanillin; S/V: ratios of syringyl to vanillyl phenols; C/V: ratios of cinnamyl to vanillyl phenols; G w : woody gymnosperms; A nw : non-woody angiosperms; and G nw : non-woody gymnosperms. ...
Terrestrial organic matter (OM) plays a key role in coastal organic carbon burial. However, few studies focus on the relationship between land use in the watershed and the transport of terrestrial OM to coasts from a long‐term perspective. In this study, we compared terrestrial OM deposition between an inlet of the Baltic Sea and an upstream lake within the same watershed over the last 500 years, using lignin biomarkers in the sediments. In combination with pollen‐based quantitative land cover reconstruction, we assessed the impacts of semicentennial‐scale changes in land use on terrestrial OM export. The results indicated that the concentration, composition, and degradation state of the lignin‐derived OM differed substantially between the two sites. The lake received larger amounts of lignin‐derived OM during periods of intensified agriculture, but the coastal site did not. The composition of lignin in the coastal sediment did not directly reflect variations in vegetation cover in the watershed. The reason could be that the OM was settled in the upstream basins. Furthermore, the terrestrial OM that did reach the coastal sediments was modified through degradation during the transport, and only the refractory component was deposited at the coast in a relatively unaltered form.
... Samples falling below the mixing line were mainly collected from channels with sandy sediments and they exhibit 13 C-depleted or low C/N signals compared to other samples. Such lower C/N signals of bulk OM have been usually observed at estuaries and coastal environments, resulting from adsorption of inorganic nitrogen with sediments due to a large amount of eutrophic fluvial input [70,74,75]. As Figure 5 shows, the extremely low C/N signals have been also observed at the delta plain proximity near the river mouth. ...
... Generally, the relative contribution of delta derived OM in total sedimentary OM of the MRE shows a high value at distal sites off the river mouth and a low value at proximal sites near the river side. Goñi et al. (1998) [74] proposed that hydrodynamic sorting caused by the resuspension and cross-estuary transport of particles may serve to physically segregate distinct pools of terrestrial OM. The result of this process is the preferential transport of C4 grassy wetland derived OM from the delta closely associated with the finer-grained mineral load (clays) to exterior regions of the MRE. ...
... Generally, the relative contribution of delta derived OM in total sedimentary OM of the MRE shows a high value at distal sites off the river mouth and a low value at proximal sites near the river side. Goñi et al. (1998) [74] proposed that hydrodynamic sorting caused by the resuspension and cross-estuary transport of particles may serve to physically segregate distinct pools of terrestrial OM. The result of this process is the preferential transport of C4 grassy wetland derived OM from the delta closely associated with the finer-grained mineral load (clays) to exterior regions of the MRE. ...
As one of the main interfaces of the Earth system, estuaries show the strongest land–sea interaction in the carbon cycle, which links terrestrial ecosystems to the marginal sea. Furthermore, estuaries are considered as one of the most active intermediate reservoirs for both terrestrial and marine matter due to complex hydrodynamic processes regulated by the river runoff, wave and tide. Processing of organic matter (OM) in tidal estuaries modifies its transfer and transformation from the river to the sea, so studies of on the source and distributions of estuarine OM can help us understand the behavior of production, exchange, transport and burial of diverse OM within this transition zone before entering the marginal sea. In this paper, we took the Minjiang River Estuary (MRE) as a typical system in which there is strong influence of the tide. The source, composition and spatial distribution of OM in surface sediments of MRE were deciphered based on multiple organic geochemical properties for source-specific biomarkers (n-alkanes, n-alkanols, sterols) and bulk OM. Results show that sedimentary organic components were negatively correlated with sediment grain size, which indicates fine particles such as silt and clay are the major carriers of the OM signals in tidal estuaries. Source-specific biomarker proxies indicate that in terms of source diversity the sedimentary OM in the MRE shows mixed signals of terrestrial and marine sources, and the proportion of terrestrial OM decreases with the increase in distance from the land. The fractional contributions of OM from the riverine (i.e., terrestrial), marine and deltaic sources were quantitatively estimated using a Monte Carlo (MC) three-end-member mixing model based on C/N and δ13C values, and the average contributions of the three sources are 40 ± 10%, 48 ± 10% and 12 ± 4%, respectively, with little contribution from deltaic sources. The dispersion of sedimentary OM from different sources in the MRE is primarily controlled by the depositional environment determined by dynamic conditions and tidal processes play a significant role in the redistribution of sedimentary OM dispersion patterns. Compared with other large estuaries in southeast China, the OM accumulation contribution in the tide dominated small and medium-sized estuaries such as the MRE which is largely dependent on riverine and marine deliveries. The MRE has a high potential for both terrestrial and marine organic carbon (OC) burial, with an accumulation rate of 3.39 ± 1.83 mg cm−2 yr−1 for terrestrial OC, and an accumulation rate of 3.18 ± 0.68 mg cm−2 yr−1 for marine OC in muddy sediment, making it an important contributor to the sedimentary carbon sink of the marginal sea.
... It is well known that the Mississippi/Atchafalaya River system has a large contribution of both C 3 and C 4 plants from the Mississippi River flood basin. Goñ i et al. (1997Goñ i et al. ( , 1998 suggest that the overprint of soil from C 4 -dominated grassland plants probably compromises the use of the two-end-member model for estimating allochthonous contributions and that terrigenous carbon contributions to offshore sediments are much greater than predicted by the simple isotope mixing model. Moreover, Goñ i et al. (1997Goñ i et al. ( , 1998 employed the classical lignin phenol analyses coupled to compound-specific stable carbon isotopic analyses to evaluate possible contribution of C 4 plants from soils. ...
... Goñ i et al. (1997Goñ i et al. ( , 1998 suggest that the overprint of soil from C 4 -dominated grassland plants probably compromises the use of the two-end-member model for estimating allochthonous contributions and that terrigenous carbon contributions to offshore sediments are much greater than predicted by the simple isotope mixing model. Moreover, Goñ i et al. (1997Goñ i et al. ( , 1998 employed the classical lignin phenol analyses coupled to compound-specific stable carbon isotopic analyses to evaluate possible contribution of C 4 plants from soils. Other work presented by Goñ i (2003, 2004), using a three-endmember mixing model, show terrOM accounts for about 65-80% of the organic matter deposited on the inner shelf of the Mississippi/Atchafalaya systems supporting Goñ i et al. (1997Goñ i et al. ( , 1998. ...
... Moreover, Goñ i et al. (1997Goñ i et al. ( , 1998 employed the classical lignin phenol analyses coupled to compound-specific stable carbon isotopic analyses to evaluate possible contribution of C 4 plants from soils. Other work presented by Goñ i (2003, 2004), using a three-endmember mixing model, show terrOM accounts for about 65-80% of the organic matter deposited on the inner shelf of the Mississippi/Atchafalaya systems supporting Goñ i et al. (1997Goñ i et al. ( , 1998. Lignin-phenol biomarkers suggested that offshore terrOM is more represented by terrestrial components, presumably due to selective removal of the more labile OM algal fractions during transport (Bianchi et al., 2002;Mead and Goñ i, 2006). ...
Sediment samples along a transect extending from the Mississippi River Birdsfoot Delta to the Mississippi Canyon on the Louisiana continental shelf were examined, by advanced analytical techniques, electrospray ionization coupled to a 12T Fourier transform ion cyclotron resonance mass spectrometer (ESI-FTICR-MS) and quantitative solid-state multiple cross polarization magic angle spinning (multi-CPMAS) ¹³C NMR, in an effort to understand the source and export of terrigenous organic matter to the Gulf of Mexico. Both NMR and mass spectral data indicate that condensed aromatics (CA) and carboxyl-containing aliphatic molecules (CCAM) are present at the mouth of the river, reflective of high contributions from terrigenous soil-like organic matter. Mass spectral peak magnitudes of CA diminish by 15%, with increasing distance offshore, and represent 30% of integrated NMR peak areas. In contrast, mass spectral and NMR CCAM peaks increase by 7 and 13%, respectively. These trends in humic acid extracts,provide novel molecular evidence of terrigenous organic matter deposition in offshore sediments.
... On the other hand, the source instruction of bulk parameters such as C/N ratios and δ 13 C showed unreliability due to their lack of sensitivity to reveal varied OM inputs, as their original distinction could be blurred by the microbial diagenetic alteration, e.g., the preferential loss of N in aquatic conditions and Meyers, 1994). In addition, the preferential sorption of inorganic N by minerals can also cause deviation to the C/N ratio (Goñi et al., 1998;and Schubert & Calvert, 2001). While the marine plankton can produce a wide range of δ 13 C (Villinski et al., 2008) and C 4 vegetation has more positive δ 13 C values (Xue et al., 2014); the Ayeyarwady Delta ; the Gulf of Martaban (Ramaswamy et al., 2008); and the GOT (Meksumpun et al., 2005). ...
... in terrestrial vegetation (Goñi et al., 1998). In addition, this uncertainty of these bulk parameters could also be related to anthropogenic influences (e.g., petroleum input and fertilization) (Yu et al., 2010). ...
... It has been reported that seasonal circumfluence plays an important role in transporting OM from multiple origins that is associated with sediments ; and the formation of the two modern fine-grained sediment depocenters in central and northeast GOT was facilitated by the hydrodynamic conditions under a long-distance dispersal (Liu et al., 2019). It has been reported that SOM bound in these fine particles has also been found to be enriched in nonwoody vascular components (i.e., lignin-poor) and pre-aged OM (soil-derived), while the lignin-rich coarser fraction of sediment was mostly deposited in nearshore areas (Bianchi et al., 2002;Goñi et al., 1998). In the present study, as shown in Figure 7, the high presence of the PC1 score with higher proportion of the terrigenous n-alkanes in the two modern depocenter indicated that the hydrodynamic sorting and long-distance lateral sediment transport could lead to a selective dispersal and accumulation of the terrigenous SOM. ...
Carbon cycling in the tropical margin is more extensive compared with other regions of the world. The goal of this study was to better understand the origins, transport, and burial of sedimentary organic matter (SOM) in the Gulf of Thailand (GOT) from the coastal margin of Southeast (SE) Asia, which serve as a major depository of fine‐grained sediments and the associated organic carbon (OC). The results revealed a variety of organic matter (OM) inputs and the selective transport of fine sediment, resulting in preferential dispersal of terrigenous SOM in the GOT. Bulk OC indices with low carbon/nitrogen ratios and enriched stable carbon isotope ratios (−24.2‰ to −20.4‰, and mean −21.4 ± 0.56‰) are likely related to the presence of marine‐derived OM and anthropogenic interference. A binary mixing model further clarified the significant contributions of terrestrial derived‐OM within the upper and central GOT. The n‐alkane compositions and principal component analysis indicated that a majority of the terrigenous SOM settles within the estuary in the upper GOT, while a selective dispersal of land‐based SOM through long‐distance transport toward the modern depocenter in the lower GOT. The characteristics of lower molecular weight n‐alkanes also suggest anthropogenic OM input from petroleum‐related contributions. Altogether, the depositional patterns and spatial heterogeneity of the SOM indicated by both the bulk and molecular signatures reveal the important roles of source variability and the selective dispersal of land‐based OM on the supply and accumulation of OC in the tropical coastal margin.
... The S/V, C/V ratios and LPVI values are used to characterize the vegetation sources in basin. S/V is used to distinguish lignin from gymnosperms (~0) or angiosperms (0.6-40); C/V is used to distinguish herbaceous tissues (> 0.20) from woody tissues (< 0.05; Goñi et al., 1998;Bianchi et al., 2011). The S/V and C/V ratios ranged from 1.04 to 2.31 and 0.22 to 0.77 (Fig. 3b), with an average value of 1.59 ± 0.27 and 0.46 ± 0.10 in this core YS-A. ...
... However, a large amount of sediment did not disperse to the nearby offshore areas, but instead accumulated in the coastal regions . In winter, the EAWM strengthens the currents as well as the vertical mixing process, and then breaks up the stratification formed in summer (Gong et al., 2017), resulting in that the sediments in the coastal Fig. 3. a, δ 13 C versus C/N ratio for all samples of core YS-A, the average values and standard deviation of δ 13 C and C/N during different periods are shown, where the background labels of organic matter sources are according to Fry and Sherr (1984), Meyers (1994), Tyson (1995) and Lamb et al. (2006); b, C/V versus S/V ratios for all samples of core YS-A, the average values and standard deviation of S/V and C/V during different periods are shown, the typical ranges of different vegetation types were from Goñi et al. (1998), Bianchi et al., 2011. areas accumulated in summer are resuspended and transported to the offshore areas by coastal currents such as YSCC and YSWC (Goñi and Hedges, 1995;Zhou et al., 2015). ...
Long-term organic matter (OM) burial in the ocean is essential to the global carbon cycle. Mud deposits, such as South Yellow Sea mud deposit (SYSMD) located in the central South Yellow Sea (SYS), are ideal for the study of long-term OM burial. A sediment core YS-A from the SYSMD was analyzed for lignin phenols and bulk OM properties to reveal the driving forces of sedimentary OM (SOM) fate during the Holocene. SOM burial was found to be dominantly influenced by sea level rise and increased East Asian summer monsoon during 11.0–7.0 ka BP. During 7.0–1.0 ka BP, the fate of SOM was controlled by El Ni˜no Southern Oscillation on the millennial time scale, and correlated with East Asian winter monsoon variability on the centennial time scale. Remarkably, anthropogenic perturbation has gradually overwhelmed long-term climate control on the fate of SOM since 1.0 ka BP, and this phenomenon became more evident after 0.4 ka BP.
... During the transportation of plant debris along coastal areas, denser woody tissues have been found to settle near the shore, and less dense non-woody particles are transported farther offshore, resulting in increased C/V ratios in that area [51,52]. In this study, non-woody tissues were transported from the Yangtze River farther offshore toward MK2, resulting in higher C/V ratios in MK2 compared to the Yangtze River estuary and the ECS. ...
... Higher (Ad/Al)v values are indicative of more highly degraded lignin materials [53]. Generally, a lignin (Ad/Al)v in the range of ∼0.1-0.3 can be considered to indicate fresh plant tissues, while a value greater than 0.6 indicates highly degraded lignin materials [51,54]. The (Ad/Al)v values along the MK2 core ranged from 0.42 to 2.04, indicating medium to high degradation of lignin materials at this location. ...
In this work, a sediment core collected from the Jiulong River estuary in southeastern China was subjected to ¹⁴C dating of foraminifera, as well as lignin, total organic carbon, and stable carbon isotope (δ¹³C) analyses in order to determine the impacts of climate change and human activities on the sedimentary organic matter in this area from around 2,000 to 1,000 years ago. The ratios of lignin parameters syringyl/vanillyl and cinnamyl/vanillyl ranged from 1.60 to 8.63 and 0.11 to 0.45, respectively, and the lignin phenol vegetation index ranged from 25.14 to 1740.14, indicating the presence of non-woody angiosperms. The ratio of vanillic acid/vanillin ranged from 0.42 to 2.04, indicating medium to high degrees of oxidative degradation. The vertical distribution profile showed a similar historical sedimentary trend with locations at higher latitudes along the Zhejiang-Fujian Mud Area, with the lower abundance of total lignin from around 2400 to 2000 BP being attributed to the decreasing temperature during this period. However, all the lignin parameters showed higher values and greater fluctuations due to increasing temperatures after 2000 BP, and human activity has probably had the greatest impact in the most recent 1,000 years.
... Close to the MR, the C/N ratio has its highest values, similar to the molar C/N ratio of 12.2 reported earlier for the MR delta (Bianchi et al., 2002). contributed to the OM-pool in areas with enriched δ 13 Corg signatures (Goñi et al., 1997(Goñi et al., , 1998Bianchi et al., 2002). If δ 13 Corg values alone would be used, this would have conventionally been interpreted as indicating a predominant marine origin of 375 the OM. ...
... Earlier studies have suggested that the transport of plant markers is 505 sensitive to hydrodynamic processes in the coastal zone due to their association with mineral particles. Molecular plant markers associated with clay particles can be transported further into the open ocean than plant remains, which are usually associated with coarser grains (Goñi et al., 1997(Goñi et al., , 1998. The spatial distribution of plant-OM in the GoM shows that, in particular, n-alkanes could be preferentially associated with clay particles and thereby transported further off-and along shore, as these mineral associations not only facilitate their transport, but also provides protection from degradation (Mayer, 510 1994a, b;Keil et al., 1997;Lalonde et al., 2012). ...
