Matter In Surface Sediments Research Articles

Distribution and Sources of Organic Matter in Surface Sediments of the Karachi Coast, Pakistan

Distribution and Sources of Organic Matter in Surface Sediments of the Karachi Coast, Pakistan

Biogeochemical controls on methylmercury distribution in a subtropical wetland ecosystem

Wetlands are widely regarded as biogeochemical hotspots of mercury methylation but little is known regarding such roles of mangrove forests. Here, we examined the detailed depth profile of mercury, methylmercury, and organic matter in surface sediments within an estuarine pond at Mai Po Nature Reserve in Hong Kong, China. There is a progressive enrichment of organic matter in sites closer to mangrove forests, and methylmercury showed a significantly positive correlation with organic matter content (p < 0.001). Methylmercury in sediments is significantly (p < 0.05) higher in the summertime when the temperature is elevated but salinity is reduced. Further, sediments at or near the mangrove forest have lower carbon to nitrogen ratio, which may imply more labile organic matter in these organic-rich sediments that can promote microbial mercury methylation. In summary, mangrove forests can enhance net methylmercury production and increase the risk to the migratory birds overwintering in this internationally important wetland.

Post-drainage vegetation, microtopography and organic matter in Arctic drained lake basins

Wetlands in Arctic drained lake basins (DLBs) have a high potential for carbon storage in vegetation and peat as well as for elevated greenhouse gas emissions. However, the evolution of vegetation and organic matter is rarely studied in DLBs, making these abundant wetlands especially uncertain elements of the permafrost carbon budget. We surveyed multiple DLB generations in northern Alaska with the goal to assess vegetation, microtopography, and organic matter in surface sediment and pond water in DLBs and to provide the first high-resolution land cover classification for a DLB system focussing on moisture-related vegetation classes for the Teshekpuk Lake region. We associated sediment properties and methane concentrations along a post-drainage succession gradient with remote sensing-derived land cover classes. Our study distinguished five eco-hydrological classes using statistical clustering of vegetation data, which corresponded to the land cover classes. We identified surface wetness and time since drainage as predictors of vegetation composition. Microtopographic complexity increased after drainage. Organic carbon and nitrogen contents in sediment, and dissolved organic carbon (DOC) and dissolved nitrogen (DN) in ponds were high throughout, indicating high organic matter availability and decomposition. We confirmed wetness as a predictor of sediment methane concentrations. Our findings suggest moderate to high methane concentrations independent of drainage age, with particularly high concentrations beneath submerged patches (up to 200 μmol l−1) and in pond water (up to 22 μmol l−1). In our DLB system, wet and shallow submerged patches with high methane concentrations occupied 54% of the area, and ponds with high DOC, DN and methane occupied another 11%. In conclusion, we demonstrate that DLB wetlands are highly productive regarding organic matter decomposition and methane production. Machine learning-aided land cover classification using high-resolution multispectral satellite imagery provides a useful tool for future upscaling of sediment properties and methane emission potentials from Arctic DLBs.

Variability of biochemical compounds in surface sediments along the eastern margin of the Arabian Sea

Different fractions of organic matter in surface sediments from three transects along the eastern margin of the Arabian Sea (AS) were quantified to determine the sources of organic matter, and also to study its impact on microbial community structure. From the extensive analyses of different biochemical parameters, it was evident that the distribution of total carbohydrate (TCHO), total neutral carbohydrate (TNCHO), proteins, lipids, and uronic acids (URA) concentrations and yield (% TCHO-C/TOC) are affected by organic matter (OM) sources and microbial degradation of sedimentary OM. Monosaccharide compositions from surface sediment was quantified to assess the sources and diagenetic fate of carbohydrates, suggesting that the deoxysugars (rhamnose plus fucose) had significant inverse relationship (r = 0.928, n = 13, p < 0.001) with hexoses (mannose plus galactose plus glucose) and positive relationship (r = 0.828, n = 13, p < 0.001) with pentoses (ribose plus arabinose plus xylose). This shows that marine microorganisms are the source of carbohydrates and there is no influence of terrestrial OM along the eastern margin of AS. During the degradation of algal material, the hexoses seem to be preferentially used by heterotrophic organisms in this region. Arabinose plus galactose (glucose free wt %) values between 28 and 64wt% indicate that OM was derived from phytoplankton, zooplankton, and non-woody tissues. In the principal component analysis, rhamnose, fucose, and ribose form one cluster of positive loadings while glucose, galactose, and mannose form another cluster of negative loadings which suggest that during OM sinking process, hexoses were removed resulting in increase in bacterial biomass and microbial sugars. Results indicate sediment OM to be derived from marine microbial source along the eastern margin of AS.

