Isotopic Analysis of Core Gases at Sites 565-570, Deep Sea Drilling Project Leg 84
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Methane carbon-isotopic compositions (1 3C values relative to the PDB standard) at Sites 565, 566, 567, and 569 were lighter (enriched in 12C) than -60, indicating a biogenic origin. In the deeper sections at Sites 568 and 570, 1 3C values were heavier, approaching - 40, and therefore suggest a thermogenic source. A significant thermogenic source was discounted, however, because the carbon dioxide 1 3C values in these sections were also anomalously heavy, suggesting that the methane may have formed biogenically by reduction of the heavy carbon dioxide. 1 3C values of ethane and higher hydrocarbons were measured in several sections from Sites 566 and 570 that contained sufficient C2-C4 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 C2-C5 hydrocarbons are thermally produced by low-temperature chemical diagenesis of indigenous organic matter. This process apparently generates isotopically heavy C2-C5 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 C2-C5 hydrocarbons.
