Sources Of Sedimentary Organic Carbon Research Articles

Changing sources and burial of organic carbon in the Chukchi Sea sediments with retreating sea ice over recent centuries

Abstract. Decreasing sea ice extent caused by climate change is affecting the carbon cycle of the Arctic Ocean. In this study, surface sediments across the western Arctic Ocean are investigated to characterize sources of sedimentary organic carbon (OC). Bulk organic parameters (total organic carbon, total nitrogen, δ13Corg, and δ15N) and molecular organic biomarkers (e.g., sterols and highly branched isoprenoids – HBIs) are combined to distinguish between sympagic, pelagic, and terrestrial OC sources. Their downcore profiles generated at the Chukchi Sea R1 core site (74∘ N) are then used to evaluate changes in the relative contribution of these components of sedimentary OC over the last 200 years with decreasing sea ice. Our data evidence that, from the 1820s to the 1930s, prevailing high sea ice cover inhibited in situ primary production, resulting in prominent land-derived material in sediments. Then, from the 1930s to the 1980s, primary production started increasing with the gradual decline of summer sea ice. The ratio of sympagic and pelagic OC began to rise to account for the larger portion of sedimentary OC. Since the 1980s, accelerated sea ice loss led to enhanced primary production, stabilizing over the last decades due to freshwater-induced surface ocean stratification in summer.

Compositions and sources of sedimentary organic carbon on the tropical epicontinental sea

The deposition of fluvially derived terrestrial organic carbon (OC) together with marine OC in marine sediments has important implications for the global carbon cycle. However, dispersal extent and fates of terrestrial soil and petrogenic OC on the continental shelf have remained unclear and debated. Here, we present OC compositions and mineral properties of marine sediments from the Gulf of Thailand and the adjacent Mekong shelf, as well as river sediments from continents surrounding them in order to determine the provenance of sedimentary OC in this tropical epicontinental sea. We find that stable carbon isotope composition (δ13C) and radiocarbon activity (Δ14C) of sedimentary OC fall between those of marine OC and of river sediments, mainly composed of C3 plant-dominated pre-aged soil and petrogenic OC. Mixing model reveals that the OC in river sediments is predominantly sourced from pre-aged deep soils, accounting for 73 ± 5%, with the remaining consisting of modern surface soil (25 ± 6%) and bedrock (2 ± 1%). The sedimentary OC on the shelf is primarily derived from marine OC, accounting for 65 ± 15%, with terrestrial soil OC and petrogenic OC contributing 24 ± 14% and 10 ± 5%, respectively. The sources, degradation, and aging of OC and mineral surface area determine the spatial pattern of sedimentary OC compositions in the Gulf of Thailand and the Mekong shelf. The contents of marine, soil, and petrogenic OC decrease with increasing distance offshore, consistent with changes in spatial patterns of sedimentation rate of terrestrial materials and of marine primary production in the overlying water column. Extensive degradation of terrestrial OC and extremely low OC burial rate in this tropical epicontinental sea suggest a minor role on OC sequestration in marine sediments, but an important CO2 source to the atmosphere in the context of the global carbon cycle.

Quantification of the sources of sedimentary organic carbon at methane seeps: A case study from the South China Sea

Large amounts of methane are stored along continental margins in the form of methane hydrate. Methane hydrate is sensitive to environmental change, resulting in the release of substantial quantities of methane upon its destabilization. In the marine subsurface, microorganisms consume most of the methane in the sulfate-methane transition zone and convert a certain amount of methane into organic matter. Burial of such organic matter deriving from methane oxidation represents a long-term carbon sink, therefore mitigating the effect of a prominent greenhouse gas on global warming. However, the controls on the formation and consumption of sedimentary organic matter at marine seeps remain poorly constrained, impeding accurate quantification of carbon burial at seeps and its role in the marine carbon cycle. To gain new insight on the effect of seeps on carbon burial, sediments from two seep sites of the South China Sea (Site F, Haiyang 4) and a nearby reference site (Jiulong canyon) were analyzed for total organic carbon contents (TOC), δ13CTOC and Δ14C values, total inorganic carbon contents (TIC) and δ13CTIC values, as well as organic nitrogen (N) contents. Depth distributions reveal that the TOC at seeps (Site F, 0.55% ± 0.08%, Haiyang 4, 0.67% ± 0.11%) is higher than in sediments not affected by seepage (Jiulong canyon, 0.50% ± 0.10%). The enrichment of total sulfur, high TIC, and negative δ13CTIC values in sediments from Site F and Haiyang 4 agrees with locally prominent sulfate-driven anaerobic oxidation of methane (AOM) and resultant precipitation of authigenic carbonate. At the two seep sites, the sediments are characterized by more negative δ13CTOC and Δ14C values and a lower TOC/N ratio, indicating a contribution of the microbial fixation of 13C-depleted methane to the local pool of sedimentary organic matter. A linear increase in 14C ages of organic carbon with sediment depth at the reference site indicates steady depositional conditions, whereas the 14C age of organic carbon at seeps is typified by contributions of methanotrophy-derived carbon. A Δ14C mass balance approach shows that carbon derived from fossil methane accounts for at least 10 to 20% of the organic carbon preserved in the sediments. By comparing seeps, hydrate-bearing sediments, modern coastal sediments, and Archaean rocks, it becomes apparent that the carbon stable isotope composition of organic carbon in seep sediments is mainly controlled by the rate of methane oxidation and marine primary productivity.

