Understanding Blake Ridge Thermal Regimes

To date, the Blake Ridge downhole temperature data set is the most complete available within the sediments of a marine hydrate reservoir. The relatively close spacing of the measurements with depth, the high precision of temperature determinations, and the fact that the data define a nearly linear gradient even when weighted by large, subjectively assigned uncertainties make it likely that the predicted temperature at the BSR is not simply incorrect. Several possible explanations for the temperature disparity at the BSR have been advanced, including: (1) Ongoing thermal equilibration of the sediments in response to Holocene climate change; (2) Lack of coincidence between the base of the HSZ and the BSR, meaning that the temperature at the BSR need not be equivalent to that predicted for the base of the HSZ; (3) Inhibition of hydrate stability owing to the presence of salts or gases other than methane; (4) Inhibition due to capillary forces; (5) Poor knowledge of the stability curves for methane hydrate at these pressures.

Temperature (°C)

450 500 550 Chloride (mM)

Temperature (°C)

Weather Ridge Symbol

Figure 4. (a) In situ equilibrium temperature measurements at Site 997 on the Blake Ridge (Ruppel, 1997). Open symbols represent temperature data collected with different probes. The small circles denote pore water chloride anomalies (bottom scale), (b) Compilation of stability curves and estimated temperatures (solid circles) at the BSR in various hydrate provinces. The 3.5% NaCl curve is from theoretical calculations by Tohidi et al. (1995). Numbers denote sites that were part of ODP legs.

Figure 4. (a) In situ equilibrium temperature measurements at Site 997 on the Blake Ridge (Ruppel, 1997). Open symbols represent temperature data collected with different probes. The small circles denote pore water chloride anomalies (bottom scale), (b) Compilation of stability curves and estimated temperatures (solid circles) at the BSR in various hydrate provinces. The 3.5% NaCl curve is from theoretical calculations by Tohidi et al. (1995). Numbers denote sites that were part of ODP legs.

The simple analysis of Ruppel (1997) demonstrated that it was relatively unlikely that ongoing adjustments of the hydrate reservoir to Holocene climate change could explain the observed temperature disparity at the BSR on the Blake Ridge. Considering the remaining hypotheses in order, we first examine the possibility that the BSR is, in fact, not coincident with the base of the HSZ. Observational evidence (Paull et al., 1996), theoretical modeling (Xu and Ruppel, 1999), and synthetic studies (Wood and Ruppel, 2000) indicate that gas hydrate may indeed be present in situ even when an underlying BSR is absent. Thus, a BSR is not a necessary condition for the presence of hydrate. In fact, a BSR is probably only present when the supply of gas exceeds a threshhold value (Figure 5) that brings the base of the HZ into coincidence with the top of the free gas zone (Xu and Ruppel, 1999). A BSR, which is defined on the basis of the negative seismic impedance contrast created by the transition from sediment ± hydrate above to free gas below, is therefore not inherently the base of the HSZ. However, if a BSR only forms when the base of the HZ is coincident with the top of free gas, then, for practical purposes, the BSR does in fact mark the stability boundary (base of the HSZ) and should be associated with predicted temperatures equivalent to the dissociation temperature.

The section on stability constraints included a detailed examination of the different factors that can inhibit or promote the stability of hydrates. On the Blake Ridge, geochemical analyses of pore waters (Paull et al., 1996) show that the low temperatures predicted at the BSR cannot be explained by anomalous concentrations of ionic compounds in pore waters. Although small amounts of gases other than methane were detected, the presence of these gases actually implies that temperatures at the dissociation boundary should be higher, not lower, for a given pressure. Strong capillary forces arising between the clay-sized particles of the Blake Ridge sediments may inhibit stability of hydrate by several degrees (Clennell et al., 1999), which is alone sufficient to explain the observed temperature disparity at the BSR on the Blake Ridge.

A final, and somewhat troubling, explanation for the temperature disparity is that good constraints on the stability field of hydrate at the pressures characteristic of marine sediments on continental margins may still be lacking. For example, the seawater stability curve shown in Figure 4b represents an extrapolation of the results of Dickens and Quinby-Hunt (1994) to higher pressures using the formulation described by Brown et al. (1996). Comparison of the predicted BSR temperature on the Blake Ridge to a stability curve calculated using a statistical thermodynamics approach (Tohidi et al., 1995) yields a much smaller estimated temperature disparity at the BSR. As experimental procedures improve for working at the pressures (-10-30 MPa) required for methane hydrate stability at realistic temperatures (0-25°C), we will gain better knowledge of the dissociation temperatures and a clearer understanding of the significance of BSR temperature estimates.

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