Natural gas hydrates are clathrates in which water molecules form a crystalline framework that includes and is stabilized by natural gas (mainly methane) at appropriate conditions of high pressures and low temperatures. The conditions for the formation of gas hydrates are met within continental margin sediments below water depths greater than about 500 m where the supply of methane is sufficient to stabilize the gas hydrate. Observations on DSDP Leg 11 suggested the presence of gas hydrates in sediments of the Blake Outer Ridge. Leg 76 coring and sampling confirms that, indeed, gas hydrates are present there. Geochemical evidence for gas hydrates in sediment of the Blake Outer Ridge includes (1) high concentrations of methane, (2) a sediment sample with thin, matlike layers of white crystals that released a volume of gas twenty times greater than its volume of pore fluid, (3) a molecular distribution of hydrocarbon gases that excluded hydrocarbons larger than isobutane, (4) results from pressure core barrel experiments, and (5) pore-fluid chemistry. The molecular composition of the hydrocarbons in these gas hydrates and the isotopic composition of the methane indicate that the gas is derived mainly from microbiological processes operating on the organic matter within the sediment. Although gas hydrates apparently are widespread on the Blake Outer Ridge, they probably are not of great economic significance as a potential, unconventional, energy resource or as an impermeable cap for trapping upwardly migrating gas at Site 533.

Leg 76 of the Deep Sea Drilling Project achieved two major scientific objectives. The first objective was met at Site 533, where on the Blake Outer Ridge, gas hydrates were identified by geophysical, geochemical, and geological studies. Gas-hydrate decomposition produced a volu-metric expansion of 20: I of gas volume to pore-fluid volume; this expansion exceeded by about a factor of four the volume of gas that could be released from solution in pore water under similar conditions. The gas hydrate includes methane, ethane, propane, and isobutane but apparently excluded normal butane and higher molecular weight hydrocarbons as predicted from gas hydrate crystallography. For the first time, marine gas hydrates were tested with a pressure core barrel. The second objective was achieved when coring at Site 534 in the Blake-Bahama Basin sampled the oldest oceanic sediments yet recovered. The sequence of oceanic basement and overlying sediments documents the geologic history of the early stages of the opening of the North Atlantic Ocean in detail. The oldest oceanic sediments are red claystones and laminated green and brown claystones of middle Cal-lovian age. This finding supports the interpretation that the beginning of the modern North Atlantic occurred in the early Callo-vian (~155 m.y. B.P.), as much as 20 m.y. later in time than often previously thought.

In an investigation of gas hydrates in deep ocean sediments, gas samples from Deep Sea Drilling Project Site 533 on the Blake Outer Ridge in the northwest Atlantic were obtained for molecular and isotopic analyses. Gas samples were collected from the first successful deployment of a pressure core barrel (PCB) in a hydrate region. The pressure decline curves from two of the four PCB retrievals at in situ pressures suggested the presence of small amounts of gas hydrates. Compositional and isotopic measurements of gases from several points along the pressure decline curve indicated that (1) biogenic methane (δ 13 C = -68‰; CJ/CJ = 5000) was the dominant gas (>90%); (2) little fractionation in the C,/ C 2 ratio or the C carbon isotopic composition occurred as gas hydrates decomposed during pressure decline experiments; (3) the percent of C 3 , i-C 4 , and CO 2 degassed increased as the pressure declined, indicating that these molecules may help stabilize the hydrate structure; (4) excess nitrogen was present during initial degassing; and (5) Cj/Q ratios and isotopic ratios of C gases were similar to those obtained from conventional core sampling. The PCB gas also contained trace amounts of saturated, acyclic, cyclic, and aromatic C 5 -C 14 hydrocarbons, as well as alkenes and tetrahydrothiophenes. Gas from a decomposed specimen of gas hydrate had similar molecular and isotopic ratios to the PCB gas (δ 13 C of -68% for methane and a q/Q ratio of -6000). Regular trends in the δ 13 C of methane ( 95 60%) and C { /C 2 ratios (-25000 -2000) were observed with depth. Capillary gas chromatography (GC) and total scanning fluorescence measurements of extracted organic material were characteristic of hydrocarbons dominated by a marine source, though significant amounts of perylene were also present.

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