METHANOGENESIS IN PARALIC CARBONATES - A POSSIBLE FEEDBACK TO GLOBAL WARMING

Siliciclastic marine sediments are not significant contributors to atmospheric methane because the "cap" of sulfate reducers and methanotrophs above the methanogenic zone results in oxidization of methane within the sediment column. Under optimum conditions, however, paralic carbonate sediments may be significant sites of methane generation and release independent of sulfate reduction. At ODP Sites 1127,1129 and 1131, in the Great Australian Bight, methanogenesis and sulfate reduction occur simultaneously in the hypersaline pore waters, leading to extremely high concentrations of both CH₄ and H₂S throughout the carbonate sediment column. Amines, derived from the proteins within the skeletal structures of carbonates and leaching into the pore waters, apparently provide a non-competitive substrate for the methanogens, allowing them to co-exist with the sulfate-reducers. Other hypersaline ODP sites in the same region exhibit negligible concentrations of both CH₄ and H₂S. These older, more slowly accumulating, carbonate sediments have lost most of their skeletal proteins and amines and are incapable of independently fueling the methanogens. As these relationships suggest, paralic carbonate sediments may be a significant and unrecognized contributor to contemporary atmospheric methane concentrations.

Widespread accumulation of carbonates in epeiric seas may have led to even greater input of methane to the atmosphere during previous times of warm climates and high sea levels. These higher levels of methane would then have been a positive feedback to the greenhouse climate, enhancing the average temperature, and leading to further increases in sea level, additional areas for carbonate accumulation, and further methane production. Eventually, extensive burial of carbonate sediments would result in a decrease in atmospheric CO₂ and global cooling.