Diagenetic processes near the sediment-water interface of long island sound. I. Decomposition and nutrient element geochemistry (S, N, P)

(1) Pore-water profiles of SO^2- ₄, alkalinity, HPO^2- ₄, and NH⁺ ₄ in the top 1 m of sediment differ radically in different depositional environments of Long Island Sound. The greatest evidence for extensive sulfate reduction and organic matter decomposition as measured by standing concentrations of pore-water metabolites is found in shallow water along the north coast of the Sound; in deeper water the buildup of decomposition products or depletion of SO^2- ₄ in sediment pore water is greatly lowered. (2) In contrast to pore-water data, analysis of the solid phase shows extensive and comparable production of sulfide fixed as FeS (acid-volatile sulfide) in all areas. Deep-water stations show extensive loss of solidphase sulfide either prior to or after conversion of FeS to FeS2 (pyrite). All stations have comparable standing crops of organic matter and all stations are capable of supporting approximately the same rate of sulfate reduction in the upper 10 cm of sediment where most decomposition takes place. (3) All stations show similar seasonal variation in pore-water profiles near the sediment-water interface. These are periodic and repeatable from year to year. During the late spring and summer, microbial activity results in high production of NH^2- ₄, HPO^2- ₄. and alkalinity throughout the sediment column but particularly near the interface. Because decomposition rates often show exponential decrease with depth below the interface, seasonally high production often results in maxima in pore-water concentrations a few centimeters below the sediment surface. In late summer and in the fall, biogenic reworking, particularly fluid transport associated with solute diffusion into burrows, results in extensive exchange of sediment pore water with overlying water and a general net lowering of NH^2- ₄, HPO^2- ₄, and alkalinity concentrations or raising of SO^2- ₄ from the earlier summer levels. During the winter, pore-water profiles reflect greatly decreased biological influence. The year can therefore be divided into a relatively warm period (spring, summer, fall) of different types of apparent biological dominance of diagenesis followed by a shorter cold period (winter) when processes are more physically controlled.