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Seismic and seafloor evidence for free gas, gas hydrates, and fluid seeps on the transform margin offshore Cape Mendocino
Seismic data and seafloor samples indicate the presence of free gas, gas hydrate, and
fluid seeps south of the Gorda Escarpment, a topographic feature that marks the eastern
end of the Gorda/Pacific transform plate boundary southwest of Cape Mendocino,
California. In spite of high sedimentation rates and high biological productivity, direct or
indirect indicators of gas hydrate presence had not previously been recognized in this
region, or along transform margins in general. Gas is indicated by a bottom simulating
reflection (BSR) observed near the Gorda Escarpment, by ââbright spotsââ and ââgas
curtainsââ scattered throughout the sedimentary basin to the south, and by ÎŽÂčÂłC and ÎŽÂčâžO
isotopes of carbonates, which are similar to those recovered from other hydrate-bearing
regions. The BSR reflection coefficient of -0.13 ± 0.04 and interval velocities as low as
1.38 km/s indicate that free gas is present beneath the BSR. Local shallowing of the BSR
toward the north facing Gorda Escarpment and beneath a channel near the crest suggests
fluid flow toward the seafloor. Integrating these various observations, we suggest a
scenario in which methane is formed in thick Miocene and Pliocene deposits of organicrich
sediments that fill the marginal basin south of the transform fault. Dissolved and free
gas migrates toward the escarpment along stratigraphic horizons, resulting in hydrate
formation and in channels, slumps and chemosynthetic communities on the face of the
escarpment. We conclude that the BSR appears where hydrate-bearing sediments are
uplifted because of current triple junction tectonics
Late Eocene to Early Oligocene magnetostratigraphic chron boundaries of ODP Hole 119-744A (Table 1)
The earliest Oligocene (~33.5 Ma) is marked by a major step in the long-term transition from an ice-free to glaciated world. The transition, characterized by both cooling and ice-sheet growth, triggered a transient but extreme glacial period designated Oi-1. High-resolution isotope records suggest that Oi-1 lasted for roughly 400,000 yr (the duration of magnetochron 13N) before partially abating, and that it was accompanied by an ocean-wide carbon isotope anomaly of 0.75â°. One hypothesis relates the carbon isotope anomaly to enhanced export production brought about by climate-induced intensification of wind stress and upwelling, particularly in the Southern Ocean. To understand how this climatic event affected export production in the Southern Ocean, biogenic silica (opal) and carbonate accumulation rates were computed for the sub-polar Indian Ocean using deep-sea cores from ODP Site 744, Kerguelen Plateau. Our findings suggest that net productivity in this region increased by several fold in response to the Oi-1 glaciation. In addition, calcareous primary producers dominant in the Late Eocene were partially replaced by opaline organisms suggesting a trend toward seasonally greater surface divergence and upwelling in this sector of the Southern Ocean. We attribute these changes to intensification of atmospheric=oceanic circulation brought about by high-latitude cooling and the appearance of a full-scale continental ice-sheet on East Antarctica. Higher terrigenous sediment accumulation rates support the idea that wind-induced changes in regional productivity were augmented by an increased supply of glacial dust and debris that provided limiting micro-nutrients (e.g., iron-rich dust particles). We speculate that the rapid changes in biogenic sediment accumulation in the Southern Ocean and other upwelling-dominated regions contributed to the ocean-wide positive carbon isotope anomaly by temporarily increasing the burial rate of organic carbon relative to carbonate carbon. The changes in burial rates, in turn, may have produced a positive feedback on climate by briefly drawing down atmospheric pCO2