HR: 14:00h
AN: OS52C-02    [PDF]
TI: Why Gas Hydrate Is Where It Is At Hydrate Ridge: The Story From Infrared Images
AU: * Weinberger, J L
EM: jlweinbe@ucsd.edu
AF: Scripps Institution of Oceanography, 9500 Gilman Dr, la Jolla, CA 9209-0244 United States
AU: Brown, K M
EM: kmbrown@ucsd.edu
AF: Scripps Institution of Oceanography, 9500 Gilman Dr, la Jolla, CA 9209-0244 United States
AU: Long, P E
EM: philip.long@pnl.gov
AF: (2) Pacific Northwest National Laboratory, PO Box 999, Mail Stop K9-33, Richland, WA 99352 United States
AU: Riedel, M
EM: mriedel@pgc-gsc.nrcan.gc.ca
AF: Geological Survey of Canada, Pacific Geoscience Centre, Sidney, BS V8L4B2 Canada
AB: A detailed analysis of infrared (IR) images from Ocean Drilling Program (ODP) Leg 204 to southern Hydrate Ridge, combined with core based observations of structure and lithology indicates that lithology plays a stronger role in influencing hydrate distribution at the ridge crest and western flank sites than on the lower eastern flanks and slope basin of southern Hydrate Ridge. Core based observations of silt layers and structural features from five sites representing the three major environments at southern Hydrate Ridge, the ridge crest, the ridge flanks, and the eastern slope basin, were documented in detail. These features were then correlated to the locations of negative temperature anomalies, indicative of decomposing gas hydrate, identified in IR images of the cores. The silt layers are typically $<$0.5 cm thick, laterally discontinuous, and compose $<$3% of the total sedimentary section in the gas hydrate stability zone (GHSZ). Structural indicators in the cores, though rare, typically consisted of shear bands and small offset faults ($<$1 cm). IR data at sites 1245, 1246, and 1250, on the western flank, high on the eastern flank, and ridge crest respectively, indicate that 40 to 60% of the temperature anomalies occur within the documented silt layers, yet show little correlation to structural features. The correlation between gas hydrate and silt layers is weaker at sites 1244 and 1251 where anywhere from 0 to 25% of the anomalies correlate with silt layers. Silt composes $<$1% of the sedimentary section at these sites east of the ridge crest. While counterintuitive at first glance, as lithology was expected to play a larger role in controlling gas hydrate distribution in the eastern slope basin than at any other site cored, these results, when placed in the regional geologic context for Hydrate Ridge should not be unexpected. Uplift and erosion of the summit and western slope of Hydrate Ridge has exposed previously deposited turbidite sequences with silty basal layers. These turbidities are sampled within 30 m of the seafloor at sites 1245, 1246, and 1250, well within the GHSZ. At sites 1244 and 1251, however, this sequence of frequent turbidite layers occurs deeper in the stratigraphic section. As a result, lithology exerts a lesser control at the core scale at sites 1244 and 1251, than at sites 1245, 1246, and 1251. It should also be noted that the steeply dipping faults and fractures likely to control fluid flow through the ridge are difficult to sample. Thus the gas hydrate imaged during the leg may be biased towards that located in sub-horizontal silt layers.
DE: 1800 HYDROLOGY
DE: 3022 Marine sediments--processes and transport
DE: 5100 PHYSICAL PROPERTIES OF ROCKS
SC: Ocean Sciences [OS]
MN: 2003 Fall Meeting