Coupling of early diagenetic processes and sedimentary dynamics in tropical shelf environments: the Gulf of Papua deltaic complex

Tropical mobile mud belts represent a major class of biogeochemical and diagenetic systems characterized by extensive and frequent physical reworking of fine-grained, organic-rich deposits underlying oxygenated waters. Large regions of the Gulf of Papua, Papua New Guinea deltaic complex are dominated by such conditions. A reworked mud belt lies within the inner shelf between similar to10 and 20 m depth on a sedimentary clinoform derived from coalescing deltas. Deposits across the topset are typically suboxic, nonsulfidic over the upper similar to0.5-1 m, and have low to moderate maximum pore water concentrations of dissolved Fe(II) and Mn(II) (similar to100-200, but up to similar to800 muM). Sediments are reactive, with surficial SigmaCO(2), production similar to0.1-0.3 mM d(-1) and benthic O-2 fluxes similar to23+/-15 mmol m(-2) d(-1) (upper similar to20cm). The highest rates occur within inner topset deposits (similar to10-20m) and near the high accumulation rollover region of the topset-foreset beds (similar to40-50m). Lower rates are found inshore along intertidal channels-mangrove fringe and within scoured or exposed consolidated deposits of the middle topset region. Remineralization rate patterns are independent of relative dominance by terrestrial or marine carbon in sediments. Dissolved O-2 usually penetrates similar to2-5 mm into surface sediments when macrofaunal burrows are absent. More than 75% of the highly reactive sedimentary Fe(III) pool (similar to350-400 mumol g(-1)) is typically diagenetically reduced in the upper similar to0.5 m. Pore water SO42- can be measureably depleted at depths >0.5m, but dissolved H2S generally remains below detection over the upper similar to1-2 m. As in other deltaic topset regions, SO42- concentration gradients often indicate that compared to many marine 4 deposits of similar sediment accumulation rates, relatively refractory C-org is Supplied to the SO4 reducing zone. Sedimentary C/S ratios are similar to4-6 within the suboxic topset regions but decrease to <3 in offshore foreset beds where sulfidic diagenesis dominates. Only similar to15-20% of the diagenetically reduced Fe(II) is pyritic and a maximum of similar to10-25% is carbonate, implying that most Fe(II) is associated with authigenic or lithogenic silicates or oxides. The dominance of suboxic, nonsulfidic diagenetic processes reflect coupling between delivery of oxide-rich terrestrial debris, remobilization and reoxidation of deposits, and repetitive entrainment/remineralization of both labile and refractory organics. Distinct sedimentary indicators of reactive, suboxic mobile mud belts within tropical climatic zones are: abundant total highly reactive Fe (SigmaFeR) > 300 mumol g(-1)'; most reactive Fe is diagenetically reduced (SigmaFe(II)/ SigmaFeRsimilar to0.7-0.8); the proportion of diagenetically reduced Fe present as pyrite is low (Py-Fe(II)<0.2); C/S similar to4-8; and C-org/particle surface area <0.4 (mgCm(-2)). These depositional environments must be most common in tropical climates during high sea stand. (C) 2004 Elsevier Ltd. All rights reserved.