College Station, Tex. : Ocean Drilling Program, Texas A & M University
Abstract
During Ocean Drilling Program Leg 204, 80 in situ measurements of
subseafloor temperature were made; 68 of these showed good frictional
pulses on insertion and extraction from the seafloor and were used to
constrain the subsurface temperature. Considering uncertainties from
various sources, uncertainties in the in situ temperatures are estimated
to generally be less than ±0.3°C. The data are consistent with a purely
conductive temperature regime at all sites, and there is no resolvable
difference in heat flow between sites on the flanks of southern Hydrate
Ridge and sites near the summit, where other data indicate that free gas
is venting into the ocean, gas hydrate is forming rapidly, and free gas
and gas hydrate coexist in the sediments. We interpret this apparent
paradox to indicate that the aqueous fluid flow is decoupled from free
gas flow and that advection of free gas does not have a significant effect
on the temperature field. The temperature data, which include several
measurements within a few meters of the predicted base of the methane
hydrate stability field (calculated for the measured pore water salinity at
each site) also indicate that the bottom-simulating reflection (BSR) corresponds
to the base of gas hydrate stability within measurement uncertainties,
although a systematic shallowing of the BSR by as much as 10
m is possible. The heat flow indicated by the Leg 204 measurements
and the regional BSR depth is significantly lower than the heat flow
predicted based on the age of the subducting plate and the thickness of
the accretionary complex. Several measurements made near the summit
at depths shallower than 60 meters below seafloor show anomalous
behavior consistent with low in situ thermal conductivity, possibly because
of the presence of free gas and/or massive gas hydrate in these
sediments
