Methane in Earth History

Abstract

Naturally occurring methane has two sources, defined as biogenic methane and thermogenic methane. Biogenic methane is the product of microbial activity, the formation of thermogenic methane is controlled by temperature, pressure and also by the composition of organic matter preserved in the sediments. Gas hydrates form the largest methane sink in the methane cycle. Methane hydrate is a mix of water molecules forming an ice structure and methane which is entrapped in frozen water. Methane hydrates are stable under well-defined temperature and pressure conditions, mainly beneath the sea floor and in soils and sediments in arctic environments. Destabilization of methane hydrates due to temperature or pressure change will trigger a rapid release of massive amounts of methane into the ocean-atmosphere system. The most reliable proxy for tracing the methane cycle through geological time is carbon isotope geochemistry. Prominent negative “spikes” identified in the Phanerozoic C-isotope record provide evidence for a rapid addition of light carbon from a volcanic or clathrate source to the Ocean-Atmosphere reservoir. Negative spikes observed only locally in C-isotope records can also be the result of diagenetic processes that occur when anaerobic oxidation of methane (AOM) triggered precipitation of diagenetically light calcite cement in hemipelagic carbonaceous sediments.