The isotopic composition of organic carbon, nitrogen and provenance of organic matter in surface sediment from the Jiangsu tidal flat, southwestern Yellow Sea

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.

Distribution Characteristics, Source Analysis, and Pollution Evaluation of Organic Matter in Surface Sediments of Qingpu District, Yangtze River Delta Integration Demonstration Area

The surface sediments were collected from 25 sampling points in the Qingpu District of the Yangtze River Delta integration demonstration area, and the total organic carbon (TOC), total nitrogen (TN), organic carbon isotope (δ13C), and nitrogen isotope (δ15N) content in the samples were analyzed. The distribution characteristics and potential sources of organic matter in the surface sediments were discussed in detail, and the organic pollution index was also evaluated. The results showed that the ω(TOC) in the surface sediments of 31 sampling points in Qingpu District was 0.21%-3.55%, with an average value of 1.18%; ω(TN) ranged from 0.02% to 0.23%, with an average value of 0.09%. The δ13C ranged from -28.04‰ to -10.80 ‰, with an average of -22.28‰, and the δ15N ranged from 2.28‰ to 11.19‰, with an average of 5.76‰. The TOC content was significantly correlated with the TN content. Source analysis showed that the organic matter in the surface sediments of the study area was mainly affected by soil organic matter, sewage organic matter, and terrestrial plants. The calculation results of the contribution rate based on IsoSource software showed that the relative contribution rate of soil organic matter was relatively high (0.3%-96.8%), and domestic sewage had a certain contribution to the organic matter source of most sampling points. In addition, the selected end-member substances had certain differences in the contribution of organic matter sources in samples of different land use types. The evaluation index of organic pollution in the surface sediments of the study area ranged from 0.006-0.745, with an average value of 0.163, indicating that the organic pollution of the study area was considered lightly polluted. Among them, the pollution of sampling points around the Jinze Reservoir was more serious than that in other areas, indicating that there was a potential risk of eutrophication in this area, which requires certain attention.

Isotopic constraints on sources of organic matter in surface sediments from two north–south oriented lakes of the Yunnan Plateau, Southwest China

Isotopic constraints on sources of organic matter in surface sediments from two north–south oriented lakes of the Yunnan Plateau, Southwest China

Restoration objectives create surface carbon cycle trade‐offs in coastal habitats

Coastal habitats are important and imperiled. To mitigate climate change impacts, carbon (C) sequestration has joined established restoration objectives, including habitat diversity, public access, and native vegetation. Changing topography, maintaining trails, and removing invasive plants may influence coastal C production and retention in different ways depending on how each activity influences the stabilization of organic matter in surface sediments. We quantified variation in litter and surface soil dynamics within and between coastal parks that varied in age from prerestoration to nearly 20 years old in Manatee County, Florida. Older parks had deeper soil organic horizons with higher levels of soil moisture and respiration, indicating that restoration sequesters soil C despite increases in microbial efficiency. However, variation within parks indicated that detrital C dynamics change depending on local management objectives. Relative to unmanaged locations, restored wetlands, including mangroves, salt marshes, and freshwater ponds, had soil organic horizons nearly 20 cm deeper, with high litter stabilization rates despite less litter mass and shorter projected soil sample C mineralization. In contrast, public access features like trails and kayak launches were associated with increased surface CO2 efflux and marginally reduced soil organic horizon depth. Removing invasives had little impact on detrital carbon pools and fluxes. Incorporating detrital C dynamics improved a multivariate model for variation in C pool sizes across sites. Overall, our results suggest that management objectives influence the C production and retention capacity of restored coastal habitats with restored uplands and public access points storing less C than relatively inaccessible wetlands.

