The amount of gas hydrate present in marine sediments is one of the more important parameters when the importance of marine gas hydrate is examined as to the environmental impact and resource potential. Gas hydrate preferentially incorporates 18O relative to ambient; therefore, the oxygen isotopic composition of water squeezed from sediment cores would provide information on the subsurface amount of gas hydrate waters (Hesse and Harrison, 1981; Ussler and Paull, 1995). Also, the isotopic fractionation between gas hydrate and water is crucial to study the effect of gas hydrate dissociation on the formation of authigenic carbonates (Matsumoto, 1989). The molecular and isotopic composition of gases in gas hydrate can provide information on the source and generation process of gases of the Blake Ridge hydrate.
Gas hydrate samples were placed in a Teflon-coated dissociation chamber and allowed to decompose at room temperature. Gas pressure of the chamber steadily increased and reached a stable value in about 10 min.; then the hydrate gas was transferred to a small gas collection chamber for gas analysis. The volume of water left in the dissociation chamber was measured and stored in a glass ampoule for determination of the chemical composition and oxygen isotopic composition. Chloride and sulfate concentrations were measured to estimate the amount of pore-water contamination. The amount of interstitial water in the sample water was calculated from chloride and sulfate concentrations, assuming that the pristine gas hydrate-derived water was salt free. As shown in Table 1, the mole fractions of gas hydrate water were 0.88 to 0.99 in the analyzed samples.
The oxygen isotopic
composition of gas hydrate-derived water was determined by the standard CO2
equilibrium method (Epstein and Mayeda, 1953), as modified by Matsuhisa and
Matsumoto (1986). A known amount (1.0-1.5 mL; 28-42 mmol of O2) of
sample water was transferred to a small flask, and the flask was evacuated after
the water was frozen by liquid nitrogen. The flask was filled with 0.10 mmol of
CO2 and placed in a vibration water bath at 25.0ºC for 15-20 hr to
attain isotopic equilibration between water and CO2. After the
completion of the isotopic exchange reaction, CO2 gas was transferred
to a cryogenic vacuum preparation line for refinement. Oxygen isotopic ratio 18O/16O
of sample gas was determined by means of a Finnigan Delta E or Delta S mass
spectrometer. The results were represented as oxygen isotopic composition 
18O
relative to standard mean ocean water (SMOW) using the per million notation.
Precision (2 sigma) of the analysis was 0.01
-0.05,
whereas the reproducibility of the measurements was about 0.10,
which was estimated from the repeated analysis of CO2 samples
prepared from the same sample water.
Chloride concentration of gas hydrate-derived waters was measured by a ICA-5000 ion-chromatograph. The standard deviation (2 sigma) of the measurements was within 2% of measured values.
Results of 18O
measurements of gas hydrate water are given in Table
1 along with gas and water volumes and chloride concentrations. The 18O
values of water taken from gas hydrate samples were corrected for mixing effect
of the interstitial water contamination.
Chloride concentrations of
five water samples of gas hydrate were between 5 and 62 mM (Table
1); thus, the mole fraction of gas hydrate water in water samples is
calculated to be 0.990-0.878. 18O
values of water within gas hydrates are 2.67 for Section 164-994C-31X-7 and 2.82-3.51
(mean = 3.2)
for Section 164-997A-42X-3. Site 997 gas hydrates are isotopically heavier than
Site 994 gas hydrates by ~0.5.
Moreover, the ~0.8
variation within a single, thick massive gas hydrate at Site 997 is significant;
however, the reasons for these isotopic variations are uncertain.
The 18O
of the interstitial water and gas hydrate at Sites 994 and 997 are given in
Matsumoto et al. (Chap. 6,
this volume). The difference in 18O
values between gas hydrate and the ambient water (
18OGH-IW)
is calculated to be 3.1
at Site 994 and 3.3-3.8
(mean = 3.6)
at Site 997. Assuming that recovered massive gas hydrates were in isotopic
equilibrium with ambient interstitial waters (T = 12º-16ºC and P = 31 MPa),
the equilibrium isotopic fractionation factor (aGH-IW) is given as
1.0034 (Site 994) and 1.0037-1.0040 (Site 997). For comparison, oxygen isotopic
fractionation of ice water is 1.0027-1.0035 (O'Neil, 1968; Craig and Hom, 1968;
Jakli and Staschewski, 1977) and that of tetrahydroflan (THF) hydrate-water
association is 1.00268 ± 0.00003 at 0º-4ºC (Davidson et al., 1983). The
values estimated for Sites 994 and 997 are similar to these values.
Molecular compositions of hydrocarbons and carbon isotope composition of methane is widely used for a genetic classification of hydrocarbon gases (Rosenfield and Silverman, 1959; Bernard, 1978; Schoell, 1980; Rice and Claypool, 1981). Schoell (1980) and Whiticar et al. (1986) have shown additionally that the hydrogen isotope composition of methane in combination with the carbon isotope composition characterizes different pathways of microbial methane formation. Here, we investigate the genetic characterization of hydrocarbons in gas hydrate samples by analyses of stable carbon and hydrogen isotope compositions in combination with molecular compositions of gases.
Dissociated gas samples
were collected from six gas hydrate samples (Table
2). Two samples were taken from 259 mbsf at Site 994 and four samples
from 330 mbsf at Site 997. The molecular compositions of hydrocarbon gases were
determined on a Shimazu GC-7A gas chromatograph. For isotope analyses, methane
was separated from the other gas components by gas chromatography and
subsequently combusted to CO2 and H2O over CuO at 850ºC,
using a vacuum preparation line (Schoell, 1980). The H2O was then
reduced to H2 by reaction with zinc in flame-sealed glass tubes at
480ºC (Vennemann and O'Neil, 1993). The stable carbon and hydrogen isotope
compositions of methane were measured using a VG Isotech Sira Series II mass
spectrometer. Isotope ratios are reported in the usual -notation
relative to the Peedee belemnite (PDB) standard for carbon and SMOW standard for
hydrogen. The reproducibility of isotope values is ±0.15
for 13C
and ±3 for D.
Hydrocarbon composition
and 13C
and D
values of methane are reported in Table
1. The 13C
values of methane range between -70
and -65 (Table
2). The C1/(C2 + C3) ratios of all
samples are higher than 5000. Those light carbon isotope values and methane-rich
hydrocarbon compositions are similar to most gas hydrates recovered on the other
DSDP and ODP cruises, indicating that the methane trapped in gas hydrate was
mainly produced by microbial activity (Fig.
11). Microbial methane formation follows two principal pathways, CO2
reduction and fermentation (Schoell, 1980; Jenden and Kaplan, 1986; Martens et
al., 1986; Whiticar et al., 1986; Burke et al., 1988). The fermentation
processes are usually more important in recent freshwater sediments and swamps,
whereas methane formed by CO2 reduction is more common in marine
sediments (Whiticar et al., 1986). The CO2 reduction-derived methane
and fermentation-derived methane can be differentiated by their deuterium
concentration. The D
values of methane in the Blake Ridge gas hydrate range between -206
and -201 (Table
2), indicating methane was generated by microbial CO2
reduction (Fig. 12).
