Early diagenetic processes in relation to river discharge and coastal upwelling in the Aru Sea, Indonesia

The Aru Sea receives massive sediment discharge (427 × 106 t yr− 1) from ≈ 30 rivers in southwestern New Guinea and upwelling of nutrient-rich water from the Banda Sea. This region is a rich fishing ground with total catch exceeding 220,000 t yr− 1, mostly in areas enriched by coastal upwelling. Reduction of Fe (range: 0–89.6 mmol Fe m− 2 day− 1) and Mn (range: 2.2–32.6 mmol Mn m− 2 day− 1) dominates Corg decomposition in homogenous silt and clay deposits and aerobic metabolism dominates in sandy deposits or sediments showing evidence of extensive physical disturbance, or both. Discrepancies between fluxes within sediments and across the sediment–water interface, diagenetic modeling, and mineral saturation states show significant loss of dissolved inorganic carbon (DIC) and highly supersaturated conditions, implying precipitation of authigenic carbonate minerals. Sedimentary δ13C values show that most total organic carbon (TOC) is marine-derived in the upwelling area, with a decline in terrestrially-derived material from inshore to the outer shelf. Rates of O2 uptake and DIC release across the sediment–water interface and depth-integrated, production rates of DIC and NH4+ are highest where upwelling enriches phytoplankton and fisheries production. Sedimentary organic matter is reactive as evidenced by rapid decomposition rates, and porewater and solid-phase stoichiometry. Massive discharge of highly weathered debris, physical disturbance, rapid deposition of vascular plant detritus inshore, and extensive trawling foster oxic and suboxic diagenesis under nonsteady-state conditions. Rates of C and N remineralization were equivalent to rates measured on other tropical shelves, supporting the notion that tropical margins are efficient ‘incinerators’ of organic matter, even in the Aru Sea where cycling of organic matter as a result of upwelling is decoupled from that resulting from riverine inputs.