The carbon isotope biogeochemistry of acetate from a methanogenic marine sediment

The δ¹³C 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 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 δ¹³C 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.