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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]

High‐resolution clay mineralogy and major element geochemistry of Core SO18383‐3 collected off the Mekong River mouth in the southern South China Sea have been investigated, aiming to reconstruct sediment provenance and chemical weathering intensity since the last glaciation. The chronology is based on foraminiferal accelerator mass spectrometry 14C dates. Clay mineral analysis suggests that the Mekong River is a major sedimentary source for the studied area. The values of chemical index of alteration (CIA) indicate moderate chemical weathering in the Mekong River basin. Smectite/(illite + chlorite) and smectite/kaolinite ratios coupled with TiO2/K2O ratio reveal the temporal variation in chemical weathering intensity in the Mekong River basin. The lower ratios around the Last Glacial Maximum (29.9–14.7 cal ka BP), the Heinrich Event 1 (~14.8 cal ka BP), the Younger Dryas interval (12.4–11.3 cal ka BP), and middle to late Holocene (7.6–1.1 cal ka BP) indicate weak chemical weathering intensity. In contrast, the higher ratios occurring during the Bølling‐Allerød period (14.7–12.7 cal ka BP) and the period of main deglaciation (14.7–7.6 cal ka BP) indicate stronger chemical weathering in the Mekong River basin. The good correlations between these proxies and the available proxy records of the East Asian summer monsoon intensity suggest that the intensity of chemical weathering in the Mekong River basin over the last 30 ka is significantly controlled by the summer monsoon. This study greatly increases our understanding of the principal forcing factor on continental weathering in the Mekong River basin since the Last Glacial Maximum. Key Points: The Mekong River is the major terrigenous sediment contributor to the southwestern South China SeaChemical weathering intensity in the Mekong River basin has been moderate since the Last Glacial MaximumIntensity of chemical weathering in the Mekong River basin is controlled by the East Asian summer monsoon evolution [ABSTRACT FROM AUTHOR]

The Gulf of Tonkin coastline migrated at an average rate of ca 60 m year−1 landward during Holocene sea-level rise (20 to 8 ka). Due to a combination of rapid coastline migration and undersupply of sand, neither coastal barriers nor tidal sand bars developed at the mouth of the Red River incised valley. Only a 30 to 80 cm thick sandy interval formed at the base of full-marine deposits. Thus, the river mouth represented a mud-dominated open funnel-shaped estuary during transgression. At the base of the valley fill, a thin fluvial lag deposit marks a period of lowered sea-level when the river did not reach geomorphic equilibrium and was thus prone to erosion. The onset of base-level rise is documented by non-bioturbated to sparsely bioturbated mud that occasionally contains pyrite indicating short-term seawater incursions. Siderite in overlying deposits points to low-salinity estuarine conditions. The open funnel-shaped river mouth favoured upstream incursion of seawater that varied inversely to the seasonal strongly fluctuating discharge: several centimetres to a few tens of centimetres thick intervals showing marine or freshwater dominance alternate, as indicated by bioturbational and physical sedimentary structures, and by the presence of Fe sulphides or siderite, respectively. Recurrent short-term seawater incursions stressed the burrowing fauna. The degree of bioturbation increases upward corresponding to increasing marine influence. The uppermost estuarine sediments are completely bioturbated. The estuarine deposits aggraded on average rapidly, up to several metres kyr−1. Siphonichnidal burrows produced by bivalves, however, document recurrent episodes of enhanced deposition (>0·5 m) and pronounced erosion (<1 m) that are otherwise not recorded. The slope of the incised valley affected the sedimentary facies. In steep valley segments, the marine transgressive surface (equivalent to the onset of full-marine conditions) is accentuated by the Glossifungites ichnofacies, whereas in gently sloped valley segments the marine transgressive surface is gradational and bioturbated. Marine deposits are completely bioturbated. [ABSTRACT FROM AUTHOR]

Aquino da Silva, A.G.; Stattegger, K.; Schwarzer, K.; Vital, H., and Heise, B., 2015. The influence of climatic variations on river delta hydrodynamics and morphodynamics in the Parnaíba Delta, Brazil. In this work, we present a baseline study on the geo-environmental, morphodynamical, and hydrodynamical conditions of the Parnaíba River and its mouth, the Parnaíba River Delta (PRD). The outline of the PRD indicates that waves are the major forcing factor for delta development. Furthermore, the shape of the delta suggests that there is a considerable degree of asymmetry in its formation. The asymmetry is the result of coastal current influence on the reworking of the sediments that are discharged by the river. To investigate the hydrodynamic conditions of the area, field measurements were conducted on the river, offshore of the PRD, and on the tidal channels of the west part of the delta. At these locations, the currents, salinity, temperature, and tidal dynamics were measured. In addition, a time series of rainfall, river discharge, and suspended sediment concentration (SSC) were obtained from Brazilian governmental agencies. The interpretation of all of the data was used to determine how they interact to characterize the hydro- and morphodynamics of the PRD. Harmonic analysis was of high importance in the analysis of the time series, and it was employed to identify longer periods presenting similarities in terms of meteorological conditions. This allowed determination of the interconnection between rainfall, river discharge, and SSC with the occurrence and intensity of the El Niño-Southern Oscillation. The influence of river discharge, rainfall, and SSC on shoreline change was demonstrated through the analysis of decadal shoreline evolution using a series of LANDSAT images covering the period from 1981 to 2009. The analysis revealed that most changes occurred westward from the PR mouth. Changes in spit migration velocity were correlated to variations in riverine sediment yielded as a result of variations of river discharge and SSC. [ABSTRACT FROM AUTHOR]