Decomposition and recycling of organic matter in muds of the Gulf of Papua, northern Coral Sea

Abstract

Most sediment and organic material transported from rivers of southern Papua New Guinea enters into the Gulf of Papua, depositing on the inner shelf as either laminated or bioturbated, silt-dominated mud. These facies are the major trawling grounds for a growing penaeid prawn fishery. In contrast to most other terrigenous shelf deposits, decomposition processes in the upper 20 cm of these Papuan silts are apparently dominated by oxic and suboxic diagenesis. Rates of surface oxygen consumption were high (mean = 26.9; range = 17.8–46.8 mmol O 2 m −2 d −1) as were bacterial numbers (range: 1–4 × 10 10 cells g −1 DW) and rates of bacterial carbon production (tritiated thymidine uptake; range: 3-f0 gC m −2 d −1). Rates of sulfate reduction were low (range: 3.6-6.8 mmol S m −2 d −1) with little (18–25%) of the total reduced 35S0 4 recovered as acid-volatile sulfide. Free sulfides were not detected in porewaters. Total solid-phase S concentrations were low (0.15-0.20% DW) indicating low net S precipitation in the upper 20 cm. Concentrations of dissolved Fe and Mn were elevated in porewaters in the laminated silts. Solid-phase Fe concentrations were moderately high (range: 4.6-5.3% DW) and measured dissolved metal and nutrient fluxes suggest active Fe and Mn reduction (at some stations) and generally high turnover of the porewater N pools. The domination of oxidants other than sulfate and probable C limitation in these moderately Ferich silts, results in aSC signature comparable to freshwater sediments. These diagenetic patterns are reminiscent of those measured in muds on the Amazon shelf and may similarly be attributed to dilution of reactive organic matter combined with intense physical reworking and/or bioturbation, promoting oxidant recharge and favoring decomposition processes other than sulfate reduction. Rapid rates of detrital decomposition lead to fast rates of benthic nutrient release that contribute, on average, 71% and 35% of the daily N and P requirements of shelf phytoplankton production. This close benthic-pelagic coupling may serve to maintain the penaeid prawn fishery in the Gulf of Papua.