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As major sites of carbon burial and remineralization, continental margins are key components of the global carbon cycle. However, heterogeneous sources of organic matter (OM) and depositional environments lead to complex spatial patterns in sedimentary organic carbon (OC) content and composition. To better constrain the processes that control OM cycling, we focus on the East Asian marginal seas as a model system, where we compiled extensive data on the OC content, bulk isotopic composition (δ13C and Δ14C), total nitrogen, and mineral surface area of surficial sediments from previous studies and new measurements. We developed a spatial machine learning modeling framework to predict the spatial distribution of these parameters and identify regions where sediments with similar geochemical signatures drape the seafloor (i.e., "isodrapes"). We demonstrate that both provenance (44%-77%) and hydrodynamic processes (22%-53%) govern the fate of OM in this margin. Hydrodynamic processes can either promote the degradation of OM in mobile mud-belts or preserve it in stable mud-deposits. The distinct isotopic composition of OC sources from marine productivity and individual rivers regulates the age and reactivity of OM deposited on the sea-floor. The East Asian marginal seas can be separated into three main isodrapes: hydrodynamically energetic shelves with coarser-grained sediment depleted in OC, OM-enriched mud deposits, and a deep basin with fine-grained sediments and aged OC affected by long oxygen exposure times and petrogenic input from rivers. This study confirms that both hydrodynamic processes and provenance should be accounted for to understand the fate of OC in continental margins. [ABSTRACT FROM AUTHOR]

Diatoms and biogenic silica (biogenic Si) in sediments are commonly analyzed as paleoceanographic environmental indicators. However, the correspondence between these sedimentary components and their counterparts in the water column above can vary over time and space. This study, undertaken in the northern South China Sea, compares diatoms and biogenic Si in sinking particles from the water column with those in surface sediments below. The results indicate substantial losses of diatoms and biogenic Si during sinking and burial, along with changes in diatom assemblages. About 6% of diatoms sinking from 1000 m depth are estimated to be preserved in sediments: 32% are recycled in the water column and 62% are lost at the sediment–water interface. Approximately, 12% of biogenic Si is preserved in sediments: 6% is remineralized in the water column and 82% is lost at the sediment–water interface. The differences between diatom and biogenic Si preservation and remineralization are likely due to differences in dissolution resistance among the various diatom species and between diatoms and other silicifiers (e.g., radiolarians), along with effects of lateral transport. This variability introduces uncertainties into paleoceanographic inferences made from these two proxies from the sediments. Thalassionema nitzschioides was found to dominate sediment assemblages, which we attribute to its great resistance to dissolution, and to be negatively correlated with diatom sinking fluxes. These properties suggest that in the northern South China Sea, T. nitzschioides may potentially be useful as an indicator of low paleoproductivity (e.g., as seen during strong El Niño events). [ABSTRACT FROM AUTHOR]

Lonar Lake is a eutrophic, saline soda lake with permanently anoxic deep water. The high pH and deoxygenation result in very elevated δ15N of suspended particulate matter (SPM) and sediments due to denitrification and pH-related loss of gaseous ammonium. SPM and sinking particles are predominantly aquatic in origin, whereas surface sediments are of mixed terrestrial plant and planktonic source. An indicator of degradation intensity was derived from a principal component analysis of the spectral distribution of amino acids and named Lonar degradation index (LI). A ratio of individual amino acids (Ox : Anox ratio) was additionally used to determine the relative degree of aerobic vs. anaerobic degradation. These two biogeochemical indicators can be used to detect changes in degradation intensity and redox conditions in the geological history, and thus the paleoclimatic interpretation of Lonar sediments. Surface sediments can be divided into three zones: (1) a nearshore, oxic zone of predominantly aquatic organic matter, in which oxidation leads to a strong diagenetic increase of δ15N; (2) an alluvial zone with a predominance of isotopically depleted land plant and soil organic matter degraded under oxic conditions; and (3) an anoxic, deep zone, which receives aquatic organic matter and land plant--derived material transported near the bottom and in which organic matter is well preserved due to anoxic diagenetic conditions. [ABSTRACT FROM AUTHOR]

Seasonal variability of diatom abundance and species composition in sinking particles in the northern South China Sea was studied using continuous time-series sediment trap moorings during a one-year period, 2009–2010. A clear seasonal pattern of diatom abundance was observed, with higher fluxes in winter than in summer. This could be related to positive response of diatom growth to increased nutrient supply from the subsurface waters as a result of enhanced vertical mixing in winter. Mesoscale eddy activities and strong Asian dust input could also stimulate the diatom growth and increase the diatom export flux to the deep South China Sea. The strong Asian dust input during spring 2010 probably accelerated the aggregation and downward flux of diatoms in the northern South China Sea. No distinctive seasonal pattern in diatom species composition was recorded, but there was a close correlation of diatom abundances with the cell size and the degree of silification. The sinking velocity of the diatom related particles from the sea surface to the deep South China Sea has been estimated to be ~30–50 m d −1 by comparing the surface chlorophyll a with the diatom abundance and species in sediment traps at 1003 m and 3226 m depths. Strong dissolution of small-sized and low-silicified diatom species and lateral advection of particles were also indicated for the deep northern South China Sea. The present results are crucial as a basis for future understanding of fossil assemblage changes in connection with palaeoclimatic and palaeooceanographic research. [ABSTRACT FROM AUTHOR]

