Early diagenetic cycling, incineration, and burial of sedimentary organic carbon in the central Gulf of Papua (Papua New Guinea)

1] The clinoform complex of the Gulf of Papua represents a major deltaic system in Oceania. Two seasons largely control seafloor dynamics and sedimentary C cycling: the relatively quiescent NW monsoon, and the SE trades, characterized by remobilization and reoxidation of topset deposits. Surface sediments (similar to 20 cm) are reactive with Sigma CO2 production fluxes similar to 35-42 mmol m(-2) d(-1) at mangrove channel and topset sites during the monsoon, and similar to 10-20 mmol m(-2) d(-1) on the foreset-bottomset (> 40 m). Fluxes decrease by a factor of similar to 0.3 on the topset during the transition period and trades. The C-13,C-14 isotopic compositions of pore water Sigma CO2 reveal diagenetic fractionation, with dominant utilization of young (Delta C-14 = 1.4-31.1 parts per thousand), terrestrial C substrates inshore (channels, topset delta C-13 = -29 to -25%) and a progressive increase of young marine C sources seaward (outer topset, foreset; bottomset delta C-13 = -22.2 to -19.5). Remineralization patterns of terrestrial and marine C-org demonstrate cross-shelf exchange. Multiple tracers show that a suboxic, mobile mud layer, similar to 10-60 cm thick (usually similar to 10-30 cm), characterizes the central gulf topset and Umuda Valley off the Fly River and unconformably overlies methanic deposits releasing old Sigma CO2 (Delta C-14 = -159 to -229 parts per thousand). Residual terrestrial C-org delivered to the bioturbated foreset continues to be remineralized slowly, generating Sigma CO2 having net Delta C-14 = -270 within sediments deposited 100-200 years ago. The reactivity of Corg below similar to 0.5 m in the foreset is similar to 10-20 times lower than expected based on accumulation rates, reflecting loss of > 50% of sedimentary C-org on the topset, which functions as a suboxic incinerator.