Characterizing the thermal regime of cold vents at the northern Cascadia margin from bottom-simulating reflector distributions, heat-probe measurements and borehole temperature data

Abstract

Several cold vents are observed at the northern Cascadia margin offshore Vancouver Island in a 10 km² region around Integrated Ocean Drilling Program Expedition 311 Site U1328. All vents are linked to fault systems that provide pathways for upward migrating fluids and at three vents methane plumes were detected acoustically in the water column. Downhole temperature measurements at Site U1328 revealed a geothermal gradient of 0.056 ± 0.004°C/m. With the measured in situ pore-water salinities the base of methane hydrate stability is predicted at 218–245 meters below seafloor. Heat-probe measurements conducted across Site U1328 and other nearby vents showed an average thermal gradient of 0.054 ± 0.004°C/m. Assuming that the bottom-simulating reflector (BSR) marks the base of the gas hydrate stability zone variations in BSR depths were used to investigate the linkages between the base of the gas hydrate stability zone and fluid migration. Variations in BSR depth can be attributed to lithology-related velocity changes or variations of in situ pore-fluid compositions. Prominent BSR depressions and reduced heat flow are seen below topographic highs, but only a portion of the heat flow reduction can be due to topography-linked cooling. More than half of the reduction may be due to thrust faulting or to pore-water freshening. Distinct changes in BSR depth below seafloor are observed at all cold vents studied and some portion of the observed decrease in the BSR depth was attributed to fault-related upwelling of warmer fluids. The observed decrease in BSR depth below seafloor underneath the vents ranges between 7 and 24 m (equivalent to temperature shifts of 0.07–0.15°C).

Appendix

Methodology of heat flow calculations from BSR depth

Travel times of the seafloor t _seaf and BSR t _bsr have to be determined from the seismic sections and are converted into BSR-depth D _bsr by using a simplified velocity-depth function V _p.

$$ \Updelta t = t_{\rm{seaf}} - t_{\rm{bsr}} $$

$$ V_{\rm{p}} = 1511 + \, 521 \times \Updelta t - 93 \times \Updelta t^{ 2} $$

$$ D_{\rm{bsr}} = 0.5 \times V_{\rm{p}} \times \Updelta t $$

The simplified velocity function (Riedel 2001) does not take the velocity increase due to hydrate into account. But the estimated heat flow is not very sensitive to velocity, since a change in BSR depth results also in a change in BSR temperature without changing the temperature gradient significantly. A change in velocity of about 180 m/s at a typical BSR depth of 200 m bsf results in heat flow values that differ by only 2–3 % (Ganguly et al. 2000).

Hydrostatic pressure was assumed at the BSR depth and was calculated by using a water density of 1,030 kg/m³ (Davis et al. 1990; Hyndman et al. 1993).

$$ P = \rho \, \times \, 9.81 \, \times \, \left( {wd \, + \, D_{\rm {bsr}} } \right)/1000000, $$

where P is pressure in MPa and wd is water depth in m (derived from t _seaf with average water velocity of 1,485 m/s).

Temperature at the BSR (T _bsr) was estimated from the phase boundary in the hydrate stability field for the pressure calculated. The following equation was adapted from Englezos and Bishnoi (1988):

$$ T_{\rm{bsr}} \, = 0.76664 \, \times \, \left( {\log \left( P \right)} \right)^{3} \, - \, 5.0075 \, \times \, \left( {\log \left( P \right)} \right)^{2} \, + \, 28.156 \, \times \, \log \left( P \right) \, - 12.136 $$

The last parameter needed is thermal conductivity (κ). An empirical expression for thermal conductivity versus depth was given by Davis et al. (1990):

$$ \kappa = \, 1.0 7 + \, 5.86 \times 10^{ - 4} \, \times \, D_{\rm{bsr}} \, - \, \left( {D_{\rm{bsr}}^{ 2} \times \, 3.24 \times 10^{ - 7} } \right) $$

with κ in W/mK, and D _bsr in m (Fig. 5).

Heat flow (H) can now be calculated assuming a linear temperature gradient and a simple conductive heat transport relation:

$$ H = \, \kappa \, \left( {T_{\rm{bsr}} - T_{\rm{seaf}} } \right) \, / \, \left( {D_{\rm{bsr}} } \right) $$

Seafloor temperatures are determined as outlined by He et al. (2007) using the following equation:

$$ T_{\rm{seaf}} \, = \, 5.9517 - 3.65226 \times 10^{{ - 3 }} \times \, wd \, + \, 7.0855 \times 10^{ - 7} \, \times \, wd^{ 2} $$