Early diagenesis of amino acids and organic matter in two coastal marine sediments

Abstract

Surface sediments of Buzzards Bay, Massachusetts (BBP) are oxic to suboxic and are extensively bioturbated by deposit-feeding infauna, while sediments of the Pettaquamscutt River Estuary, Rhode Island (PRE) are reducing and overlain by anoxic bottom water. Sediments from these two sites were analyzed for total organic carbon and nitrogen, total hydrolyzable amino acids, dissolved free amino acids, and porewater dissolved organic carbon, ammonium, total carbon dioxide, and sulfide, to describe and compare the early diagenesis. Based on model calculations, organic matter remineralization rates in the upper meter of BBP and PRE sediments are of similar magnitude, at 14 and 10 g C/m 2· year, respectively. The surface sediment organic carbon accumulation rates at the two sites are also similar, 36 (BBP) and 30 (PRE) g C/m 2 · year. Thus, the unusually high organic content of PRE sediments (>12% by weight organic carbon) does not result from much greater deposition rates or from much lower decomposition rates than in BBP sediments, which contain only 2% organic carbon. Total hydrolyzable amino acids made up 11 to 23% of the total carbon remineralized in BBP sediments. Decomposition did not result in detectable changes in hydrolyzable amino acid composition at either site. Glutamic acid and β-aminoglutaric acid were major constituents of the sediment dissolved free amino acids. Dissolved free amino acid distributions in these sediments are probably the net result of production and consumption by bacteria. Macrofauna and adsorption to sediments are additional sinks for dissolved amino acids in Buzzards Bay sediments, but they do not appear to be responsible for the major features of the dissolved free amino acid distributions.