Interest in marine gas hydrates has grown as earth science researchers look at the potential influence of gas hydrates on global climatic change, long-term energy resources, and natural hazards such as slope instability and submarine methane expulsion. The most critical and fundamental question in gas hydrate study is: how much gas hydrate is stored in marine sediments? Variable estimates have been reported (e.g., McIver, 1981; Kvenvolden, 1988; Dillon et al., 1995), but global inventory of gas hydrate is believed to be 104 Gt (1019 g) as carbon (Kvenvolden, 1988). This large estimate has highlighted the potential importance of marine gas hydrates, but the global inventory of gas hydrate requires further refinement through the assessment of local marine gas hydrate amounts.
Assessment of gas hydrate amounts in hydrate-bearing sediments has been made using various techniques such as seismic investigations, well logging, geochemical investigations, and temperature measurements. Chloride anomalies in interstitial water are believed to be a reliable method of estimating gas hydrate amounts, as documented during Ocean Drilling Program (ODP) Leg 164 (Paull, Matsumoto, Wallace, et al., 1996). Recently, however, a serious question has been raised concerning the effects of selective filtration/adsorbtion on some chemical elements during mechanical squeezing (Cave et al., 1998). This problem seems to require reassessment of the chloride anomaly technique.
The water within gas
hydrate is enriched in isotopically heavy oxygen (18O) relative to
ambient waters (e.g., Hesse and Harrison, 1981; Harrison and Curiale, 1982;
Ussler and Paull, 1995). Consequently, gas hydrate should be an enormous sink of
18O in the earth's surface, analogous to polar ice. Matsumoto and
Matsuda (1987) and Matsumoto (1989) have argued for a genetic relationship
between heavy-oxygen-containing siderites and in situ dissociation of gas
hydrates, and have suggested that diagenetic carbonates would provide a unique
tool to identify fossil gas hydrate horizons in ancient sedimentary rock
sequences. Interstitial waters squeezed from gas hydrate-bearing sediments are
expected to be variably enriched in 18O, depending on the amount of
gas hydrate present, hence, deviations in 
18O
of pore waters from baseline values (the 18O
anomaly) should be an independent technique in estimating gas hydrate amount.
However, the isotopic fractionation factor for oxygen has not been determined
yet for any clathrate system, except for the experimental study on the THF (tetrahydrofuran)
hydrate and water (Davidson et al., 1983).
Leg 164 coring recovered
several massive gas hydrate samples and collected hundreds of interstitial water
samples from Blake Ridge sediments, offering a unique opportunity to estimate
the oxygen isotopic fractionation factor (
GH-IW)
for naturally occurring methane gas hydrate. In this study, we measured 18O
of gas hydrates and ambient interstitial waters to determine GH-IW.
We then estimated the gas hydrate amount in Blake Ridge sediments on the basis
of observed 18O
anomalies and compared the results with those of the chloride anomaly technique.
