Subseafloor Temperature Variations Influenced by Variations in Bottom Water Temperature and Pressure: New High Resolution Observations and Implications

A new instrument developed for monitoring acceleration, tilt, and pressure at the ocean floor also measures sediment temperature 1 m below the seafloor. Four deployments have been completed and connected to the Ocean Networks Canada cabled observatory, one on the inner Cascadia accretionary prism, two on the outer prism, and one on the sedimented eastern flank of the Juan de Fuca Ridge. Relative amplitudes and phases of temperature variations measured at the seafloor and in the sediment at periods greater than roughly 1 week constrain the thermal diffusivity of the upper meter of subseafloor sediment to be 4 × 10−7 m2/s. Clear ±0.1‐mK amplitude tidal sediment temperature variations are also resolved. These are too large and regular to be the consequence of downward thermal diffusion from the seafloor and too large to be the consequence of fluid migration driven along the sediment geotherm by poroelastic response to tidal loading. The variations are closely correlated with tidal pressure variations, however, and we infer that these temperature signals reflect adiabatic heating and cooling. The lapse rates inferred from the observations at two of the sites are close to the values for seawater but significantly higher than predicted for a mixture that includes sediment grains. The values observed by both instruments at the outer prism site, located near methane‐bearing‐fluid springs, are particularly high, 20% higher than predicted for a sediment‐seawater mixture. This discrepancy could be reconciled if free gas or methane hydrate were present within the pore volume. Plain Language Summary A new tool developed for deep ocean seismic and geodynamic monitoring also measures sediment temperature 1 m below the seafloor to very high precision. Temperature variations that have diffused from week‐ to month‐long temperature variations at the ocean bottom provide a direct measurement of the thermal diffusivity of the sediments. Small (0.0001 °C) sediment temperature variations are also observed at tidal periods, and these are synchronized with ocean tidal pressure variation. These observations provide a new method for the in situ determination of the adiabatic lapse rate for the sedimentary material. Key Points A new tool for geodynamic monitoring documents temperature variations 1 m below the seafloor with a few tens of microkelvin precision Sediment‐temperature response to bottom water temperature variations leads to a determination of thermal diffusivity of 4 by 10