Molecular and Isotopic Analysis of Core Gases and Gas Hydrates, Deep Sea Drilling Project Leg 96

Abstract

Molecular and isotopic compositions of void gases from Sites 618 and 619 were determined. The gases were predominantly methane with the C L /C 2 ratios averaging 4,000 and 26,000 at Sites 618 and 619, respectively. The δ 13 C-Cj values at Site 618 were nearly constant, ranging from -70.1 to -73.7‰. δ 13 C-CHj values for Site 619 between 76 and 178 meters sub-bottom ranged from -94.8 to -70.8‰, becoming progressively heavier with depth. The molecular and isotopic compositions at both sites are characteristic of biogenic methane.Small spheres (~ 1-2 mm diameter) of gas hydrates were observed in the Orca Basin between 20 and 40 m sub-bottom and were often associated with sandy sections. The hydrate was predominantly methane with a δ 13 -Cj of -71‰ and trace amounts of ethane, propane, and carbon dioxide. The isotopic value for the methane was similar to that of the nearest gas pocket, indicating that hydrate was formed from predominantly biogenic methane without isotopic fractionation.

Organic carbon content, bacterial methanogenesis, and accumulation processes of gas hydrates in marine sediments.

“…These similar patterns in both hy drate-bearing and hydrate-free sediments suggest that the hydrate formation causes no 13C fraction ation in methane (Claypool et al, 1985). The S13C values of methane in gas hydrates are similar to those of interstitial gases in sediments at the depth where the gas hydrates were found, also suggest ing that hydrate formation involves no 13C frac tionation in methane (Pflaum et al, 1986;Kennicutt et al, 1993). Furthermore, no obvious isotopic effect has been observed during controlled decomposition of CH4 hydrate (Claypool et al, 1985).…”

Subsurface gas hydrates in the northern Gulf of Mexico

“…Perhaps the initial report of gas hydrates being encountered below the GoM seafloor, with no obvious connection to seafloor anomalies, was the visual observation of mm-to cm-scale "crystals" of gas hydrate in sediment recovered from 20 to 40 mbsf at Deep Sea Drilling Project Leg 96 Site 618 in the Orca basin (Pflaum et al, 1986) in 1983. The nature and form of the gas hydrate observed in situ is not clear, although the sediments were described as primarily mud-rich with potential preferred occurrence of the gas hydrate within minor interbedded biogenic (primarily shell fragments) sands.…”

Fault system and thermal regime in the vicinity of site NGHP-01-10, Krishna–Godavari basin, Bay of Bengal

“…northwestern Gulf of Mexico MacDonald et al, 1994;Sassen, 2000, 2001;Sassen et al, 1999Sassen et al, , 2001, Hydrate Ridge (Hovland et al, 1995;Suess et al, 1999Suess et al, , 2001Trehu et al, 1999), and Haakon Mosby (Bogdanov et al, 1999;Ginsburg et al, 1999). In stratigraphic accumulation gases are transported along permeable horizons, e.g., Blake ridge (Xu and Ruppel, 1999;Dickens et al, 1997), Gulf of Mexico minibasins (Milkov and Sassen, 2001;Pflaum et al, 1986), Nankai trough (Matsumoto et al, 2001), and Mallik (Dallimore et al, 1999). A combination of both structural and stratigraphic transport and trapping mechanisms are also possible (Diaconescu and Knapp, 2000;Diaconescu et al, 2001).…”