Sources of sedimentary organic carbon in coastal ecosystems from the Tien Yen Bay, Quang Ninh

Sources of sedimentary organic carbon in coastal ecosystems from the Tien Yen Bay, Quang Ninh

A 120-year record of burial fluxes and source apportionment of sedimentary organic carbon in Alian Bay, China: Implication for the influence of mariculture activities, and regional environment changes

Sediment cores collected from the mariculture area in Ailian Bay, China, and adjacent area were analyzed using a variety of bulk organic chemical and chronological techniques, including grain size composition, elemental composition (C and N), C/N ratio, stable isotopic composition (δ13C and δ15N), and 210Pb activities. We aimed to investigate the response of burial and sources of sedimentary organic carbon (SOC) to the mariculture activities and regional environmental changes on a centennial scale. Sediment chronology results showed that the sediment cores in the abalone culture area and control area covered a period of 45 years (1971–2016), and 120 years (1896–2016). The sediment accumulation rate in the abalone culture area and control area were 0.95 cm·a−1 and 0.30 cm·a−1, respectively. And the TOC and TN burial fluxes in the abalone culture area (35.41 ± 0.25 g·m−2·a−1 and 4.58 ± 1.70 g·m−2·a−1, respectively) were about 5 and 7 times higher than those in the control area (7.03 ± 1.39 g·m−2·a−1 and 0.72 ± 0.17 g·m−2·a−1, respectively), respectively. The source contributions and increasing trend of marine-sourced OC were significantly higher in the abalone culture area (range: 47–126%; average: 89 ± 20%) than in the control area (range: 35–73%; average: 59 ± 10%). In conclusion, our results indicated the burial process and sources of SOC in the mariculture areas and its adjacent areas were significantly enhanced by the mariculture activities and were influenced by the environmental changes.

Stable carbon isotopes of sedimentary kerogens and carbonaceous macrofossils from the Ediacaran Miaohe Member in South China: Implications for stratigraphic correlation and sources of sedimentary organic carbon

Ediacaran stratigraphic correlation depends on the integration of paleontological and δ13Ccarb data. However, these data often come from different lithologies (e.g., shales vs. carbonates), making it difficult to directly link biostratigraphic and chemostratigraphic markers. Ediacaran successions in South China consist of mixed carbonates and shales, offering an opportunity for integrated biostratigraphic and chemostratigraphic investigations. Black shales of the Ediacaran Miaohe Member in the Yangtze Gorges area contain macroalgae and Ediacara-type macrofossils, both preserved as carbonaceous compressions. The Miaohe Member has been traditionally regarded as an equivalent to Member IV of the uppermost Doushantuo Formation that records part of the Shuram negative δ13Ccarb excursion in South China. Thus, the Miaohe Member may have the potential to clarify the stratigraphic relationship between Ediacara-type fossils and the Shuram excursion. Recently, however, the Miaohe Member has been partially correlated with the Shibantan Member of the Dengying Formation (the “A” correlation) or partially with Member IV (the “Z” correlation). These correlations have different implications for the relationship between the Miaohe Member and the Shuram excursion. Here we test these two correlations using δ13C data of sedimentary kerogen or total organic carbon (δ13CorgTOC), taking advantage of the distinct δ13CorgTOC values between the Shibantan Member and Member IV. Our data show that δ13CorgTOC of the Miaohe Member is more similar to that of Member IV than to Shibantan Member, providing tentative support for the “Z” correlation. In addition, we also analyzed δ13Corg of individual carbonaceous compression fossils (δ13Corgtaxon), which have lower carbon isotope values than TOC. The data suggest that oxygenic photosynthesizers (e.g., macroalgae, cyanobacteria, and eukaryotic phytoplankton) were not the only contributors to sedimentary TOC. Methylotrophs, photoautotrophs, and chemoautotrophs living in anoxic waters or at the oxycline may have fixed recycled CO2 derived from organic carbon remineralization, thus contributing a significant amount of 13C-depleted organic carbon to sedimentary kerogen in the Miaohe Member.

