Gas hydrate growth, methane transport, and chloride enrichment at the southern summit of Hydrate Ridge, Cascadia margin off Oregon

At the summit of Hydrate Ridge (ODP Sites 1249 and 1250), pore fluids are highly enriched in dissolved chloride (up to 1370 mM) in a zone that extends from near the sediment surface (similar to1 mbsf) to depths of 25 5 mbsf. Below this depth, brines give way to chloride values approaching seawater concentrations with lower chloride anomalies superimposed on baseline values. We developed a one dimensional, non-steady state, transport reaction model to simulate the observed chloride enrichment at Site 1249. Our model shows that in order to reach the observed high chloride values, methane must be transported in the gas phase from the depth of the BSR to the seafloor. Methane transport exclusively in the dissolved phase is not enough to form methane hydrate at the rates needed to generate the observed chloride enrichment. Methane transport in the gas phase is consistent with geophysical and logging data, estimates of gas pressure beneath the BSR, and observations of bubble plumes at the seafloor. In order to reproduce the observed chloride and gas hydrate distributions, the model requires an enhanced rate of hydrate formation in near surface sediments, which we implement through depth-dependent kinetic constants. We argue that this is justified by changes in geomechanical properties of the sediment. At depths shallower than 25 mbsf the force of crystallization can overcome effective overburden stress, and hydrate growth proceeds by particle displacement, thus minimizing capillary inhibition effects. Our calculations indicate the hydrates in the upper sediments of the ridge summit are probably younger than 1500 years, although the age is difficult to constrain. Independent estimates based on seafloor observations at this site yield gas hydrate formation rates at the ridge crest on the order of 10(2) mol m(-2) year(-1). These rates are several orders of magnitude higher than those estimated for Site 997 on the Blake Ridge. (C) 2004 Elsevier B.V. All rights reserved.