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

During Ocean 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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