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 ΣCO2 production ∼0.1–0.3 mM d−1 and benthic O2 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 O2 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 SO 4 2 - can be measureably depleted at depths >0.5 m, but dissolved H2S generally remains below detection over the upper ∼1–2 m. As in other deltaic topset regions, SO 4 2 - concentration gradients often indicate that compared to many marine deposits of similar sediment accumulation rates, relatively refractory Corg is supplied to the SO4 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 Corg/particle surface area <0.4 (mg C m−2). These depositional environments must be most common in tropical climates during high sea stand.