Tracing the source of sedimentary organic carbon in the Loess Plateau of China: An integrated elemental ratio, stable carbon signatures, and radioactive isotopes approach

Soil erosion, which will induce the redistribution of soil and associated soil organic carbon (SOC) on the Earth's surface, is of critically importance for biogeochemical cycling of essential elements and terrestrial carbon sequestration. Despite the importance of soil erosion, surprisingly few studies have evaluated the sources of eroded carbon (C). This study used natural abundance levels of the stable isotope signature (13C) and radioactive isotopes (137Cs and 210Pbex), along with elements ratio (C/N) based on a two end member mixing model to qualitatively and quantitatively identify the sources of sedimentary OC retained by check dam in the Qiaozigou small watershed in the Loess Plateau, China. Sediment profiles (0–200 cm) captured at natural depositional area of the basin was compared to possible source materials, which included: superficial Loess mineral soils (0–20 cm) from three land use types [i.e., grassland (Medicago sativa), forestland (Robinia pseudoacacia.), shrubland (Prunus sibirica), and gully land (Loess parent material.)]. The results demonstrated that SOC in sediments showed significantly negative correlation with pH (P < 0.01), and positive correlation with soil water content (SWC) (P < 0.05). The sedimentary OC was not derived from grasslands or gullies. Forestland and shrubland were two main sources of eroded organic carbon within the surface sediment (0–60 cm deep), except for that in the 20–40 cm soil layer. Radionuclides analyses also implied that the surface sediments retained by check-dams mainly originated from soils of forestland and shrubland. Results of the two end-member mixing model demonstrated that more than 50% SOC (mean probability estimate (MPE) 50.13% via 13C and 60.53% via C/N) in surface sediment (0–20 cm deep) derived from forestland, whereas subsurface sedimentary SOC (20–200 cm) mainly resulted from shrubland (MPE > 50%). Although uncertainties on the sources of SOC in deep soils exist, the soil organic δ13C and C/N is still an effective indicator for sources of sedimentary organic carbon in the deposition zone in the short term (<10 years).

Seasonal variations and sources of sedimentary organic carbon in Tokyo Bay

Total organic carbon (TOC), total nitrogen (TN) contents, their stable C and N isotope ratio (δ13C and δ15N), and chlorophyll a ([Chl a]sed) of surface sediments were investigated monthly to identify the seasonal variations and sources of organic matter in Tokyo Bay. The sedimentary TOC (TOCsed) and TN (TNsed) contents, and the sedimentary δ13C and δ15N (δ13Csed and δ15Nsed) values were higher in summer than other seasons. The seasonal variations were controlled by high primary production in the water column and hypoxic water in the bottom water during summer. The fraction of terrestrial and marine derived organic matter was estimated by Bayesian mixing model using stable isotope data and TOC/TN ratio. Surface sediments in Tokyo Bay are dominated by marine derived organic matter, which accounts for about 69±5% of TOCsed.

Organic carbon burial in fjords: Terrestrial versus marine inputs

Fjords have been identified as sites of enhanced organic carbon (OC) burial and may play an important role in regulating climate change on glacial–interglacial timescales. Understanding sediment processes and sources of sedimentary OC are necessary to better constrain OC burial in fjords. In this study, we use Fiordland, New Zealand, as a case study and present data on surface sediments, sediment down-cores and terrestrial end-members to examine dynamics of sediments and the sources of OC in fjord sediments. Sediment cores showed evidence of multiple particle sources, frequent bioturbation and mass-wasting events. A multi-proxy approach (stable isotopes, lignin-phenols and fatty acids) allowed for separation of marine, soil and vascular plant OC in surface sediments. The relationship between mass accumulation rate (MAR) and OC contents in fjord surface sediments suggested that mineral dilution is important in controlling OC content on a global scale, but is less important for specific regions (e.g., New Zealand). The inconsistency of OC budgets calculated by using MAR weighted %OC and OC accumulation rates (AR; 6 vs 21–31 Tg OC yr−1) suggested that sediment flux in fjords was likely underestimated. By using end-member models, we propose that 55% to 62% of total OC buried in fjords is terrestrially derived, and accounts for 17±12% of the OCterr buried in all marine sediments. The strong correlation between MAR and OC AR indicated that OC flux will likely decrease in fjords in the future with global warming due to decrease in sediment flux caused by glacier denudation.