Rivers play a key role in the global carbon cycle by transporting terrestrial organic matter (TerrOM) from land to the ocean. Upon burial in marine sediments, this TerrOM may be a significant long-term carbon sink, depending on its composition and properties. However, much remains unknown about the dispersal of different types of TerrOM in the marine realm upon fluvial discharge as the commonly used bulk OM parameters do not reach the required level of source- and process-specific information. Here, we analysed bulk OM properties, lipid biomarkers (long-chain n-alkanes, sterols, long-chain diols, alkenones, branched and isoprenoid glycerol dialkyl glycerol tetraethers (GDGTs)), pollen, and dinoflagellate cysts in marine surface sediments along two transects offshore the Mississippi and Atchafalaya Rivers (MAR), as well as one along the 20 m isobath in the direction of the river plume. We use these biomarkers and palynological proxies to identify the dispersal patterns of soil-, fluvial, higher plant, and marine produced OM in the coastal sediments of the northern Gulf of Mexico (GoM). The Branched and Isoprenoid Tetraether (BIT) index and the relative abundance of C32 1,15-diols indicative for freshwater production show high contributions of soil and fluvial OC near the Mississippi River mouth (BIT = 0.6, FC32 1,15 >50 %), which rapidly decrease further away from the river mouth (BIT <0.1, FC32 1,15 <20 %). In contrast, concentrations of long-chain n-alkanes and pollen grains do not show this stark decrease along the path of transport, and especially n-alkanes are also found in sediments in deeper waters. Proxy indicators show that marine productivity is highest close to shore, and reveal that marine producers (diatoms, dinoflagellates, coccolithophorids) have different spatial distributions, indicating their preferential niches. Close to the coast, where food supply is high and waters are turbid, cysts of heterotrophic dinoflagellates dominate the assemblages. The dominance of heterotrophic taxa in shelf waters in combination with the rapid decrease in the relative contribution of TerrOM towards the deeper ocean, suggests that TerrOM input may trigger a priming effect that results in its rapid decomposition upon discharge. In the open ocean far away from the river plume, autotrophic/phototrophic dinoflagellates dominate the assemblages, indicating more oligotrophic conditions. Our multi-disciplinary approach reveals that different types of TerrOM have distinct dispersal patterns, suggesting that the initial composition of this particulate OM influences the burial efficiency of TerrOM on the continental shelf and open ocean.
... Ces derniers n'ont été ultérieurement que très partiellement abordés par Relexans et al. (1992) puis Le 13 C est un marqueur d'origine de la MOP . Au large des grands fleuves, ce traceur montre habituellement des valeurs croissantes depuis la côte vers le large ; ce qui traduit la modification des contributions relatives des sources continentales et marines , Goñi et al. 1998, Blair et al. 2003, Gordon & Goñi 2003, Cathalot et al. 2013. Ceci n'est pas le cas dans la VOG pour laquelle ce rapport ne varie pas significativement avec la profondeur dans les sédiments de surface ( . ...
... Une autre spécificité des résultats obtenus lors de mon travail réside dans l'augmentation des concentrations en carbone organique particulaire (COP) le long du gradient de profondeur, ce qui est contradictoire avec la dilution d'apports continentaux massifs généralement observée aux débouchés des (grands) fleuves (Goñi et al. 1998, Blair et al. 2003, Gordon & Goñi 2003. De manière générale, dans les marges continentales sous l'influence des grands fleuves, le mouvement des particules sédimentaires vers le large résulte le plus souvent de cycles de sédimentation/resuspension successifs qui s'accompagnent d'une dégradation de la MOP sédimentée (Gordon et al. 2001 ; phénomène que l'on observe également dans la VOG. ...
Les grands fleuves influencent fortement certaines régions côtières (i.e., les Riverdominated Ocean Margins, ou RiOMar), qui présentent de forts taux de sédimentation et auxquelles sont associés des communautés benthiques et des processus biogéochimiques dont le fonctionnement varie en fonction de la dynamique temporelle des flux particulaires et de leur interaction avec l'hydrodynamisme. La Vasière Ouest-Gironde (VOG) constitue un modèle pertinent pour ce type de systèmes puisqu’elle constitue la principale zone de dépôt primaire des particules issues de l'estuaire de la Gironde, et qu’elle est située dans un environnement hautement énergétique. Bien que sa dynamique sédimentaire ait fait l’objet de nombreux travaux, l’étude des caractéristiques (dont la matière organique associée) des sédiments superficiels, et de la macrofaune benthique y a été jusqu'à présent négligée. L’objectif de cette thèse consiste à mieux décrire la structuration spatio-temporelle de l’écosystème benthique de la VOG, via l’étude de la matière organique particulaire (MOP) sédimentée ainsi que de la composition de la macrofaune benthique et de son activité. Une comparaison a par ailleurs été effectuée avec le prodelta du Rhône qui a déjà beaucoup été étudié. Bien qu’également situé en zone tempérée, celui-ci diffère en effet de la VOG à la fois par la saisonnalité plus marquée des apports fluviaux ainsi que par la plus faible intensité de l'hydrodynamisme dans la zone réceptrice. Une campagne synoptique (juin 2018, 32 stations) et 4 campagnes saisonnières (5 stations le long d’un gradient côte-large, octobre 2016-avril 2018) ont été réalisées sur la VOG dans des conditions de débits et d’hydrodynamisme contrastées. Une large gamme de paramètres a été mesurée : (1) caractéristiques des sédiments superficiels (granulométrie, surfaces spécifiques, descripteurs quantitatifs et qualitatifs de la MOP), (2) composition de la macrofaune, et (3) traces d’activité biologique (imagerie de profils sédimentaires). Sur la base de l’analyse de la distribution spatiale de ces paramètres, les résultats obtenus confirment la subdivision de la VOG en une zone proximale et une zone distale qui avait déjà été mise en évidence par des études sédimentologiques. Ils montrent l’existence de gradients de profondeur (i.e., entre zones proximale et distale et à l’intérieur de la zone distale) marqués pour la plupart de ces paramètres. L’analyse des corrélations entre ces variations spatiales et celles de plusieurs facteurs de contrôle potentiels suggère le rôle prédominant de l’hydrodynamisme comparé à celui du débit de la Gironde et du chalutage de fond. Mes résultats montrent également l’existence de variations temporelles dont la composante saisonnière est liée à l’efflorescence printanière, et à laquelle se superpose une tendance interannuelle entre 2016 et 2018 pour la composition de la macrofaune benthique. Dans le cas de cette dernière, et pour les 3 stations déjà échantillonnées en 2010, mes résultats montrent enfin l’existence d’importants changements temporels entre 2010 et 2016-2018. Ces changements sont attribués à la succession de tempêtes exceptionnelles intervenues durant l’hiver 2013/2014, qui aurait profondément perturbé l’écosystème benthique de la VOG et initié une séquence de cicatrisation. De manière générale, une différence importante avec le prodelta du Rhône réside dans le rôle majeur joué par l’hydrodynamisme (i.e., par rapport aux apports fluviaux) dans le contrôle de la structuration spatio-temporelle des paramètres étudiés. Cette différence tend à valider la transposition aux zones tempérées de la typologie des RiOMar jusqu’ici établie sur des bases biogéochimiques et principalement à partir d’exemples tropicaux et subtropicaux.
... Furthermore, the TOC and TN contents display a good positive linear correlation ( Figure 3) with a coefficient of 0.8637. The linear correlation between TOC and TN has no intercept indicating that nitrogen in the sediment mainly exists in the form of organic compounds (Goni et al., 1998;Schubert and Calvert, 2001). ...
... For example, C 3 vascular plants are typically characterized by isotopic compositions that are relatively depleted in 13 C (−22‰∼−33‰, with an average of −27‰; Fry and Sherr, 1984;Ramaswamy et al., 2008), while C 4 plants display significant more enriched 13 C value (−9‰∼−16‰, with an average of −13‰; Pancost and Boot, 2004). For the typical marine phytoplankton, it exhibits intermediate δ 13 C values (−18‰∼−21‰, Goni et al., 1998). The values of δ 13 C org in the surface sediments from the coastal Leizhou Peninsula range from −22.47‰ to −19.18‰, with an average of −20.61 ± 0.71‰ ( Figure 4B; Table 1), is similar to that of marine organic matter. ...
A total of 43 seafloor surface sediment samples collected from the offshore region surrounding the Leizhou Peninsula were analyzed in terms of the total organic carbon content (TOC), total nitrogen (TN), organic carbon isotope (δ¹³Corg) and sediment grain size. Our results showed that the organic carbon content in the samples ranged from 0.12 to 0.79%, with an average of 0.42% which was lower compared with other offshore regions of China, whereas the δ¹³Corg was in the range of −22.47‰ to −19.18‰. The total nitrogen content ranged from 0.011 to 0.100% which was also low. The combination of δ¹³Corg and TOC/TN ratio (5.1–14.3) suggested that the organic matter is dominant by marine authigenic source input, as marine sourced organic matter accounts for 71%, 82%, and 75% in the sediments from the offshore areas of the east side, west side and the south (Qiongzhou Strait) of the Leizhou Peninsula, respectively. The weak/poor correlations between the organic carbon and clay, silt content, the pH and Eh value suggested that organic matter abundance and distribution were influenced by the source input of organic matter, the seafloor sediments’ redox status, and seawater’s hydrodynamic condition.
... Organic matter (OM) is an important component of riverine sediments, and is a major participant in biogeochemical cycles in the aquatic environment (Stevenson, 1982;Mayer, 1995;Huang and Weber, 1997;Goi et al., 1998). Sedimentary OM mainly enters rivers via endogenous and exogenous pathways. ...
... The treated samples were further processed to separate the OM fraction. Briefly, HCl, HF, methanol, and acetone were used to dissolve and remove minerals and hydrolyzable lipids using multiple rounds of mixing, heating in a water bath, and shaking (Gélinas et al., 2001;Song et al., 2002;Zhang et al., 2016aZhang et al., , 2016b. The percentages of N, C, H, and O were determined using an elemental analyzer (Vario EL III, Elementar, Germany). ...
Organic matter (OM) is an important component of riverine environments and a major factor in the migration and transformation of hydrophobic organic substances, such as polycyclic aromatic hydrocarbons (PAHs), to sediments. We studied the distributions, sources, and correlations between PAHs and OM in sediments from the Duliujian and the Beiyun rivers in North China. Sixteen PAHs were detected in the surface sediments at total concentrations ranging from 356 to 4652 ng·g⁻¹ dry weight, which caused a moderate to high level of pollution. The PAH distributions were significantly and positively correlated with OM (p < 0.01) and higher concentrations were detected downstream of areas affected by human activity. Petroleum, coal, and wood combustion were the main sources of PAHs in riverine sediments, and the sources of OM in sediment included terrestrial and aquatic higher plants, soil, and sewage discharge. The OM accumulated and aged along the river, with increases in the degree of aromaticity and condensation, which led to stronger adsorption of PAHs. Our results will help to promote the management and restoration of contaminated riverine sediments.
... The inorganic nitrogen fraction in sediments mostly consists of ammonium (Das et al., 2008), and the capacity of sediments to bind inorganic N depends on the type of clay minerals, which bind the ammonium ions. A Cross-plot between TN and C org (from elemental analysis) may look for inorganic nitrogen (Choudhary et al., 2009;Goñi et al., 1998) in organic matter-rich sediments. If the regression line between the C org and the TN values intercepts the X-axis (TN axis), then that intercept calculates the percentage of inorganic nitrogen within the sediments (Goñi et al., 1998). ...
... A Cross-plot between TN and C org (from elemental analysis) may look for inorganic nitrogen (Choudhary et al., 2009;Goñi et al., 1998) in organic matter-rich sediments. If the regression line between the C org and the TN values intercepts the X-axis (TN axis), then that intercept calculates the percentage of inorganic nitrogen within the sediments (Goñi et al., 1998). This plot, in the present study, shows a positive correlation between C org and TN (r = 0.84) and the regression line intercepts on the Yaxis (C org axis) (Fig. 4b), which may reflect absence of inorganic nitrogen in these samples. ...
The Palaeocene-Eocene organic sedimentary archives comprising lignites, carbonaceous shales, and shales from the Palana Formation of the Bikaner-Nagaur Basin, Rajasthan, India, are investigated in this study using elemental and biogeochemical proxies. The objectives of this investigation are to elucidate the sources of organic matter and their responses to palaeoredox and palaeoclimatic conditions, as well as to reconstruct the palaeoenvironment of the organic matter deposition. Elemental distributions, Rock Eval pyrolysis, stable carbon isotope, n-alkane and its ratios, acyclic isoprenoids and terpenoid biomarkers are employed as the tools to carry out this research. The hydrogen index shows that the samples contain type III, admixed type II-III and type II kerogen. The presence of admixed type II-III kerogen in the samples may indicate mixing of terrestrial plant-derived organic matter with autochthonous organic matter in a coastal depositional setting. Deposition in a coastal setting may also raise the possibility of organic matter supply from coastal vegetation. The large HI values (>300 mg HC/g TOC) may result from selective preservation of hydrogen-rich biomolecules derived from terrestrial and coastal vegetation. The stable carbon isotopic composition of bulk organic matter (−30.66 to −25.51‰) and total carbon to organic nitrogen ratio (26.62–131.12) may suggest that the organic matter was sourced primarily from C3 plants. The integration of these two parameters may also indicate that mangroves supplied significant amount of organic matter to the coastal peatland. The n-alkane distributions and carbon preference indices show the derivation of organic matter from waxy terrestrial plants, aquatic submerged vegetation, and microbial communities. The palaeohydrological proxies, i.e., proxy-aqueous ratio (0.24–0.86), proxy-wax ratio (0.33–0.84), and average chain length (26.32–28.30) may illustrate intermittent changes in organic matter sources (mangrove-dominated emergent vegetation and waxy plants to aquatic submergent plants and vice-versa) and their relations with shifts in palaeohydrological conditions of mire and associated palaeoclimatic oscillations between wet (high rainfall) and dry spells (low rainfall). Additionally, terpenoid distributions reveal organic matter input from angiosperms (including mangroves) (olean-12-ene, de-A-olean-13(18)-ene, de-A-lupane, and other de-A-triterpenoids), conifers (C18 diterpane, abietane, and ent-beyerane) and microbial sources (hopanoids). Besides, regarding the palaeodepositional environment, acyclic isoprenoid distributions (pristane and phytane), relative hydrocarbon potential, and the newly introduced palaeoredox factor may put forward mostly suboxic to oxic redox conditions of organic matter deposition. The total organic carbon and sulfur contents may further imply deposition of organic matter in topogenous-transitional mire conditions under a marginal marine/coastal environment. In complementary, the ranges of production index (0.01–0.19), Tmax (388–422 °C) and the presence of 17β(H),21β(H)-hopanes (ββ hopanes), 17α(H),21β(H)-homohopane, hop-17(21)-ene, ββ-hopanes, norhopanes and angiosperm-derived unsaturated triterpenoids may suggest thermal immaturity of organic matter in the studied samples.
... Stable isotopes and biomarkers are often combined in a multi-proxy approach in coastal and estuarine biogeochemical studies in order to determine the sources and transport of particulate and dissolved organic matter (Goñi et al., 1998;McCallister et al., 2006), and also to show shifts in the abundance of C3 and C4 marsh plants (Wang et al., 2003;Johnson et al., 2007). Lignin-phenols are one of the more useful biomarker suites of terrestrial OC in aquatic systems, and are used to trace vascular plant inputs to sedimentary and aquatic organic matter pools (Ertel et al., 1984;Alberts et al., 1992;Goñi et al., 1998;Bianchi et al. 2009Bianchi et al. , 2011. ...
... Stable isotopes and biomarkers are often combined in a multi-proxy approach in coastal and estuarine biogeochemical studies in order to determine the sources and transport of particulate and dissolved organic matter (Goñi et al., 1998;McCallister et al., 2006), and also to show shifts in the abundance of C3 and C4 marsh plants (Wang et al., 2003;Johnson et al., 2007). Lignin-phenols are one of the more useful biomarker suites of terrestrial OC in aquatic systems, and are used to trace vascular plant inputs to sedimentary and aquatic organic matter pools (Ertel et al., 1984;Alberts et al., 1992;Goñi et al., 1998;Bianchi et al. 2009Bianchi et al. , 2011. Previous studies of lignin in coastal wetlands have focused mainly on its alteration during early diagenesis (Benner et al., 1991b;Opsahl and Benner, 1995;Dittmar and Lara, 2001). ...
Organic matter (OM) accumulation in marsh soils affects marsh survival under rapid sea-level rise (SLR). This work describes the changing organic geochemistry of a salt marsh located in the Blackwater National Wildlife Refuge on the eastern shore of Chesapeake Bay that has transgressed inland with SLR over the past 35–75 years. Marsh soils and vegetation were sampled along an elevation gradient from the intertidal zone to the adjacent forest, representing a space-for-time substitution of the process of marsh transgression. Stable carbon isotope analysis of bulk OM gives evidence for a transition from C3 upland-sourced OM to C4-dominated marsh vegetation over time. The vegetative source of the OM changes along a marsh-upland mixing line from herbaceous angiosperm-sourced lignin in the lower elevation marsh to a woody gymnosperm signature at the upper border of the marsh. The results of stable isotope and lignin analyses illustrate that landward encroachment of marsh grasses results in deposition of herbaceous tissues exhibiting relatively little decay. This presents a possible mechanism for OM stabilization as marshes migrate inland.
