Unsteady diagenetic processes and sulfur biogeochemistry in tropical deltaic muds: Implications for oceanic isotope cycles and the sedimentary record

Sedimentary S cycling is usually conceptualized and interpreted within the context of steadily accreting (1-D) transport-reaction regimes. Unsteady processes, however, are common in many sedimentary systems and can result in dramatically different S reaction balances and diagenetic products than steady conditions. Globally important common examples include tropical deltaic topset and inner shelf muds such as those extending from the Amazon River similar to 1600 km along the Guianas coast of South America. These deposits are characterized by episodic reworking of the surface seabed over vertical depths of similar to 0.1-3 m. Reworked surface sediments act as unsteady, suboxic batch reactors, unconformably overlying relict anoxic, often methanic deposits, and have diagenetic properties largely decoupled from net accumulation of sediment. Despite well-oxygenated water and an abundant reactive organic matter supply, physical disturbance inhibits macrofauna, and benthic communities are dominated by microbial biomass across immense areas. In the surficial suboxic layer, molecular biological analyses, tracer experiments, sediment C/S/Fe compositions, and delta S-34, delta O-18 of pore water SO42- indicate close coupling of anaerobic C, S, and Fe cycles. delta O-18-SO42- can increase by 2-3 parts per thousand during anaerobic recycling without net change in delta S-34-SO42-, demonstrating SO42- reduction coupled to complete anaerobic reoxidation to SO42- and a delta O-18-SO42- reduction + reoxidation fractionation factor >= 12 parts per thousand (summed magnitudes). S reoxidation must be coupled to Fe-oxide reduction, contributing to high dissolved Fe2+ (similar to 1 mM) and Fe mobilization-export. The reworking of Amazon-Guianas shelf muds alone may isotopically alter delta O-18-SO42- equivalent in mass to >= 25% of the annual riverine delivery of SO42- to the global ocean. Unsteady conditions result in preservation of unusually heavy delta S-34 isotopic compositions of residual Cr reducible S, ranging from 0 parts per thousand to >30 parts per thousand in physically reworked deposits. In contrast, bioturbated fades adjacent to physically reworked regions accumulate isotopically light S (delta S-34 to -20 parts per thousand) in otherwise similar decomposition regimes. The isotopic patterns of both physically and biologically reworked regions can be simulated with simple diagenetic models. Heavy S isotopic signatures are largely a consequence of unsteady diffusion and progressive anaerobic burndown into underlying deposits, whereas isotopically depleted bioturbated deposits predominantly reflect biogenic diffusive scaling and isotopic distillation/diffusive pumping associated with reoxidation in burrow walls immediately adjacent to reduced zones. The S isotopic transition from unsteady physically controlled regions of the Amazon delta moving laterally into bioturbated facies mimics the transition of S isotopic patterns temporally in the geologic record during the rise of bioturbation. No special role for S disproportionation is required to explain these differences. The potential role of unsteady, suboxic diagenesis and dynamic reworking of sediments has been largely ignored in models of the evolution of surficial elemental cycling and interpretations of the geologic record. (C) 2010 Elsevier Ltd. All rights reserved.