Science-based dating in archaeology

Abstract

Microbial methanogenesis is a major source of the greenhouse gas methane (CH4). It is the final step in the anaerobic degradation of organic matter when inorganic electron acceptors such as nitrate, ferric iron, or sulfate have been depleted. Knowledge of this degradation pathway is important for the creation of mechanistic models, prediction of future CH4 emission scenarios, and development of mitigation strategies. In most anoxic environments, CH4 is produced from either acetate (aceticlastic methanogenesis) or hydrogen (H2) plus carbon dioxide (CO2) (hydrogenotrophic methanogenesis). Hydrogen can be replaced by other CO2-type methanogenesis, using formate, carbon monoxide (CO), or alcohols as substrates. The ratio of these two pathways is tightly constrained by the stoichiometry of conversion processes. If the degradation of organic matter is complete (e.g., degradation of straw in rice paddies), then fermentation eventually results in production of acetate and H2 at a ratio of > 67% aceticlastic and < 33% hydrogenotrophic methanogensis. However, acetate production can be favored when heterotrophic or chemolithotrophic acetogenesis is enhanced, and H2 production can be favored when syntrophic acetate oxidation is enhanced. This typically occurs at low and elevated temperatures, respectively. Thus, temperature can strongly influence the methanogenic pathway, which may range from 100% aceticlastic methanogenesis at low temperatures to 100% hydrogenotrophic methanogenesis at high temperatures. However, if the degradation of organic matter is not complete (e.g., degradation of soil organic matter), the stoichiometry of fermentation is not tightly constrained, resulting, for example, in the preferential production of H2, followed by hydrogenotrophic methanogenesis. Preferential production of CH4 by either aceticlastic or hydrogenotrophic methanogenesis can also happen if one of the methanogenic substrates is not consumed by methanogens but is, instead, accumulated, volatilized, or utilized otherwise. Methylotrophic methanogens, which can use methanol as a substrate, are widespread, but it is unlikely that methanol is produced in similar quantities as acetate, CO2, and H2. Methylotrophic methanogenesis is important in saline environments, where compatible solutes are degraded to methyl compounds (trimethyl amine and dimethyl sulfide) and then serve as non-competitive substrates, while acetate and hydrogen are degraded by non-methanogenic processes, e.g., sulfate reduction.