Tidal pumping of fluids within and from the oceanic crust: new observations and opportunities for sampling the crustal hydrosphere

Seven boreholes established by the Ocean Drilling Program in sedimented ridge-crest and ridge-flank environments have been sealed to stop the exchange of formation water and seawater via the holes after drilling, and instrumented to allow long-term hydrologic monitoring. Records of seafloor and basement-fluid pressures in these holes document tidal-frequency formation-pressure variations that are attenuated and phase-shifted relative to variations at the seafloor. Spatial variations in the response to seafloor tidal loading create oscillatory pressure gradients which drive fluid flow within the crust, and tidal pressure differentials between the crust and the water column modulate flow through the seafloor. Both may have significant geochemical and biological consequences. The magnitude of the tidal pressure differential observed across sediment sections is in many instances larger than the average differential pressure created by hydrothermal buoyancy forces, and this causes the pressure difference across fluid sampling ports incorporated in the borehole seals to reverse sign periodically. This can create a problem for obtaining undiluted crustal fluid samples at the seafloor from moderately overpressured holes, but it also provides an opportunity for fluids to be pumped passively from moderately underpressured holes through the use of uni-directional valves. Potential production rates, estimated using pre- and post-seal thermal, fluid-flow, and pressure data, indicate that significant fluid fluxes can be established, of the order of 10 m3 (typically one hole-volume) per month.