Geothermal heating and the properties of bottom water in Cascadia Basin

Abstract

[1] A new compilation of CTD data sheds light on the circulation and properties of bottom water within Cascadia Basin. A relatively high salinity water mass is revealed in a region of likely bottom water inflow east of the Blanco Fracture Zone. Water mass distributions and the deep geostrophic shear field are consistent with a general cyclonic circulation within the basin and suggest a northward-flowing boundary current next to the deep continental slope. Shifts in temperature-salinity relationships are consistent with a vertical transition from waters of northern origin to waters of southern origin approaching the seafloor. This vertical water-mass transition complicates the interpretation of temperature anomalies. We redefine the temperature anomaly due to geothermal activity and the thickness of the layer influenced by seafloor heating. Simple heat budget calculations lead to a residence time of about one year for the bottom water, shorter than suggested by previous studies. Citation: Hautala, S. L., H. P. Johnson, and T. Bjorklund (2005), Geothermal heating and the properties of bottom water in Cascadia Basin, Geophys. Res. Lett., 32, L06608,

Quantifying the North Pacific silica plume

“…As with the North Pacific silica plume flux estimates, we can use a simple box model to provide an independent estimate of the silica source strength within Cascadia Basin. The heat budget for Cascadia Basin indicates a residence time of a less than 1 year for the bottom water, consistent with geostrophic estimates of northward bottom water currents of 1 to 2 cm/s relative to reference levels between 1500 to 2200 dbar [HJB05]. The anomalously high geothermal heating on the eastern flank of the Juan de Fuca Ridge decreases the density of the bottom water mass, so that circulation within this constrained basin flows “uphill” as the depth of the seafloor systematically shallows from south to north.…”

“…This former near‐bottom water then becomes the general source for the North Pacific silica plume at 2300 meters depth [TJ92; Lupton , 1998]. While it is clear that additional observations of currents, mixing and seafloor heat flux are needed to verify and better quantify Cascadia Basin's deep circulation, consistency between water mass properties, the deep geostrophic shear field and the timescale constrained by seafloor heating [HJB05] suggest that the basin‐scale picture is likely to be correct to first‐order.…”

“…This upward migration of Pacific bottom water, from near 4000 meters depth near the Mendocino FZ to a depth near 2300 meters when it exits Cascadia Basin, is unusual for the global ocean, and was first suggested by TJ92. The decrease in bottom water depth is driven by the buoyancy associated with the 0.01 to 0.04 TW of geothermal heat that results from the rapid coverage of newly formed crust at the Juan de Fuca and Gorda Ridges by sediments from the near‐by continental margin [HJB05].…”

“…The temperature anomaly can be quantified by comparing water parcels from different points on the θ/ S curves, keeping either salinity or density constant; the latter measure was used by TJ92. As in HJB05, the temperature anomaly (Δθ) is defined for Cascadia Basin bottom water as the difference between the observed potential temperature and that of a water parcel having the same potential density (referenced to 2500 dbar) sampled in a CTD station collected in 2004 just north of Blanco Saddle. A ΔSi/Δθ ratio of 800 μmol/L/°C is then estimated from Figure 5 for the hydrostations along the margin.…”

Sediment gravity flows triggered by remotely generated earthquake waves

“…Again, although the geographical setting is different, a similar mechanism may also be responsible for the drop in temperatures that we observe following the observed thermal spikes. Along the Cascadia margin, a chronic geothermally heated bottom boundary layer is found east of Cascadia Sea Channel [ Hautala et al ., , ]. Colder bottom water is located both to the south of this site in the Blanco Saddle and to the west in the Cascadia Sea Channel.…”