Inter-specific and inter-individual trait variability matter in surface sediment reworking rates of intertidal benthic foraminifera

Although benthic foraminifera are an important component of meiofauna and contribute to carbonate production and carbon/nitrogen cycles, their role in bioturbation processes remains poorly known. Five dominant intertidal benthic foraminiferal species were recently classified into functional bioturbator groups according to their sediment reworking mode and intensity. Our study aimed at identifying potential drivers (i.e. size and/or travelled distance) of species-specific surface sediment reworking rate. The travelled distance and surface sediment reworking rate of Haynesina germanica, Cribroelphidium williamsoni, Ammonia tepida, Quinqueloculina seminulum and Miliammina fusca were assessed through image analysis. Our results show that the surface sediment reworking performed by these species is not size-dependent, but dependent on their motility traits through interspecific differences in the travelled distance. Smaller species (i.e. Quinqueloculina seminulum and Haynesina germanica) contributed more to surface sediment reworking than larger ones (i.e. Ammonia tepida, Cribroelphidium williamsoni and Miliammina fusca). These observations stress the critical role of motion behaviour in surface sediment reworking processes by intertidal foraminifera. Finally, we stress that the high inter-individual variability observed in conspecific motion behaviour may be important to decipher the role of foraminifera in sediment bioturbation. Noticeably, the species characterized by a strong inter-individual variability are also the species that have the highest surface sediment reworking rates. This last observation may inform on the species-specific phenotypic plasticity and, therefore, the potential for the functional role of these species to be maintained in their natural environment. This is particularly relevant in an era of global change where ecosystem balance is increasingly threatened by various stressors such as heat-waves, ocean acidification and pollution.

Ecological indication based on source, content, and structure characteristics of dissolved organic matter in surface sediment from Dagu River estuary, China.

Dissolved organic matter (DOM) controls the fate of a variety of nutrients and trace metals in river estuary systems. The aim of our study is to explore the sources, structure characteristics of sediment DOM, and potential ecological release risk (heavy metal, N and P) under the environmental conditions of Dagu River estuary. The relative contribution of DOM source was calculated by carbon stable isotope (combining δ13CDOM and C/N ratios). The structural characteristics of sediment DOM in Dagu River estuary were determined by ultraviolet-visible spectroscopy (UV-vis) absorbance and fluorescence spectroscopy. The potential ecological risks of heavy metals and N and P release were also assessed. Results show that the relative contributions of rural area and aquaculture are high in Dagu River estuary. The humification degree of DOM in downstream river is higher than that in the estuary, and the sediment DOM in Dagu River estuary is influenced by both terrestrial input and biological metabolism. The potential risk of eutrophication in the estuary is high. Moreover, under the effect of aquaculture and rural area, there is higher potential ecological release risk of heavy metal in the estuary. And samples in the middle transect have the highest potential ecological risks of heavy metal. Therefore, a framework has been proposed to predict the ecological status of the estuary by analyzing the sources, content, and structural characteristics of sediment DOM. These results provide a new insight on ecological indication of DOM in Dagu River estuary.

Spatial and temporal distributions of terrestrial and marine organic matter in the surface sediments of the Yangtze River estuary

