Diagenetic cycling of arsenic in Amazon shelf sediments

In order to help elucidate major natural controls on the diagenetic behavior of As, vertical profiles of total dissolved and solid phase As were measured in Amazon shelf deposits; a deltaic region little influenced by human activity. Arsenic distributions are typically characterized by subsurface porewater maxima and surficial solid phase enrichment. Maximum porewater As concentrations (∼2-4 μM) are an order of magnitude greater than levels reported in most other coastal marine environments, and elevated concentrations extend several meters below the sediment surface. Dissolved As varies inversely with easily leachable As (∼40-140 nmol·g^-1) in the solid phase and directly with increases in solid phase Fe(II). These patterns reflect a strong redox coupling with Fe whereby oxidized As is associated with Fe oxyhydroxides in surface sediments, then subsequently reduced and released upon burial in the extensive suboxic zone. Upward diffusion and intense physical reworking of sediments presumably releases the dissolved arsenic fraction to the water column or allows for re-adsorption in surface sediments. Arsenic porewater concentrations generally increase away from the river. In some cases regular seasonal variations are evident, apparently corresponding to nonsteady state ingrowth of diagenetic profiles following extensive sediment reworking during rising river flow. The massive cycling of As in Amazon shelf sediments is likely due to the combined effects of intense chemical weathering in regions of the drainage basin (production of oxides) and a turbulent physical regime present on the shelf, both of which promote sedimentary cycling of Fe and coupled elements during early diagenesis.