Geochemistry of a sealed deep-sea borehole on the Cascadia Margin

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, NO 3, or PO 4 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 CH 4. The hydrocarbons are characterized by C 1 C 2+ ratios of 170–200 and δ 13 C values of − 59.5 to − 62.4%o which indicates a considerable admixture of thermogenic hydrocarbon gases. The occurrence of methane of partly thermogenic origin demonstrates that CH 4 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 CH 4-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.