Early diagenetic remineralization of sedimentary organic C in the Gulf of Papua deltaic complex (Papua New Guinea): Net loss of terrestrial C and diagenetic fractionation of C isotopes

Oceania supplies similar to40% 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 similar to4 Mt yr(-1) particulate terrestrial organic carbon with initial particle C-org loading similar to0.7 mg m(-2). C-org loading is reduced to similar to0.3 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 (delta(3)C, Delta(14)C) of SigmaCO(2) and C-org demonstrate rapid (<100 yr) remineralization of both terrestrial (delta(13)C <-28.6) and marine C-org (delta(13)C similar to-20.5) ranging in average age from modern (bomb) (Delta(14)C similar to60) to similar to1000 yr (Delta(14)C similar to - 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 SigmaCO(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 SigmaCO(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 C-14 activity independent of radioactive decay. In the Gulf of Papua topset-Upper foreset, Delta(14)C of pore water SigmaCO(2) averaged similar to 300parts per thousand greater than C-org sediment between similar to 1-3 m depth in deposits. Diagenetic fractionation and decomposition aging of sedimentary C-org compromises simple application of C-14 for determination of sediment accumulation rates in diagenetically reactive deposits. Copyright (C) 2004 Elsevier Ltd.