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

Author(s): Schlüter, Michael; Linke, Peter; Suess, Erwin
Author Affiliation(s): Primary:
GEOMAR Research Center for Marine Geosciences, Kiel, Federal Republic of Germany
Volume Title: Marine Geology
Source: Marine Geology, 148(1-2), p.9-20. Publisher: Elsevier, Amsterdam, Netherlands. ISSN: 0025-3227 CODEN: MAGEA6
Note: In English. 42 refs.; illus., incl. sketch map
Summary: 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, NO3, 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 C1/C2+ ratios of 170-200 and δ13C values of -59.5 to -62.4per mil which indicates a considerable admixture of thermogenic hydrocarbon gases. The occurrence of methane of partly thermogenic origin demonstrates that CH4 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 CH4-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.
Year of Publication: 1998
Research Program: ODP Ocean Drilling Program
Key Words: 02 Geochemistry; 07 Marine Geology and Oceanography; Accreting plate boundary; Aliphatic hydrocarbons; Alkanes; Alvin; Boreholes; C-13/C-12; Carbon; Cascadia Basin; Chemical composition; Concentration; Deep-sea environment; East Pacific; Geochemistry; Hydrocarbons; Hydrochemistry; Isotope ratios; Isotopes; Leg 146; Marine environment; Methane; Nitrate ion; North Pacific; Northeast Pacific; ODP Site 892; Ocean Drilling Program; Organic compounds; Oxygen; Pacific Ocean; Phosphate ion; Sea water; Solutes; Stable isotopes; Submersibles
Coordinates: N444026 N444032 W1250705 W1250709
Record ID: 1998040018
Copyright Information: GeoRef, Copyright 2019 American Geosciences Institute. Reference includes data from CAPCAS, Elsevier Scientific Publishers, Amsterdam, Netherlands