Gas hydrate decomposition and carbonate
precipitation on the Carolina Continental Rise:
Results from Ocean Drilling Program Leg 164
Thomas Naehr, Ph.D.
Monterey Bay Aquarium Research Institute
Wednesday, March 24, 1999
3:00 p.m.Pacific Forum
Drilling Program (ODP) Leg 164, authigenic carbonates were recovered from gas hydrate
bearing sediments that overlay the Blake Ridge Diapir on the Carolina Continental Rise
(southeastern USA). The presence of gas hydrate nodules near the sediment surface
indicates that, at present, gas hydrates are stable under seafloor p/T conditions in this
area. However, active chemosynthetic communities on the seafloor are apparently fed by
methane-rich fluids. These fluids, which migrate upward along a fault extending through
the gas hydrate seal, are most likely derived from the decomposition of gas hydrates below
the base of the gas hydrate stability zone. As there is a largely unassessed potential for
significant diagenetic changes to occur as a consequence of gas hydrate formation and
decomposition, one objective of this study was to investigate if dissociation of gas
hydrate at this site produces such changes at depth or if authigenic mineral precipitation
is restricted to near-surface sediments.
This presentation will focus on the petrographic, mineralogical and geochemical
characteristics of authigenic carbonate nodules, recovered from zero to 52 mbsf (meters
below seafloor). Carbon and strontium isotopic data of carbonate precipitates and
associated pore fluids suggest that these carbonates had formed on or near the seafloor.
There is no evidence of continuing carbonate precipitation with depth. As a consequence,
with the exception of their seafloor expression as carbonate crusts, fossil vent sites
will not be preserved. As these authigenic features apparently form only at the seafloor,
their vertical distribution and sediment age imply that seepage has been going on in this
area for at least 600,000 years.
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Last updated: December 19, 2000