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

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 (∼20 cm.) are reactive with ΣCO₂ production fluxes ∼35-42 mmol M^-2 d^-1 at mangrove channel and topset sites during the monsoon, and ∼10-20 mmol m^-2 d^-1 on the foreset-bottomset (>40 m). Fluxes decrease by a factor of ∼0.3 on the topset during the transition period and trades. The ^13,14C isotopic compositions of pore water ΣCO₂ reveal diagenetic fractionation, with dominant utilization of young (Δ ¹⁴C = 1.4-31.1‰), terrestrial C substrates inshore (channels, topset δ¹³C = -29 to -25‰) and a progressive increase of young marine C sources seaward (outer topset, foreset; bottomset δ¹³C = -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, ∼10-60 cm thick (usually ∼10-30 cm), characterizes : the central gulf topset and Umuda Valley off the Fly River and unconformably overlies methanic deposits releasing old ΣCO₂ (Δ ¹⁴C = -159 to -229‰). Residual terrestrial C_org delivered to the bioturbated foreset continues to be remineralized slowly, generating ΣCO₂ having net Δ¹⁴C = -270 within sediments deposited 100-200 years ago. The reactivity of C_org below ∼0.5 m in the foreset is ∼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.