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 Corg loading ∼0.7 mg m-2. Corg 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 (δ13C, Δ14C) of ΣCO2 and Corg demonstrate rapid (<100 yr) remineralization of both terrestrial (δ13C <-28.6) and marine Corg (δ13C ∼-20.5) ranging in average age from modern (bomb) (Δ14C ∼60) to ∼1000 yr (Δ14C ∼-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 Corg often preferentially dominates pore water ΣCO2 relative to bulk Corg 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 ΣCO2 reflects both rapidly cycled marine and terrestrial sources, terrestrial material dominates the slower overall net loss of Corg from particles in the topset-upper foreset zone (i.e. recycled marine Corg leaves little residue). Preferential utilization of Corg 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 Corg composition can produce changes in 14C activity independent of radioactive decay. In the Gulf of Papua topset-upper foreset, Δ14C of pore water ΣCO2 averaged ∼ 300‰ greater than Corg sediment between ∼1-3 m depth in deposits. Diagenetic fractionation and decomposition aging of sedimentary Corg compromises simple application of 14C for determination of sediment accumulation rates in diagenetically reactive deposits.
ASJC Scopus subject areas
- Geochemistry and Petrology