CORK Borehole Observatories record driving forces for
off-axis hydrothermal circulation in young oceanic crust

Keir Becker
University of Miami – RSMAS

Wednesday, July 29, 1998
3:30 p.m.—Pacific Forum

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"CORKs" or "Circulation Obviation Retrofit Kits" are Ocean Drilling Project (ODP) boreholes sealed and instrumented as long-term hydrologic observatories with subseafloor pressure and temperature sensors and fluid samplers. Since 1991, 13 CORKs have been installed in 11 eastern Pacific and Atlantic holes, in three characteristic subseafloor hydrologic settings: the Middle Valley sedimented ridge hydrothermal system on the Juan de Fuca Ridge, accretionary prisms of the Cascadia and Barbados subduction margins, and ridge-flank hydrothermal systems in young oceanic crust east of the Juan de Fuca Ridge and west of the Mid-Atlantic Ridge. Results from the first two settings have been summarized in prior seminars at MBARI; this seminar will focus on the most recent results from off-axis systems.

Although high-temperature hydrothermal venting at ridge crests is much more spectacular, the effects of lower-temperature off-axis circulation, when integrated over time and space, are equally important to the chemical and physical evolution of the oceanic crust. All five CORKs installed in young oceanic crust beneath sediment cover show that the driving forces for off-axis circulation—lateral pressure gradients in upper basement—are very small. Despite the small pressure gradients, one of these CORKs, located in a sediment-buried basement high, produces warm crustal fluids at about 5 liter/min, presently being filtered to sample the "subsurface biosphere." These CORKs also show very small levels of attenuation and phase lag of seafloor tidal signals, as sensed from permeable basement covered by sediment. In combination, these observations imply very high permeability in uppermost basement, interconnected over long distances to isolated basement exposures, supporting extensive lateral fluid flow in sediment-covered basement over unexpectedly large distance scales up to many tens of kms. These results also directly document previous inferences that off-axis circulation patterns are strongly influenced by topography of the most permeable layer - the upper oceanic basement beneath less permeable sediments.

A recent workshop on "Advanced CORKs for the 21st Century" has recommended major changes to CORK design, to allow for multi-level monitoring and greatly enhanced serviceability by wireline control vehicles and ROV's. Preliminary engineering designs for advanced CORKs will be presented, and budding plans for future CORK installations will be discussed.

Next: An introduction to the Marine Mammal Center

Last updated: December 19, 2000