ELSEVIER Marine Geology 148 (1998) 9-20 Geochemistry of a sealed deep-sea borehole on the Cascadia Margin Michael Schliiter *, Peter Linke, Erwin Suess zyxwvutsrqponmlkjihgfedcbaZY GEOMAR Research Center for Marine Geosciences, W ischhofstr: 1-3, D-24148 Kiel, German) Received 6 September 1996; accepted 29 January 1998 Abstract The deep-sea borehole seal CORK was deployed for the first time on a modern accretionary prism during ODP Leg 146 to the Cascadia Margin. Ten months after the deployment the fluid flow and geochemistry of the borehole fluids was investigated during several dives by DSRV Alvin. The chemical analysis of the borehole fluids revealed methane concentrations of more than 3.5 mM, whereas oxygen and dissolved ions as Cl, N03, or PO4 are still close to the ambient seawater composition. The exceedingly high methane content measured at the top of the sealed borehole and the observed degassing during the ascent of the submersible indicates that the sampled fluid was initially saturated or close to saturation with respect to CH4. The hydrocarbons are characterized by Cl/C *+ ratios of 170-200 and S”C values of -59.5 to -62.4%0 which indicates a considerable admixture of thermogenic hydrocarbon gases. The occurrence of methane of partly thermogenic origin demonstrates that CH1 enters the sealed borehole in the lower, perforated section (94-178 mbsf) and accumulates at the top of the borehole. This suggests the occurrence of free gas within the encapsulated borehole. Considering the stability field of CHA-hydrates, the formation of these ice-like structures may take place and potentially results in a clogging of the top of the borehole. Such precipitates could result in a decoupling of the top of the borehole from the hydraulic and geochemical regime of the accretionary complex, an important aspect for future plans of CORK deployments. 0 1998 Elsevier Science B.V. All rights reserved. Keywords: Cascadia Basin; accretionary wedge; ODP Site 892; hydrate; BSR; thermogen gas 1. Introduction Geochemical and geophysical investigations at convergent continental margins established the global distribution of tectonically induced fluid flow and its importance for hydrological and geochemical budgets (Suess et al., 1985; Han and Suess, 1989; Kastner et al., 1991; Moore and Vrolijk, 1992). Tectonically induced dewatering is caused by gravi- tational and tectonic stress, associated with the for- mation of an accretionary wedge by subduction pro- * Corresponding author. Fax: +49 413 600 2928; E-mail: mschlueter@geomar.de cesses, which reduces considerably the porosity and affects fluid advection (Carson, 1977; von Huene and Scholl, 1991; Le Pichon et al., 1993). Additionally, fluids derived from diagenetic reactions, dehydration of smectites and external sources like meteoric wa- ter may contribute to fluid expulsion (Kastner et al., 199 1; Orange and Breen, 1992; Moore and Vrolijk, 1992). These processes cause a substantial release of fluids and dissolved components (e.g. calcium, ammonium, hydrogen sulphide, or methane) through the sediment/water interface (Han and Suess, 1989; Carson et al., 1990; Linke et al., 1994). First direct evidence for tectonic dewatering and methane expulsion was massive barite and calcare- 0025.3227/98/$19.00 @ 1998 Elsevier Science B.V. All rights reserved. PII SOO25-3227(98)00016-4