Regional index terms
USA, Maryland, Chesapeake Bay, Blackwater National Wildlife Refuge.
... Lignin and its copper oxidation products have been employed to discriminate sediment sources (angiosperms vs gymnosperms, terrestrial vs marine, and land-uses) (Goñi et al. 1998;Goñi et al. 2000;Kuzyk et al. 2008;Rezende et al. 2010). However, the application of lignin copper oxidation products has limitations. ...
Purpose
Soil erosion models are essential to improving sediment management strategies. Sediment source fingerprinting is used to help validate erosion models. Fingerprinting sediment sources with organic isotopic tracers faces challenges from aquatic sources and co-linearity. To address these complexities, integrating another land-use-specific tracer is essential. Suess corrections incorporating multiple mean-residence-times are necessary to accurately model historical sediment apportionments. In previous studies, compound specific isotopic tracers indicated forest as the dominant source. We hypothesize that there is an overestimation of forest contribution, attributed to the misclassification of particulate organic matter as forest.
Methods
In this study, we utilize stable carbon isotope (δ¹³C) values of fatty acids and the average chain length in combination with the δ¹³C values of lignin-derived methoxy groups as an additional tracer. We apply different Suess corrections to explore the effect of the changing atmospheric δ¹³CO2 values on sediment apportionment. The performance of the unmixing model is evaluated with 300 mathematical mixtures. To determine shifts in sediment sources throughout the last 130 years, particulate organic matter contributions are determined and removed to apportion sediment soil sources. We investigate the potential misclassification of forest contributions by merging particulate organic matter and forest sources to simulate tracers which are unable to discriminate.
Results
The inclusion of δ¹³C values of lignin methoxy groups and the alkane average chain length as additional tracers successfully removed tracer co-linearity. Additionally, we used an updated concentration dependent point in polygon test to identify sediment with increased potential for incorrect source apportionments. Changes in the dominant sediment sources over time (Forest: pre-1990, Pasture: 1910–1940, Arable: post 1940) highlight the effect of policy-induced land-use changes. Additionally, the inability to discriminate particulate organic matter and forest sources was revealed to cause a 37% overestimation of forest contributions from 1944 to 1990.
Conclusion
Using δ¹³C values of lignin methoxy groups as an additional tracer, we identified critical points in the 130-year sediment history of Lake Baldegg. Furthermore, we highlight the importance of incorporating multiple Suess effects. Through mathematical mixtures, we assessed the confidence that should accompany apportionment estimates. While merging forest and particulate organic matter sources did not result in forest as the dominant source over the last 130 years, separating these sources resulted in more accurate apportionment. These insights offer valuable information to enhance the accuracy of sediment fingerprinting, which can then be used to assist soil erosion models employed for sediment mitigation policies.
... However, the terrigenous OM in HZB is deeper than that of the Pearl River estuary, as measured using organic carbon isotope analysis (shown in Figure 3B). When the results are compared to those regarding the contemporary sedimentary OM at different water depths in ECSS, a linear relationship for the mixture of marine and riverine OM is obtained, which aligns with the terrestrial-marine OM mixing hypothesis (Hedges and Oades, 1997;Goñi et al., 1998;Perdue and Koprivnjak, 2007). Therefore, a two-end-member mixed model with organic carbon isotope is employed to calculate the relative proportion of terrestrial and marine OM sources (Equations 1, 2). ...
The correlation between the amount of organic carbon (OC) and sulfur (S) in sediments has been widely used as a paleosalinity indicator to distinguish between marine and freshwater environments. However, whether the ratio of total OC to total S (TOC/TS) can be used to identify unsteady or dynamic marine environments across sedimentary strata is still contended. An HZW1907 sediment core of 80 m in length was successfully collected in the middle of Hangzhou Bay (HZB), serving as one of the few boreholes that are crucial for the study of geologic and geo-environment changes in the coastal regions of eastern China since the Last Glacial Maximum (LGM). Total OC (TOC), stable carbon isotope, and TS of 82 subsamples from the HZW1907 core were analyzed to reconstruct the history of the shallow water biological pump and sulfur preservation record in the bay since the Late Pleistocene. Our results indicate that the samples had low concentrations of TOC (0.21%) and total nitrogen (TN) (0.02%), high mass ratio of TOC/TN (10.8), low δ¹³C (−24.9‰), low TS content (0.06%), and a high ratio of TOC/TS (9.1) from 33.6 ka BP to 12.3 ka BP, implying that freshwater organic matter (OM), algae, and C3 plant fragments were the main sources of OM in a relatively cold environment. The abundances of TOC, TN, and TS increased to 0.56%, 0.07%, and 0.4%, respectively, while δ¹³C (−23.9‰) increased and TOC/TS (2.7) decreased in the Holocene sediments, suggesting that seawater began to influence the composition of the sediments of HZB. Climate warming, which is likely to have impacted the results, was experienced from 12.3 ka BP. An OC isotope mixing model indicated that since the Mid-late Holocene, more than 70% of riverine OM accounted for the total OM. The TOC/TS ratio was identified as an effective indicator of seawater intrusion, with C/S ratios of 1–6 being considered to indicate a “sea–land transitional zone” sedimentary environment, a C/S >6 indicating freshwater, and a C/S<1 indicating normal marine facies. These findings provide crucial evidence for using TOC/TS to distinguish freshwater from marine environments and enhance our understanding of past climate changes. Therefore, these geochemical indicators can be used in conjunction with other sedimentary records to obtain accurate results about sedimentary evolution.
... However, the δ 15 N may be inadequate for source estimation due to the additional fractionation of nitrogen fixation and the denitrification and degradation of OM [37]. The positive correlation of the TOC and TN (Supplementary Figure S1c) indicated that most nitrogen was organic [38]. The BIT index (0.02 to 0.21, average 0.06, Equation (1) in Table 1a) revealed a lower contribution of land OM to the aquatic environment, since the BIT generally ranges 0.0-0.1 in a marine environment and 0.8-1.0 in land soil [39]. ...
One gravity core retrieved from the Niger Delta was used to explore the origin of deposited organic matter (OM) and the paleo-climatic and environmental conditions over the Holocene in equatorial West Africa. The geochemical properties of sediments including glycerol dialkyl glycerol tetraethers (GDGTs) and elemental (%OC, %N, C/N) and isotopic (δ13Corg, δ15N) signatures were determined. The determination constrained the age of the column and revealed that the sediment OM was mainly derived from a marine source. The isoprenoid (iso)GDGTs were the dominant GDGTs, with a small amount of branched (br)GDGTs, which led to a low-branched and isoprenoid tetraether index (BIT, 0.02–0.21) and represented a low terrestrial input. Most isoGDGTs and OH-GDGTs were produced in situ by Marine Group I (MG-I) Thaumarchaeota, while the brGDGTs were mainly transported from land. A two-endmember model quantified the contribution of terrestrial OM, as 0.9–19.9% by BIT and 1.1–32.6% by δ13C. Accordingly, the millennium-scale sea surface temperatures (SSTs) were reconstructed based on the cyclopentane ring distribution (TEX86H) and the ring index of OH-GDGTs (RI-OH). The top core SSTs were lower than the modern mean annual SST due to the growth season and habitat depth of Thaumarchaeota. The reconstructed SSTs clearly revealed the four stages of paleoclimate change, in particular, the drought episode of 8.2 kyr and the following humid period. The above research has enhanced our understanding of the paleoclimate change in river outflow during the Holocene at the millennium scale.
... Particles with high POC/TSS contribute less to accretion after deposition, as organic matter in the particles can be removed through mineralization. Variability in POC/TSS also impacts the sequestration of terrigenous (non-algal) organic matter, an important ecosystem service provided by coastal estuary systems (Galy et al., 2015;Goñi et al., 1998;Hedges et al., 2001). Particle size also affects sediment fate. ...
... While the undisturbed zones on land (AL and PF) and in the basin appear relatively constant in OC partitioning, keeping the limited sample size in mind, the "disturbed zone" and the "nearshore zone" show a wide variation in partitioning. The trend in partitioning along the pathway of transport is as follows: the HDF fraction-mineral bound carbon-is transported farthest away, with the largest part of it ending up in the basin; the HDC fraction remains relatively close to shore in the high-energy nearshore zone, where the fine material is winnowed out by wave action, and the LD fraction-matrix-free organic debris-appears, somewhat surprisingly, most prominent in the disturbed zone and the shallow nearshore zone, likely due to the large size of particles in this fraction, causing them to settle rapidly (Bianchi et al., 2002;Goñi et al., 1998;Tesi et al., 2016;Wakeham et al., 2009). ...
Erosion of permafrost coasts due to climate warming releases large quantities of organic carbon (OC) into the Arctic Ocean. While burial of permafrost OC in marine sediments potentially limits degradation, resuspension of sediments in the nearshore zone potentially enhances degradation and greenhouse gas production, adding to the “permafrost carbon feedback.” Recent studies, focusing on bulk sediments, suggest that permafrost OC derived from coastal erosion is predominantly deposited close to shore. However, bulk approaches disregard sorting processes in the coastal zone, which strongly influence the OC distribution and fate. We studied soils and sediments along a transect from the fast‐eroding shoreline of Herschel Island—Qikiqtaruk (Yukon, Canada) to a depositional basin offshore. Sample material was fractionated by density (1.8 g cm⁻³) and size (63 μm), separating loose OC from mineral‐associated OC. Each fraction was analyzed for element content (TOC, TN), carbon isotopes (δ¹³C, Δ¹⁴C), molecular biomarkers (n‐alkanes, n‐alkanoic acids, lignin phenols, cutin acids), and mineral surface area. The OC partitioning between fractions changes considerably along the transect, highlighting the importance of hydrodynamic sorting in the nearshore zone. Additionally, OC and biomarker loadings decrease along the land‐ocean transect, indicating significant loss of OC during transport. However, molecular proxies for degradation show contrasting trends, suggesting that OC losses are not always well reflected in its degradation state. This study, using fraction partitioning that crosses land‐ocean boundaries in a way not done before, aids to disentangle sorting processes from degradation patterns, and provides quantitative insight into losses of thawed and eroded permafrost OC.
... Lower POC may be due to increased turbidity from sediment resuspension resulting in lower primary productivity (Lehrter et al., 2009). This would be expected closer to the river mouth and therefore the higher organic content seen in core A7 must be attributed to terrigenous inputs from the MR (Duan et al., 2007;Ghaisas et al., 2021;Goñi et al., 1998). Ghaisas et al. (2021) demonstrated that reactive iron and TOC distribution in the top 20 cm of these sediments is nearly homogenous and may not show variability downcore. ...
Many studies use traditional alkaline leach digestions to quantify biogenic silica (bSiO2) in sediments. Studies from river-plume systems demonstrate this methodology precludes information on other operational Si reactive pools, missing potentially valuable information on early diagenetic products. We build on previous studies by broadly examining the reactive Si pools in Mississippi River plume-influenced coastal sediments. The magnitude of Si released among operational pools across the region revealed large spatial variations in the conservation of detrital materials. Contrary to other analyses using different approaches, we suggest the availability of detrital sourced components (i.e. Al, Fe and Mn) may be a factor limiting authigenesis and resulting Si storage capacity in coastal sediments by this chemical mechanism. Overall, these data suggest authigenic precipitates may be forming as a function of bSiO2 accumulation, thereby implying that bSiO2 acts as the primary substrate for formation of authigenic products. The amount of authigenic Si products, relative to bSiO2, appears to be larger in this region than other systems studied to date, and this may be due to regional eutrophication which has led to an increase in sediment bSiO2 compounding the substrate available for authigenic precipitates. Thus, regions which have all experienced increased sediment bSiO2 accumulation coincident with eutrophication, may be primed for substantial authigenic product formation and increased Si sequestration.
... Mayer (1994). (Goñi et al., 1998(Goñi et al., , 2000, although this can also be affected by hydrodynamic sorting of particles enriched in SP and CP relative to VP (Bianchi et al., 2002;Pasqual et al., 2013). Similarly, the different proportions of acid and aldehydes within the vanillyl and syringyl phenolic groups can also provide an indication of the degree of degradation of terrestrial organic matter given the higher reactivity of aldehydes with respect to acids (Gordon and Goñi, 2004;Tesi et al., 2012). ...
Marine sediments play a crucial role in the global carbon cycle by acting as the ultimate sink of both terrestrial and marine organic carbon. To understand the spatiotemporal variability in the content, sources, and dynamics of organic carbon in marine sediments, a curated and harmonized database of organic carbon and associated parameters is needed, which has prompted the development of the Modern Ocean Sediment Archive and Inventory of Carbon (MOSAIC) database (http://mosaic.ethz.ch/, last access: 26 July 2023; https://doi.org/10.5281/zenodo.8322094, Paradis, 2023; https://doi.org/10.5168/mosaic019.1, Van der Voort et al., 2019 ). MOSAIC version 2.0 has expanded the spatiotemporal coverage of the original database by >400 % and now holds data from more than 21 000 individual sediment cores from different continental margins on a global scale. Additional variables have also been incorporated into MOSAIC v.2.0 that are crucial to interpret the quantity, origin, and age of organic carbon in marine sediments globally. Sedimentological parameters (e.g. grain size fractions and mineral surface area) help understand the effect of hydrodynamic sorting and mineral protection on the distribution of organic carbon, while molecular biomarker signatures (e.g. lignin phenols, fatty acids, and alkanes) can help constrain the specific origin of organic matter. MOSAIC v.2.0 also stores data on specific sediment and molecular fractions, which provide further insight into the processes that affect the degradation and ageing of organic carbon in marine sediments. Data included within MOSAIC are continuously expanding, and version control will allow users to benefit from updated versions while ensuring reproducibility of their findings.
... wt%, respectively. TOC contents are positively and significantly correlated with TN contents (TOC = 15.89 × TN − 0.03; R 2 = 0.78; p < 0.01), which highlights the minor influence of inorganic nitrogen (Goñi et al., 1998). C/N values vary from 3.8 to 17.1, with an average of 33.0 ± 5.4. ...
... We know that some of our watersheds, especially those in the Gulf of Mexico, could be receiving terrestrial organic matter with a variety of sources and δ 13 C values. One study found that ≥ 50% of the organic matter in continental shelf sediments in the northern Gulf of Mexico ultimately originated from C 4 plants like Spartina alterniflora and the grassland plants in associated watersheds (Goni et al. 1998) These C 4 plants have δ 13 C values similar to seagrasses; our analyses did not include such plants as a possible contributor of OM to seagrass soils. ...
The organic carbon (Corg) stored in seagrass meadows is globally significant and could be relevant in strategies to mitigate increasing CO2 concentration in the atmosphere. Most of that stored Corg is in the soils that underlie the seagrasses. We explored how seagrass and soil characteristics vary among seagrass meadows across the geographic range of turtlegrass (Thalassia testudinum) with a goal of illuminating the processes controlling soil organic carbon (Corg) storage spanning 23° of latitude. Seagrass abundance (percent cover, biomass, and canopy height) varied by over an order of magnitude across sites, and we found high variability in soil characteristics, with Corg ranging from 0.08 to 12.59% dry weight. Seagrass abundance was a good predictor of the Corg stocks in surficial soils, and the relative importance of seagrass-derived soil Corg increased as abundance increased. These relationships suggest that first-order estimates of surficial soil Corg stocks can be made by measuring seagrass abundance and applying a linear transfer function. The relative availability of the nutrients N and P to support plant growth was also correlated with soil Corg stocks. Stocks were lower at N-limited sites than at P-limited ones, but the importance of seagrass-derived organic matter to soil Corg stocks was not a function of nutrient limitation status. This finding seemed at odds with our observation that labile standard substrates decomposed more slowly at N-limited than at P-limited sites, since even though decomposition rates were 55% lower at N-limited sites, less Corg was accumulating in the soils. The dependence of Corg stocks and decomposition rates on nutrient availability suggests that eutrophication is likely to exert a strong influence on carbon storage in seagrass meadows.
... Previous studies have demonstrated that the C/N ratios of marine organic matter range from 5 to 7, while those of terrestrial organic matter are generally higher than 12, and δ 13 C maintains a variation of − 22 ~ − 20‰ for marine organic carbon and approximately − 27‰ for terrestrial organic carbon (Meyers, 1997;Lamb et al., 2006). It should be noted that C/N ratios might be misleading due to the influence of early diagenetic processes and the grain size of sediments (Meyers, 1997), and the carbon isotopic signal is more complex when coastal areas receive organic carbon from both C3 and C4 plants (Goñi et al., 1998). Therefore, C/N ratios with δ 13 C values are usually used together, in combination with other geochemical approaches, to trace environmental evolution. ...
