Unraveling Environmental Forces Shaping Surface Sediment Geochemical "Isodrapes" in the East Asian Marginal Seas

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 (delta 13C and Delta 14C), 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. Plain Language Summary This study focuses on carbon cycle processes occurring in marine sediments of the East Asian marginal seas. We compiled extensive data on the organic carbon content and composition of surface sediments in these seas and developed a machine learning model to predict their spatial patterns and identify the environmental conditions that drive their distribution. We found that the spatial distribution of organic matter is governed by the resuspension of different grain size fractions due to water current intensity as well as the contrasting origin of the organic matter (marine, terrestrial, and rock-derived) that influences its reactivity. We also identified three main areas where sediments with similar composition drape the seafloor: shelves with strong bottom current with less organic matter, mud deposits rich in organic matter, and a deep basin with aged organic matter. Understanding the factors that control the distribution of organic matter in these areas is important for accurately assessing their contribution to the global carbon cycle.