THE CARBON ISOTOPE BIOGEOCHEMISTRY OF ACETATE FROM A METHANOGENIC MARINE SEDIMENT

The delta-C-13 value of porewater acetate isolated from the anoxic sediments of Cape Lookout Bight, North Carolina, ranged from -17.6 parts per thousand in the sulfate reduction zone to -2.8 parts per thousand in the underlying methanogenic zone. The large C-13-enrichment in the sulfate-depleted sediments appears to be associated with the dissimilation of acetate to CH4 and CO2. Fractionation factors for that process were estimated to be 1.032 +/- 0.014 and 1.036 +/- 0.019 for the methyl and carboxyl groups. A subsurface maximum in delta-C-13 of the total acetate molecule, as well as the methyl and carboxyl carbons at 10-15 cm depth within the sediment column indicates that changes in the relative rates of acetate cycling pathways occur in the methanogenic zone. The methyl group of the acetate was depleted in C-13 by 7-14 parts per thousand relative to the carboxyl moiety. The intramolecular heterogeneity may be the result of both synthetic and catabolic isotope effects.