Abstract: The sediments in the upwelling area off central Vietnam are totally bioturbated and display a low-diverse assemblage of bioturbational structures. During interglacial times (Marine Isotope Stage MIS 1, 5a, 5c, 5e), summer monsoon leads to pronounced upwelling and seasonally pulsed arrival of organic matter on the seafloor. These deposits are characterized by a 4-tier bioturbated zone with Zoophycos. Zoophycos producers used this seasonal food source and show a cache behavioral strategy. During glacial periods upwelling was weak and oxygenation of bottom water decreased as evidenced by bioturbational structures that show a decrease in size, penetration depth and diversity. Enlarged freshwater influx during glacial times fuelled primary production and led to estuarine circulation. The resultant oxygen minimum layer favored an increased deposition of organic matter that in turn affected sediment properties. During MIS 2 and 5b oxygenation was probably a little lower than today. During MIS 3 and 5d Chondrites-like burrows point to a stronger oxygen deficiency. During MIS 4 bottom water oxygenation was even lower and only biodeformational structures occur in soft sediment rich in benthic food. Based on these findings an ichnofabric model for food-rich, fairly soft deposits experiencing oxygen deficiency is suggested having biodeformational structures as low-oxygen end member. [Copyright &y& Elsevier]

Abstract: The German Bight/SE North Sea is considered a hot-spot of river-induced eutrophication, but the scarce observational data of river nitrate loads prior to the 1970s complicate the assessment of target conditions for environmental management and legislation. Stable nitrogen isotope ratios (δ15N) in sediment records can be used to decipher historical river nitrate contributions. To better constrain pre-1970s conditions, we determined δ15N in archive sediment samples (1950–1969) and dated cores from the Helgoland depositional area. We also modeled the δ15N in past situations (1960 and 1860) using an N-isotope-tracking ecosystem model. The modeled spatial distribution of δ15N in sediments for 1960 conditions and the observed spatial pattern of δ15N in archive sediment samples (1950–1969) represent a period of moderate eutrophication. The modeled spatial distribution of δ15N in sediments for 1860 conditions (pre-industrial) showed a moderate δ15N gradient from the Elbe river mouth (δ15N<4‰) to the open sea (δ15N∼5‰). This pattern contrasts with the δ15N pattern in modern surface sediments, which exhibits a steep and inverted δ15N gradient from the Elbe river mouth (δ15N>9‰) to the open sea (δ15N<7‰). Modeled δ15N for 1860 conditions are consistent with δ15N values observed in dated sediment cores that span the last 900 years. Value of δ15N in sediment cores increased from approximately 1860 to 2000 by 2.5‰. The increasing trend reflects changes in the abundance and isotopic composition of riverine nitrate loads caused by anthropogenic activities. Sensitivity tests suggest that loads and isotopic ratios of nitrogen forms other than nitrate (ammonium and organic nitrogen) have minor impact on the modeled surface sediments, despite their higher abundance in the riverborne loads in the past. Our results suggest that eutrophication of the German Bight predates the 1960 period of documented rapidly increasing river loads. Pre-industrial levels of δ15N modeled with 28% of the modern annual (1990–1999) atmospheric loads and 10% of the modern annual river loads agree best with levels of δ15N (∼6‰) observed in sediments of the cores dated to 1860. [Copyright &y& Elsevier]

• Eddy current controls the movement of particles within the mesoscale eddies. • The source funnel of sediment traps within the mesoscale eddy is tilted vertically. • A great number of particles can be transported from eddy's edge to the center. • Slow-sinking particles collected by traps could be originated from hundreds of kilometers away. The mooring tethered time-series sediment traps (TS-traps) collect sinking particles in the ocean, enabling the estimation of biological pump (BP) efficiency based on the assumption of vertical settling. However, the advection of sea water can disperse particles over long distance during the sinking process, introducing uncertainties into the estimates of BP efficiency. In the South China Sea, mesoscale eddies generate significant lateral transport above the mesopelagic zone, which has a large impact on the collecting area and source funnel of TS-traps. We studied the role of eddies in the lateral transport of sinking particles using a Lagrangian Particle Tracking Model (LPTM) which was forced by three-dimensional currents of eddy composites derived from the global Hybrid Coordinate Ocean Model (HYCOM). The eddy reanalysis using the HYCOM outputs was validated using occurrence, genesis and dissipation statistics of eddies. Backward particle tracking was performed at three stations located in the northern, central and southwestern SCS, respectively. The results indicate that the vertical sinking assumption is invalid when the particle sinking velocity is relatively low. Furthermore, the fraction of surface sources from the edge of the eddies depends on both the sinking velocity and seasonal variability of the background current. The eddy current controls the movement of particles within the eddy, and many of the particles with a sinking velocity <80 m d−1 can be transported from the edge to the eddy center. The source funnel within the mesoscale eddy is tilted vertically, leading to an asymmetrical distribution of the particle source around the trap location. Finally, for slow-sinking particles, trap-collected particles could originate from the subsurface located tens to hundreds of kilometers away from the trap location. Overall, our results highlight the role of mesoscale eddies in interpreting trap measurements. [ABSTRACT FROM AUTHOR]