Walter S. Borowski

In situ methane concentrations at Hydrate Ridge, offshore Oregon: New constraints on the global gas hydrate inventory from an active margin

The widespread presence of bottom-simulating reflectors (BSRs) on continental margins has bolstered suggestions that gas hydrates and free gas constitute a large dynamic reservoir of CH4 carbon and a vast potential source of energy. However, only a few hydrate-bearing areas have been drilled, and of these, the amount of CH4 has only been directly quantified in 18 discrete samples from 3 holes on Blake Ridge, east of Georgia. Here we report and discuss 30 direct measurements of CH4 concentration in sediments above and below the BSR at Hydrate Ridge on a tectonically active margin offshore Oregon. High CH4 concentrations (71–3127 m M ) support abundant gas hydrate (occupying an average of ∼11% of porosity) and free gas (occupying ∼4% of porosity in 1 sample) in a restricted area where hydrocarbon gases migrate from the deep accretionary complex to the seafloor. In a larger area lacking this hydrocarbon supply, lower CH4 concentrations (10–893 m M ) indicate less gas hydrate (average ∼1% of porosity) and little or no free gas. Overall, the amount of CH4 at Hydrate Ridge is significantly less than that at Blake Ridge. These results challenge certain interpretations, including the global volume of hydrate-bound CH4, which though large, may be four to seven times less than widely cited estimates. Speculations on the distribution and role of gas hydrate and free gas need revision.

Relationship of pore water freshening to accretionary processes in the Cascadia margin: Fluid sources and gas hydrate abundance

[1] Drilling in the Cascadia accretionary complex enable us to evaluate the contribution of dehydration reactions and gas hydrate dissociation to pore water freshening. The observed freshening with depth and distance from the prism toe is consistent with enhanced conversion of smectite to illite, driven by increase in temperature and age of accreted sediments. Although they contain gas hydrate -as evidenced by discrete low chloride spikes- the westernmost sites drilled on Hydrate Ridge show no freshening trend

Factors affecting the diagenesis of Quaternary sediments at ODP Leg 172 sites in western North Atlantic: evidence from pore water and sediment geochemistry

Abstract Pore waters and Quaternary sediments at ODP Leg 172 sites on the Carolina Slope (CS; Site 1054), Blake Outer Ridge (BOR; Sites 1057 and 1060), Bahama Outer Ridge (BAOR; 1062) and Bermuda Rise (BR, Site 1063) were studied. The sediments are mainly clayey and silty mud with intercalations of nannofossil-rich and lutite-rich beds towards the top. Sedimentation rate ranges from 4.2 cm/ky at Site 1054 to 23 cm/ky at Site 1060. Average total organic carbon (TOC) contents of the upper sediment units range from 0.40% to 0.58% at Sites 1057, 1060, 1062 and 1063, and from 0.86% to 1.25% at Site 1054. The organic matter at all the sites is mostly degraded, except at Site 1054. Sediments at Sites 1062 and 1063 have lower total sulfur (TS) (0.00–0.62%; average: 0.06%) than those at the other sites. Site 1054 has the highest TS (0.8–1.4%) and the lowest reactive iron (0.003–0.074%) contents among all the sites. The deep-water Sites 1062 and 1063 have concave-down pore-water sulfate concentration/depth profiles, relatively deep sulfate/methane interfaces (SMIs) (65 and 38 meters below sea floor (mbsf), respectively) and steep methane gradients at the base of the SMI, whereas the shallow-water Sites 1054, 1057 and 1060 have near-linear sulfate and relatively shallow methane gradients. The near-linear sulfate/depth profiles and shallow SMIs (17 and 14 mbsf, respectively) at Sites 1057 and 1060 are primarily due to high sulfate consumption by anaerobic methane oxidation (AMO) at the SMI. Concave-down profiles and deep SMIs at Sites 1062 and 1063 are the result of low rates of sulfate consumption through organic matter degradation, due to a limitation of metabolizable organic matter. Site 1054 is unusual in having an 8-m thick oxic–suboxic zone, a 48-m deep SMI and a linear sulfate/depth profile. Despite the high organic matter content of sediments at this site, the thick oxic–suboxic zone is mainly due to intense bioturbation, while the linear sulfate/depth profile and the deep SMI are likely caused by low sedimentation rate and by “reactive” iron limitation. Pore-water magnesium, calcium and strontium profiles indicate that carbonate precipitation has occurred near the SMI. This is confirmed by the presence of authigenic dolomite rhombs and nodules especially at and below SMI at all sites.

Effect of massive gas hydrate formation on the water isotopic fractionation of the gas hydrate system at Hydrate Ridge, Cascadia margin, offshore Oregon

Because gas hydrate is preferentially enriched in the heavy water isotopes, the δ18O and δD values of pore waters collected from gas hydrate–bearing sediment can provide information on the abundance and mechanisms of gas hydrate formation. Pore waters sampled from deep-seated (40 to 125 mbsf) gas hydrate deposits in Hydrate Ridge during ODP Leg 204 show depletion in dissolved Cl− and enrichments in 18O and D due to gas hydrate destabilization during core recovery. The oxygen and hydrogen isotopic fractionation factors (αO = 1.0025 and αH = 1.022) estimated from an extensive data set (n = 30 samples) correspond to experimentally determined values. In contrast, pore waters from shallow samples (<25 mbsf) at the ridge summit (n = 32) are highly enriched in dissolved Cl− and depleted in 18O and D, consistent with formation of massive gas hydrate deposits at rates faster than those at which these anomalies would be removed by advection or diffusion. The water isotopic fractionation factors in the brine are significantly lower than those experimentally determined, with αO of 1.0010 (average value of 1.0012) and αH of 1.008 (average value of 1.008). We discuss several factors that may be causing this anomalous fractionation and suggest that low gas occupancy in hydrate lattice (high hydration number) may be responsible for the observed small fractionation. If this were the case, the oxygen and hydrogen fractionation may serve as an indicator of hydration number during formation of gas hydrate in natural systems.