Source and composition of sedimentary organic matter in the head of Three Gorges Reservoir: a multiproxy approach using δ13C, lignin phenols, and lipid biomarker analyses
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Sedimentary organic matter
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Sediment–water interface
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δ15N
Isotopes of nitrogen
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Sedimentary organic matter
Oxygen minimum zone
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Use of sedimentary organic carbon concentrations as a record of paleoceanographic conditions is complicated by an insufficient understanding of the mechanisms controlling present‐day variations in the organic matter content of surface open ocean sediments. This paper is a review of organic carbon distributions in marine sediments, the global marine balance of particulate and dissolved organic carbon and the processes controlling organic matter diagenesis. The discussion focuses on the last topic with the intention of bringing together mass balance and organic chemical evidence for mechanisms that control the preservation of organic matter in open ocean sediments.
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The temporal variations of the superficial (0–1 cm) sedimentary organic matter were studied at a depth of 26 m on the continental shelf of the Gulf of Lions (northwestern Mediterranean). The samples were analyzed for total organic carbon, coarse organic carbon (>40 μm), hydrolyzable organic carbon, nitrogen, total amino acids, total and individual sugars (HPLC), lignin-derived compounds (HPLC) and kerogens (acid-soluble, humic substances and humin). Seasonal variations of the organic compounds are related to the sedimentological, hydrodynamical and physico-chemical environmental conditions. The mean annual values of the different organic compounds analyzed show the low quantities and their evolved character at the sediment-water interface: 0.5% total org C (TOC) (d.w.), 0.049% N (d.w.), CN: 11.2, coarse org C (COQ: 62% of TOC, hydrolyzable org C: 45% TOC. The labile compounds represent a low percentage of the total organic matter (TOM), amino-acids: 12% of TOM and sugars: 5% of TOM. The relative proportions of soluble (humic) and insoluble kerogens (humin), respectively 6% and 94% of TOC are typical of a highly evolved organic matter. The large contribution of plant remains confirmed by the high proportion of COC, corresponds to a low proportion of humic substances and a high degree of condensation (HC = 1.3). The infrared spectroscopy determination of the functional groups of the humic substances permits us to confirm both autochtonous (marine) and allochtonous (terrestrial) sources of organic matter in the Têt prodeltaïc accumulation area. Numerous functional groups identified reveal the fresh quality of the organic inputs at the sediment-water interface. Aliphaticity is well marked and nitrogenous compounds (1 and 2 amines) correspond to autochtonous production (in spring: phyto- and zoo-planktonic blooms in the euphotic zone; in summer: primary production under the thermocline and phytobenthic blooms). Sugars are well represented, but from two origins: phytoplanktonic, and issue from terrestrial high plants (ligno-cellulosic complexes). The aromatic functional groups are well identified (aromatic ether, phenol) as are the markers of a large contribution of terrestrial organic matter. The active hydrodynamical (resuspension) and biological, (consumption, bioturbation) events participate in the frequent reworking of the superficial deposits (muddy-silts) and, consequently, the active degradation (oxidization) of the organic matter. The more labile organic matter is rapidly consumed by the intense biological benthic activity. The organic C content is low (0.5% d.w.) except during the autumnal input (2.5% d.w.). This input (8 cm thick) disappears one month later, because of the resuspension induced by littoral currents. This same phenomenon induces during all the year the reworking of the first centimeter of the deposit. In terms of budget of the carbon cycle in the coastal area, this active hydrodynamical environment shows the supremacy of the carbon degradation over its burial.
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Sedimentary organic matter
Biogeochemical Cycle
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Organic carbon deposited in marine sediments is an important part of the global carbon cycle. The knowledge concerning the role of shelf seas (including the Baltic Sea) in the carbon cycle has increased substantially, however organic carbon accumulation rates in the Baltic sediments still require clarification. This paper describes methods used for assessing organic carbon and nitrogen accumulation rates in six sediment cores collected in the sediment accumulation areas in the Baltic Sea. Mass sediment accumulation rates were based on 210Pb method validated by 137Cs measurements. The organic carbon accumulation rates ranged from 18 to 75 g·C·m−2·yr−1. The C/N ratios and δ13C were used to access sedimentary organic matter provenance. The C/N ratios in the investigated cores vary in the range from 7.4 to 9.6, while δ13C ranged from −24.4‰ to −26.4‰. Results of the terrestrial organic matter contribution in the sedimentary organic matter were calculated basing on δ13C using the end member approach. Large proportion (41–73%) of the sedimentary organic carbon originates on land. The obtained results indicate the Baltic Sea sediments as an important sink for organic carbon. Substantial fraction of the sedimentary load originates on land.
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Carbon fibers
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δ15N
Paleoclimatology
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