~(13)C enrichments in periglacial biogenic carbonates may be due to acetogenic processes instead of methanogenesis

摘要

Biomineralized secondary carbonate deposits in periglacial limestone fissures typically show enrichments in heavy carbon (~(13)C) compared to the host rock. Previous investigations have implicated the role of methanogenic bacteria in creating these enrichment signals. Acetogenic bacteria may play an equal or greater role in cycling carbon. Our research proposes a new "mixotrophic" acetogenic reaction where CO_2 and a partially-reduced organic compound (such as methanol) combine to reduce the dependence of acetic acid production on molecular hydrogen (H_2). Furthermore, acetogenesis appears to enhance limestone dissolution and hence reduce the depletion of residual DIC by biogenic CO_2. Radiogenic age dating using biogenic carbonates needs to account for the different mixing ratios of new and old carbon-bearing materials.