Organic matter — A key factor in controlling mercury distribution in estuarine sediment

Organic matter (OM) was found to play an important role in controlling mercury (Hg) distribution and speciation in estuarine sediment of the Vembanad Lake. The sedimentary organic carbon (OC) from the northern part of the lake was influenced mainly by marine in situ biological production. However, the sedimentary OC was mainly of terrestrial origin in the southern part of the lake. The average concentrations of Hg in the sediments were relatively higher in the northern part (ranging from ~6.5 to 172.0μg/kg, with an average concentration of 59.6μg/kg) than the southern part of the lake (ranging from ~6.8 to 114.0μg/kg with an average concentration of 49.7μg/kg). This study suggests that nature and source of sedimentary OC probably control Hg speciation in the Vembanad Lake system. Speciation study of Hg suggests that a major part (55–90%) of the total Hg was mainly associated with organic phases in the sediment. The variation in concentrations of organic matter bound Hg with the increasing Hg/TOC ratio suggests that more Hg binding sites were available in the sediment from the northern part of the lake but not in the southern part of the lake. Anthropogenic activities were responsible for the higher Hg content in the sediment collected from the northern part of the lake. It is suggested that the strong affinity of sedimentary total nitrogen (TN) for Hg is independent of its source. The sedimentary OM was found to control Hg distribution in different size fraction of the estuarine sediment.

Bulk organic geochemistry of sediments from Puyehue Lake and its watershed (Chile, 40°S): Implications for paleoenvironmental reconstructions

Since the last deglaciation, the mid-latitudes of the southern Hemisphere have undergone considerable environmental changes. In order to better understand the response of continental ecosystems to paleoclimate changes in southern South America, we investigated the sedimentary record of Puyehue Lake, located in the western piedmont of the Andes in South–Central Chile (40°S). We analyzed the elemental (C, N) and stable isotopic (δ 13C, δ 15N) composition of the sedimentary organic matter preserved in the lake and its watershed to estimate the relative changes in the sources of sedimentary organic carbon through space and time. The geochemical signature of the aquatic and terrestrial end-members was determined on samples of lake particulate organic matter (N/C: 0.130) and Holocene paleosols (N/C: 0.069), respectively. A simple mixing equation based on the N/C ratio of these end-members was then used to estimate the fraction of terrestrial carbon (ƒ T) preserved in the lake sediments. Our approach was validated using surface sediment samples, which show a strong relation between ƒ T and distance to the main rivers and to the shore. We further applied this equation to an 11.22 m long sediment core to reconstruct paleoenvironmental changes in Puyehue Lake and its watershed during the last 17.9 kyr. Our data provide evidence for a first warming pulse at 17.3 cal kyr BP, which triggered a rapid increase in lake diatom productivity, lagging the start of a similar increase in sea surface temperature (SST) off Chile by 1500 years. This delay is best explained by the presence of a large glacier in the lake watershed, which delayed the response time of the terrestrial proxies and limited the concomitant expansion of the vegetation in the lake watershed (low ƒ T). A second warming pulse at 12.8 cal kyr BP is inferred from an increase in lake productivity and a major expansion of the vegetation in the lake watershed, demonstrating that the Puyehue glacier had considerably retreated from the watershed. This second warming pulse is synchronous with a 2 °C increase in SST off the coast of Chile, and its timing corresponds to the beginning of the Younger Dryas Chronozone. These results contribute to the mounting evidence that the climate in the mid-latitudes of the southern Hemisphere was warming during the Younger Dryas Chronozone, in agreement with the bipolar see-saw hypothesis.

Multi-biomarker characterization of sedimentary organic carbon in small rivers draining the Northwestern United States

The sources of sedimentary organic carbon in various river systems draining the Pacific Northwest (USA) were assessed using a biomarker approach. Sediment samples were collected in creeks and small rivers from Oregon and Washington states (Cascades and Coastal Range ecoregions) and analyzed as total and silylated total extracts using gas chromatography–mass spectrometry (GC–MS). In general, the individual organic compounds were similar in the river samples from both the Cascades and the Coast Range, with apparent higher averaged abundances (normalized to organic carbon content) observed for the Cascades sediments. Major contributions in the river samples were from sterols and triterpenoids derived mainly from the vegetation bordering the rivers, followed by n-alkanols and n-alkanes from epicuticular plant wax. Fatty acids (16:0, 16:1, 18:0, 18:1, 18:2), cholesterol, monoacylglycerols (e.g., 1- O-hexadecanoyl glycerol) and glucose were important organic tracers detected especially in higher altitude systems, presumably due to autochthonous production, namely from algal blooms during late spring. Resin acids (e.g., dehydroabietic acid, a biomarker for conifers) were expected to be the major tracer input to these sediments. However, diterpenoids were detected in significant amounts only at sites directly bordering conifer forests, suggesting that those compounds are not transported downstream during normal river flow.