Paleoenvironmental reconstructions of coastal areas during geological history are critical for understanding past responses to marine transgression. Identifying transgressive layers in sedimentary strata is therefore of great importance. To this end, we examined the content and isotopic composition of pyrite sulfur, in combination with classical organic geochemical indicators (e.g., C/N ratio and the δ13C of organic carbon), to trace the marine transgressions along the southeast coast of China since Marine Isotope Stage (MIS) 5. Our results suggest that the mass ratio of total organic carbon and pyrite sulfur (C/S ratio) is sensitive to seawater intrusion. Three transgressive layers were identified by low C/S ratios in core NDGK2, which correspond to sea level rise during MIS5, MIS3, and MIS1. Sediments deposited during MIS1 are represented by low C/S ratios with an average of 2.31, while those deposited during MIS5 and MIS3 have slightly higher C/S ratios, indicating a less extensive influence of seawater. Pyrite production in sediments deposited during MIS5 and MIS3 may have been limited by sulfate supply, as reflected by more positive values of δ34Spyr, whereas pyrites deposited during MIS1 were likely formed without sulfate limitation, as reflected by more negative values of δ34Spyr. The isotopic signals of pyrite sulfur
thus further confirm the inference based on the C/S ratio. The magnitude of the transgression increased from MIS5 to MIS1, which is in agreement with the findings from classical organic geochemical indicators examined in this study, as well as geochemical and micropaleontological salinity indicators from previous studies. Our findings provide a promising method for identifying marine transgressions and their magnitude from the
depositional record, especially when sedimentary strata lack diagnostic micropaleontological and sedimentological characteristics.
... A strong linear relationship exists between TOC and TN in both saturated (R 2 = 0.82; p value < 0.05) and unsaturated (R 2 = 0.65; p value < 0.05) TAL sediments. Following the method of Goni et al. (1998), total inorganic nitrogen (TIN) can be estimated from TOC by applying a linear regression to a TN vs. TOC plot. The concentration of TIN (C TIN ) is mathematically represented by the y-intercept. ...
The clastic sediments that accumulate in cave settings can be an important storage reservoir for organic carbon. This study reports on grain size, total organic carbon (TOC) concentrations, and total organic carbon: total organic nitrogen (TOC:TON) ratios measured in sediments from two caves in Puerto Rico. El Tallonal Cave (TAL) is a small cave with a flowing stream; the sediments in TAL were collected from a deposit that is being actively eroded. Clara Cave (CAM) is an upper level of the Río Camuy Cave System; the sediments from CAM were newly deposited by an internal river that rose in response to Hurricane Maria. Sediments collected from both caves were poorly sorted and contained no apparent stratigraphic correlation. CAM sediments contained a larger range in TOC concentrations but were overall lower than TOC in the TAL sediments. In TAL, the TOC concentrations were higher in sediments collected from below the erosional terrace. TOC:TON ratios from sediments at both caves were highly variable, highlighting the heterogeneous deposition and storage of organic matter. Despite the observed variation, TOC concentrations in both cave systems could cause retardation of organic contaminants by up to two orders-of-magnitude, implying that deposited sediments influence the fate of organic contaminants in the groundwater; therefore, cave sediments could facilitate long term storage of organic carbon and associated contaminants.
... The study of combined isotopic signatures (δ 13 C and Δ 14 C) of DIC in pore waters is a powerful tool to assess the sources and the mineralization mechanisms of OM deposited in deltas (Bauer et al. 1995;Aller et al. 2008). The δ 13 C signatures of the dissolved organic carbon (DIC) of sediment pore waters provide information about the origin of the OM mineralized in the sediment, with different signatures for terrestrial sources (δ 13 C around -25‰) and marine sources (δ 13 C around -20‰) (Goñi et al. 1998;Burdige 2006). The Δ 14 C signatures provide information about the age of the different OM but can also be used as a tracer for the freshly produced OM of the river (Aller et al. 2008;Aller and Blair 2004). ...
Estuaries and deltas are crucial zones to better understand the interactions between continents and oceans, and to characterize the mineralization and burial of different sources of organic matter (OM) and their effect on the carbon cycle. In the present study, we focus on the continental shelf of the northwest Mediterranean Sea near the Rhône river delta. Sediment cores were collected and pore waters were sampled at different depths at one station (Station E) located on this shelf. For each layer, measurements of dissolved inorganic carbon concentration (DIC) and its isotopic composition ( δ ¹³ C and Δ ¹⁴ C) were conducted and a mixing model was applied to target the original signature of the mineralized OM. The calculated δ ¹³ C signature of the mineralized organic matter is in accordance with previous results with a δ ¹³ C OM of marine origin that is not significantly impacted by the terrestrial particulate inputs from the river. The evolution with depth of Δ ¹⁴ C shows two different trends indicating two different Δ ¹⁴ C signatures for the mineralised OM. In the first 15 cm, the mineralized OM is modern with a Δ ¹⁴ C OM = 100 ± 17‰ and corresponds to the OM produced during the nuclear period of the last 50 years. Deeper in the sediment, the result is very different with a depleted value Δ ¹⁴ C OM = –172 ± 60‰ which corresponds to the pre-nuclear period. In these two cases, the marine substrate was under the influence of the local marine reservoir effect with more extreme Δ ¹⁴ C results. These differences can be largely explained by the influence of the river plume on the local marine DIC during these two periods.
... The d 13 C of total organic carbon (d 13 C TOC ) is readily influenced by sources (Hedges et al., 1997), plant type (Goñi et al., 1998) and microbial OM contribution (O'Leary, 1988). The d 13 C values in aquatic algae are around -20‰, while the average d 13 C values is -27‰ in terrestrial C 3 plants and -14‰ in terrestrial C 4 plants (Meyers, 1997). ...
... The d 13 C of total organic carbon (d 13 C TOC ) is readily influenced by sources (Hedges et al., 1997), plant type (Goñi et al., 1998) and microbial OM contribution (O'Leary, 1988). The d 13 C values in aquatic algae are around -20‰, while the average d 13 C values is -27‰ in terrestrial C 3 plants and -14‰ in terrestrial C 4 plants (Meyers, 1997). ...
Long-chain alkyl diols (LCDs) can be used as organic geochemical proxies for paleoceanographic change, especially in marginal sea areas where large volumes of sediments are deposited rapidly and continuously. However, little is known about the applicability and response on a millennium scale in relation with existing records in those sediments. We reconstruct changes in upwelling and terrestrial organic matter (OM) input in core sediments from the Zhejiang Fujian coastal station (T08) and Yangtze River Estuary station (T06) in the inner shelf of the East China Sea (ECS) over the last millennium, using the LCD based proxies: diol index 2 (DI-2), and FC321,15-diol. Our results show that DI-2 values ([(C28+C30)1,14-diols]/([(C28+C30)1,13-diols]+[(C28+C30)1,14-diols])) at T08 decrease significantly during 600–400 yr BP but increase gradually after 400 yr BP. The FC321,15-diol proxy ([C321,15-diol]×100/([(C28+C30)1,13-diols]+[(C30+C32)1,15-diols])) at T06 shows marked fluctuations during 1000–800 yr BP, followed by a significant decline during 800–500 yr BP but a subsequent increase from 500 to 300 yr BP. We find that variations in DI-2 values are broadly consistent with changes in the strength of the East Asian Summer Monsoon (EASM) and the Kuroshio Current and are likely linked to changes in the frequency and intensity of the El Niño-Southern Oscillation (ENSO). The increased strength of the EASM causes greater offshore movement of the upper layer of seawater, which in turn triggers upwelling of bottom waters formed by Kuroshio subsurface waters. We find that variations in FC321,15-diol proxy are controlled mainly by the East Asian Winter Monsoon (EAWM) and the Yangtze River discharge. By increasing the strength of the EAWM, southward transportation of material deposited in the estuary of the Yangtze River by the ECS coastal currents is promoted. In addition, we synthesize records of other organic geochemical indicators nearby core sediments in the ECS; these records emphasize the importance of reconstructing the evolutionary history of upwelling and subdividing the relative inputs of terrestrial OM. Our study provides a new means for reconstructing the evolution of upwelling and terrestrial OM input in the inner shelf of the ECS over the last millennium.
... Many field-based studies have been conducted on the effects of rivertransported materials on biogeochemical processes in coastal marine systems. Based on data collected from multiple locations along transects, these studies demonstrated that the quantity of materials declined from the river mouth towards offshore locations (e.g., Lapointe and Clark, 1992;Goñi et al., 1998;Washburn et al., 2003). Spatiotemporal variations in biogeochemical conditions near river mouths are often described using process-based models, that deductively explain relevant processes, such as advection, diffusion, and biological reactions, and can be adequately predicted (e.g., Kourafalou et al., 1996;Fennel et al., 2011;Wolff et al., 2018). ...
The analytical spatial scale and selection of biogeochemical indicators affect interpretations of land-use impacts on coastal marine environments. In this study, nine biogeochemical indicators were sampled from 36 locations of coral reefs fringing a subtropical island, and their relationships with watershed land use were assessed by spatial autoregressive models with spatial weight matrixes based on distance thresholds of a few to 30 km. POM-relevant indicators were associated with agricultural and urban lands of watersheds within relatively small ranges (6–14 km), while the concentrations of inorganic nutrients were associated with watersheds within 20 km or more. The macroalgal δ¹⁵N showed a strong relationship with agricultural lands of watersheds within 7 km and urban/forest lands of watersheds within 24 km. These results demonstrate significant effects of land use on the coral reef ecosystems of the island, and the importance of appropriate combinations of analytical scales and biogeochemical indicators.
... They presented restricted spatial changes in surface sediment granulometry, but conversely a clear increase in bulk organic contents (i.e., Particulate Organic Carbon and Total Hydrolysable Amino Acids) with station depth, which supports previous observations by Lamarque et al. (2021) and Massé et al. (2016). Such a pattern is rather uncommon in River-dominated Ocean Margins (RiO-Mar; e.g., Bonifácio et al., 2014;Cathalot et al., 2013;Goñi et al., 1998;Gordon et al., 2001;Keil et al., 1997) and has been attributed to a particle sieving process cued by particle density rather than size (Hedges and Keil, 1995;Keil et al., 1998;Mayer, 1994a, b) during the succession of sedimentation/resuspension cycles governing the transfer of particles offshore in RiOMar (Blair and Aller, 2012). The results of the DISTLM/dbRDA analysis support the predominant role of local hydrodynamics as the driving factor of the spatial distribution of surface sediment characteristics in the WGMP as already put forward by Lamarque et al. (2021) based on a synoptic spatial survey. ...
The benthic compartment of River-dominated Ocean Margins (RiOMar) is largely affected by sedimentary processes, as well as by natural and anthropogenic disturbances. Recent studies have confirmed the major importance of riverine inputs and local hydrodynamics in the spatial structuration of low- and high-energy temperate RiOMar, respectively. Differences in the nature of these structuring factors could also affect the temporal dynamics of these two types of systems. The present study is aiming at: (1) quantifying spatiotemporal changes in surface sediment and benthic macrofauna within the West Gironde Mud Patch (WGMP; high-energy system) over both short (2016–2018) and long (2010–2018) time scales, (2) identifying the main environmental factors controlling those changes, and (3) achieving a comparison with the Rhône River Prodelta (RRP; low-energy system) in view of further characterizing the functioning of the benthic components of these two temperate RiOMar. Surface sediment characteristics (grain size, quantitative and qualitative descriptors of particulate organic matter) and benthic macrofauna compositions were assessed based on 4 seasonal sampling of 5 stations located along a depth gradient within the WGMP. Results highlighted the existence of spatial patterns for both surface sediment and benthic macrofauna, which are cued by local hydrodynamics. Most variables presented seasonal changes. Benthic macrofauna compositions also showed pluri-annual changes, which were attributed to a cicatrization process following a major disturbance caused by the 2014 series of severe winter storms, which underlines the major role of local hydrodynamics in controlling long-term temporal changes in WGMP benthic macrofauna compositions. The comparison with the RRP highlighted major discrepancies between the two systems in the main processes (i.e., hydrodynamics versus river hydrological regime) controlling surface sediment characteristics and benthic macrofauna compositions, which further supports RiOMar typologies derived from meta-analyses mainly achieved on tropical and subtropical systems.
... However, the AMO may act as a key pacemaker that the western tropical Pacific multidecadal climate variability is forced by the AMO instead of PDO in interdecadal time scales over the last century Zheng and Wang, 2021). In this study, the correlations between terrestrial OC parameters (e.g., F terr , Table 5) and both 7-year running mean of climate oscillation (Kao et al., 2003;Li et al., 2012;Wu et al., 2013;Kao et al., 2014;Bao et al., 2018b) Blair et al., 2003;Komada et al., 2005;White, 2006;Mollenhauer and Eglinton, 2007;Drenzek et al., 2009;Wakeham et al., 2009;Griffith et al., 2010;Feng et al., 2013) Mackenzie River shelf 0.38 0.17 1.23 12 (Goñi et al., 2005;Drenzek et al., 2007;Hilton et al., 2015) North Gulf of Mexico 0.79 0.09 0.92 9 (Goñi et al., 1998;Gordon and Goñi, 2003;Gordon and Goñi, 2004) Amazon River Coast 0.84 0.03 0.62 5 (Aller and Blair, 2006;Williams et al., 2015) index were not significantly correlated (AMO, R = 0.28, p <0.01; PDO, R=0.27, p <0.01) (Figure 7 and Table 5) through the whole core. However, there is a significant correlation between F terr and PDO than that with AMO before 1957 (R=0.43, ...
Estuaries have experienced significant changes due to global climate change and human perturbations since the last century. However, the climate and anthropogenic influence on the burial of sedimentary organic carbon (OC) in estuaries is still not understood well yet. Here, a 3-meter sediment core was taken from the Pearl River Estuary (PRE) in China. Depth profiles of both bulk OC and lignin biomarker data indicated three stages with different features of buried OC during the 130-year sediment deposition. The 1893-1957 stage showed 20% more burial of marine derived OC, which was mostly adsorbed on finer minerals compared to the years after 1957. The 1957-1980 period witnessed 4.6 times higher burial rate of petrogenic OC, which made the radiocarbon age of total organic carbon 42% older than before due to soil erosion and carbonate rock weathering. The 7-year running average variation of terrestrial OC input based on endmember mixing model was correlated with the Pacific Decadal Oscillation index before 1957, but correlated with the Atlantic Multidecadal Oscillation between 1957 and 1980 in the region. The reduction of land derived OC content after 1980s was mostly affected by human perturbations such as deforestation and dam construction which corresponded to the beginning of Economic Reform and Open Up in China. The overall increase of lignin content from bottom to surface sediment indicated increased vascular plant derived OC due to deforestation activities during the urbanization process. The study suggested different time periods when climate or human disturbance dominantly affected the OC burial in the PRE, which have significant indications for local and global carbon cycling and environmental ecology.
... A strong linear relationship exists between TOC and TN in both saturated (R 2 = 0.82 p value < 0.05) and unsaturated (R 2 = 0.65 p value < 0.05) TAL sediments. Following the method ofGoni et al. (1998), total inorganic nitrogen (TIN) can be estimated from TOC by applying a linear regression to a TN vs. TOC plot.The concentration of TIN (C TIN ) is mathematically represented by the y-intercept. C TIN was estimated to be 0.036 (C TIN =0.08C TOC + 0.036) and 0.042 (C TIN =0.10C TOC + 0.042) for saturated and unsaturated sediments from TAL, respectively. ...
The clastic sediments that accumulate in cave settings can be an important storage reservoir for organic carbon, affect contaminant fate and transport, and contribute to ecosystem processes. This study reports on grain size, total organic carbon (TOC) concentrations, and total organic carbon:total organic nitrogen (TOC:TON) ratios measured in sediments from two caves in Puerto Rico. El Tallonal Cave (TAL) is a small cave with a flowing stream; the sediments from TAL were collected from a deposit that is being actively eroded. Cueva Cave (CAM) is an upper level of the Río Camuy Cave System; the sediments from CAM were newly deposited by an internal river that rose in response to Hurricane Maria. Sediments collected from both caves were poorly sorted and contained no apparent stratigraphic correlation. CAM sediments contained a larger range in TOC concentrations but were overall lower than TOC in the TAL sediments. In TAL, the TOC concentrations were higher in sediments collected from below the usual water level. TOC:TON ratios from sediments at both caves were highly variable, highlighting the heterogeneous deposition and storage of organic matter. Despite the observed variation, TOC concentrations in both cave systems cause retardation of organic contaminants by up to two orders-of-magnitude, implying that deposited sediments influence the fate of organic contaminants in the groundwater; therefore, cave sediments could facilitate long term storage of organic carbon and associated contaminants.
