In situ measurements of thermal diffusivity in sediments of the methane-rich zone of Cascadia Margin, NE Pacific Ocean

Abstract

Thermal diffusivity (TD) is a measure of the temperature response of a material to external thermal forcing. In this study, TD values for marine sediments were determined in situ at two locations on the Cascadia Margin using an instrumented sediment probe deployed by a remotely operated vehicle. TD measurements in this area of the NE Pacific Ocean are important for characterizing the upslope edge of the methane hydrate stability zone, which is the climate-sensitive boundary of a global-scale carbon reservoir. The probe was deployed on the Cascadia Margin at water depths of 552 and 1049 m for a total of 6 days at each site. The instrumented probe consisted of four thermistors aligned vertically, one sensor exposed to the bottom water and one each at 5, 10, and 15 cm within the sediment. Results from each deployment were analyzed using a thermal conduction model applying a range of TD values to obtain the best fit with the experimental data. TD values corresponding to the lowest standard deviations from the numerical model runs were selected as the best approximations. Overall TDs of Cascadia Margin sediments of 4.33 and 1.15 × 10-7 m 2 s-1 were calculated for the two deployments. These values, the first of their kind to be determined from in situ measurements on a methane hydrate-rich continental margin, are expected to be useful in the development of models of bottom-water temperature increases and their implications on a global scale.

On the Importance of Advective Versus Diffusive Transport in Controlling the Distribution of Methane Hydrate in Heterogeneous Marine Sediments

“…(3) Thermal perturbations due to enhanced fluid flow may be neglected. This assumption is appropriate given that the thermal Péclet number at the length scale over which pore size‐driven changes in solubility are expected to occur is sufficiently small (10 −2 to 10 −4 assuming that vertical fluid flow rates are 1–100 mm/year; the effective thermal diffusivity in marine sediments is 10 −7 m 2 /s, Homola et al, , and the length scale is 1 m). (4) While fluid transport requires the presence of a nonhydrostatic pressure gradient, we neglect the influence of such pressure perturbations on the solubility of methane.…”

Thermal environment of the Southern Washington region of the Cascadia subduction zone

“…The required parameters include the sediment accumulation rate ( v s , in m yr −1 ), depth of the BSR below seafloor ( Z BSR , in meters), and sediment accumulation time interval ( t s , in years). A sediment thermal diffusivity ( κ s ) of 1.15 × 10 −7 m 2 s −1 was determined in situ within the midslope terrace of the COAST Line 4 at 1049 m water depth [ Homola et al ., ]. Q B is the bathymetrically corrected heat flow, resulting in Q BSR , the basal heat flow with effects of sediment accumulation removed.…”

Analysis of bubble plume distributions to evaluate methane hydrate decomposition on the continental slope

“…Using an experimentally determined thermal diffusivity of 4.33 × 10 −7 m 2 s −1 for margin surface sediments, Homola et al . [] calculate thermal penetration depths of 4, 12, and 23 m for intervals of 1, 10, and 40 years, respectively. However, thermal diffusion into a conducting infinite half‐space is the slowest method of propagation for water column temperature changes, and rough surface topography, the presence of faults and shallow fluid circulation cells in high porosity turbidites can extend these depths by an order of magnitude [ Henry et al ., ; Tryon et al ., ; Solomon et al ., ].…”