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

Abstract

[1] Because gas hydrate is preferentially enriched in the heavy water isotopes, the d 18 O and dD 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 À and enrichments in 18 O and D due to gas hydrate destabilization during core recovery. The oxygen and hydrogen isotopic fractionation factors (a O = 1.0025 and a H = 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 À and depleted in 18 O 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 a O of 1.0010 (average value of 1.0012) and a H 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. Tomaru, H., M. E. Torres, R. Matsumoto, and W. S. , Effect of massive gas hydrate formation on the water isotopic fractionation of the gas hydrate system at Hydrate Ridge,

Sources of fluids and gases expelled at cold seeps offshore Georgia, eastern Black Sea

“…Compared to the seawater value of d 37 Cl = 0&, d 37 Cl values of pore fluids of Pechori Mound indicate significant Cl isotope fractionation either in the formation depth or on its migration pathway. Similar to observations by Dählmann et al (2004) for low-Cl fluids from the Anaximander Mountains and the Milano dome site (Mediterranean), all Pechori Mound samples carry a Na/Cl molar ratio of up to 1.5, which is well above that Cl including theoretical reaction pathways for gas hydrate dissociation (fractionation factor according to Tomaru et al, 2006) and smectite-illite transformation (fractionation factor according to Aloisi et al, 2004a). (c) d 18 O vs. K + normalized to Cl À and Mg 2+ normalized to K + .…”

“…5b). The fractionation pathways of smectite-illite transformation and gas hydrate dissociation were calculated according to Aloisi et al (2004a) and Tomaru et al (2006), respectively, assuming closed system Rayleigh fractionation. The Pechori Mound samples plot between the gas hydrate dissociation and the smectite-illite transformation pathways, but somewhat closer to the latter one, suggesting that both processes affect the d 18 O pw of these samples with a predominance of the smectite-illite transformation.…”

Pore fluid geochemistry from the Mount Elbert Gas Hydrate Stratigraphic Test Well, Alaska North Slope

“…Conversely, during ice melting and gas hydrate decomposition, the water released will dilute the pore fluids and shift the d 18 O and dD of the water to more positive values (e.g. Ussler and Paull, 1995;Tomaru et al, 2006).…”

“…The expected change in the oxygen and hydrogen isotopic composition of the pore water (Dd) can be estimated by: (Tomaru et al, 2006). The oxygen and hydrogen equilibrium fractionation (a O À ice, a D À ice) between ice and water were determined to range from 1.0029 to 1.0031 and 1.0178 to 1.0195, respectively (O'Neill, 1968).…”

Data report: d18O in pore fluids from NanTroSEIZE Expeditions 322 and 333

“…Drilling at Site C0012 penetrated into the igneous basement to 630.5 mbsf, recovering the sediment/basalt interface intact at 537.8 and 525.7 Oxygen isotope geochemistry has been widely applied to the study of sedimentation and diagenetic processes in continental and oceanic environments because the isotopic fractionation depends on both the temperature and the chemical composition of the minerals undergoing diagenesis. Examples of applications of the 18 O/ 16 O ratio in pore fluids include ion filtration processes, alteration of volcanogenic tephras and oceanic crust, evidence of diagenetically evolved fluid fluxes, gas hydrate dynamics, opal transformations to cristobalite, and quartz and clay dehydration reactions (Coplen and Hanshaw, 1973;Lawrence et al, 1975;Kastner et al, 1993;Tomaru et al, 2006;Kashiwaya et al, 2013;Kim et al, 2013). Isotopic geothermometry laws based on experimental, empirical, and theoretical data are available in scientific literature for most carbonate, silicate, and oxide minerals (Friedman and O'Neil, 1977;Savin and Lee, 1988;Sheppard and Gilg, 1996).…”