Gkdtimican zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA Camachimiur Acia Vol. 51. pp. I-IS zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA 63 papmon Jounulr Ltd. 1987.Printed in U.S.A. 001~7037/8763.00 + .a? Early diagenesis of amino acids and organic matter in two coastal marine sediments* SUSAN M. HENRICHS Institute of Marine Science, University of Alaska, Fairbanks, zyxwvutsrqponmlkjihgfedcbaZYXW AK 99701, U.S.A. and JOHN W. FARRINGTON Chemistry Department, Woods Hole Oceanographic Institution, Woods Hole, MA 02543, U.S.A. (Received June 11, 1985; accepred in r~jsed~~ September 15, 1986) AM-Surface sediments of Buzzards Bay, Massachusetts @BP) are oxic to suboxic and are. extensively biomrbated by deposit-feeding infauna, while sediments of the Pettaquamxutt 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 caltitions, organic matter remineralization rates in the upper meter of BBP and PRE sediments are of similar magnitude, at 14 an& 10 g C/m2 * year, mspectivtly. The surface sediment organic carbon a~mu~tion rates at the two sitea are also similar, 36 (BBP) and 30 (PRE) g C/m” l year. Ths, the unusually high organic content of PRE sediments (~12% by weight organic carbon) does not result from much greaterdeposition rates or from muchlower decomposition’rates than in BBP sediments, which containonly 2%organic carbon. Total hydrolyzabk! aminoacids madeup i 1 to 23% of the total carbon remineralized in BBP sediments. &compositiondid not resultin detectable &ages in blip amino acid composition at eithersite. Glutamic acidand ~rn~u~~ acid were major ~n~~~ of the sediment dissolved freeaminoacids. Dissolved f’ree amino acid distributioas in thesesediments arc probably the net result of production and consumption by bacteria. hfacrofauna and adsozpdon to sediments are additional sinks for dissolved amino acids in Buzzards Bay sediments, but they do not am to be responsible for the major features of the zyxwvutsrqpo dissolved free aminoaciddistributions. INTRODUCTION Bu- BAY IS located southwest of Cape Cod, Massachusetts and covers an area of a~~~~ly 460 km2 to an average depth of 13 to 16 m in the central basin (HOUGH,1940). Buzzards Bay, Station P (BBP) is located at a depth of 17 zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA m in the southern part of the bay, near open water. Primary p~u~~~ shows a large seasonal variation, from 100 to greater than 500 mg C/m’*day, with the productivity maxi- mum extending from June to November or December (ROMAN and TENORE, 1978).Sediments at this station have been the subject of several previous organic geo- chemical studies dealing with fatty acid (FARRINGTON et uf., 1977a), hydrocarbon (FARRINGTON et ai., 1977b),and sterol (LIZ et al.. 1977) ~~butio~ and diagenesis. The sediment at BBP is a ciayey silt pop ulated primarily by deposit-feeding infauna which are predominantly two species, zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA Nephtys incisu (a poly- chaete worm) and N~u proxima (a bivalve moliusc) (SAWERS,1958). Animals biotu~te the upper two to three centimeters of sediment extensively; Nephrys may burrow up to 10 cm depth (RHOADS, 1967). The pcitaquamscutt River (PRE) is a shallow, drowned-valley type estuary which flows south into Rhode Island Sound. Two basins are located at its northern end: the upper basin which has an area of * W.H.O.I. contribution no. 6327. I 0.3 km” and a maximum depth of 13.5 m, and the lower basin which has an area of 0.7 km* and ii max- imum depth of 19.5 m. Seawater enters the basins via the shallow river channel (about I m deep) and is trapped beneath the freshwater outflow. This saline bottom water stagnates and very high concentrations of sulfideaccumulate, up to 4.5 mM (ORR and GAINSZS, 1974; GAINES and PUON, 1972). Autumn mixing events partially ventilate the water column at intervals averaging three to four years. The bottom sediments are a highly organic diatomaceous ooze, with very low aluminosilicate mineral content. The PRE has been described in detail by GAINES ( 1975). This paper presents results of a study of organic matter dia8enesis in Buzzards Bay and Pettaquamscutt River Estuary sediments. Dia8enesisis discussed &om the perspectiveof three data sets:total sediment organic carbon (TOC) and nitrogen (TN), total hydrolyzable amino acids (THAA), and dissolved free amino acids (DFAA).Hyping amino acids usually decompose more rapidly in sediments than TOC (e.g., WHIXAN, 1977; ROSENFELQ 1979b; MAITA et al., 1982; HEN- RKHS et al., 1984). However, in the upper meter of nearshore and continental shelf sediments the relative decrease in THAA is usually less than two or three times that of TOC, and the composition of THAA rarely changes significantly with depth (ROSENFELD, 1979b; MAITA et al., 1982; HENRICHS ef al,, 1984). In contrast, both the concentration and camposition of