... Comparison of the downcore (Ad/Al)v ratios determined in this study with that of fresh plants and humid substances suggested that most of the vascular plant material in the core was highly degraded. The negligible decrease of (Ad/Al)v and (Ad/Al)s values with increasing Fig. 4. Scatter plots of (a) TN (%) versus TOC (%) for all subsamples of core ST2, red line represents the geometric mean regression line; (b) δ 13 C versus C/N ratio for all subsamples of core ST2, where the background labels of organic matter sources were modified from Lamb et al. (2006), with the grey area representing 95% confidence band; (c) S/V versus C/V ratios for all subsamples of core ST2, where the different vegetation types were typical ranges were from Goñi et al. (1998), Bianchi et al. (2011), Yu (2007 and modified from Gong et al. (2017), and purple circle represents the characteristic of surface sediments in outer shelf of the ECS , and green and yellow circles represent the surface sediments in inner shelf of the ECS. depth down the core (Fig. 3h) supports the fact that polymeric lignin is difficult to biodegrade under oxygen-deprived environments (Kirk and Farrell, 1987). ...
Sediment core ‘ST2’ collected from southern Zhejiang-Fujian muddy coastal area (ZFMA) was analyzed for grain size, bulk elemental composition (TOC and TN), stable carbon isotopes (δ¹³C), and lignin-derived phenols. The C/N ratios (6.2–10.1) and δ¹³C values (−21.7‰ to −23.0‰) demonstrated the predominance of marine organic matter. This was supported by a three end-member mixing model generated based on N/C ratio, δ¹³C, and Ʌ (total lignin in mg/100 mg of TOC). The results showed that marine and soil-derived organic matter accounted for 68.0% and 29.7%, respectively, of the total organic matter. The syringyl/vanillyl (S/V; 0.32 to 1.07) and cinnamyl/vanillyl (C/V; 0.13 to 0.91) ratios, together with the estimated range of lignin phenol vegetation index (LPVI) from 22.39 to 1451.91, indicated the predominance of non-woody angiosperms. The high vanillic acid/vanillin and syringic acid/syringaldehyde ratios suggested that the sediments were degraded prior to deposition. Our results showed that the variation of soil-derived organic matter along the core was affected by the East Asian monsoons, suggesting an anti-phase relationship between East Asian Winter Monsoon and East Asian Summer Monsoon. On the other hand, the temperature variations affected the abundance and type of vegetation. In addition, the variation of soil-derived organic matter showed remarkable consistency with other climate factors, such as solar irradiance, El Niño Southern Oscillation, North Atlantic Oscillation and North Atlantic ice drift. Meanwhile, the coherency relationship between the soil-derived organic matter and paleotyphoon activity indicates a possible linkage with paleotyphoon activity. Thus, the southern ZFMA was affected by both regional and global climatic factors, as well as extreme events-typhoon activity.
... Ouellet et al. (2009), for example, used lignin to demonstrate the importance of terrigenous OM as a vector for mercury from watersheds to lakes, and Van Metre et al. (1997) used organochlorine compounds to trace historic declines in water quality in reservoirs adjacent to human population centers. Although single biomarker classes have proven useful in ecosystems where few sources of OM dominate, multi-biomarker approaches have been more successful in resolving OM sources in complex systems (e.g., Goñi et al. 1998;Yunker et al. 2005;Canuel and Hardison 2016). For example, using lipid biomarker and stable isotope data, He et al. (2014) were able to identify three different sources of OM (terrestrial plants, estuarine, and marine diatoms) to the Shark River Estuary in south Florida, and quantify the contribution of each OM source to the surface sediments from the estuary. ...
Organic matter in soils and sediments derives from a mixture of biological origins, often making it difficult to determine inputs from individual sources. Complicating the determination of source inputs to soil and sedimentary organic matter (OM) is the fact that physical and microbial processes have likely modified the initial composition of these sources. This study focused on identifying the composition of watershed-derived OM to better understand inputs to inland waters and improve our ability to resolve between terrigenous and aquatic sources in downstream systems, such as estuaries and coasts. We surveyed OM sources from the Yuba River watershed in northern California to identify specific biomarkers that represent aquatic and terrigenous OM sources. Multiple classes of organic proxies—including sterols, fatty acids (FA), lignin phenols and stable carbon and nitrogen isotope values (δ13C, δ15N)—were measured in soils, vegetation, charcoal, and freshwater plankton to characterize representative source endmembers. Sterols—including 27-nor-24-cholesta-5,22-dien-3β-ol, cholesta-5,22-dien-3β-ol, 24-methylcholesta-5,22-dien-3β-ol and cholesta-5-en-3β-ol, and positive δ15N values—were associated with aquatic OM (plankton, suspended particulate OM), whereas lignin phenols, long chain FA, and diacids characterized terrigenous sources (soils, charcoal, vegetation). Trends in organic carbon and biomarker signatures in soil samples showed a response to environmental disturbance (i.e., mining, agriculture) through an inverse relationship between OM content and land use. Results from this study demonstrate the utility of multi-biomarker studies for distinguishing between OM from different sources and land uses, offering new insights for biogeochemical studies in aquatic systems.
... Ouellet et al. (2009), for example, used lignin to demonstrate the importance of terrigenous OM as a vector for mercury from watersheds to lakes, and Van Metre et al. (1997) used organochlorine compounds to trace historic declines in water quality in reservoirs adjacent to human population centers. Although single biomarker classes have proven useful in ecosystems where few sources of OM dominate, multi-biomarker approaches have been more successful in resolving OM sources in complex systems (e.g., Goñi et al. 1998;Yunker et al. 2005;Canuel and Hardison 2016). For example, using lipid biomarker and stable isotope data, He et al. (2014) were able to identify three different sources of OM (terrestrial plants, estuarine, and marine diatoms) to the Shark River Estuary in south Florida, and quantify the contribution of each OM source to the surface sediments from the estuary. ...
Organic matter in soils and sediments derives from a mixture of biological origins, often making it difficult to determine inputs from individual sources. Complicating the determination of source inputs to soil and sedimentary organic matter (OM) is the fact that physical and microbial processes have likely modified the initial composition of these sources. This study focused on identifying the composition of watershed-derived OM to better understand inputs to inland waters and improve our ability to resolve between terrigenous and aquatic sources in downstream systems, such as estuaries and coasts. We surveyed OM sources from the Yuba River watershed in northern California to identify specific biomarkers that represent aquatic and terrigenous OM sources. Multiple classes of organic proxies—including sterols, fatty acids (FA), lignin phenols and stable carbon and nitrogen isotope values (δ13C, δ15N)—were measured in soils, vegetation, charcoal, and freshwater plankton to characterize representative source endmembers. Sterols—including 27-nor-24-cholesta-5,22-dien-3β-ol, cholesta-5,22-dien-3β-ol, 24-methylcholesta-5,22-dien-3β-ol and cholesta-5-en-3β-ol, and positive δ15N values—were associated with aquatic OM (plankton, suspended particulate OM), whereas lignin phenols, long chain FA, and diacids characterized terrigenous sources (soils, charcoal, vegetation). Trends in organic carbon and biomarker signatures in soil samples showed a response to environmental disturbance (i.e., mining, agriculture) through an inverse relationship between OM content and land use. Results from this study demonstrate the utility of multi-biomarker studies for distinguishing between OM from different sources and land uses, offering new insights for biogeochemical studies in aquatic systems.
... A close linear correlation between TOC and TN suggests constant terrestrial and marine OC sources with changing proportions (Schubert and Calvert, 2001;Lamb et al., 2006). The linear correlation between TOC and TN has a significant positive intercept, suggesting the presence of total inorganic nitrogen (TIN) in sediment samples (Goñi et al., 1998;Schubert and Calvert, 2001), and the positive Y-intercept was considered as the content of TIN. However, in core HH12, the plot of TOC versus TN showed two different regression lines before and after 1855 AD (Figure 7). ...
The Huanghe (Yellow River) supplies large amount of sediments and terrestrial organic carbon (OC) to the eastern Chinese marginal seas. A relocation of the Huanghe outlet from the southern Yellow Sea (YS) to the Bohai Sea occurred in 1855 AD, however, detailed knowledge about the impact of this relocation on sedimentary source and OC burial in Chinese marginal seas is still critically lacking. In this study, we present total OC content and its isotope (δ¹³C), along with bulk total organic carbon (TOC)/total nitrogen (TN) molar ratio and lipid biomarker contents, in a sediment core HH12 from the southern YS with sediment age spanning the last 300 years. We find that TOC and terrestrial lipid biomarker mass accumulation rates were lower between 1855 AD and 1950 AD than that prior to 1855 AD in core HH12; and in accordance, both TOC/TN ratio and δ¹³C records indicate a gradual decrease of terrigenous source contributions to sedimentary OC. This suggests that the relocation of the Huanghe outlet reduced the transport of terrestrial OC to the southern YS. However, the δ¹³C record also indicates a relative increase of terrestrial OC contribution to sedimentary OC after 1950 AD, and the most likely explanation is increased contributions from the old Huanghe delta erosion and Korean rivers. Future studies should focus on better constraining the variations of terrestrial and marine endmembers with δ¹³C and Δ¹⁴C analyses of specific biomarkers to examine these linkages.
... where C4-derived OC with less negative δ 13 C org values may be overrepresented, as fine fractions are transported farther offshore and in greater quantities than coarse-grained particles (Goñi et al., 1997(Goñi et al., , 1998Bianchi et al., 2002). (Fig. 1b,c, 2) (Giosan et al., 2017;Usman et al., 2018). ...
The large difference in the fractionation of stable carbon isotopes between C3 and C4 plants is widely used in vegetation reconstructions, where the predominance of C3 plants suggests wetter and that of C4 plants drier conditions. The isotopic composition of organic carbon (OC) preserved in soils or sediments may be a valuable (paleo-)environmental indicator, based on the assumption that plant-derived material retains the carbon isotopic signature of its photosynthetic pathway during transfer from plant to sediment. In this study, we investigated the carbon isotopic signature of C3 and C4 plants (δ13C) and of organic carbon (δ13Corg) in soils, river Suspended Particulate Matter (SPM) and riverbed sediments, to gain insight in the control of precipitation on C3 and C4 plant δ13C values and to assess changes in δ13Corg values along the plant–soil–river continuum. This information allows us to elucidate the implications of different δ13C end-members on C3/C4 vegetation reconstructions. Our analysis was performed in the Godavari River basin, which has mixed C3 and C4 vegetation and is situated in the Core Monsoon Zone in peninsular India, a region that integrates the hydroclimatic and vegetation changes caused by variation in monsoonal strength. The Godavari C3 and C4 plants revealed more negative δ13C values than global average vegetation values, suggesting region-specific plant δ13C signatures. Godavari C3 plants confirmed a strong control by Mean Annual Precipitation (MAP) on their δ13C values, with an isotopic enrichment of ~2.2 ‰ for the interval between ~500 and 1500 mm y-1. Tracing δ13Corg values from plant to soils and rivers revealed that soils and riverbed sediments reflected the transition from mixed C3 and C4 vegetation in the dry upper basin to more C3 vegetation in the humid lower basin. Soil degradation and stabilisation processes and hydrodynamic sorting within the river altered the plant-derived δ13C signal. Phytoplankton dominated the δ13Corg signal carried by SPM in the dry season and year-round in the upper basin. Our analysis revealed that the reconstructed C3/C4 vegetation composition was sensitive to the plant δ13C end-members used as mixing model input. The %C4 plants in the different subbasins was ~10–19 % higher using Godavari-specific end-members than using global averages, and including a correction for drought enrichment in Godavari C3 plants resulted in a 2–10 % lower estimated C4 plant cover. Hence, incorporating region-specific plant δ13C end-members and drought correction of the C3 end-member in mixing models need to be considered to determine C3 and C4 distributions of modern- and paleo-vegetation in monsoonal regions.
Rivers play a key role in the global carbon cycle by transporting terrestrial organic matter (TerrOM) from land to the ocean. Upon burial in marine sediments, this TerrOM may be a significant long-term carbon sink, depending on its composition and properties. However, much remains unknown about the dispersal of different types of TerrOM in the marine realm upon fluvial discharge since the commonly used bulk organic matter (OM) parameters do not reach the required level of source- and process-specific information. Here, we analyzed bulk OM properties, lipid biomarkers (long-chain n-alkanes, sterols, long-chain diols, alkenones, branched and isoprenoid glycerol dialkyl glycerol tetraethers (brGDGTs and isoGDGTs)), pollen, and dinoflagellate cysts in marine surface sediments along two transects offshore the Mississippi–Atchafalaya River (MAR) system, as well as one along the 20 m isobath in the direction of the river plume. We use these biomarkers and palynological proxies to identify the dispersal patterns of soil–microbial organic matter (SMOM), fluvial, higher plant, and marine-produced OM in the coastal sediments of the northern Gulf of Mexico (GoM). The Branched and Isoprenoid Tetraether (BIT) index and the relative abundance of C32 1,15-diols indicative for freshwater production show high contributions of SMOM and fluvial OM near the Mississippi River (MR) mouth (BIT = 0.6, FC321,15 > 50 %), which rapidly decrease further away from the river mouth (BIT < 0.1, FC321,15 < 20 %). In contrast, concentrations of long-chain n-alkanes and pollen grains do not show this stark decrease along the path of transport, and especially n-alkanes are also found in sediments in deeper waters. Proxy indicators show that marine productivity is highest close to shore and reveal that marine producers (diatoms, dinoflagellates, coccolithophores) have different spatial distributions, indicating their preferred niches. Close to the coast, where food supply is high and waters are turbid, cysts of heterotrophic dinoflagellates dominate the assemblages. The dominance of heterotrophic taxa in shelf waters in combination with the rapid decrease in the relative contribution of TerrOM towards the deeper ocean suggest that TerrOM input may trigger a priming effect that results in its rapid decomposition upon discharge. In the open ocean far away from the river plume, autotrophic dinoflagellates dominate the assemblages, indicating more oligotrophic conditions. Our combined lipid biomarker and palynology approach reveals that different types of TerrOM have distinct dispersal patterns, suggesting that the initial composition of this particulate OM influences the burial efficiency of TerrOM on the continental margin.
In this study, five sediment cores (N57, N59, N60, N61, and N63) were recovered from the deep-sea area of the northeastern Gulf of Mexico (~1851 m, ~3057 m, ~3542 m, ~3726 m, and ~3738 m water depth, respectively). The grain size, mineralogy, geochemistry, and radiocarbon ages of the sediment cores were analyzed. N57 and N59 cores were characterized by detrital components enriched in SiO2, Al2O3, and Fe2O3 contents. On the other hand, the N60, N61, and N63 cores were abundant with planktonic foraminifer and depleted in detrital components. The weathering indices like Chemical Index of Alteration (CIA), Chemical Index of Weathering (CIW), and Plagioclase Index of Alteration (PIA) revealed a low intensity of weathering in the source area. The enrichment Factor (EF), Pollution Load Index (PLI), and Geo-accumulation Index (Igeo) indicated that the sediments were moderately contaminated. 14C ages of foraminifer species indicated a maximum age of 1683 ± 52 calibrated years B.P.KeywordsDeep seaWeatheringForaminiferPollution indexRadiocarbonMicrofossil
Groundwaters dominated by sodium-bicarbonate represent an important end-member continental water type. The Southern Hills aquifer system in southwestern Mississippi and southeastern Louisiana, USA provides an excellent field laboratory for studying the origin and evolution of Na–HCO3 groundwaters in siliciclastic aquifers. Our research, which is based on establishing spatial variations in water chemistry, mass balance relations, and thermodynamic considerations, supports the hypothesis that the process of generating Na–HCO3 groundwaters in the Southern Hills aquifer system is driven by irreversible dissolution-precipitation reactions primarily involving silicate minerals, not by calcite dissolution and cation exchange, as has been proposed by others. The evidence supports the following scenario: oxidation of organic carbon produces organic acids which react with detrital plagioclase feldspars having an average bulk composition of approximately Ab0.70An0.30. In the up-gradient part of the system, the incongruent dissolution of plagioclase produces a kaolinite phase and releases dissolved silica, Na, Ca, and HCO3 into solution. As the silica concentration and the sodium/hydrogen ion activity ratio in the waters increase down gradient, the waters become saturated with respect to a (Na,Ca)-smectite. As plagioclase hydrolysis continues as a result of production of carbonic acid, there is a progressive increase in (Na⁺/H⁺) and (Ca²⁺/(H⁺)²) activity ratios and now a progressive decrease in dissolved H4SiO4⁰ as a result of buffering by kaolinite-smectite. It is likely that some Ca is preferentially removed from solution over Na by adsorption on newly-formed smectite. In the most distal parts of the aquifer system the high bicarbonate alkalinity and high pH result in the waters becoming saturated with respect to calcite, and the precipitation of calcite is another probable sink for Ca. Using dissolved bicarbonate as a reaction progress variable provides a useful technique for evaluating controls on groundwater compositions. It is also clear that the evolution of groundwater compositions in this and similar systems cannot be understood without understanding the controls on dissolved silica.
