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Offshore Mexico and Guatemala slope sediments are classic sites of deep-sea gas hydrate occurrence. Gassy, frozen sediment was recovered in cores from this region on Legs 66, 67, and 84 of the Deep Sea Drilling Project. In addition, a massive 3 to 4-m thick layer of nearly pure methane hydrate at a depth of 250 m was cored on Leg 84 and preserved for study. The gas from the hydrate is 99+% methane with a few tenths percent carbon dioxide and traces (10-4v/v) of ethane. Most of the sites with gas hydrate in the sediments have methane with ^dgr13C of -70 to -60 ^pmil, indicating origin from methane-generating bacteria. The massive gas hydrate contained methane with ^dgr13C of -40 ^pmil, and the surrounding sediment had bicarbonate in the por water with ^dgr13C of +35 ^pmil. The 75 ^pmil separation in ^dgr13C between coexisting methane and bicarbonate is consistent with kinetic fractionation during bacterial reduction of carbon dioxide to methane, with continuous replenishment of carbon dioxide by fermentation processes.
The areal extent of the massive gas hydrate is not known, but the single point yields a gas-in-place estimate of 5.2 × 108m3/km2 or 48 bcf/mi2.
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