Simulation of Arctic Gas Hydrate Dissociation in Response to Climate Change: Basin-Scale Assessment

Abstract

Abstract Paleooceanographic evidence has been used to postulate that methane from oceanic hydrates may have had a significant role in regulating climate. However, the behavior of contemporary oceanic methane hydrate deposits subjected to rapid temperature changes, like those now occurring in the arctic and those predicted under future climate change scenarios, has only recently been investigated. Field investigations have discovered substantial methane gas plumes exiting the seafloor along the Arctic Ocean margin, and the plumes appear at depths corresponding to the upper limit of a receding gas hydrate stability zone. It has been suggested that these plumes may be the first visible signs of the dissociation of shallow hydrate deposits due to ongoing climate change in the arctic. We simulate the release of methane from multiple oceanic deposits, including the effects of fully-coupled heat transfer, fluid flow, hydrate dissociation, and other thermodynamic processes for systems representative of segments of the Arctic Ocean margin. The model encompasses a cross section of marginal sediments from the upper extent of the hydrate stability zone down to depths beyond the expected range of century-scale temperature changes. We impose temperature changes corresponding to predicted rates of climate change-related ocean warming and examine the possibility of hydrate dissociation and the release of methane. The simulation results are consistent with the hypothesis that dissociating shallow hydrates may contribute to the formation of undersea methane plumes, and that dissociating hydrate along can result in significant methane fluxes at the seafloor. However, the methane release is likely to be confined to a narrow region of high dissociation susceptibility, defined by depth and temperature, and that any release will be continuous and controlled, rather than explosive. These results also indicate the possibility that hydrate dissociation and methane release may be both a result and a cause of climate change. This modeling also establishes the first realistic bounds for methane release along the arctic continental shelf for potential hydrate dissociation scenarios, and ongoing work may help confirm whether climate change is already impacting the stability of the vast oceanic hydrate reservoir. Introduction Gas hydrates are solid crystalline compounds in which gas molecules are lodged within the lattices of water clathrate crystals1. Natural gas hydrate deposits occur in geologic settings where the necessary low temperatures and high pressures exist for their formation and stability--in the permafrost and in deep ocean sediments. The existing literature indicates that the global methane hydrate reserves are enormous. These estimates began with an initial ?consensus value? of 10,000 Gt through work by various investigators.2,3,4 More recently, Milkov5 proposed a total of 500–2,500 Gt of methane-derived carbon, but two of the most recent studies have produced a wider range, from an upper estimate of 74,400 Gt of methane carbon in hydrate form6 (27,300 Gt along continental margins) a lower estimate of 3,000 Gt of methane in hydrate and 2,000 Gt of gaseous methane existing in a stable state under current climate conditions7.