Two-dimensional modeling of gas hydrate decomposition in the northwestern Gulf of Mexico: significance to global change assessment

Abstract

Thinning of the gas hydrate stability zone (GHSZ) in response to bottom water temperature increases and drops in sea level similar to those during Pliocene and Pleistocene time was modeled on two-dimensional (2D) regional scale in the northwestern Gulf of Mexico. A sea level drop of 100 m is unlikely to significantly influence the stability of gas hydrate, especially when coupled with an expected decrease in water temperature. A bottom water temperature increase of 4 °C may lead to appreciable (∼30%) thinning of the GHSZ. Neither a 100-m drop in sea level nor a 4 °C bottom water temperature increase is hypothesized to initiate significant gas flux from decomposition of gas hydrate. Hydrocarbon gases may have been released from decomposed gas hydrate to sediment at a rate considerably lower than the preliminary estimated late Pleistocene–Holocene total gas seepage from a leaky subsurface petroleum system. Potential input of greenhouse gases into the ocean is suggested to be less significant. Several processes such as recrystallization of gas hydrate in the GHSZ, trapping of free gas below the GHSZ, and microbial oxidation of hydrocarbons in sediment by bacteria and archaea contribute to sequestration and destruction of gas from gas hydrate decomposition. Gas hydrate in the Gulf of Mexico is frequently associated with enormous volumes of authigenic carbonate rock, depleted in 13C, that sequesters a large pool of carbon in sediment and perturbs the carbon cycle. These factors appear to significantly decrease the role of hydrate-derived gas in global change. An improved understanding of how the gas hydrate system of the Gulf of Mexico responds to natural variation will contribute to better assessment of gas hydrate as an agent of global change.