Large volumes of methane are sequestered in marine sediments along deep continental margins by an icy solid known as methane clathrate (or methane hydrate). Geologic data from the Paleocene document a methane release at that time which is comparable to the size of the current inventory of traditional fossil fuels (the Paleocene Eocene thermal maximum event, or PETM). The timing of the methane release coincides with deep sea warming which is not radically different from the long-term global warming forecast. Thus the prospect of an eventual release of methane from clathrate in response to anthropogenic climate change in the future warrants serious consideration. The clathrate carbon reservoir contains more carbon than the atmosphere, the biosphere, soils, and fossil fuels combined, but the stability of this reservoir appears precarious. The physical stability of the clathrates relies on the cold temperatures and high pressures of the deep sea. Clathrate is buoyant in water, like regular water ice, and is held in the deep sea by the weight of overlying sediment. The dissolved methane that the clathrate equilibrates with is chemically unstable against oxygen and sulfate in the modern ocean. In this proposal we show a preliminary model result that on geologic time scales the size of the steady-state clathrate reservoir may be extremely sensitive to the temperature and oxygenation of the deep ocean. This model combines two previously existing components: (1) the Muds model for early diagenesis of organic matter in sediments, and (2) the Davies and Buffett model of the physics and chemistry of methane clathrate formation. The model reproduces the present-day estimates of clathrate in the ocean, but predicts almost no clathrates as the ocean temperature is raised by 6C, the condition before the PETM. The model is able to store enough clathrate in the Paleocene ocean to account for the PETM if we assume a cool and anoxic Arctic ocean. This analysis however tells us nothing about the transition from one steady state condition to another, a shortcoming we propose to address. We will devote particular attention to the structural stability of the sediment column under conditions of methane phase change, by incorporating pore pressure and metastable clathrate behavior into the clathrate model. Intellectual Merit: In summary, we propose to characterize the long- and short-term stability of the clathrate reservoir in the ocean as a component of the climate and carbon cycle of the earth. (1) What is the sensitivity of the steady-state clathrate reservoir to temperature and oxygenation of the deep ocean? (2) How quickly does the clathrate reservoir respond to changes in forcing? (3) Under what conditions might we expect catastrophic release of methane, as in the PETM? (4) Is there a positive feedback between temperature and the clathrate reservoir? Broader Implications: Global warming will undoubtedly destabilize clathrate in the future, but the extent and consequences of any possible methane release are not known.
