HR: 0830h
AN: OS51B-0858    [PDF]
TI: Temperature Time-Series from a Seafloor Gas Hydrate Deposit on the Gulf of Mexico Slope
AU: * MacDonald, I R
EM: imacdonald@falcon.tamucc.edu
AF: Texas A&M University - Corpus Christi, 6300 Ocean Dr. ST320, Corpus Christi, TX 78412 United States
AU: Bender, L
EM: les@gergx.gerg.tamu.edu
AF: Texas A&M University, GERG 727 Graham Rd., College Station, TX 77845 United States
AU: Vardaro, M
EM: vardaro@gerg.tamu.edu
AF: Texas A&M University, GERG 727 Graham Rd., College Station, TX 77845 United States
AB: Gas hydrate deposits at the seafloor in the Gulf of Mexico are subjected to frequent ambient temperature changes as bottom water temperatures fluctuate. The effect of temperature changes on hydrate formation and dissociation will depend on thermal diffusion between bottom water, the sediments at the seafloor interface, and the interior mass of hydrate deposits. During a two year study, we recorded multiple temperature time-series in a gas hydrate mound at a depth of 550m. Temperatures were recorded using Antares autonomous thermistors and data loggers installed at each end of a 50 cm probe. Probes were implanted 7 cm deep into tight-fitting holes that we bored in the hydrate mass with a specially-designed drill and 50 cm deep into unconsolidated sediment next to the mound. This allowed synoptic measurement of the ambient water temperature (top) and the hydrate or sediment temperature (bottom). The integrity of probe insertion was verified by detailed photographic inspection after installation and upon recovery. We obtained temperature time-series of 327 and 404 days, as well as shorter intervals. The range of bottom water temperature was 3.2 C over the two years of observation. Mean temperatures for the water, hydrate, and sediment, respectively were 7.90 C (sd 0.437), 7.81 C (sd 0.344), and 7.81 C (sd 0.156). Spectra of the temperature records showed significant, high-frequency peaks for in-water data corresponding to K1, M2 and M3 lunar tides. Of these peaks, only the K1 (23.9 h) was evident for in-hydrate records and none of the tidal peaks were evident for in-sediment records. All three records showed significant low-frequency periodicity at about 288 h. In-hydrate temperatures lagged the in-water temperatures by 6 h with high correlation. In-sediment temperatures lagged in-water temperatures by 288 h with weak correlation. Careful examination of the temperature records does not show any separation of the hydrate and sediment temperatures from the bottom water cycles and trends. We therefore infer that geothermal heat flow was not perturbed by episodic gas or fluid venting. Analysis of the records can therefore provide the following estimates for the thermal diffusivity of hydrate and sediments. Hydrate: 2.57 cm$^{2}$ h$^{-1}$ and Sediment 6.36 cm$^{2}$ h$^{-1}$. These values are significantly lower than published estimates of thermal diffusivity for gas hydrates created in the laboratory (9.32 cm$^{2}$ h$^{-1}$). Although the hydrate deposit is active biological substratum and is gradually increasing in size, on a yearly time scale it appears to be a relatively stable component of the seep environment and is relatively unaffected by changes in ambient water temperature.
DE: 1615 Biogeochemical processes (4805)
DE: 3015 Heat flow (benthic) and hydrothermal processes
DE: 3045 Seafloor morphology and bottom photography
DE: 3094 Instruments and techniques
DE: 4804 Benthic processes/benthos
SC: Ocean Sciences [OS]
MN: 2003 Fall Meeting