GH-IW)
AND 
18O
ANOMALIES OF THE INTERSTITIAL WATERS: LEG 164, BLAKE RIDGE1GH-IW)
AND 18O
ANOMALIES OF THE INTERSTITIAL WATERS: LEG 164, BLAKE RIDGE1The 
18O
values of interstitial waters from Site 994 and Site 997 sediments, Blake Ridge,
western Atlantic, tend to decrease with depth from 0.3
to -0.5
Standard Mean Ocean Water in the upper 200 mbsf, then fluctuate with significant
positive spikes of 
= 0.2-0.5
in the gas hydrate zone (200 to 450 mbsf), and finally increase from -0.4
to -0.2
toward 700 mbsf. Positive shifts of 18OIW
in the gas hydrate zone are probably caused by the dissociation of gas hydrates
originally contained in sediment cores. Gas hydrates recovered from the sites
are enriched in 18O, 18O
ranging between 2.7
and 3.5. 18O
values of gas hydrates and ambient interstitial waters give an oxygen isotopic
fractionation factor of 1.0034-1.0040 at 12º-16ºC and ~31 MPa (3 km below sea
level). Based on this fractionation and observed isotopic anomalies in the gas
hydrate zone, gas hydrates occupy 6% to 12% of pore-space volume within Blake
Ridge sediments.
1Paull,
C.K., Matsumoto, R., Wallace, P.J., and Dillon, W.P. (Eds.), 2000. Proc. ODP,
Sci. Results, 164: College Station, TX (Ocean Drilling Program).
2Geological
Institute, University of Tokyo, Hongo, Tokyo 113-0033, Japan. ryo@geol.s.u-tokyo.ac.jp
3Department of
Geology, University of North Carolina, Chapel Hill, NC 27599-3315, U.S.A.
(Present address: Exxon Exploration Company, P.O. Box 4778, Houston, TX
77210-4778, U.S.A.)
Date of
initial receipt: 12 May 1998
Date of acceptance: 3 November 1998
Ms 164SR-206
