Hydrothermal venting and geothermal heating in Cascadia Basin

Abstract

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

Site surveys related to IODP Expedition 301: ImageFlux (SO149) and RetroFlux (TN116) expeditions and earlier studies

“…It is this basement temperature and the typical sediment thickness, accumulation rate, and properties in the area that define the characteristic heat flow of ~180 mW/m 2 . The total heat output of Baby Bare outcrop is 1-3 MW Thomson et al, 1995;Wheat et al, 2004); this much heat could be accounted for by suppressing seafloor heat flow by 40 mW/m 2 to a radial distance of 4-5 km from the outcrop, but low values extend far beyond this distance.…”

“…We focused this effort on the Second Ridge area, where there is the greatest distribution of high-quality, colocated heat flow and seismic data across similarly aged seafloor. There are distinct, local thermal anomalies associated with both discharging and recharging outcrops in this area (e.g., Davis et al, 1992;Fisher et al, 2003a;Thomson et al, 1995;, but their influences appear to extend only a few kilometers from individual basement features.…”

“…The thermal influence of both recharging and discharging outcrops appears to extend only a few kilometers from the limit of basement exposure. Considering the forces available to drive recharge, discharge, and transport between the two outcrops (the difference in fluid pressure at the base of recharging and discharging columns of fluid) and the volume flux of fluid exiting Baby Bare outcrop Thomson et al, 1995;, the bulk permeability of the basement rocks along the flow path must be on the order of 10 -12 to 10 -9 m 2 (Fisher et al, 2003a). These values are consistent with independent estimates made from packer and open hole thermal tests and results of numerical models of coupled heat and fluid flow in basement (e.g., Becker and Davis, 2003;Becker and Fisher, 2000;Davis et al, 1997b;Fisher et al, 1997;).…”

Geothermal heating and the properties of bottom water in Cascadia Basin

“…Geothermal heat flux from this small basin alone provides energy to heat deep water at a rate of 0.01 to 0.04 TW [Moran and Lister, 1987;Thomson et al, 1995]. This heating rate is $1% of the total input from all sources required to maintain the global thermohaline circulation (2.1 TW) [Munk and Wunsch, 1998], a large number for a basin that represents only 0.03% of the global ocean area.…”

Influence of sedimentation, local and regional hydrothermal circulation, and thermal rebound on measurements of seafloor heat flux

“…[42] The eastern flank of the Juan de Fuca Ridge near 48°N has been the focus of numerous surface-ship, drilling, and submersible programs exploring the nature, driving forces, and influences of hydrothermal circulation within young ( 3.6 Ma) seafloor (Figure 12a) [Davis et al, 1989[Davis et al, , 1996[Davis et al, , 1992a[Davis et al, , 1997a[Davis et al, , 1999[Davis et al, , 1997b[Davis et al, , 1992bFisher et al, 2005;Mottl et al, 1998;Thompson et al, 1995;Wheat and Mottl, 1994;Wheat et al, 1997;Zuehlsdorff et al, 2005]. This region has also yielded many observations that provide a basis for analytical and numerical models of coupled fluid, heat, and/or solute transport [Davis et al, 1999[Davis et al, , 1997cElderfield et al, 1999;Fisher et al, 2003a;Giambalvo et al, 2000;Hutnak et al, 2006;Rudnicki et al, 2001;Spinelli and Fisher, 2004;Stein and Fisher, 2003].…”