Riverine supply of largely terrestrial carbon is important to the carbon cycle chemistry of the oceans. Along a river-ocean continuum, sedimentary organic matter composition and distribution can provide insights to the balance of terrestrial input and in situ production of an estuary. In the Yangtze River estuary (YRE), this balance is especially important to understand as human impacts and climate change continue to modify processes along the continuum. Here we discuss the sources and spatio-temporal distributions of organic matter in the coastal ocean surface sediments and how environmental factors have affected changes there. The isotopic compositions (δ13C and δ15N), total organic carbon (TOC), and total nitrogen (TN) of surface sediments were investigated along with sediment grain size and bottom water temperature, salinity, and total suspended matter. Evidence suggests that fine sedimentary grain size, bottom water temperature, and the circulation dynamics of water masses such as Changjiang Diluted Water (CDW) and Taiwan Warm Current (TWC) were the main factors affecting spatial and temporal distributions, with organic matter concentrations in autumn showing higher than that in spring and nearshore areas higher than offshore. An isotopic-based two end-member mixing model suggests that terrigenous inputs were the main source (about 53%) of organic matter in the YRE, followed by autochthonous marine organic matter, in both seasons. Terrestrial organic matter distribution, controlled by the CDW, decreased seaward from the mouth, as expected, and the proportion of terrestrial organic carbon increased in autumn compared to spring. Intrusions of the TWC water likely contributed to the increase of marine components in a tongue-shaped area during both seasons.

Sources and preservation dynamics of organic matter in surface sediments of Narmada River, India – Illustrated by amino acids

Abstract The preservation process of organic matter (OM) in estuarine environments determines the recycling and sinking of nutrients. This process requires the identification of sources, degradation states and the main processes affecting OM transformations. Unfortunately, our understandingof the sources, degradation and factors affecting OM distribution in tropical rivers experiencing strong seasonality and monsoonal influence is still limited. This study examined the sources, degradation and factors affecting OM distribution along the Narmada River and its estuary during different seasons. Surface waters and sediments were analyzed seasonally for selected physico-chemical parameters and bulk compositions of sediments, together with amino acids (AA, including the bacterial biomarker, D-AA). The sources of OM were soils containing detrital terrestrial plant material, with C4 and C3 plants dominating the estuarine and riverine stations, respectively. The other sources of OM were in-situ production, together with bacteria and their remnants. Strong seasonality and monsoonal conditions control the sources and distribution of OM in the river. Higher concentrations of total hydrolysable amino acids (THAA) were observed in riverine stations, suggesting the presence of relatively fresher OM. The lower OC:SA ratios recorded in the estuarine sediments indicated a limited OM preservation in the studied river. Positive degradation index (DI) values were obtained during the pre-monsoon season, suggesting seasonal changes in OM diagenesis. Physical (strong tidal currents, rainfall, reduced water flow due to seasonal variations and shallow water depth within the estuary) and geochemical (mineral surface adsorption processes) factors control the distribution and transport of OM. Taken together, the sources, preservation and diagenesis of terrestrial OM along the Narmada River was controlled differentially by the strong seasonal variability of the region. Thus, under variable temporal conditions, tropical estuaries and rivers form important realms for examining, determining, evaluating and assessing OM in order to better interpret nutrient budgets of the seas and oceans.

Cross shelf transport of terrigenous organic matter in surface sediments from outer shelf to Okinawa Trough in East China Sea

Identification of sources and distribution of terrigenous organic matter (TOM) are significant to understand the carbon transport in East China Sea (ECS). However, sources of TOM buried in the ECS are diverse and complex, which are likely derived from the Changjiang River, Yellow River and/or small rivers from China mainland and Taiwan. In order to evaluate the fate of TOM offshore in the ECS, the study sampled 80 surface sediments from outer shelf-slope-Okinawa Trough (OT) region in the ECS. Both terrigenous and marine organic biomarkers were detected. The concentration (ng/g dry weight sediment) of terrigenous biomarkers including ƩC27 + C29 + C31 n-alkanes ranged from 12.2 to 990.9; while the content of marine biomarkers varied from 1.3 to 3891.2 for C37 alkenones, 2.6 to 922.6 for brassicasterol and 7.1 to 1535.8 for dinosterol. Bulk organic carbon and TOM distributions were controlled by the hydrodynamic conditions. The terrigenous and marine biomarker ratio (TMBR) (ƩC27 + C29 + C31 n-alkanes)/(ƩC27 + C29 + C31 n-alkanes + (brassicasterol + dinosterol +alkenones)) increased first and decreased from the outer shelf to OT spatially, showing the distribution pattern of relative proportion of TOM. Hotspots of significant deposition of TOM were observed in the ECS by δ13C and TMBR proxies that the mechanism was unclear yet. Binary model revealed higher percentage of TOM based on TMBR than δ13C for both slope (32.1% vs. 20.0%) and trough (23.1% vs. 15.2%) region. An underestimation of C4 vegetation contribution likely biased the bulk carbon model. Carbon preference index suggested a zonation distribution of TOM that outer shelf had different sources from the slope and OT attributed to cross shelf transport of terrigenous organic matter.

