TROPICAL DELTAIC SYSTEMS AS UNSTEADY DIAGENETIC REACTORS, GLOBAL C INCINERATORS, AND REVERSE WEATHERING CENTERS

The central role of deltas as storage sites for sedimentary debris can obscure their major function in the dynamic processing and biogeochemical cycling of material along continental margins. Tropical deltas in particular receive ~60% of global river water, terrestrial organic carbon, and sediment fluxes to the oceans. Subaqueous topset regions in these highly productive systems are often characterized by episodic reworking and remobilization of immense areas of seafloor. Bottom waters are typically well oxygenated and sediment mobilization reoxidizes topset muds over vertical depths of 0.1 – 3 m annually. In contrast to traditional diagenetic concepts, these dynamic deposits act as unsteady, suboxic batch reactors, often unconformably overlying relict methanic deposits, with diagenetic properties largely decoupled from net sediment accumulation. Physical disturbance inhibits macrofaunal bioturbation, and benthic communities are dominated by microbial biomass. More than 90% of both terrestrial and marine carbon entering these suboxic diagenetic systems is commonly incinerated, leaving C_org / particle surface area loadings comparable to the oligotrophic deepsea. δ¹³C and Δ¹⁴C of ΣCO₂ and C_org demonstrate efficient remineralization of ancient and modern marine and terrestrial organics prior to export into foreset regions. Sedimentary biogenic silica is rapidly converted into Fe-rich clays within mobile suboxic layers, consuming alkalis, F^-, and to a lesser extent Mg²⁺. Distinctive Fe mineral properties of these deposits are: elevated reactive Fe (tropical weathering source), high fraction of reduced reactive Fe (carbonate, clay), a low proportion of total diagenetic Fe(II) present as pyrite, and high C/S (4 – 6). Although topset deposits are suboxic, molecular microbiological analyses, tracer experiments, and δ³⁴S - δ¹⁸O of pore water SO₄ suggest close coupling of anaerobic C, S, and Fe cycles. Unsteady diagenetic conditions can result in unusually heavy δ³⁴S isotopic compositions of residual Cr reducible S, seldom < 0 per mil and ranging to > 30 per mil. The critical role of unsteady, suboxic diagenesis and the dynamic exchange of sedimentary debris between muddy diagenetic facies has been largely ignored in models of elemental cycling and interpretations of the sedimentary record.