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 ∼10 and 20 m depth on a sedimentary clinoform derived from coalescing deltas. Deposits across the topset are typically suboxic, nonsulfidic over the upper ∼0.5-1 m, and have low to moderate maximum pore water concentrations of dissolved Fe(II) and Mn(II) (∼100-200, but up to ∼800 μM). Sediments are reactive, with surficial ΣCO ₂ production ∼0.1-0.3 mM d ^-1 and benthic O ₂ fluxes ∼23±15 mmol m ^-2 d ^-1 (upper ∼20 cm). The highest rates occur within inner topset deposits (∼10-20 m) and near the high accumulation rollover region of the topset-foreset beds (∼40-50 m). 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 ∼2-5 mm into surface sediments when macrofaunal burrows are absent. More than 75% of the highly reactive sedimentary Fe(III) pool (∼350-400 μmol g ^-1) is typically diagenetically reduced in the upper ∼0.5 m. Pore water SO42- can be measureably depleted at depths >0.5 m, but dissolved H ₂S generally remains below detection over the upper ∼1-2 m. As in other deltaic topset regions, SO42- concentration gradients often indicate that compared to many marine deposits of similar sediment accumulation rates, relatively refractory C _org is supplied to the SO ₄ reducing zone. Sedimentary C/S ratios are ∼4-6 within the suboxic topset regions but decrease to <3 in offshore foreset beds where sulfidic diagenesis dominates. Only ∼15-20% of the diagenetically reduced Fe(II) is pyritic and a maximum of ∼10-25% 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 (ΣFeR) >300 μmol g ^-1; most reactive Fe is diagenetically reduced (ΣFe(II)/ΣFeR∼0.7-0.8); the proportion of diagenetically reduced Fe present as pyrite is low (Py-Fe(II)<0.2); C/S ∼4-8; and C _org/particle surface area <0.4 (mg C m ^-2). These depositional environments must be most common in tropical climates during high sea stand.