International audienc

Observations in Cascadia Basin on the eastern flank of the northern Juan de Fuca Ridge reveal significant bottom water modification as a result of regional conductive heating and local hydrothermal venting. Seafloor conductive heating occurs throughout the sedimented basin while hydrothermal fluid discharge is confined to small (• 1 km 2) isolated igneous basement outcrops. In the northern sector of the plateaulike basin, the vertical fluxes of heat and mass associated with these seafloor processes lead to the formation of a 250-to 350-m-thick "geothermal boundary layer" characterized by anomalously high temperature, reduced vertical stability, and high dissolved silicate concentration. Using a basinwide average lithospheric heat flux of 0.3 W m -2 and the observed thermal anomaly structure of the water column, we obtain a mean residence time of I to 2 years for the deep water over Cascadia Basin. Detailed water property data collected in 1992 and 1993 within the immediate vicinity of three isolated igneous basement outcrops in the north-central sector of the basin indicate that local bottom-water heating arises from low-temperature venting through the summit and flanks of the outcrops. Near the smallest edifice, especially well-defined layers of anomalously warm, particleladen water were found within +20 m of the outcrop summit depth of 2610 m. Maximum anomalies of temperature, light attenuation coefficient, and dissolved silicate concentration in the layers were 0 040øC, 0.015 m -• and 5 pmol L -• respectively. We estimate the local heat flux, Fo •, from the smallest outcrop to be (2.4 q-0.8)U x 109 W, where U (m s -•) is the mean horizontal current at the venting depth. For reasonable mean currents in the range 10 -3 to 10 -2 m s -•, we find Fo • m 0.2 to 2.4 x 10 7 W. Assuming that the depressed conductive heat flow of -0.05 W m -2 observed through the sedimented seafloor surrounding the smallest outcrop reflects the advective loss of heat through the outcrop, the radial distance over which crustal fluids must collect heat and converge on the outcrop is about 10 km. 108 to 1010 W [Rosenburg et al., 1988; Baker et al., 1989; Kadko et al., 1990; Schultz et al., 1992; Thomson et al., 1992a]. This heat flux, together with trace metals, dissolved gases, and other associated property 6121 6122 THOMSON ET AL.: HYDROTHERMAL VENTING AND GEOTHERMAL HEATING

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The episodic magmatic intrusions that accompany seafloor spreading can profoundly affect the discharge of hydrothermal heat and fluid. A rapid field response to the acoustic detection of seismic activity on the CoAxial segment of the Juan de Fuca Ridge, June/July 1993, provided the opportunity to conduct a unique series of observations of hydrothermal event plumes. Between July 3 and 26 we detected at least three event plumes ranging in plume volume from 1.3 to 4 × 1010 m³, and in excess heat from 2.2 to 12.4 × 1015 J. Two were discovered directly above a new lava extrusion. One of these was first sampled within days of its release, as evidenced by its asymmetric shape, complex structure, and a steadily increasing light attenuation anomaly apparently generated by ongoing precipitation of hydrothermal Fe. We hypothesize that event plumes were produced intermittently in response to successive volcano‐tectonic events that produced pulses in seismic activity between June 28 and July 14.

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