Divergent Responses of Organic Carbon to Sedimentary Environment Transformation in a River‐Dominated Marginal Sea

Abstract The fate of organic carbon (OC) in most river‐dominated ocean margins (RiOMars) has undergone a noticeable transformation with the increased sediment retention engineering in watersheds. In the East China Sea (ECS), transformation in sediment and the influence of bulk OC have been broadly studied. However, the response of different mechanisms of OC protection under transformation has not been investigated, hindering our understanding of the factors that control OC deposition. In this study, we isolated different OC fractions, analyzed the basic parameters of the sediments, and compared the previous study's data to reveal how OC deposition responded to transformation. Our research indicates that transformation leads to the reduction of OC associated with minerals and sorting of OC occluded by plant debris and OC associated with minerals resulting in increased decomposition and mineralization of OC. The transformation affects the mechanism of OC binding with reactive iron (Fe R ), increasing Fe R ‐protected OC content. Still, the co‐precipitation mechanism and the intense redox environment in the mud deposit decrease the Fe R ‐protected OC stability. Taken together, the impact of transformation is to increase the risk of OC decomposition and to weaken the OC preservation ability in RiOMars as carbon sinks. This study has implications for river‐dominated passive margins subject to increased sediment retention engineering in watersheds worldwide and deserves more attention.