This research is part of the AmasSeds Project (A Multi- disciplinary Amazon Shelf Sediment Study), which seeks to understand the physical, chemical and geological processes that affect, or are affected by, particle fluxes on the Amazon shelf. High particulate fluxes occur on the Amazon shelf as a result of the enormous sediment discharge from the Amazon River and the extensive resuspension of fine-grained shelf sediment by tidal processes. This component of the research examines Fe and Mn reduction reactions occurring in Amazon inner shelf muds. The dominance of Fe and Mn reactions over SO4 reactions apparently results from a favorable balance between low organic C, moderately Fe-rich particles, and massive nonsteady-state deposition and physical reworking of the seabed. These properties promote the formation of nonsulfidic authigenic minerals (Fe-carbonates, Fe, Al-silicates) more typical of low SO4 deposits, strongly influence export and storage of minor/trace elements during diagenetic remobilization/ precipitation of Fe carrier phases, inhibit benthic communities and shell-bearing fauna, and have major consequences for the nature of the preserved sedimentary record. These characteristics suggest the Amazon shelf may be an analogue to ancient environments of ironstone or greensand formation. Relationships between the variables affecting these processes will be investigated in the Amazon shelf and delta-shelf region. By understanding the processes by which chemical phases of sediment are deposited in this estuary, other river/ocean systems can be better characterized. In particular, Fe and Mn reduction reactions occurring in Amazon inner shelf muds depend upon a balance between low organic C, inner shelf muds depend upon a balance between low organic C, moderately Fe-rich particles, massive nonsteady-state deposition and physical reworking of the seabed. The processes by which in situ precipitates enter the historical geological record will be investigated in the Amazon shelf and delta-shelf region.
