Floods Enhanced the Terrestrial and Marine Organic Carbon Burial in the East China Sea

Abstract Floods deliver large amounts of terrestrial organic carbon (TerrOC) and nutrients, potentially influencing long‐term organic carbon (OC) burial. Flood‐induced pulses of terrestrial materials could form distinct event layers in sediments, serving as valuable archives for understanding how floods regulate carbon sequestration over time. We investigate the multi‐proxy records (e.g., carbon isotopes and lipid biomarkers) from a 300‐year sediment core in the East China Sea inner shelf to explore the impacts of floods on terrestrial and marine OC burial. Flood signatures in distal muddy sediments are amplified by increased flood magnitude and frequency, particularly for floods originating upstream. Although the OC burial regime shifted from hydrological forcing (East Asian Monsoon and floods) to increasing anthropogenic forcing (e.g., dam construction and coastal eutrophication) after the 1970s, episodic extreme floods remained a critical mechanism coupling terrestrial and marine OC burial over the past 300 years, reflected by synchronous peaks in MultiTerrOC and MultiMarine OC indices during major flood intervals. These coincident OC burial peaks, together with coarser particles and high hypoxia intensity, suggest that flood‐driven nutrient pulses stimulated phytoplankton blooms, promoted hypoxic conditions, and consequently enhanced OC preservation. Overall, episodic extreme floods promote long‐term OC burial on large river‐dominated continental margins by redistributing land‐derived OC and stimulating atmospheric CO 2 fixation via enhanced marine OC burial. This century‐scale sedimentary record provides critical insights into the reconstruction of extreme hydrological events in ancient oceans and highlights the increasingly important role of marginal seas in carbon fixation under future scenarios of intensified flooding.