Early diagenesis and authigenic mineral formation in mobile muds of the Changjiang Estuary and adjacent shelf.

Large-river delta-front estuaries (LDEs) and their adjacent shelf margins are sites of dynamic diagenetic processes that play a significant role in coastal biogeochemical cycling. In this study, we used dissolved inorganic carbon (DIC), redox sensitive elements (Fe 2 + and Mn 2 + ), dissolved inorganic nitrogen (DIN) nutrients (NH 4 + , NO 3 − , and NO 2 − ), major cations and anions (K + , Ca 2 + , Mg 2 + , SO 4 2 − , and Cl − ) in bottom-water and sediment pore-waters, to investigate the early chemical diagenesis and authigenic mineral formation in mobile-mud deposits of the Changjiang Estuary and adjacent inner shelf of the East China Sea (ECS). Vertical profiles of DIC and NH 4 + in pore-waters had similar trends at most sites, showing a significant increase with depth near the Changjiang Estuary and being relatively constant at offshore sites. Higher pore-water DIC and NH 4 + concentrations were observed in nearshore sites in winter, which were likely attributed to exposure of deeper deposits by winter coastal erosion. Nitrification was observed at most sites, and AOB (ammonia-oxidizing bacteria) played a leading role in ammonia oxidation in the study areas. The nitrification-denitrification was likely important in contributing to the loss of DIN in offshore sites during summer. Large inputs of organic carbon (OC) and terrestrial materials from Changjiang River resulted in intense sulfate reduction and Fe and Mn reduction in nearshore sites. Lower C/N and C/S ratios coupled with an apparent decrease in pore-water Ca 2 + and Mg 2+ concentrations with depth near the Changjiang Estuary, which indicated that authigenic carbonate formation occurs in these sediments. Decreases in K + and Mg 2 + with depth reflected that reverse weathering was an important process of authigenic mineral formation in these sediments. We conclude that adsorption process, seasonal erosion-redeposition, and summer hypoxic conditions of bottom-waters may play an important role in early diagenesis processes and remineralization of SOC in the Changjiang LDE. [ABSTRACT FROM AUTHOR]