The principles of chemical oceanography provide insight into the processes regulating the marine carbon cycle. The text offers a background in chemical oceanography and a description of how chemical elements in seawater and ocean sediments are used as tracers of physical, biological, chemical and geological processes in the ocean. The first seven chapters present basic topics of thermodynamics, isotope systematics and carbonate chemistry, and explain the influence of life on ocean chemistry and how it has evolved in the recent (glacial-interglacial) past. This is followed by topics essential to understanding the carbon cycle, including organic geochemistry, air-sea gas exchange, diffusion and reaction kinetics, the marine and atmosphere carbon cycle and diagenesis in marine sediments. Figures are available to download from www.cambridge.org/9780521833134. Ideal as a textbook for upper-level undergraduates and graduates in oceanography, environmental chemistry, geochemistry and earth science and a valuable reference for researchers in oceanography.
This volume provides a state-of-the-art summary of biogeochemical dynamics at major river-coastal interfaces for advanced students and researchers. River systems play an important role (via the carbon cycle) in the natural self-regulation of Earth's surface conditions by serving as a major sink for anthropogenic CO2. Approximately 90 percent of global carbon burial occurs in ocean margins, with the majority of this thought to be buried in large delta-front estuaries (LDEs). This book provides information on how humans have altered carbon cycling, sediment dynamics, CO2 budgets, wetland dynamics, and nutrients and trace element cycling at the land-margin interface. Many of the globally important LDEs are discussed across a range of latitudes, elevation and climate in the drainage basin, coastal oceanographic setting, and nature and degree of human alteration. It is this breadth of examination that provides the reader with a comprehensive understanding of the overarching controls on major river biogeochemistry.
The large difference in the fractionation of stable carbon isotopes between C3 and C4 plants is widely used in vegetation reconstructions, where the predominance of C3 plants suggests wetter and that of C4 plants drier conditions. The stable carbon isotopic composition of organic carbon (OC) preserved in soils or sediments may be a valuable (paleo-)environmental indicator, based on the assumption that plant-derived material retains the stable carbon isotopic value of its photosynthetic pathway during transfer from plant to sediment. In this study, we investigated the bulk carbon isotopic values of C3 and C4 plants (δ13C) and of organic carbon (δ13Corg) in soils, river suspended particulate matter (SPM) and riverbed sediments to gain insight into the control of precipitation on C3 and C4 plant δ13C values and to assess changes in δ13Corg values along the plant–soil–river continuum. This information allows us to elucidate the implications of different δ13C end-members on C3 / C4 vegetation reconstructions. Our analysis was performed in the Godavari River basin, located in the core monsoon zone in peninsular India, a region that integrates the hydroclimatic and vegetation changes caused by variation in monsoonal strength. The basin has distinct wet and dry seasons and is characterised by natural gradients in soil type (from clay-rich to sandy), precipitation (∼ 500 to 1500 mm yr−1) and vegetation type (from mixed C3 / C4 to primarily C3) from the upper to the lower basin. The δ13C values of Godavari C3 plants were strongly controlled by mean annual precipitation (MAP), showing an isotopic enrichment of ∼ 2.2 ‰ from ∼ 1500 to 500 mm yr−1. Tracing δ13Corg values from plant to soils and rivers revealed that soils and riverbed sediments reflected the transition from mixed C3 and C4 vegetation in the dry upper basin to more C3 vegetation in the humid lower basin. Soil degradation and stabilisation processes and hydrodynamic sorting within the river altered the plant-derived δ13C signal. Phytoplankton dominated the δ13Corg signal carried by SPM in the dry season and year-round in the upper basin. Application of a linear mixing model showed that the %C4 plants in the different subbasins was ∼ 7 %–15 % higher using plant end-members based on measurement of the Godavari vegetation and tailored to local moisture availability than using those derived from data compilations of global vegetation. Including a correction for the 13C enrichment in Godavari C3 plants due to drought resulted in maximally 6 % lower estimated C4 plant cover. Our results from the Godavari basin underline the importance of making informed choices about the plant δ13C end-members for vegetation reconstructions, considering characteristics of the regional vegetation and environmental factors such as MAP in monsoonal regions.
An outcrop section of rocks from the Upper Devonian, providing plant and animal microfossils that register a time of global post-anoxia and major marine flooding events, was investigated by applying palynofacies techniques and organic geochemistry. The primary purpose of this work is the palynofacies characterization of a Neo-Devonian section of the Parnaíba Basin, belonging to the Pimenteiras Formation. Thirteen outcrop samples were collected in the western edge of the Parnaíba Basin, Tocantins State - Brazil, where a possible contact between the Pimenteiras and Cabeças formations was identified. Through optical/visual analysis of organic content and organic groups (amorphous organic matter - AOM/Palynomorphs/Phytoclasts), as well as their relative abundance, associated with previous total organic carbon (TOC) data, it was possible to individualize four (4) distinct palynofacies throughout the section. The variation in the proportions of marine and continental palynomorphs suggests sea-level fluctuation during deposition, characterizing a transitional environment, varying from the most distal to the most proximal deposition platform. Based on the predominance of palynomorphs and phytoclasts, the kerogen types II and III were recognized, corroborating with the hydrogen and oxygen indexes obtained through pyrolysis data. The predominance of yellow to chestnut colors exhibited by the prasinophyte and acritarch components under ultraviolet light excitation indicates good organic matter preservation and thermal immaturity following the organic geochemistry data.
The Jiangsu tidal flat is a significant organic matter reservoir, but quantitative studies of organic matter sources are scarce. In this study, total organic carbon (TOC) and total nitrogen (TN) concentrations and δ¹³Corg and δ¹⁵Ntotal values of surface sediment from Jiangsu tidal flat were investigated for their distributions, influencing factors, and sources of organic matter. TOC and TN were high in the center of study area and correlated well with grain size, indicating hydrodynamic influence on organic matter. High TOC/TN and low δ¹³Corg and δ¹⁵Ntotal in estuaries were characteristic of C3 plants, soil, and fertilizer sources, suggesting source effect on the distribution of organic matter. The MixSIAR model revealed that marine sources were dominant with a contribution reaching 56.9 %, followed by uniform of C3 plants, soil and fertilizer, while domestic sewage was least prominent. This study enriched theories of the biogeochemical cycle and ecological protection in the southwestern Yellow Sea.
This paper investigates the negative third-party effects of the fisheries industry on the oceanic Ecosystem in Sri Lanka. A semi-Structured interview method was employed to collect data. Since the negative externalities of the fisheries industry were not directly quantifiable a sample of related authorities in Sri Lanka was chosen to conduct interviews in order to collect qualitative data. The sample was limited to 7 government authoritative institutions operating in the Southern Marine Region and Colombo of Sri Lanka. As per findings, the biggest oceanic Ecosystem damage due to the fisheries industry is caused during the "Point-of-catch" stage of the fisheries process. Modification of the coastal environment, the environmental damage caused by illegal fishing practices, disposal of plastic to the ocean, the release of diesel from boats and chemicals, over-fishing result over-exploitation of marine ecosystem services, and voluntary dumping or loss of fishing gear were found as major harmful negative third-party effects of fisheries industry during "Point-of-Catch" stage. In addition to that, minor negative-third party effects on oceanic Ecosystems are found in other stages of the fisheries process namely "Pre-Catch", "Point-of-Processing", "Point-of-Purchase" and "Point-of-Purchase". The negative third-party effect is not significant in the final stage of "Integration of Sea Food Supply Chain" according to this study. Introducing a strong monitoring system to mitigate illegal fishing habits and strengthening legislation against them, enhancing the linkage among responsible government institutions and unifying them, standardizing material and equipment usage in fishing activities, and setting a national policy framework for oceanic Ecosystem conservation can be cited as few policy recommendations to control negative third-party effect of fisheries on oceanic Ecosystem in Sri Lanka.
This paper investigates the negative third-party effects of the fisheries industry on the oceanic Ecosystem in Sri Lanka. A semi-Structured interview method was employed to collect data. Since the negative externalities of the fisheries industry were not directly quantifiable a sample of related authorities in Sri Lanka was chosen to conduct interviews in order to collect qualitative data. The sample was limited to 7 government authoritative institutions operating in the Southern Marine Region and Colombo of Sri Lanka. As per findings, the biggest oceanic Ecosystem damage due to the fisheries industry is caused during the "Point-of-catch" stage of the fisheries process. Modification of the coastal environment, the environmental damage caused by illegal fishing practices, disposal of plastic in the ocean, the release of diesel from boats and chemicals, over-fishing result over-exploitation of marine ecosystem services, voluntary dumping or loss of fishing gear were found as major harmful negative third-party effects of fisheries industry during "Point-of-Catch" stage. In addition to that, minor negative-third party effects on the oceanic Ecosystems are found in other stages of the fisheries process namely "Pre-Catch", "Point-of-Processing", and "Point-of-Purchase" and "Point-of-Purchase". The negative third-party effect is not significant in the final stage of "Integration of Sea Food Supply Chain" according to this study. Introducing a strong monitoring system to mitigate illegal fishing habits and strengthening legislation against them, enhancing the linkage among responsible government institutions and unifying them, standardizing material and equipment usage in fishing activities, and setting a national policy framework for oceanic Ecosystem conservation can be cited as few policy recommendations to control negative third-party effect of fisheries on oceanic Ecosystem in Sri Lanka.
Hydrodynamic sorting exerts a first-order control on the quantity, type and molecular composition of sedimented organic matter. Although this physical mechanism has been invoked to explain the varying content and composition of organic matter in global continental margin sediments, it is rarely mentioned in the interpretation of the heterogeneity of source rocks in petroliferous basins. We investigate the controls of hydrodynamic sorting on the differentiation of organic matter content, type and molecular composition, and on the hydrocarbon generation capacity of source rocks in the lower part of the first member of the Eocene Shahejie Formation in the Qibei slope of the Bohai Bay Basin, by using the Rock-Eval pyrolysis, gas chromatography-mass spectrometry and logging interpretation tools. The source rock samples in the northeastern part of the slope generally present a relatively coarse grain size, less organic matter content, and are rich in humic material, whereas the source rocks in the southwestern slope area show opposite trends in these characteristics. Biomarker data demonstrate reducing sedimentary conditions and that the proportion of organic matter input from aquatic organisms versus terrestrial higher plants increases with a reduction in grain size. Pyrolysis data with a hydrogen index model shows that this organic matter-grain size relationship causes a difference in hydrocarbon generation capacity of the source rocks in various regions of the slope. This relationship also plays a role in a migration of the hydrocarbon generation center from a low-lying area in the northeastern slope to a relatively higher area in the southwest. In summary, hydrodynamic sorting is important in the distribution of organic matter pools in the slope area, causing changes in the abundance, type and composition of organic matter in the source rocks in the slope area and profoundly affecting the overall pattern of hydrocarbon generation.
Oil seepage is widespread in the Gulf of Mexico, which also applies to the Bay of Campeche in the southern Gulf. The ascension of crude oil and oil-derived hydrocarbons poses a challenge to most seep-dwelling organisms. On the other hand, short- and long-chain hydrocarbons have been shown to be potentially degraded by a range of heterotrophic sulfate-reducing bacteria. Here we present lipid biomarker and carbon isotope data from authigenic carbonates from the Campeche Knolls in the southern Gulf of Mexico and use methane-seep carbonates from the Makran convergent margin (southern Pakistan and Iran) for comparison. The Campeche Knolls and Makran were selected for such comparison as endmember types for oil versus methane seepage in an attempt to identify molecular signatures of oil seepage. The Campeche carbonates yielded on average higher δ¹³C values (−31.3‰ to −21.9‰) than the Makran methane-seep carbonates (−54.3‰ to −39.8‰) and revealed a particularly high content of ether lipids. The Campeche carbonates revealed a greater variety of alkyl chains in non-isoprenoidal dialkyl glycerol diether lipids (DAGE) and a higher relative proportion of long alkyl chains (DAGEs >C36) compared with the Makran methane-seep carbonates. Bacterial biomarkers showed an on average distinctively heavier δ¹³C signature than at the studied methane seeps, indicating bacterial oxidation of oil-derived hydrocarbons. Mixing calculations indicate that up to 73% of the local sulfate reduction is coupled to the oxidation of non-methane hydrocarbons at the oil seeps, contributing as much as 40% to carbonate precipitation. These data imply that oil-seep carbonates can be discriminated from methane-seep carbonates by their respective biomarker inventories in modern and, in cases of moderate to good biomarker preservation, ancient environments, setting the stage for the reconstruction of the influence of fluid composition on the compositions of chemosynthesis-based communities at seeps.
Comparison of rates of accumulation or organic carbon in surface marine sediments from
the central North Pacific, the continental margins of Northwest Africa, northwest and southwest America, the Argentine Basin, and the western Baltic Sea with primary production rates suggests that the fraction of primary produced organic carbon preserved in the sediments is universally related to the bulk sedimentation rate. Accordingly, less than 0.0l% of the primary production becomes fossilized in slowly accumulating pelagic sediments [[2 to 6 mm (1000 y) -1] of the Central Pacific, 0.1 to 2%, in moderately rapidly accumulating [(2 to 13 cm (1000 y) -1] hemi-pelagic sediments off northwest Africa, northwest America (Oregon) and southeast America (Argentina) and 11 to 18 % in rapidly accumulating [66 to 140 cm (1000 y) -1] hemi-pelagic sediments off southwest America (Peru) and in the Baltic Sea.
The empirical expression:
0.0030 · R · S 0.30 / ps(1- Θ) = % org-C
implies that the sedimentary organic carbon content (% org-C) doubles with each 10-fold increase in sedimentation rate (S), assuming that other factors remain constant; i.e. primary production (R), porosity (Θ) and sediment density (ps). This expression also predicts the sedimentary organic carbon content from the primary production rate, sedimentation rate, dry density of solids, and their porosity; it may be used to estimate paleo-productivity as well. Applying this relationship to a sediment core from the continental rise off northwest Africa (Spanish Sahara) suggests that productivity there during interglacial oxygen isotope stages 1 and 5 was about the same as today but was higher by a factor of 2 to 3 during glacial stages 2, 3 and 6.
Focuses on isotopic fractionation of carbon and nitrogen and the implications for modern and ancient sediments. A review of recent models for carbon isotope fractionation is followed by a proposed biological model for the fractionation of nitrogen isotopes in aquatic photosynthetic organisms. Discusses fractionation of oxygen, hydrogen, and sulfur isotopes during photosynthesis -from Authors
In order to examine the transport and burial of terrigenous organic matter along the coastal zones of large river systems, we assessed organic matter dynamics in coupled river/delta systems using mineral surface area as a conservative tracer for discharged riverine particulate organic matter (POM). Most POM in the rivers studied (n = 6) is tightly associated with suspended mineral material; e.g., it is sorbed to mineral surfaces. Average organic loadings in the Amazon River (0.67 ± 0.14 mg C m-2), the river for which we have the largest dataset, are approximately twice that of sedimentary minerals from the Amazon Delta (∼0.35 mg C m-2). Stable carbon isotope analysis indicate that approximately two-thirds of the total carbon on the deltaic particles is terrestrial. The combined surface-normalized, isotope-distinguished estimate is that >70% of the Amazon fluvial POM is not buried in the delta, consistent with other independent evidence (Aller et al., 1996). Losses of terrestrial POM have also been quantified for the river/delta systems of Columbia in the USA, Fly in New Guinea, and Huange-He in China. If the losses of riverine POM observed in these river/delta systems are representative of rivers worldwide, then the surface-constrained analyses point toward a global loss of fluvial POM in delta regions of ∼0.1 X 1015 g C y-1.
