Open AccessEffect of massive gas hydrate formation on the water isotopic fractionation of the gas hydrate system at Hydrate Ridge, Cascadia margin, offshore Oregon
Author(s) -
Tomaru Hitoshi,
Torres Marta E.,
Matsumoto Ryo,
Borowski Walter S.
Publication year - 2006
Publication title -
geochemistry, geophysics, geosystems
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.928
H-Index - 136
ISSN - 1525-2027
DOI - 10.1029/2005gc001207
Subject(s) - clathrate hydrate , geology , fractionation , equilibrium fractionation , hydrate , carbonate , saturation (graph theory) , mineralogy , isotope fractionation , chemistry , chromatography , organic chemistry , mathematics , combinatorics
Because gas hydrate is preferentially enriched in the heavy water isotopes, the δ 18 O and δ D 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 18 O and D due to gas hydrate destabilization during core recovery. The oxygen and hydrogen isotopic fractionation factors ( α O = 1.0025 and α 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 − 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 α O of 1.0010 (average value of 1.0012) and α 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.