A compilation of estimated BSR temperatures for a variety of settings in which downhole measurements have been collected as part of ODP research is shown in Figure 4b. Note that most of the estimated BSR temperatures are lower than the theoretically- or experimentally-constrained dissociation temperatures for methane hydrate. The disparity may imply either imprecision in the P-T curves for in situ conditions or a systematic problem with the downhole temperature data from which the BSR temperatures are estimated through extrapolation.
Because the BSR coincides with the base of the HSZ, the seismically-constrained depth of the BSR can be used in combination with the stability curve and water depth to estimate first the temperature at the BSR and thence the thermal gradient and heat flux in the overlying sediment column. This type of interpretation, first detailed by Yamano et al. (1982), has been successfully applied in the Cascadia hydrate province, where the heat flux values predicted based on BSR depths are similar to true surface heat flux values measured using traditional marine heat flow probes (Davis et al., 1990). In such settings, seismic surveys, which are far less time consuming than heat flow studies, provide an important proxy (BSR depth) that constrains the thermal state of the reservoir.
In the Blake Ridge hydrate province, which has also been well-characterized thermally through the acquisition of coincident marine heat flow data (Ruppel et al., 1995), seismic data (e.g., Paull et al., 1996), and downhole temperature measurements (Ruppel, 1997), prediction of heat flux from the BSR depth is more problematic. Using the BSR depth in combination with a range of acceptable stability curves yields a prediction of surface heat flux significantly lower than the direct measurements of surface heat flux. In addition, thermal calculations based on the BSR depth predict a thermal gradient at least 11 % higher than that determined directly from downhole temperature measurements. For this data set, the obvious corollary is that there is a significant disparity (-30% in some places) between thermal gradients measured by traditional heat flow methods and those determined from downhole temperature measurements.
6. TEMPERATURE PERTURBATIONS: SEDIMENTARY PROCESSES AND CLIMATE CHANGE
Under static (steady-state) conditions, the thickness of the zone in which hydrate forms in marine sediments can be predicted by combining the gas solubility curve, the geothermal gradient, and three fluxes: energy (heat) flux, fluid flux, and methane flux. Figure 5 summarizes how changing the flux rates will alter the relative thicknesses of the gas hydrate zone, the free gas zone, and the dissolved gas zones. Higher heat flux causes the base of the stability zone to migrate to shallower depths and reduces the thickness of the hydrate-bearing sediments. Higher fluid flux causes the top of the hydrate zone to lie closer to the seafloor. For a constant value of fluid flux, higher methane flux will increase the depth to the base of the gas hydrate zone until it becomes coincident with the bottom of the stability zone.
The sensitivity of the thickness of the gas hydrate zone to variations in flux rates underscores the dynamic nature of the reservoir, which will respond to changes that affect thermal regimes, fluid flux rates, or the rate of methane supply. While the effects of these fluxes on the reservoir have been here presented as largely separable, there are actually complex feedback loops. For example, superposing an upward advective fluid flux on a pre-existing conductive geotherm will lead to increased thermal gradients near the surface and lower gradients at depth (Figure 2a). In this section, we simplify the approach and focus only on the response of the gas hydrate reservoir to the temperature changes that may accompany climate change events, sedimentation, erosion, or catastrophic slumping.
The qualitative impact of temperature perturbations on the gas hydrate reservoir can be deduced by considering the effects of various processes in redistributing heat in the system (Figure 6). During climate change events, perturbations to oceanic circulation patterns or the temperature of deep currents cause long-term changes in the average BWT. Such BWT fluctuations scafloqr Dissolved (jis Zone increitjiaK fluid flux]
Oistolved Gas Zone
Oistolved Gas Zone
increasing methane flux
increasing methane flux
Figure 5. Effects of changes in energy (heat) flux, fluid flux, and methane flux on the thickness and positions of the HZ and free gas zone. Increasing heat flux causes the base of the HZ to lie closer to the seafloor. After Ruppel and Kinoshita (2000), based on Xu and Ruppel (1999).
propagate into the sediments on a time scale controlled by the sediment's thermal diffusivity. An increase in BWT (Figure 6a) will eventually cause temperature changes that may destabilize hydrate near the base of the HSZ and lead to partial degassing of the reservoir into the overlying sediments. If the entire reservoir is dissociated or if significant gas is emitted from the seafloor, measurable isotopic anomalies may result (Dickens et al., 1997; Ruppel and Dickens, 1999).
Erosional and depositional events may also have an important impact on the thermal state of the hydrate reservoir if re-equilibration of thermal gradients cannot keep pace with the changes caused by these processes. For typical sediment thermal diffusivity values and marine thermal gradients, even sedimentation or erosion rates as rapid as 50 mm yr"1 will not produce an appreciable change in the depth of the BSR if the initial geotherm is conductive and if addition or removal of sediment is perfectly isostatically compensated (no net change in water depth). On the other hand, slope failure events, which remove a substantial thickness of sediment from one area and deposit it in another, may result in a sudden change in the BSR depth. However, after thermal equilibration, the final BSR depth should be equivalent to the original BSR depth (Figures 6b and 6c), unless some other factor (e.g., BWT or background heat flux) changes. Following slumping events, individual sediment particles may cross from the hydrate zone to the free gas zone (sedimentation) or vice versa (erosion).
