Limnol. Oceanogr., 44(5), 1999, 1323–1330

Proteinaceous material usually accounts for much of the total nitrogen and organic carbon in marine sediments. Thus, decomposition of protein is frequently investigated as a measure of labile organic matter turnover. The fraction of protein that escapes remineralization to CO2 undergoes transformations that may reflect pathways of preservation in sedimentary environments. We analyzed the molecular weight distribution of sedimentary proteinaceous material extracted with NaOH and evaluated the results in terms of diagenetic changes experienced by this pool. Total protein concentrations were determined in Long Island Sound sediments as total hydrolyzed amino acids (THAA), the sum of concentrations of individual amino acids measured after acid hydrolysis. Although the total fraction of THAA extracted by NaOH decreases slightly downcore, from 55% to 40%, the molecular size of that THAA increases. Proteinaceous material .100 kD increases in relative importance with depth and age of sediment, reaching 70% of the total THAA pool at 3 m depth, or roughly 1,000 yr of sediment accumulation. The fraction of THAA between 10 and 100 kD decreased from 25% to 5% of the total over this depth. The relative abundance of the peptide-size fraction (,3 kD) remained essentially unchanged. The processes of bacterial production, selective adsorption, geopolymerization, and selective preservation were explored to explain these changes. Most of the characterized organic nitrogen in organisms, seawater, and marine sediments is in amino acids, many of which are present as proteins. Protein nitrogen accounts for 42–72% of the nitrogen in marine plankton (Degens and Mopper 1976), 40–65% of sinking particulate organic nitrogen (Lee and Cronin 1982), and 40–60% of total nitrogen in coastal surface sediments (Henrichs et al. 1984; Burdige and Martens 1988). Determination of protein concentration in sediments is of interest in organic geochemistry because amino acid nitrogen is one of the most labile fractions of total nitrogen in sediments, and amino acid degradation supports both microbial production and regeneration of NH (Blackburn 1986; Burdige and Martens 1988). Because 4 some protein is preserved, however, there is also interest in evaluating structural changes as the protein pool undergoes diagenesis. Protein concentration was measured as total hydrolyzed amino acids (THAA) in surface and downcore sediments in Long Island Sound. THAA was measured in surface sediments over a 1-yr cycle, covering high and low phytoplankton productivity periods in the water column of the Sound (prebloom, bloom, and postbloom). The nature of the sedimentary protein was further explored by examining molecular weight (MW) distribution of proteinaceous material as a function of sediment depth and season. The MW distri1 Present address: Department of Marine Chemistry and Geochemistry, Woods Hole Oceanographic Institution, Woods Hole, Massachusetts 02543-1543.