Organic Carbon in the Surface Sediments from the Intensive Mariculture Zone of Sanggou Bay: Distribution, Seasonal Variations and Sources

Mariculture has rapidly grown worldwide, which might greatly influence the biogeochemical cycle of organic carbon in coastal seas. In this study, several geochemical parameters, including grain size composition, sedimentary total organic carbon (TOC), total nitrogen (TN), stable carbon (δ13C) and nitrogen (δ15N) isotopic compositions, were analyzed for surface sediments collected from different mariculture zones of Sanggou Bay and in different seasons. We investigated the composition and distribution of organic matter in surface sediments and further evaluated the contribution of mariculture activities to TOC sources. The TOC and TN contents (mass percentage) in the bay were in the range of 0.14% to 1.45% and 0.03% to 0.20%, respectively. The spatial distribution indicated that sedimentary TOC and TN contents in shellfish monoculture and shellfish-kelp polyculture zones were higher than in other mariculture zones, which might be related to grain size composition and mariculture organisms. Seasonal variations of TOC contents were observed in different mariculture zones. The TOC/TN atomic ratio (C/N), δ13C and δ15N were in the ranges of 5.97 to 10.97, −21.76‰ to −13.14‰ and 2.13‰ to 8.08‰, respectively, implying that sedimentary organic matter in Sanggou Bay was the mixture of marine phytoplankton, terrestrial and maricultural sources. A simple mixing model based on δ13C was applied and the results indicated that the relative contributions of organic carbon sources in Sanggou Bay followed the order kelp (36.6%) > marine phytoplankton (28.7%) > shellfish bio-deposition (23.8%) > terrestrial input (10.9%). Surface sediments in Sanggou Bay were dominated by mariculture-derived organic carbon, which on average accounted for 60.4% of TOC.

Distribution and Sources of Organic Matter in Surface Sediments of the Northern Bering and Chukchi Seas by Using Bulk and Tetraether Proxies

The organic matter (OM) preserved in Arctic Ocean sediments is of great importance to the global carbon budget. However, works that apply multiple proxies to determine the distribution and concentration of organic carbon (OC) in the surface sediments of the northern Bering and Chukchi Seas remain limited. Here a multiproxy approach based on bulk OM parameters and the branched vs. isoprenoid tetraether (BIT) index was used to investigate the distribution and sources of OM in the surface sediments of the northern Bering and Chukchi Seas. Binary and ternary mixing models were applied to trace the contribution of different OC sources to the total OC in the study area. The δ13C values of the sediments provided by the binary model showed that the proportion of terrestrial OC fell in the range of 27.4%−79.8% (46.2% on average). The BIT index returned the lowest fraction (4.8%−27.3%, 12.0% on average). The ternary mixing model was employed to determine the plant-, soil-, and marine-derived fractions of the total OM. The ternary model showed that 11.5%±6.3%, 31.4%±9.5%, and 57.1%±12.4% of OM in the sediment of the study area was derived from soil, plants, and marine sources, respectively. The differences in OM composition between the west and east sides of the Chukchi Sea were controlled by OM inputs from key water masses (i.e., Anadyr Water and Alaska Coastal Water), river discharge, and the nutrient supply from the Pacific inflow that supports marine productivity.

