Oceania supplies ∼40% of the global riverine flux of organic carbon, approximately half of which is injected onto broad continental shelves and processed in shallow deltaic systems. The Gulf of Papua, on the south coast of the large island of New Guinea, is one such deltaic clinoform complex. It receives ∼4 Mt yr −1 particulate terrestrial organic carbon with initial particle C org loading ∼0.7 mg m −2. C org loading is reduced to ∼0.3 mg m −2 in the topset-upper foreset zones of the delta despite additional inputs of mangrove and planktonic detritus, and high net sediment accumulation rates of 1–4 cm yr −1. Carbon isotopic analyses (δ 13C, Δ 14C) of ΣCO 2 and C org demonstrate rapid (<100 yr) remineralization of both terrestrial (δ 13C <−28.6) and marine C org (δ 13C ∼−20.5) ranging in average age from modern (bomb) (Δ 14C ∼60) to ∼1000 yr (Δ 14C ∼−140). Efficient and rapid remineralization in the topset-upper foreset zone is promoted by frequent physical reworking, bioturbation, exposure, and reoxidation of deposits. The seafloor in these regions, particularly <20 m, apparently functions as a periodically mixed, suboxic batch reactor dominated by microbial biomass. Although terrestrial sources can be the primary metabolic substrates at inshore sites, relatively young marine C org often preferentially dominates pore water ΣCO 2 relative to bulk C org in the upper foreset. Thus a small quantity of young, rapidly recycled marine organic material is often superimposed on a generally older, less reactive terrestrial background. Whereas the pore water ΣCO 2 reflects both rapidly cycled marine and terrestrial sources, terrestrial material dominates the slower overall net loss of C org from particles in the topset-upper foreset zone (i.e. recycled marine C org leaves little residue). Preferential utilization of C org subpools and diagenetic fractionation of C isotopes supports the reactive continuum model as a conceptual basis for net decomposition kinetics. Early diagenetic fractionation of C isotopes relative to the bulk sedimentary C org composition can produce changes in 14C activity independent of radioactive decay. In the Gulf of Papua topset-upper foreset, Δ 14C of pore water ΣCO 2 averaged ∼ 300‰ greater than C org sediment between ∼1–3 m depth in deposits. Diagenetic fractionation and decomposition aging of sedimentary C org compromises simple application of 14C for determination of sediment accumulation rates in diagenetically reactive deposits.