The sources and burial processes of organic matter in marine sediments
are not well understood, yet they are important if we are to have a
better understanding of the global carbon cycle. In particular, the
nature and fraction of the terrestrial organic carbon preserved in
marine sediments is poorly constrained. Here we use the chemical and
stable carbon isotope signatures of oxidation products from a
macromolecular component (lignin) of the terrigenous organic matter
preserved in offshore surface sediments in the Gulf of Mexico to
complement similar data from an existing onshore transect in this
region. The complete onshore-offshore data set, along with radiocarbon
dates of the bulk organic material at the same sites, allows the
differentiation of material originating from plants that photosynthesize
using the C4 mechanism from those that undergo C3
photosynthesis. We conclude that the offshore lignins derive from
erosion of the extensive grassland (C4) soils of the
Mississippi River drainage basin, and that the nearshore lignins
originate largely from C3 plant detritus from coastal forests
and swamps. This distribution is probably due to the hydrodynamic
sorting of the different source materials during their seaward
transport. These results suggest that previous studies have
significantly underestimated the terrigenous fraction of organic matter
in offshore sediments by not recognizing the contribution of
C4 vegetation to the carbon-isotope composition. Such an
underestimate may force revisions in the assessment of past marine
primary productivity and associated organic carbon fluxes, and of
organic matter preservation/remineralization and nutrient cycling in
marine sediments.
Molecular carbon isotopic (13C/12C) ratios of a selected marine organic biomarker compound (a C37 alka-dienone) from the Pigmy Basin (Deep Sea Drilling Program Hole 619) in the northern Gulf of Mexico varied by 5.3% in sychrony with the last ~100 ka glacial-interglacial cycle. This carbon isotopic (delta13C) variation is likely due to the effect of climatically linked variations in concentrations of CO2(aq) on photosynthetic fractionation of carbon isotopes by phytoplankton. Measurement of the delta13C of the alkadienone (delta37:2) to reconstruct the carbon isotopic composition of total marine organic carbon permits improved estimates of the time-varying delta13C of preserved marine organic carbon end members. With the time series record of marine end members, the concentrations and net accumulation rates of terrigenous Corg and marine Corg are calculated and their paleoceanographic significance is considered. Although this method may be improved by consideration of additional biomarkers representative of diverse marine and terrigenous inputs, it is the first application of molecular isotopic analysis to quantifying marine and terrigenous Corg concentrations. Further application of such techniques will be become valuable in reconstructing regional and global budgets of marine and terrigenous Corg accumulation.
This study examines the regional distribution of the stable isotopes of organic carbon in the surface soils (SOC) of a variety of biomes including forests, savannas, and grasslands. A transect through tropical/subtropical biomes in northern Australia demonstrates that forest and grassland soils exhibit comparatively small variations in delta13C value on both local and regional scales. Savanna soil delta13C values exhibit extreme variability at all spatial scales with samples separated by only a few meters differing by up to 6.60/00, and a total range of values for savanna samples from -15.9 to -26.60/00. Forest surface SOC has an average delta13C value of -28.4+/-0.70/00(1sigma), while tropical grasslands (C4-dominated) have an average delta13C value of -15.5+/-0.80/00(1sigma) and temperate grasslands (C3-dominated) -26.0+/-1.10/00(1sigma). Despite extreme variability between savanna samples, there is a consistent relationship between delta13C value and SOC content in all samples from northern Australia, with savanna soils forming a continuum between forests with low delta13C values and high SOC contents, and tropical grasslands with high delta13C values and low SOC contents. The relationship suggests that an integrated regional delta13C value for SOC is a useful proxy for terrestrial carbon storage. River sediment delta13C values from the transect region reflect the delta13C values obtained for the regional soils, with a bias toward the C3 end-member. Size-fractionated ``average'' soils from a variety of biomes suggest that little isotopic fractionation accompanies degradation but that in mixed C3/C4 biomes, C3-derived carbon is preferentially incorporated into the coarse size fractions, while C4-derived carbon is preferentially added to the fine size fractions.
Cultivation of C‐3 (Calvin cycle) plants after turning under a native prairie of grasses possessing C‐4 metabolism (Hatch‐Slack cycle) allows partitioning of soil organic matter as to origin. Contrasting natural ¹³ C abundance for the two different contributing kinds of plant residues provides an in situ labeling of the soil organic matter. A study to exploit this approach used soil from a long‐term experimental field. Samples were collected under different continuous crops and at different dates and were analyzed for total C and for natural isotope composition of C. The results demonstrated an initial rapid loss of prairie C, suggesting the presence of an easily mineralizable component that was exhausted in 30 to 40 yr. The extent of this was dependent upon the type of cultivation. A pool of stable organic matter of prairie origin persisted under all systems during the approximate 100 yr of cultivation, which was no less than 50% of the current level. The half‐life, under cultivation, of the easily mineralizable compartment was shown to be 10 to 15 yr. Mechanical fractionation of soil mineral particles and associated organic matter showed a heterogeneity with regard to stability. The clay fraction included both the most stable C, and some labile C, whereas clay associated with fine silt demonstrated a turnover that suggested slow replacement of the whole of the fraction. The overall results demonstrate the potential value of long‐term field experiments for studying soil organic matter turnover by the natural ¹³ C abundance technique.
New data on the vegetational history and Quaternary geology of Amazonia permit an improved reconstruction of past environments in Amazonia during the last glacial period. Although limited, data from Rondonia, Carajas and Guyana show that, in certain areas, savanna-type vegetation and savanna forest had replaced the rain forest during the late Pleniglacial (ca. 22,000–13,000 yr B.P.). The Amazonian forest may have been split up into one major west Amazonian and several other medium-size forest areas. This suggests a decline in rainfall of 500 to 1000 mm (a reduction of 25 to 40%). Temperatures may also have been 2° to 6°C (4±2°C) lower than today, possibly substancially influencing Amazonian vegetation. During the humid middle Pleniglacial (55,000–26,000 yr B.P.), rivers carried a lot of water and sediment, resulting in the deposition of lower terrace sediments with one dry interruption around 40,000 yr B.P. (Carajas and Katira). In Carajas, there is evidence of dry periods ocurring at about 40,000 yr B.P., and during the early Pleniglacial (ca. 60,000 yr B.P.). Rivers carried little water and incised into the low terrace sediments during the dry late Pleniglacial. Water levels rose during the late glacial (13,000–10,000 yr B.P.) or at the beginning of the Holocene (10,000 yr B.P.). Sedimentation in (and of) the present inundation valleys commenced after that.
Solid-state 13C NMR (nuclear magnetic resonance) spectroscopy, analytical pyrolysis, and chemical degradative methods are combined to elucidate the structural changes in different plant-derived biomacromolecules during biodegradation and humification in forest soils. The humification processes operating with different intensity on the plant compounds are selective preservation, direct transformation, and resynthesis by microorganisms. Polysaccharides of plant litter are decomposed intensively and substituted by microbial polysaccharides. Lignin is partly mineralized and the remnant molecule is transformed directly by ring cleavage and side-chain oxidation. Further modifications of the aromatic carbon in forest soils lead to an increase of C-substituted aromatic carbon and a concomitant loss of phenolic structures in the humic acid fraction during humification. The results indicate that the refractory alkyl carbon moieties in humified forest soil organic matter do not result from a selective preservation of plant-derived biomacromolecules. Cutin and suberin are easily mineralized or transformed and do not accumulate with depth. The recently described “aliphatic biopolymers” cutan and suberan were also not found to accumulate in the forest soils investigated. The process occurring most likely in the soils investigated is a transformation and increase in cross-linking of lipid- and/or cutin-type and suberin-type materials.
This chapter reviews the important factors controlling primary and secondary production in grasslands, particularly at the subsystem level of organization. This evidence is drawn largely from the previous chapters and from the experience and data accumulated under the IBP Grassland Biome studies conducted from 1968 through 1976.
The relationship of physical factors to vegetation distribution and fishery harvest was analyzed using data from the Gulf of Mexico. Data from 64 estuaries were used to investigate the relationships with vegetation. Fishery harvest in the southern Gulf of Mexico was analyzed using harvest statistics, estuarine area, and river discharge by state. Results show that the fishery harvest and area of an estuary are strongly related to freshwater input and physiography. Intertidal area is correlated to coastal land slope, length of coastline occupied by the estuary, and inshore open water area. The area of emergent vegetation is related to intertidal area and rainfall, but not to riverflow, because there are large areas of emergent vegetation with low riverflow (e.g., south Florida). Salt flat area is related to intertidal area and rainfall deficit. Fishery harvest per unit open water area in the southern Gulf is highly correlated to river discharge (r = 0.98).
A new equation to calculate the local ocean paleoproductivity has been developed. The formula is based on the relationships between carbon accumulation rate, water depth, and carbon-free bulk sedimentation rates (as a 'sealing factor') of deep-sea sediments, and is independent of a large range of bottom water O2 concentration. For comparison with paleoproductivity, delta 13C fluctuations of Cibicidoides wuellerstorfi serve to record the total CO2 dissolved in North Atlantic Deep Water. In the region of coastal upwelling off northwest Africa, paleoproductivity increased by a factor of three from interglacial to glacial stages depicting a clear 100 000 year cycle. In oceanic 'deserts', the productivity varied much less. Phase relationships during the last two Terminations show that trade wind strength and related productivity due to upwelling in the east Atlantic started to decrease slightly prior to or simultaneously with global ice melting, synchronously with a drastic increase in atmospheric pCO2. CO2 depletion in the NADW only followed after some 2500 to 4500 years and cannot have caused the change in atmospheric CO2. The high latitude insolation balance which causes changes in sea-ice cover and thus, of meridional trade wind intensity, is regarded as the prime factor responsible for this massive feedback mechanism for climatic change. - from Authors
Interest in primary production on a global scale has flourished in the past two decades (Westlake 1963, Bazilevich et al 1971, Lieth 1973, Cooper 1975, Lieth and Whittaker 1975, Rodin et al 1975). Although reviews are available for forests (Kira 1975), grazing lands (Caldwell 1975), tundra (Wielgolaski 1975), and aquatic ecosystems (Bunt 1975, Likens 1975), no similar review is available in English for grasslands. The work presented here is an initial step towards a synthesis of grassland primary production. Before proceeding to the detailed examination of North American grasslands in the succeeding chapters, it is well to place their production in perspective with other grasslands and with other ecosystems.
Soft-rot fungal degradation of archaeological wood was investigated by comparing the micromorphological characteristics, elemental compositions and monomeric and dimeric phenols derived from CuO oxidation of lignins in degraded boxwood, maple, juniper and pine woods from Tumulus MM, Turkey, thought to be the ancient tomb to King Midas dated 2700 years before present (BP). Chemical compositions of these soft-rotted archaeological woods were compared to those of modern undegraded counterparts and to those of contemporary white- and brown-rotted woods. The degraded angiosperm woods showed Type 1 (cavity) and Type 2 (erosive) forms of soft-rot, whereas the degraded gymnosperm samples exhibited only Type 1 decay. All soft-rotted woods contained higher weight percentages of ash and nitrogen than their fresh counterparts. Polysaccharides were preferentially attacked by soft-rot fungi as indicated by decreased atomic ratios of hydrogen/carbon in all degraded woods and lower atomic ratios of oxygen/carbon in remnant boxwood, maple, and pine woods. In addition, soft-rot fungi extensively attacked lignin as evidenced by the lower carbon-normalized yields of lignin monomers from most degraded samples. All soft-rotted woods had elevated acid/aldehyde ratios indicative of oxidative microbial degradation of remnant lignin sidechains. Soft-rot degradation resulted in greater carbon cross-linking of the remnant lignin and enrichment of ring-ring dimers in the degraded angiosperm samples, indicating preferential attack of sidechain-linked structures within the lignin polymer. While soft-rot fungi share some degradative characteristics with both white- and brown-rot fungi, the physical and chemical effects of soft-rot degradation are unique.
Onshore-offshore trends in phosphorus (P), organic carbon (OC), and total nitrogen (TN) concentration, P distribution, elemental organic C:N:P ratios, and stable carbon isotopic composition of OC (δ13Coc) of surficial sediments, are presented from three river-dominated coastal regimes: the Mackenzie River/Beaufort shelf in the Canadian Arctic; the Mississippi Delta and Louisiana shelf in the temperate Gulf of Mexico; and the tropical Amazon shelf. These parameters, measured in surficial sediments from the three sites, are used to assess changes in the importance of terrestrial and marine organic matter sources to sediments as a function of distance from the locus of riverine discharge.
River particulates have been collected from twenty-three rivers from throughout the continental USA. The rivers drain mostly large basins (basin areas range from 2.1 × 103 to 2970 × 10³ km2) composed of mixed lithologies, and spanning a wide range of climatic conditions as evidenced by a large variation in air and ground water temperatures, precipitation, and runoff. Suspended particulates have been analyzed for organic and inorganic carbon, as well as the major elements Al, Fe, Mn, K, Si, Ca, Mg, and Na. Also determined were suspended sediment loads at the time of sampling, particle surface areas, and grain size distributions. Five-year average river water chemistry and suspended sediment concentrations, obtained from USGS water supply reports, are also included as supporting information. Particle chemistry systematically varies with rates of runoff, with high runoff rivers transporting the most heavily altered particulates and low runoff rivers carrying the least altered. Degrees of alteration are indicated by the extent to which the refractory, nonmobile elements Al and Fe are concentrated into the particulates and the extent to which the most easily weathered elements Na, Ca, and Mg have been leached. Overall, the susceptibility of elements towards leaching is consistent with numerous previous descriptions of element mobility on weathering.
In order to examine relationships of organic matter sourer, composition, and diagenesis with particle size and mineralogy in modern marine depositional regimes, sediments from the continental shelf and slope along the Northwest Pacific rim (Washington coast, USA) were sorted into hydrodynamic size fractions (sand: >250, 63-250 mu m; silt: 35-63, 17-35, 8-17, 3-8 mu m; and clay-sized: 1-3, 0.5-1, <0.5 mu m). The size fractions were then density fractionated to separate distinct organic debris from mineral-associated organic matter, and the various separates were analyzed for their amino acid, aldose, and lignin compositions. The composition of organic matter in the separates changes markedly as a function of particle size and density. Large compositional differences were observed between the clay sized fractions (dominated mineralogically by smectites), the sand-sized mineral-associated isolates (quartz-rich), and floated coarse organic matter (dominated by vascular plant debris). Organic matter intimately associated with the clay-sized fractions shows the most extensive diagenetic alteration, as reflected in high abundances of nonprotein amino acids (especially beta-alanine), elevated lignin phenol acid/aldehyde ratios, and high relative concentrations of the deoxyhexoses fucose and rhamnose. Organic matter in the silt fractions, though degraded, is not as diagenetically altered as in the clay fractions. Enrichment of pollen gains in the silt-size material is reflected by high cinnamic acid to ferulic acid lignin phenol ratios. The highest pollen biochemical signal is observed in the silt fractions of the deepest station (1835 m), where pollen abundances are also highest. Organic matter tightly bound in the silt and sand-sized fractions are enriched in aldoses and show indications of enhanced microbial biomass as reflected by high weight percentages of ribose. Distinct organic debris was composed of relatively unaltered vascular plant remains as reflected by high lignin phenol yields and low acid/aldehyde ratios. Clay-size fractions are enriched in nitrogenous components, as reflected by elevated yields of total and basic amino acids (especially lysine). Silt- and sand-size fractions rich in quartz and albite show slightly higher yields of neutral amino acids. Consistent trends across all size classes and among the different depositional settings illustrates that only a small portion of the organic matter is present as distinct organic debris (e.g. pollen, vascular plant tissues, etc.), but that this debris can be isolated in specific size classes. The data for surface-associated organic matter are consistent with, but not conclusive of, selective partitioning of some organic matter to specific mineral surfaces. The dominant size class-specific trends in organic matter composition are due to changes in both source and diagenetic alteration. Copyright (C) 1998 Elsevier Science Ltd.
We conducted a study of relative gas composition changes of CO2, CO and CH4 during the formation of graphite targets using different temperatures, catalysts and methods. Reduction with H2 increases the reaction rate without compromising the quality of the AMS target produced. Methane is produced at virtually any temperature, and the amount produced is greater at very low temperatures. The reduction of CO to graphite is very slow when H2 is not included in the reaction.
Stable isotope ratios (δ13C) of total organic carbon were measured in surface sediments from the continental margins of the northern and western Gulf of Mexico, the north coast of Alaska and the Niger Delta. Gulf of Mexico outer-shelf isotope ratios were in the same range as has been reported for Atlantic coastal shelf sediments, −21.5 to −20‰. Off large rivers including the Mississippi, Niger and Atchafalaya (Louisiana), δ13C values increased from terrigenous-influenced (around −24‰) to typically marine (∼−20‰) within a few tens of kilometers from shore. This change was accompanied by a decrease in the amount of woody terrigenous plant remains in the sediment. Alaskan continental margin samples from the cold Beaufort Sea had isotopically more negative carbon (−25.5 to −22.6‰) than did warmer-water sediments. The data indicate that the bulk of organic carbon in Recent sediments from nearshore to outer continental shelves is marine derived.
