13. Geological and Geochemical Constraints on the Isotopic Composition of Interstitial Waters from the Hydrate Ridge Region, Cascadia Continental Margin¹

H. Tomaru,^{2, 3} R. Matsumoto,² M.E. Torres,⁴ and W.S. Borowski⁵

ABSTRACT

The isotopic compositions of interstitial waters collected from Hydrate Ridge during Ocean Drilling Program Leg 204 were measured to evaluate the fluid evolution of this accretionary prism. At shallow depths, the dissolved Cl^– concentrations and D and ¹⁸O values of the interstitial water reflect changes in the salinity and the isotopic compositions of seawater from the Last Glacial Maximum to the present. The presence of disseminated gas hydrates, which is well identified by discrete low Cl^– anomalies within the gas hydrate stability zone, is accompanied by high D and ¹⁸O values of the freshened fluids. This is consistent with incorporation of heavy isotopes into the gas hydrate lattice, which is also apparent in the signals observed at the ridge summit. Here, massive gas hydrate formation in the upper 20 meters below seafloor leads the formation of brines with dissolved Cl^– concentrations as high as 1400 mM. The interstitial waters sampled near massive gas hydrates at the ridge summit are extremely depleted in D and ¹⁸O. Clay mineral dehydration within the deep prism results in a progressive decrease in Cl^– and D with depth. Dehydration temperature estimates based on those data likely suggest a progressive increase in the temperature of isotopic fractionation between clay and water with distance from the prism toe. The oxygen isotope data probably reflect the combined effects of clay dehydration, carbonate precipitation, and alteration of oceanic basement; however, there are not enough data to constrain the relative contribution of these processes to the observed signals.

¹Tomaru, H., Matsumoto, R., Torres, M.E., and Borowski, W.S., 2006. Geological and geochemical constraints on the isotopic composition of interstitial waters from the Hydrate Ridge region, Cascadia Continental Margin. In Tréhu, A.M., Bohrmann, G., Torres, M.E., and Colwell, F.S. (Eds.), Proc. ODP, Sci. Results, 204: College Station, TX (Ocean Drilling Program), 1–20. doi:10.2973/odp.proc.sr.204.109.2006

²Department of Earth and Planetary Science, University of Tokyo, 7-3-1 Hongo, Bunkyo-ku Tokyo 113-0033, Japan.

³Present address: Department of Earth and Environmental Sciences, University of Rochester, 227 Hutchison Hall, Rochester NY 14627, USA. hitoshi@earth.rochester.edu

⁴College of Oceanic and Atmospheric Sciences, Oregon State University, 104 COAS Admin. Building, Covallis OR 97331-5503, USA.

⁵Department of Earth Sciences, Eastern Kentucky University, 521 Lancaster Avenue, Roark 103, Richmond KY 40475-3102, USA.

Initial receipt: 27 January 2005
Acceptance: 7 February 2006
Web publication: 6 July 2006
Ms 204SR-109