Thermal Conductivity

Thermal conductivity K (units of W rrf1 K"1) describes a material's ability to transfer heat. Materials like salt (K~6 W m"1 K"1) transfer heat efficiently (conductors), while air (K~0.025 W m"1 K'1) and other gases are insulators. Most modern heat flow probes measure the thermal conductivity of the sediments in situ immediately following determination of equilibrium temperatures. Combining these conductivity values with thermal gradient measurements through the application of Fourier's law of heat conduction yields heat flux q=-K'dT/dz (units of mW m"2). The negative sign indicates flux of heat out of the seafloor, in the direction opposite the increase in thermal gradient with increasing depth. If q is constant within a vertical column of the seafloor, the low thermal conductivity of hydrate (-0.5 W m"1 K"1) relative to that of typical saturated marine sediments (0.7 to 1.3 W m"' K"1) implies that thermal gradients within the hydrate-bearing zone must be larger than those in the zone lacking hydrate, as shown in Figure 2a.

Two types of laboratory measurements are relevant to the determination of the thermal conductivity of hydrate and hydrate-bearing sediments. The first kind of measurement is conducted on sediments recovered during conventional piston coring or ocean drilling and uses needle probes (Von Herzen and Maxwell, 1959) inserted into the sediment through the core liner. After testing to ensure that the sediments are in thermal equilibrium, a known amount of heat is introduced and the temperature response of the sediments is measured as a function of time. Interpretation of temperature records as a function of time yields an estimate of thermal conductivity (Jaeger, 1956, 1958). Because these measurements can only be completed after the core has thermally equilibrated to ambient conditions, the thermal conductivity value applies only to the sediment matrix and associated pore fluids after hydrate dissociation. To estimate the conductivity of the original three component (hydrate, pore fluid, sediment) mixture requires additional information about the concentration of hydrate in situ, the thermal conductivity of pure hydrate, and the effective media models that govern the thermal conductivity of multicomponent mixtures.

Temperature

Temperature k «-o

O propane hydrate Osand & methane hydrate'

- pure ice pure water

Temperalure (°C)

Figure 2. (a) Examples of a (1) a conductive thermal gradient, (2) a thermal gradient affected by upward advection of fluids, (3) a thermal gradient that increases within a zone of hydrate (low thermal conductivity) to maintain constant heat flux q. (b) Updated compilation of thermal conductivity results for hydrate, hydrate-sediment mixtures, water ice, and other substances after Sloan (1998). Measurements denoted by an asterisk were completed by deMartin et al. (1999) or provided by deMartin (pers. comm.).

The second type of laboratory thermal conductivity measurement is conducted on synthesized hydrate either in pure form or as part of hydratesediment mixtures. Until recently, most laboratory thermal conductivity measurements have used Structure II propane (Stoll and Bryan, 1979) or tetrahydrofuran (Ross and Anderson, 1982) hydrates or Structure I methane hydrates maintained at P-T conditions not characteristic of typical marine sediments (e.g., Stoll and Bryan, 1979). These experiments and others on hydrates containing various guest gas molecules (Sloan, 1998) confirm that hydrate has low thermal conductivity, with an estimated value of ~0.5 W m"1 K'1. In contrast, water ice, a substance to which hydrate is often compared, has thermal conductivity of 2.23 W m"1 K"1. Recently, deMartin et al. (1999) have reported on the first modern experiments to constrain the thermal conductivity of methane hydrate and hydrate-sand mixtures at P-T conditions more characteristic of those on continental margins, using hydrate synthesized with the Stern et al. (1996) technique. Experiments on pure hydrate and calculations to understand how thermal conductivity depends on the proportion of gas, sediment, and hydrate in the system remain in progress. Figure 2b summarizes the existing thermal conductivity data for hydrate, hydrate-sediment mixtures, and ice.

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