Diagenetic processes near the sediment-water interface of long island sound. II. fe and mn

(1) Pore-water profiles of Fe²⁺ and Mn²⁺ from three stations in Long Island Sound have general depth-dependent concentration distributions similar to those reported from other sedimentary basins: concentrations rise above seawater values to a maximum below the interface and then decrease again or remain constant deeper in the deposit. Beyond these general features, specific features of the profiles reflect the internal transport- reaction regime effective at each station. (2) The production of Mn²⁺ in pore waters is directly related to the rate of reduction of Mn oxides during the decomposition of organic matter, both as a function of depth in the sediment as well as seasonally. Fe²⁺, on the other hand, is produced both by the reduction of Fe oxides and by abiogenic or biogenic oxidation of Fe sulfides. The result of different sources for the two dissolved metals is a different seasonality of the interstitial water profiles near the sediment-water interface. The temporal changes in both Mn²⁺ and Fe²⁺ profiles are repeatable from year to year. During the summer, pore-water Mn²⁺ in the top few centimeters reaches the highest concentration of the year. In the fall, Mn²⁺ concentrations are lowered in magnitude throughout the sediment column as a result of both decreased production and a relative increase in the effect of biogenic transport processes that exchange sediment solutes with overlying waters. During the winter, Mn²⁺ profiles reflect lowered rates of production. Concentrations near the sediment-water interface decrease because of the increased dominance of precipitation reactions and diffusive loss to overlying water compared with production rates. Overall Mn²⁺ production mimics the seasonal and, to a lesser extent, the depth-dependent production patterns of metabolites such as NH: (Part I). Fe²⁺ is also formed in abundance near the sediment-water interface during the early summer when the overlying water first begins to warm. Because of the associated increase in sulfide production, Fe²⁺ does not always rise to its maximum yearly concentration at this time. In the fall, like many other ions whose source is in the sediment, Fe²⁺ concentrations below the top few centimeters drop to their lowest value of the year. Unlike Mn²⁺, during the winter Fe²⁺ may reach its maximum standing concentration of the year at some stations. This presumably results from a net oxidation and loss of FeS and FeSz from surface sediment and an associated release of Fe²⁺ that does not immediately depend on strongly temperature-controlled microbial metabolism.