The effects of clay mineral reactions on dissolved Al distributions in sediments and waters of the Amazon continental shelf

Reactions involving clay minerals may exert a major control on some aspects of marine water and sediment chemistry. The potential of clay mineral reactions in this regard was investigated in Amazon continental shelf muds and overlying waters using highly sensitive dissolved Al analyses. Data are restricted to low-Fe pore waters from undisturbed and incubated (4-11 days, T = 28 ± 1 °C) surface sediment at 9 stations, a surface water transect through the Amazon River plume, and water column profiles determined at coring sites. Approximately constant relations between dissolved Al, Si, and H⁺ in pore waters imply that aluminous authigenic clays (Si/Al = 0.83, H⁺/Al = 0.43) are forming in muddy regions of the Amazon shelf. Equilibrium models based upon the pore water data also predict the correct magnitude of dissolved A1 concentrations in the Amazon River plume in the absence of high biological productivity, indicating that authigenic clays control some characteristics of overlying water chemistry. In water column profiles, dissolved Al increases with depth at high salinities apparently because of sediment resuspension into low dissolved Si waters and subsequent clay dissolution. The results of this study confirm predictions based upon previous laboratory and field studies of dissolved Al behavior. They also point out some of the possible complexities of clay reconstitution reactions where aluminous authigenic clays form from more siliceous precursors in nearshore sediments and waters.