Tapping into the sub-seafloor: examining diffuse flow and temperature from an active seamount on the Juan de Fuca Ridge

A unique sampling strategy involving the cementing of a permanent fluid sampler directly to the seafloor on Axial Seamount, Juan de Fuca Ridge, has recently allowed the first long-term direct coupling to a low-temperature hydrothermal vent on a mid-ocean ridge. Using the hydrologically sealed sampler, direct measurement of fluid volume and heat flux from a diffuse hydrothermal vent on the seafloor was obtained over a period of 206 days. Enhanced variance at both tidal and lower frequencies was recorded with a high coherence between the temperature and flow data. We estimate a volume flux of 48 m3/yr and a heat flux of 260 W/m2 for the square meter of seafloor sampled by the instrument. Measurement of the Darcy flow velocities of the effluent fluid are approximately 1.5×10−6 m/s and indicate fluid velocities within cracks in the substrate of 1–4 m/day, with an effective upper crustal permeability of 10−11–10−12 m2. While the measured variability in fluid flow is driven primarily by changes in the thermal buoyancy of upwelling fluid, there is also a significant (above hydrostatic) pressure gradient contribution to the measured flux. This overpressure produces roughly four times the driving force compared to that attributed to thermal buoyancy alone. Small variations in the volume flux and the effluent fluid temperature (∼1.5×10−8 m/s and ∼0.5°C, respectively) also occurred on approximately tidal time scales and appear to be related to poroelastic control of the velocity of hydrothermal fluid through the seafloor boundary (i.e. tidal pumping).