A kinetic model for the methane hydrate precipitated from venting gas at cold seep sites at Hydrate Ridge, Cascadia margin, Oregon

Abstract

[1] We develop a kinetic model for hydrate crystallization from methane gas venting through shallow sediments at Hydrate Ridge on the Cascadia margin of Oregon that predicts how pore water chlorinity, temperature, and crystallized hydrate evolve after the onset of steady venting. Predictions are compared to observations at Ocean Drilling Program Site 1249. In the preferred model, calculated gas hydrate saturation and chloride concentrations reach those observed at depths less than 20 m below seafloor (bsf) under the southern summit of Hydrate Ridge in~650 years, and the vertical water flux must be less than 50 kg/m 2 /yr. Hydrate accumulates more slowly between 20 m bsf and the base of the hydrate stability zone where there is no free gas, accumulating to observed levels of a few volume percent of hydrate in 10 5 to 10 6 years, depending on the water flux that is assumed through this zone. This dichotomy means that the presently observed gas venting must have been diverted to this area~650 years ago, or be episodic and infrequent. If the gas venting for the last 650 years has been as observed today, the latent heat of hydrate precipitation in the upper 20 m of sediments would have caused the temperature to increase~0.8°C at~20 m bsf and~0.2°C at~100 m bsf. This would have caused a~5 m rise in the elevation of the base of hydrate stability zone, and decreased the rate of hydrate crystallization from 1.

An Areal Assessment of Subseafloor Carbon Cycling in Cold Seeps and Hydrate-Bearing Areas in the Northern South China Sea

“…Gas hydrates are usually closely coupled with cold seeps at continental slopes. On the continental margins, methane produced through either microbial methanogenesis or pyrolysis of organic matter can precipitate as hydrates when the dissolved methane concentration exceeds methane hydrate solubility within the gas hydrate stability zone [2,[5][6][7]. The gas hydrates will decompose and release large amounts of methane into the water column upon the pressure and/or temperature changes induced by global warming and sea level changes.…”

Synchrotron X-ray computed microtomography study on gas hydrate decomposition in a sedimentary matrix

“…Furthermore, the revealed decomposition regime with hydrate surfaces exposed to pore gas ablating toward the quartz grain surface, rather than evolving at the quartz‐hydrate interface, will also have positive effects on the further improvements of the sediment permeability during decomposition; because the latter scenario will eventually result in a pore‐filling pattern of hydrates in pores. As an example gas venting and water flux via the pore network was studied for cold‐vent shallow GHs at Cascadia margin with implications for their formation age [ Cao et al ., ]; the transport could take place within the sediments as well as along the liquid layers at the hydrate‐grain interfaces.…”

Impact of anaerobic oxidation of methane on the geochemical cycle of redox-sensitive elements at cold-seep sites of the northern South China Sea

“…Masuzawa et al, 1992;Chen et al, 2010;Luo et al, 2013). Methane that ascends upwards along seabed sedimentfractures is intermediately stored in hydrate deposits (Tryon et al, 2002;Cathles, 2003, 2005;Cao et al, 2013aCao et al, , 2013b, which host about 500-2000 Gt of carbon worldwide (Wallmann et al, 2012). The total reservoir of methane along continental slopes is even larger, as much of the methane can be found dissolved or in gas bubbles below and above the hydrate stability zone (Buffett and Archer, 2004).…”