Methane production in the sulfate-depleted sediments of two marine basins

Abstract

Rates of methane production via both acetate fermentation and CO 2 reduction were directly measured with radiotracer techniques in the sulfate-depleted sediments of Saanich and Princess Louisa Inlets. Comparison of measured and modelled rates suggests that these two pathways account for the majority of methane produced below the sulfate reduction zone in the sediments of both basins. Methane production via CO 2 reduction was slightly more important than acetate fermentation with 57–58% of the methane in Saanich Inlet and 52–57% in Princess Louisa Inlet being produced from bicarbonate. The results from Saanich Inlet, a seasonally anoxic basin, are compared with Princess Louisa Inlet, with a permanently oxic hypolimnion. Although the two basins have comparable organic-carbon rain rates, the rates of methanogenesis are much lower in Princess Louisa Inlet. This decrease in methane production can be attributed to the consumption of organic carbon via aerobic respiration occurring in the surface sediments of Princess Louisa Inlet, thereby decreasing the actual input of organic carbon to the zone of methane production. The relative importance of CO 2 reduction and acetate fermentation in the production of methane was the same in both basins, suggesting that prior aerobic degradation of the organic matter has little influence on the pathways of methane production. The results from this study in the two marine systems (high sulfate) are also compared to published studies in freshwater environments (low sulfate) where acetate fermentation is the predominant pathway of methane production.