[Department of Ocean Science] PhD Qualifying Examination Defense Seminar: Decoding Molecular Fingerprints of Sedimentary Dissolved Organic Matter in Coastal Margins
 

Abstract:

Marine sediments constitute a major reservoir of organic carbon but are also active biogeochemical reactors where organic matter is continuously degraded, transformed, mobilized, and preserved. These processes are particularly dynamic at estuarine–coastal margins, where terrestrial inputs, marine production, sediment resuspension, redox gradients, and mineral reactions interact over short spatial and temporal scales. Sedimentary dissolved organic matter (SDOM) occupies a central position in this system by linking particulate organic matter degradation, microbial processing, sediment–water exchange, and carbon burial. Focusing on the Yangtze Estuary–East China Sea and Pearl River Estuary systems, this work integrates bulk geochemistry, optical properties, carbon isotopes, and ultrahigh-resolution mass spectrometry to resolve the molecular controls on SDOM mobility and persistence. The results support a view of coastal sediments as reactive molecular filters. Hydrodynamic reworking selectively redistributes molecular components across the sediment–water interface, while chemically unresolved features represent a structured pool that contributes to benthic–pelagic connectivity. Reactive Fe phases further impose source- and composition-dependent selectivity on organic matter retention, regulated in part by OC/Fe stoichiometry. These findings reveal that SDOM cycling emerges from the interplay among physical transport, molecular transformation, and mineral association. Resolving these interconnected processes at the molecular level provides a mechanistic basis for understanding how estuarine–coastal sediments control organic carbon redistribution, retention, and long-term preservation along the river-to-ocean continuum.