The carbon isotope biogeochemistry of acetate from a methanogenic marine sediment

The δ 13C value of porewater acetate isolated from the anoxic sediments of Cape Lookout Bight, North Carolina, ranged from −17.6% in the sulfate reduction zone to −2.8% in the underlying methanogenic zone. The large 13C-enrichment in the sulfate-depleted sediments appears to be associated with the dissimilation of acetate to CH 4 and CO 2. 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 δ 13C 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 13C by 7–14% relative to the carboxyl moiety. The intramolecular heterogeneity may be the result of both synthetic and catabolic isotope effects.