Lignin is one of the most abundant polymeric organic constituents of forest litter. Due to its molecular structure and heterogeneity the isolation of lignin in an unchanged form and its exact determination in forest humus have not proved possible. The oxidative degradation with CuO provides a specific method for the characterization of intact lignin structures in forest humus layers. The sum of phenolic CuO oxidation products gives an overall pattern of lignin decomposition. The degree of alteration of the remnant lignin is described by the acid-to-aldehyde ratio (Ac:Al) of syringyl and vanillyl units for angiosperm and gymnosperm lignin and the ratio of syringyl-to-vanillyl units (S:V) for angiosperm lignin.The lignin component of three different forest humus layers investigated was partly decomposed. The residual lignin fraction had undergone extensive modification during microbial decomposition. The chemical changes occurring in the lignin molecule during decomposition in forest humus layers suggest similar mechanisms of lignin degradation (“white-rot”) are shared by wood and forest litter.
This book presents the papers given at a conference on the atmospheric chemistry of carbon dioxide. Topics considered at the conference included the carbon cycle, the tropospheric methane cycle, the last deglaciation, reef growth, climatic change, carbon deposition rates in the Atlantic Ocean, low-latitude biomass, carbon isotopes, geochemistry, charcoal fluxes, volcanism, geologic ages, tectonics, carbonate rocks, marine surveys, and biogeochemistry.
Systematic side-scan sonar and high-resolution seismic records from the shallow-water offshore areas of the Mississippi Delta have revealed widespread sub aqueous slope failures in bottom sediments. These failures have resulted in damage and loss to offshore structures and pipelines. The features occur on slopes with very low inclinations (ranging from 0.2° to 1.5°) and in water depths of 5-100 m. The types of features include collapse depressions, bottleneck slides, elongate slides and slumps, mudflow gullies, and overlapping mudflow lobes. Although movements include both vertical and rotational displacements, the bas1c mechanism can be approximated as down slope translation of shallow slabs of debris. Although movement rates of up to several hundred meters/year have been documented, it is postulated that large-magnitude surged may be inherent in these features. These submar1ne landslides result from complex temporal and spatial combinations of wave-induced stresses, sediment loading, and generation of high pore water and methane gas pressures.
INTRODUCTION
Numerous detailed marine surveys (high-resolution geophysical, side-scan sonar, and bathymetric data) have been conducted in the area offshore of the Mississippi River. These surveys have revealed the presence of a large number of distinct types of sub aqueous slope instabilities that can be generally classed as submarine landslides. Most of these bottom sediment mass movements are of sufficient magnitude to severely endanger bottom-emplaced petroleum facilities such as offshore drilling and production platforms, wellheads, and pipelines.
The modern bird-foot (Balize) delta of the Mississippi River displays three major distributaries (Fig. I) and has formed within the past 600-800 years.1 Seaward progradation rates of the distributaries vary from in excess of 100 m/yr to less than 50 m/yr, depending upon the specific distributary monitored. Sedimentation rates seaward of the delta shoreline vary spatially as well as temporally. Near the mouths of the distributaries, accumulation rates in excess of 1 m/yr have been documented, whereas in adjacent interdistributary bays accumulation rates rarely exceed a few centimeters per year. In offshore waters, in front of the delta, accumulation rates vary from a few tens of centimeters per year in 50-m-depth water to a few millimeters per year in water depths approaching 200 m.
Offshore slopes of the entire delta front are extremely low, rarely exceeding 1.5°. In the interdistributary bays, bottom slopes are generally less than 0.5° and are rarely greater than 0.2°. In water depths of approximately 5-80 m, bottom slopes range from 0.7° to 1.5°, and in depths of 80-150 m the slopes are less than 1°. In general, hydrographic maps indicate extremely irregular topography, the bottom displaying a large number of radial trending submarine gullies 2 in water 20-80 deep and broad, flat terraces seaward to water depths as great as 200 m (Fig. 1).
Analyses of organic carbon, total nitrogen, and inorganically bound ammonium (exchangeable and fixed ammonium) in two oxic deep-sea sediment cores from the Central Pacific Ocean revealed insufficiently high inorganic ammonium contents of these sediments to explain the low C/N ratios, although representing 20-45% of the total nitrogen. Both, organic carbon/total nitrogen ratios (ranging from 3.9-1.3) and organic carbon/organic nitrogen ratios (5.6-1.9) decrease with increasing sediment depth, the latter indicating a real enrichment of organic nitrogen compounds during diagenesis relative to total organic matter. Organic matter/alumina relationships indicate that this unusual preservation of organic nitrogen compounds is probably caused by sorption to clay minerals protecting them against bacterial attack.
The magnitude and timing of the response of the soil carbon reservoir to changes in land use or climate is a large source of uncertainty in global carbon cycle models. One method of assessing soil carbon dynamics, based on modeling the observed increase of 14C in organic matter pools during the 30 years since atmospheric weapons testing ended, is described in this paper. Differences in the inventory and residence time of carbon are observed in organic matter from soils representing tropical (Amazon Basin, Brazil) and temperate (western slope of the Sierra Nevada mountains, California) forest ecosystems. The majority of the organic carbon in the upper 22 cm of the tropical soil (7.1 kgC m-2) has residence times of 10 years or less, with a minor component of very refractory carbon. The estimated annual flux of carbon into and out of the soil organic matter in this horizon of the mineral soil, based on modeling of the 14C data, is between 1.9 and 5.5 kgC m-2 yr-1. In contrast, organic matter in the temperate soil over a similar depth interval (0-23 cm; 5.2 kgC m-2), is made up of approximately equal amounts of carbon with residence times of 10, 100, and 1000 years. The estimated annual flux of carbon into and out of this soil is 0.22 to 0.45 kgC m-2 yr-1. Rapid turnover of organic matter with density 1.6-2.0 g cm-3 removed 14C-enriched components from the temperate soil but had no effect on the 14C content of the residue in 0-22 cm layer of the tropical soil.The results presented here show that carbon cycle models which treat soil carbon dynamics as a single reservoir with a turnover rate based on radiocarbon measurements of bulk soil organic matter underestimate the annual fluxes of organic matter through the soil organic matter pool, particularly in tropical regions.
Vertical accretion and sediment accumulation rates were determined from the distribution of ¹³â·Cs in cores collected from fresh water, intermediate, brackish, and salt marshes in the Barataria Basin, Louisiana. Vertical accretion rates vary from about 1.3 cm.yrâ»Â¹ in levee areas to 0.7 in backmarshes. Mineral sediment content of the marsh soil profile decreased with distance from the coast. Except in natural levee areas, marsh accretion rates are less than subsidence measured by water level data, however this alone cannot account for observed land-loss patterns in the basin area.
The stable carbon isotope compositions of vegetation and soils of marshes of Louisiana's Barataria Basin were determined. The δ13C-values of organic soils taken from fresh, intermediate, brackish and salt marshes are similar to the δ13C-value of its vegetation. The δ13C of a sediment profile taken from a salt marsh in a rapidly subsiding distributary of the Lafourche delta, an older distributary of the Mississippi River, showed that the δ13C signature of buried peats represents original organic sources (e.g., fresh marsh plant species). The δ13C signature at depth representing a C-3 plant-dominated freshwater environment corresponded to reported depositional reconstruction using paleo-environmental techniques. Vertical accretion rates determined by 137Cs dating support historical and measured δ13C-values in the profiles corresponding to earlier freshwater environments.
Three major rivers supply most of the clay-mineral detritus that the northeastern Gulf of Mexico receives. The mineralogy of the clay supplied by each river is a product of the weathering versus parent-rock interplay in the drainage basins. In the western drainage basins, erosion and transportation of essentially unaltered montmorillonitic sediments prevails. Eastward, weathering becomes more effective, and kaolinite gradually becomes more abundant in the soils and river clays. Consequently, the Mississippi River is contributing a montmorillonitic clay-mineral suite, and the Apalachicola River is contributing a kaolinitic suite. The Mobile River, between these two rivers, is contributing an intermediate clay-mineral suite. The river sediments pass through the various bays and estuaries with only minor alterations in their clay-mineral suite which are of too small a magnitude to affect significantly the gross regional distribution pattern. Within the Gulf of Mexico, that portion of the clay not flocculated by saline water at the river mouths is distributed first by the wind-driven shallow water currents and then by the semi-permanent oceanic currents. A gradational facies pattern is developed in which the sources of supply, their magnitudes, and the distributional directions are clearly evident. Clay-mineral distributional patterns in the modern Apalachicola River area are similar to those in the Texas lower Eocene (Wilcox) sediments. Similar weathering and current factors may have produced the analogous clay-mineral facies patterns.
The amount of 14C produced by nuclear bomb testing that entered the Atlantic Ocean by late 1972 was 1.71×10-8 mumol/cm2 of ocean surface area for the west Atlantic (36°S-45°N) and 1.18×10-8 mumol/cm2 for the east Atlantic (50°S-28°N) Geochemical Ocean Sections Study stations. There are strong latitudinal differences in the integrated amount of bomb 14C content in Atlantic waters. Bomb-produced 14C is mostly encountered near the center of the large mid-latitude gyres, whereas the equatorial region has a lower 14C inventory. The average ocean wide vertical distribution of bomb 14C in the Atlantic can be explained by a vertical eddy diffusion coefficient of 4.0 cm2/s in the surface mixed layer plus thermocline gyre reservoirs. The average 14C activity per unit area measured in the Atlantic yields an atmosphere-ocean CO2 exchange rate of 23 mol/m2 yr, which is equivalent with an atmospheric CO2 residence time of 6.8 years.
Amino acid analyses yielded no source-indicative compositional parameters, but microbes produced markedly higher carbon-normalized yields than did vascular plant tissues. Sediment trap samples collected monthly for a year at 30 and 60 m in Dabob Bay, Washington, were also analyzed, as were subsamples from a sediment core taken at the same site. Amino acid yields from the trap samples exhibited spring and fall maxima that coincided with blooms, and a winter minimum that resembled yields in underlying sediments. Diagenesis was apparently the primary control on calculated reactivities. Among the amino acids, reactivity patterns indicated relative preservation of diatom cell-wall protein. Within the sediments, amino acids were again degraded preferentially relative to organic C and N. -from Authors
The predepositional stability of lignin in the salt marsh cordgrass Spartina alterniflora was examined in two different degradation studies: one was a traditional litterbag study carried out using post-senescent brown leaves in a North Carolina marsh creek, and the other was a study in which senescing, standing plants were tagged and allowed to undergo in situ degradation in a Sapelo Island, Georgia, salt marsh. Based on results from lignin oxidation product (LOP) analysis of leaves, lignin in the S. alterniflora was shown to be significantly degraded in both studies, with 13 [plus minus] 2% and 25 [plus minus] 12% of the total lignin mass loss occurring over the 496-day litterbag and 146-day tagged studies, respectively. A comparison of the results from both studies suggests that most of the calculated lignin loss (> 90%) occurs early in the degradation history of the plant, with a significant portion occurring while the plant is still standing in the salt marsh. Further detailed evaluation of this loss demonstrates that selective lignin degradation occurs in S. alterniflora, deriving from the preferential loss of labile lignin moieties. Based on measured changes in both the lignin mass loss and the LOP acid/aldehyde ratio, as well as evidence suggesting that degradation occurred under oxic conditions, it is proposed that aromatic ring cleavage was the predominant mechanism of lignin degradation in both studies. In light of these results and those from other recent lignin degradation studies, the authors discuss the geochemical consequences regarding the usefulness of lignin oxidation products as quantitative tracers of vascular plant-derived organic matter being transported, deposited, and buried in aquatic environments.
Physically intact fir, hemlock and cedar needles were isolated from different horizons of a sediment core from a coastal marine bay (Dabob Bay, Washington State, U.S.A.) and from nearby trees and forest litter. Green fir, hemlock and cedar needles were all characterized by glucose-rich aldose mixtures (~30% of tissue carbon), the production of vanillyl and cinnamyl CuO-derived phenols (~8% of tissue carbon) and the presence of both pinitol and myo-inositol (1–2% of tissue carbon). Needles from forest litter were enriched in lignin phenols and non-glucose aldoses and depleted in glucose and cyclitols.
A vertical profile of lignin in the upper 700 m layer of a 1400 m sediment core of Lake Biwa, an oligotrophic freshwater lake in Japan, was determined using a CuO oxidative degradation method. The results indicated that lignin is found throughout the core, demonstrating lignin to be very stable for over 0.6 million years. Moreover, the upper 250 m (approximately 0.6 million years old) segment of the sediment core was investigated to determine the apparent long term degradation rate of lignin. A downward lignin concentration decrease is observed over the upper 250 m of the core which corresponds to a calculated half life of at least approximately 40 × 10 4 years, assuming that lignin decrease is due to its in situ degradation (diagenesis).
Submarine landslides play a major role in the development of distributary-mouth bars and are of major importance in transporting sediment from the bar front to deeper water along the Mississippi delta front. Historic maps of the South Pass of the Mississippi delta show that the bar advanced seaward more than 1 mi (1.6 km) between 1867 and 1953. Details of the growth of the bar have been elucidated using an elaborate computer modeling program to analyze these historic maps. The analysis has shown that the geometry of the bar was controlled by the dynamics of the freshwater plume of river water as it mixed with saline Gulf water. Approximately half the sediment deposited on the bar was moved into deeper water by submarine landslides. The underlying causes of bar failure were established during major floods with the deposition of thick blankets of unstable, watersaturated sediments on the bar front. Failure occurred one to four years later in response to a variety of triggering mechanisms, which either changed the shear strength of the sediment or modified local bottom slope. The triggering mechanisms include: major storms and hurricanes, mudlump activity, and possibly, increased pore pressures resulting from generation of biogenic gas. Bar growth and basinward movement of sediment thus represent a multilvariate problem that can be approached by means of a computer analysis of bathymetric data.
Calibration curves spanning several millennia are now available in this special issue of R adiocarbon . These curves, nearly all derived from the 14 C age determinations of wood samples, are to be used for the age conversion of samples that were formed through use of atmospheric CO 2 . When samples are formed in reservoirs (eg, lakes and oceans) that differ in specific 14 C content from the atmosphere, an age adjustment is needed because a conventional 14 C age, although taking into account 14 C (and 13 C) fractionation, does not correct for the difference in specific 14 C activity (Stuiver & Polach, 1977). The 14 C ages of samples grown in these environments are too old, and a reservoir age correction has to be applied. This phenomenon has been referred to as the reservoir effect (Stuiver & Polach, 1977).
Laboratory study of the bacterial decomposition of Long Island Sound plankton in oxygenated seawater over a period of 2 years shows that the organic material undergoes decomposition via first‐order kinetics and can be divided into two decomposable fractions, of considerably different reactivity, and a nonmetabolizable fraction. This planktonic material, after undergoing varying degrees of oxic degradation, was added in the laboratory to anoxic sediment taken from a depth of 1 m at the NWC site of Long Island Sound and the rate of bacterial sulfate reduction in the sediment measured by the ³⁵ S radiotracer technique. The stimulated rate of sulfate reduction was in direct proportion to the amount of planktonic carbon added. This provides direct confirmation of the first‐order decomposition, or G model, for marine sediments and proves that the in situ rate of sulfate reduction is organic‐matter limited. Slower sulfate reduction rates resulted when oxically degraded plankton rather than fresh plankton was added, and the results confirm the presence of the same two fractions of organic matter deduced from the oxic degradation studies.
Near‐surface Long Island Sound sediment, which already contains abundant readily decomposable organic matter, was also subjected to anoxic decomposition by bacterial sulfate reduction. The decrease in sulfate reduction rate with time parallels decreases in the amount of organic matter, and these results also indicate the presence of two fractions of organic carbon of distinctly different reactivity. From plots of the log of reduction rate vs. time two first‐order rate constants were obtained that agree well with those derived from the plankton addition experiment. Together, the two experiments confirm the use of a simple multi‐first‐order rate law for organic matter decomposition in marine sediments.
An extensive suite of thirty lignin-derived phenolic dimers and fourteen additional monomers has been identified among the CuO reaction products of twenty-four different vascular plant tissues. The various lignin dimers are characterized by five different types of linkages between phenolic units, including direct 5,5'-ring-ring bonding, as well as ,1-diketone, ,1-monoketone, ,5-monoketone, and ,2-methyl sidechain-ring couplings. The new lignin-derived monomeric CuO reaction products include vanillyl and syringyl glyoxalic acids and vanillyl phenols with formyl and carboxyl functional groups attached at various ring positions. The distribution of all these novel compounds in twenty-four different vascular plant tissues indicates important differences in the structure and chemical composition of the lignin macromolecule among these sources. The abundances of these compounds in a selected set of sedimentary samples suggest that the lignin dimers and novel lignin monomers can characterize the ultrastructure, sources, and diagenetic state of sedimentary lignin in ways not possible from the routinely utilized lignin monomers alone.