Dissociation of hydrate is endothermic (AH--401 kJ kg"1), meaning that heat is consumed during the breakdown of the hydrate lattice to its constituent gas and water components. The endothermic nature of dissociation reduces the impact of increasing temperatures within the reservoir, since some of the heat introduced must necessarily be consumed in the dissociation process. Conversely, hydrate formation is exothermic (AH-614 kJ kg"1), and cooling of an arbitrary depth within the reservoir by an externally imposed perturbation will result in net cooling by an amount smaller than the perturbation (Tzirita, 1992). Such thermodynamic effects have often been ignored by researchers in determining the impact of thermal perturbations on the hydrate reservoir and are clearly important in modulating the outcome of perturbations such as those associated with global climate change.
Figure 6. Qualitative effects of BWT variations and slumping (massive removal and re-deposition of sediments). In each panel, the gray line S represents the phase equilibrium for hydrate, and the BWT is shown by the circle. The path followed by a sediment parcel is shown by the squares, with the final position of the parcel after thermal equilibration denoted by the open square, (a) An increase in BWT with constant background heat flux causes the initial geotherm (solid line) to move to the position of the dashed line. Intermediate temperatures as a function of depth are shown by the dotted curves. The increase in BWT will lead to shoaling of the base of the HSZ. At some depths, the hydrate contained in a sediment parcel may dissociate during thermal equilibration, (b) Instantaneous, catastrophic deposition of a large thickness of sediment (e.g., during slumping) with no change in water depth (perfect isostatic compensation) will produce the perturbed geotherm indicated by the dashed line. The geotherm eventually equilibrates to its initial position (solid line), resulting in no net change in the BSR depth, (c) The effects of instantaneous erosion (during slope failure) are the opposite. Panels (b) and (c) show catastrophic events. For rates of normal sedimentation or erosion as high as several centimeters per year, thermal equilibration can easily keep pace with the slight thermal perturbations associated with the addition or removal of sediment. In such cases, there is little or no change in the BSR depth throughout the entire deposition or removal event.
This section has primarily focused on perturbations to the thermal state of the hydrate reservoir from above. However, in some settings, the thermal perturbation may be derived from below or within the reservoir. The most likely mechanism for such thermal perturbations in marine settings is likely to be disruption of the flow of energy or fluids (including gas) and establishment of new pathways for channeling fluid and energy in response to tectonic activity or even pressure fluctuations associated with oceanographic phenomena. Rapid changes in fluid flux rates and patterns (Tryon et al., 1999) and near-bottom temperatures (Macdonald et al., in review) have been observed in hydrate settings characterized by a high degree of spatial and temporal variability (e.g., Gulf of Mexico and the Hydrate Ridge feature on the Cascadia margin). Such hydrate reservoirs must clearly be characterized by a highly three-dimensional regime of energy (heat), fluid, and gas flux.
Temperature is one of the most fundamental parameters controlling the stability of the hydrate reservoir and is easily affected by a variety of processes (e.g., BWT variations, erosion, sedimentation, slumping, subsidence, and uplift) and physical parameters (e.g., bulk thermal conductivity of hydrate-bearing sediments). The best hope for constraining the thermal state of the hydrate reservoir probably remains direct measurement of equilibrium temperatures in the bottom of seafloor boreholes during drilling through hydrate-bearing sediments. In some settings, though, seismic constraints on the depth to the BSR can be combined with knowledge of hydrate stability curves to roughly constrain average heat flux and thermal gradients in the overlying hydrate-bearing sediments (Yamano et al., 1982). Observed disparities between the predicted hydrate dissociation temperature and the temperature inferred at the BSR from direct measurements in a variety of marine settings remain difficult to fully explain. If the measurements themselves are correct, then factors such as high capillary pressures (Clennell et al., 1999) or poor knowledge of the hydrate stability curves at elevated pressures may account for the disparities. Future research should provide not only higher quality in situ temperature measurements, but also better constraints on hydrate stability curves and on such important parameters as the thermal conductivity and diffusivity of hydrate.
Acknowledgements Research related to this article has been supported by the National Science Foundation (OCE-9730846), the Petroleum Research Fund of the American Chemical Society (AC8-31351), the Joint Oceanographic Institutions (F000319 and F000555), and the Ocean Drilling Program. I am grateful to R. Von Herzen for introducing me to thermal measurements, C. Paull and R. Matsumoto for supporting the in situ temperature program on ODP Leg 164, G. Dickens, W.S. Holbrook, I. Pecher, and B. Clennell for many fruitful discussions, and S. Kirby and W. Durham for their generous collaboration on the laboratory thermal measurements program. B. deMartin and J. Nimblett provided comments that improved this chapter, and B. deMartin permitted use of unpublished results obtained with W. Waite, S. Kirby, and others.
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