Sources, reactivity and burial of organic matter in East China Sea sediments as indicated by a multi-geochemical proxy approach

Abstract. Large-river estuaries and adjacent seas play an important role in material exchanges and interactions in the land-ocean continuum and thereby impact global marine biogeochemistry. Due to the highly dynamic and complex transport and transformation processes of organic matter (OM), its distribution, sources, and reactivity in this region, especially under the multiple pressures of intense human activities and climate change, are not fully understood. An East China Sea transect, ranging from the mouth of the Changjiang River to the Okinawa Trough (OT), was selected to investigate the sources and reactivity of OM in surface sediments using multi-geochemical proxies. Carbon (C) to nitrogen (N) ratios and stable isotopic signatures, total hydrolyzable amino acids, neutral sugars and organic carbon (OC)-normalized total lignin-phenol indicated that OM in nearshore surface sediments derived primarily from terrestrial vascular plants, while offshore OM was dominantly derived from marine production. In the estuary vegetation mainly consisted of gymnosperms, whereas nonwoody angiosperms were dominant in offshore regions. Hydrodynamics, i.e., Changjiang Diluted Water and the Kuroshio Current markedly impacted sediment characteristics along this transect. The degree of OM degradation increased seaward, and sedimentation rates and OC burial fluxes were highest in Changjiang prodeltaic sediments and lowest in the OT. These results based on multi-biomarkers will advance our understanding of OC sources and burial during transport and deposition processes from estuaries to the deep oceans.