Upward shifts in the southern Hydrate Ridge gas hydrate stability zone following postglacial warming, offshore Oregon

Abstract

High‐resolution three‐dimensional (3‐D) seismic reflection data acquired on the R/V Thomas G. Thompson in 2000 reveal a pair of bottom simulating reflections (BSRs) across a broad region of southern Hydrate Ridge, offshore Oregon. The primary BSR (BSR p ) is a regionally extensive reflection that lies 120–150 m below seafloor and exhibits typical characteristics of a gas hydrate BSR. We also imaged a second weaker BSR (BSR s ), 20–40 m below BSR p , with similar characteristics. BSR s is interpreted as a remnant of a BSR that probably formed during the Last Glacial Maximum 18,000 years ago, when the base of the gas hydrate stability zone (GHSZ) was deeper. An increase in bottom water temperatures of 1.75°–2.25° and a corresponding sea level rise of 120 m could have produced the BSR shift. The preservation of BSR s for at least 5000 years, which is the time since subseafloor temperatures stabilized following ocean warming after the Last Glacial Maximum, implies very slow upward advective and diffusive flow of methane (<1 m/1000 years in the vicinity of BSR s ). BSR s appears where there are no resolvable steeply dipping faults and fractures, consistent with very low advective flow rates, and has dispersed where vertical fractures are visible. Free gas released by the shift in the BSR either migrates so slowly that it remains stable beneath the GHSZ or is directed upward along fractures to reform as hydrate in the GHSZ. There is no evidence for release of this free gas into the ocean or atmosphere.