Geochemical discrimination of bulk organic matter in surface sediments of the Cross River estuary system and adjacent shelf, South East Nigeria (West Africa)

Knowledge of the sources, distribution and fate of organic matter (OM) in estuarine and adjacent shelf sediments are important for the understanding of the global biogeochemical cycles. Bulk organic carbon (C-org), total nitrogen (TN), biogenic silica (BSi), stable carbon (δ13C-org) and nitrogen (δ15N) isotopes, and sediment grain sizes were measured to study the spatial distributions and sources of sediment OM in the Cross River estuary system (CRES) and adjacent shelf. Surface sediments in the CRES were composed of clayey silt and sandy silt, while the adjacent shelf sediments were mainly silty sand. The range of the studied parameters was −28.79‰ to −22.20‰ for δ13C-org, −1.32‰–6.31‰ for δ15N, 6.7–29.2 for C-org/N ratios, 0.08%–0.33% for TN, 0.24‰–0.74‰ for BSi, and 0.47%–5.28% for C-org, and their spatial distributions showed a general decreasing trend in both the terrestrial and estuarine OM from the riverine regions to the adjacent shelf. Based on the three-end-member mixing model using the δ13C and δ15N isotopic values, ~58.01 ± 15.32% of sediment OM are derived from terrestrial sources dominated by C3 vascular plants, while ~26.34 ± 9.71% are attributed to estuarine sources dominated by aquatic macrophytes, and ~15.65 ± 12.37% for marine plankton source. Other sources of OM identified included soils underlain C3 vascular plants and agricultural farms enriched with N, sewage, and petroleum hydrocarbons. The relationship between C-org vs. BSi, and the atomic BSi/Corg ratios suggested that diatoms also play an important role in OM sequestration in surface sediments of the CRES and adjacent shelf. The correlations of the δ13C-org and δ15N isotopic values vs. C-org/N ratios resulted in scatter plots, indicating that the distributions of sediment OM in the CRES and adjacent shelf are influenced by post depositional processes, fixed inorganic N adsorbed on fine-grained sediments, microbial degradation, as well as sediment grain size.

Discriminating sources and preservation of organic matter in surface sediments from five Antarctic lakes in the Fildes Peninsula (King George Island) by lipid biomarkers and compound-specific isotopic analysis

Lakes are important paleoenvironmental archives retaining abundant information due to their typical high sedimentation rates and susceptibility to environmental changes. Here, we scrutinize the organic matter (OM) composition, origin and preservation state in surface sediments from five lakes in a remote, warming-sensitive, and poorly explored region partially covered by the retreating Collins Glacier in King George Island (Antarctica), the Fildes Peninsula. Lipid biomarkers of terrestrial origin (i.e. high-molecular weight n-alkanes, n-alkanoic acids, and n-alkanols; β-sitosterol, campesterol, and stigmasterol) were detected in the five Fildes Lakes, with the smallest basin (i.e., Meltwater) showing a particularly strong moss imprint. Aquatic source indicators such as low C/N and terrestrial over aquatic ratios (TAR), or less negative δ13C values were preferentially found in the mid-sized lakes (i.e., Drake and Ionospheric). Sedimentary carbon in the larger lakes (i.e., Uruguay and Kitezh) displayed a largely biogenic origin (i.e., values of carbon preference index, CPI, ≫1), whereas the three lakes close to Collins Glacier (i.e., Drake, Meltwater, and Ionospheric) showed certain contribution from petrogenic sources (CPI ~ 1). The results suggest that the geochemical signature of the surface sediments in the five Fildes lakes is determined by factors such as the distance to the retreating Collins Glacier, the proximity to the coast, or the lake depth. This study illustrates the forensic interest of combining lipid biomarkers, compound-specific isotopic analysis, and bulk geochemistry to reconstruct paleoenvironments and study climate-sensitive regions.