The origin of organic matter in Holocene sediments in the Bay of Koper (Gulf of Trieste, northern Adriatic Sea)

Three cores, V-3, V-5 and MK-6, drilled in the inner part of the Bay of Koper were used to reconstruct the paleoenvironmental conditions occurring during the Holocene. Based on stable isotope results two depositing environments can be distinguished in the cores: the upper, marine and lower, brackish sedimentation. Marine sedimentation prevailed over fluvial sedimentation at depths of 15 m, 19 m and 10 m in the V-3, MK-6 and V-5 cores respectively. The marine part of the core V-3 was influenced by varying amounts of land-derived organic carbon transported by the River Rižana, while in MK-6 and V-5 cores the marine algae and/or microphytes constituted the main source of sedimentary organic carbon. The fluvial sedimentation has typically lower δ13Corg and higher C/N ratios, ~ -26 ‰ and > 12 respectively and based on stable carbon isotope mass balance the terrestrial organic carbon present up to 70 to 100 % of the sedimentary organic carbon. On the other hand, the higher δ15N values observed at some depths in the fluvial sedimentation indicated that nitrogen could be of marine origin. The observed data in parallel with previous studies are in good agreement with the simultaneous rise of the sea level in the Northern Adriatic.

Sources of organic matter in seagrass-colonized sediments: A stable isotope study of the silt and clay fraction from Posidonia oceanica meadows in the western Mediterranean

The origin of sedimentary organic matter in 22 sandy beds of Posidonia oceanica (L.) Delile on the coast of the Iberian Peninsula and the Balearic Islands in the northwestern Mediterranean Sea was investigated using natural abundance stable isotope measurements of carbon and nitrogen (δ 13C and δ 15N) in fine (<63 μm grain size) surface sediments and in the local primary sources of detrital organic matter, seston, above- and below-ground seagrass tissues and bulk epiphytes. The δ 13C measurements provided the greatest power for resolving primary sources of the fine sedimentary organic matter in these shallow coastal ecosystems. Their use in an end member, isotopic mass balance approach showed that sestonic particles were as important as seagrass-derived detrital material as a source of sedimentary organic carbon in the P. oceanica meadows. The δ 15N measurements for the sediments exhibited patterns of isotopic depletion on a regional scale relative to the local primary sources of organic matter, which could be associated with occurrence of microbial fixation of molecular nitrogen (N 2), indicating potential contribution of this process to the pool of sedimentary nitrogen in the P. oceanica meadows.

Terrestrial plant biopolymers in marine sediments

The vascular land plant biopolymers lignin and cutin were surveyed in the surface sediments of coastal and open ocean waters by controlled alkaline CuO oxidation/reaction. Two contrasting oceanic regimes were studied: the northwest Mediterranean (NWM) Sea, which receives significant particulate terrigenous debris through riverine discharge; and the northeast Atlantic (NEA) Ocean, with poorly characterised terrestrial carbon inputs. In the NWM products of lignin and cutin co-occurred at all stations, elevated levels (ca. 0.5–3.0 mg lignin phenols/100 mg organic carbon; ca. 0.01–0.09 mg cutin acids/100 mg organic carbon) were observed for near-shore deltaic and shelf sediments. The influence of terrestrial land plant inputs extended across the shelf and through the slope to the abyssal plain, providing molecular evidence for advective offshore transfer of terrestrial carbon. Mass balance estimates for the basin suggest riverine inputs account for the majority of surface sedimentary lignin/cutin, most of which (>90%) is deposited on the shelf. Products of CuO oxidation of lignin and cutin were also detected in NEA surface sediments, at levels comparable to those observed for the NWM continental slope, and were detectable at low concentrations ( ca. 0.5 μg g −1 in the sediments of the abyssal plains (>4,000 m depth). While atmospheric deposition of lignin/cutin-derived material cannot be discounted in this open ocean system, lateral advective transfer of enriched shelf sediments is inferred as a possible transport process. A progressive enrichment in cutin-derived material relative to lignin was observed offshore, with evidence of an increase in the degree of oxidative alteration of lignin residues. To account for these observations, preferential offshore transport of finer and more degraded material is proposed. Nonspecific oxidation products dominated the gas chromatograms of NEA sediments, which appear to originate from marine sources of sedimentary organic carbon. Preliminary mass balance calculations applied to the global ocean margin suggest riverine sources of both particulate lignin and cutin are important and that most (>95%) deposition of recognisable land plant biopolymers occurs in shelf seas.