The potential volume of oceanic methane hydrates with variable external conditions

Abstract

Gas hydrate abundance in marine sediment depends on gas concentration and the available pore space within certain stability limits. Potentially, gas hydrates can occur between the seafloor and a locus of subbottom depths where geothermal gradients intersect gas–gas hydrate-pore water equilibrium curves. Perpendicular to a given continental margin, the lens shaped area between these two bounding surfaces ( A sl) varies according to seven basic parameters: gas composition, water activity ( a w), bottom water temperature ( T b), geothermal gradient ( G), slope depth ( z slb), slope gradient ( Z) and sea level relative to the shelf break ( z 0). Assuming pure CH 4 gas, ∼35 km 2 of sediment can host gas hydrate across an average continental margin at a Pleistocene lowstand ( a w =0.981, T b=0 °C, G=0 °C; z slb=4000 m; Z=0.04; z 0=0). However, this potential area would decrease with smaller a w, higher T b, greater G, shallower z slb, steeper Z and lower z 0, and increase with opposite external conditions. Of the basic parameters, temperature ( T b and G) and bathymetry ( z slb and Z) can particularly influence the distribution of gas hydrate on continental slopes. A hydrothermal gradient with e.g. surface temperatures> T b will also decrease A sl, although minimally, especially if T b is warm. The sum of parallel cross-sectional areas along a margin combined with porosity ( φ) gives the potential volume of gas hydrate ( V). Assuming ∼200,000 km of continental margin with a φ of 50%, ∼3.5×10 6 km 3 of pore space can contain gas hydrates, at present-day, a volume that compares favorably with previous estimates (1.2 to 6.4×10 6 km 3) although underlying approaches differ fundamentally. Since the Triassic, V Glob probably has increased significantly because T b has cooled while total margin length has grown. This trend was likely punctuated by at least one major decrease (nominally 1.5 to 0.7×10 6 km 3) when T b suddenly rose by ∼5 °C during the latest Paleocene thermal maximum (LPTM). A prominent global negative δ 13C excursion across the LPTM may signify massive release of CH 4 associated with this theoretical drop in V Glob.