Isotopic Analysis of Core Gases at Sites 565-570, Deep Sea Drilling Project Leg 84

Abstract

Methane carbon-isotopic compositions (δ 13 C values relative to the PDB standard) at Sites 565, 566, 567, and 569 were lighter (enriched in 12 C) than-60‰, indicating a biogenic origin. In the deeper sections at Sites 568 and 570, δ 13 C values were heavier, approaching-40‰, and therefore suggest a thermogenic source. A significant thermogenic source was discounted, however, because the carbon dioxide δ 13 C values in these sections were also anomalously heavy, suggesting that the methane may have formed biogenically by reduction of the heavy carbon dioxide. δ 13 C values of ethane and higher hydrocarbons were measured in several sections from Sites 566 and 570 that contained sufficient C 2-C 4 hydrocarbon concentrations. Ethane values in six sections (245-395 m sub-bottom) from Site 570 were fairly uniform, ranging from-24 to-26%o. These values are among the heaviest ethane values reported for natural gases. The isobutane/n-butane and isopentane/«-pentane ratios of the core gases suggested that the C 2-C 5 hydrocarbons are thermally produced by low-temperature chemical diagenesis of indigenous organic matter. This process apparently generates isotopically heavy C 2-C 5 hydrocarbons. High gas concentrations in the serpentinite basement rocks at Sites 566 and 570 appear to have resulted from migrated biogenic methane gas containing small amounts of immature C 2-C 5 hydrocarbons.

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

“…The 613C values of methane in marine sedi ments become generally heavier with depths par allel to depth-trends of 613C values of co-existing carbon dioxide in regions where gas hydrates are found (Claypool and Kaplan, 1974;Galimov and Kvenvolden, 1983;Jeffrey et al, 1985;Claypool et al, 1985;Kvenvolden and Kastner, 1990). Similar patterns of the 613C change with depth are also observed in regions where gas hydrates are not stable, such as Cariaco Trench (Claypool and Kaplan, 1974).…”

“…The more abundant organic carbon should relate to prolonged and vigorous methanogenesis, resulting in 13C_ depletion in the CO2 reservoir from which the CH4 was formed. The isotopically heavy methane (--40%0) in gas hydrate was found at Site 570 on DSDP Leg 84 offshore Guatemala (Table 1), ac companied by isotopically heavy CO2 (Jeffrey et al, 1985;Claypool et al, 1985).…”

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

“…At sub-bottom depths greater than those sampled at Leg 96 sites, ethane and other C 2 -C 5 hydrocarbons were detected (Galimov et al, 1980;Schoell, 1982). In contrast, core gas-pocket studies in the Japan Trench (Whelan and Sato, 1980), California Basin (Galimov and Simoneit, 1982), and Middle America Trench (Jeffrey et al, 1985) found C 2 -C 5 hydrocarbons at most depths where gas pockets were sampled. The processes controlling the amount of C 2 -C 5 hydrocarbons in shallow sediments containing primarily biogenic gas are unclear.…”

Isotope Compositions of Gases in Sediments from the Chile Continental Margin

“…δ 13 C values of ethane were measured on 13 samples below 200 mbsf at Sites 860 and 861, and one sample at 70 mbsf at Site 863, ranging between -70%o and -AA‰ (Table 1). The δ 13 C values of ethane of core gases from ODP Leg 84, Site 570 are fairly uniform, ranging from -24‰ to -26%o in six sections, indicating a thermogenic origin (Jeffrey et al, 1985), and those values measured on sorbed gases from ODP Leg 112, Sites 679, 680, and 682 range from -38‰ to -25%e for 22 samples except for one data of -48.3‰, also indicating a thermogenic origin (Whiticar and Suess, 1990). Ethanes of this study are depleted in 13 C compared to these data, and the carbon isotope compositions are among the lightest values reported for ethane in natural gas.…”