Exploring the composition of macromolecular organic matter in Arctic Ocean sediments under a changing sea ice gradient

Arctic sea ice has been affected by climate change, leading to reductions in summer sea ice extent over the past few decades, impacting nutrient dynamics, ocean temperature fluxes and the biological communities present in the ocean. Marine organic matter is a complex mixture of differentially degraded terrestrial and marine organisms from a range of sources and time periods. In this study pyrolysis-gas chromatography-mass spectrometry (Py-GC–MS) has been used to quantify the solvent-insoluble component of marine organic matter in surface sediments to gain a more holistic understanding of the macromolecular composition at five stations along a south to north transect in the Arctic Barents Sea, east of Svalbard (depths 288–334 m.b.s.l). Two methods were compared to identify the effectiveness of rapid screening, in contrast to grouping similar pyrolysis products. There were changes in macromolecular composition of marine surface sediments across the S-N transect using both methods, highlighting the varying benthic and pelagic communities north of the Polar Front and across the variable sea ice margin, corresponding to differing biological communities (e.g. fish, phytoplankton, ice algae, zooplankton). All five stations across the changing sea ice transect were interpreted as having primarily marine surface sediment macromolecular signatures, given their locations far from major terrestrial inputs and the more subtle changes when compared with previous investigations on the East Siberian Artic shelf. Fluctuations in macromolecular compositions across the transect included increasing N-containing compounds (including pyridines) and n-alkene/n-alkane doublet pyrolysis products from sediments collected in stations with the greatest average ice cover. If the future position of the Polar Front moves northwards then deposition of labile organic matter which appears to be efficiently processed will move further north, meaning greater deposition of organic carbon under areas of open ocean. Future research needs to understand how this OC will be buried and if it is regionally significant, given anticipated weakening stratification and a more Atlantic influenced northern Barents Sea.

Riverine input of organic carbon and nitrogen in water-sediment system from the Yellow River estuary reach to the coastal zone of Bohai Sea, China

The temporal-spatial distribution of the carbon and nitrogen contents and their isotopic compositions of suspended matter and sediments from the Yellow River estuary reach (YRER), the estuary to the offshore area were measured to identify the source of organic matter. The higher relative abundances of suspended and sedimentary carbon and nitrogen (POC, TOC, PN and TN) in the offshore marine area compared to those of the riverine and estuarine areas may be due to the cumulative and biological activity impact. The organic matter in surface sediments of YRER, the estuary and offshore area of Bohai Sea is basically the mixture of continental derived material and marine material. The values of δ13Csed fluctuate from values indicative of a land source (− 22.50‰ ± 0.31) to those indicative of a sea source (− 22.80‰ ± 0.38), which can be attributed to the fine particle size and decrease in terrigenous inputs to the offshore marine area. Contrary to the slight increase of POC and PN during the dry season, TOC and TN contents of the surface sediments during the flood season (October) were higher than those during the dry season (April). The seasonal differences in water discharge and suspended sediment discharge of the Yellow River Estuary may result in seasonal variability in TOC, POC, TN and PN concentrations in some degree. Overall, the surface sediments in the offshore area of Bohai Sea are dominated by marine derived organic carbon, which on average, accounts for 58–82% of TOC when a two end-member mixing model is applied to the isotopic data.

Using Isotopes to Identify the Sources of Organic Carbon and Nitrogen in Surface Sediment in Shallow Lakes Alongside Poyang Lake

Water exchange in shallow lakes alongside Poyang Lake is weak because of sluggish water flow, and thus these lakes are susceptible to the effects of human activities and nutrient oversupply. Therefore, it is meaningful to study the sources of organic matter in these lakes, which is supposed to provide theoretical basis for the control of eutrophication of the lakes. The sources of organic matter in surface sediment in shallow lakes (Bang, Sha, Dahuchi, and Zhu lakes) alongside Poyang Lake and urban lakes (Qingshan and Xiang lakes) in Nanchang City were investigated by measuring the δ15N and δ13C values, total organic carbon and total organic nitrogen contents, and C/N ratios of sedimented organic matter (SOM). The δ15N, δ13C, and C/N ratios indicated that SOM in Sha and Dahuchi lakes mostly originated in phytoplankton, SOM in Bang and Zhu lakes was supplied by phytoplankton and soil organic matter, SOM in Qingshan Lake was strongly affected by sewage organic matter, and SOM in Xiang Lake mainly came from aquatic macrophytes. The results demonstrate that stable isotopes in SOM in lakes can be used to discriminate between different sources of organic material.