The effects of iron reduction and nonsteady-state diagenesis on iodine, ammonium, and boron distributions in sediments from the Amazon continental shelf

The sediments off the mouth of the Amazon River are unique in terms of modern depositional environments because Fe reduction dominates redox properties for several meters into the seabed. This unique character offers the opportunity to investigate the effects of Fe reduction on reactive solutes like iodine and boron. Also, direct measurements of solute distributions and reaction rates, reported in this paper for iodine and NH₄⁺, constrain the timing of major physical disturbances, which are partly responsible for poising the sediments in the Fe reduction stage. Major enrichments of dissolved iodine and boron occur in 1-3 m cores from the region, in the absence of high organic matter decomposition rates. Coherent relationships between boron and iodine, high dissolved I/N and I/Br production ratios, and the lack of a strong correlation of iodine production with dissolved NH₄⁺ all indicate that Fe reduction exerts the major control on iodine and boron distributions. Nonsteady-state models of dissolved iodine and NH₄⁺ and solid-phase iodine distributions in long cores, using measured surface (0-20 cm) reaction rates, indicate that the surface mixed layer (as defined by ²¹⁰Pb; which includes the upper 1.5 m of sediment in some areas) was formed within 50-250 days prior to our sampling (May-June 1983; high Amazon River flow). These data imply that physical sediment reworking on the Amazon shelf has a strong seasonal character, with greatest disturbances occurring during rising river flow. The full implications of these reworking events, for the persistence of the unique authigenic mineral assemblages found in the sediments and for the shelf sediment budget, will require future seasonal studies.