METHANE HYDRATE IN THE GLOBAL ORGANIC CARBON CYCLE--THE BIOGEOLOGIC CONTROL OF METHANE

The global occurrence of methane hydrate in outer continental margins and in polar regions, and the amount of methane in hydrate form suggest that methane hydrate is an important component in the global organic carbon cycle. However, the role of methane in this cycle is complex, having two possible sources: biologic and geologic. Biologic methane is microbially derived in the shallow geosphere from recent organic matter, whereas geologic methane comes from the thermal alteration of ancient organic matter deep within the geosphere. Methane hydrate, being both a sink and source of microbial and thermal methane, is a major reservoir that geologically controls the amount of methane that leaks into the hydrosphere/atmosphere system. A second control is biological methane oxidation, both anaerobic and aerobic, which converts leaking methane to carbon dioxide. Thus methane-hydrate storage and methane oxidation operate in concert to limit the leakage of methane. These controls have implications for global climate change because the storage and conversion of biogeological methane strongly limit the amount of methane that ever reaches the atmosphere where it can act as a strong greenhouse gas. To capture in a simplistic way the possible role of methane hydrate in global change, including global climate change, the global organic carbon cycle is visualized as being analogous to the workings of an electric circuit. In this circuit, the methane hydrate is a condenser and the global-change consequences of methane hydrate formation and decomposition are depicted as a resistor and inductor, which are connected to a timer via a feedback loop. Possible global changes influenced by methane hydrate include changes in ocean chemistry, temperature, climate, glacial history, sea level, and sediment stability.