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 (Δ14C = 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 Corg 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 (Δ14C = −159 to −229‰). Residual terrestrial Corg 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 Corg below ∼0.5 m in the foreset is ∼10–20 times lower than expected based on accumulation rates, reflecting loss of >50% of sedimentary Corg on the topset, which functions as a suboxic incinerator.