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TitleSubseafloor temperature variations influenced by variations in bottom water temperature and pressure: new high resolution observations and implications
AuthorDavis, E E; Villinger, H
SourceJournal of Geophysical Research, Solid Earth vol. 124, 2019 p. 76-87, Open Access logo Open Access
Alt SeriesNatural Resources Canada, Contribution Series 20190026
PublisherAmerican Geophysical Union (AGU)
Mediapaper; on-line; digital
ProvinceBritish Columbia; Western offshore region
AreaPacific Ocean; Vancouver Island
Lat/Long WENS-130.0000 -125.0000 49.0000 47.0000
Subjectsgeophysics; marine geology; surficial geology/geomorphology; tectonics; Science and Technology; Nature and Environment; marine sediments; ground temperatures; water temperature; seabottom temperatures; in-field instrumentation; seismology; pressure; tectonic setting; subduction zones; seismological network; thermal analyses; fluid migration; tides; gas seeps; methane; hydrate; porosity; Cascadia Subduction Zone; Cascadia Accretionary Prism; Barkley Canyon; Clayoquot Slope; Cascadia Basin; Juan de Fuca Ridge; North American Plate; Ocean Networks Canada; monitoring
Illustrationslocation maps; tables; time series; plots
ProgramPublic Safety Geoscience Assessing Earthquake Geohazards
Released2019 01 12
AbstractA new instrument developed for monitoring acceleration, tilt, and pressure at the oceanflooralso measures sediment temperature 1 m below the seafloor. Four deployments have been completedand connected to the Ocean Networks Canada cabled observatory, one on the inner Cascadia accretionaryprism, two on the outer prism, and one on the sedimented easternflank of the Juan de Fuca Ridge. Relativeamplitudes and phases of temperature variations measured at the seafloor and in the sediment at periodsgreater than roughly 1 week constrain the thermal diffusivity of the upper meter of subseafloor sediment tobe 4 × 10\'017m2/s. Clear ±0.1-mK amplitude tidal sediment temperature variations are also resolved. Theseare too large and regular to be the consequence of downward thermal diffusion from the seafloor and toolarge to be the consequence offluid migration driven along the sediment geotherm by poroelastic responseto tidal loading. The variations are closely correlated with tidal pressure variations, however, and we inferthat these temperature signals reflect adiabatic heating and cooling. The lapse rates inferred from theobservations at two of the sites are close to the values for seawater but significantly higher than predicted fora 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 asediment-seawater mixture. This discrepancy could be reconciled if free gas or methane hydrate werepresent within the pore volume.
Summary(Plain Language Summary, not published)
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.

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