Isotopic analysis of core gases at DSDP Leg 84 Holes

In the deeper sections at Sites 568 and 570, d13C values were heavier, approaching -40 per mil, and therefore suggest a thermogenic source. A significant thermogenic source was discounted, however, because the carbon dioxide d13C values in these sections were also anomalously heavy, suggesting that the methane may have formed biogenically by reduction of the heavy carbon dioxide. d13C 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 per mil. These values are among the heaviest ethane values reported for natural gases. The isobutane/ n-butane and isopentane/n-pentane ratios of the core gases suggested that the C2-C5 hydrocarbons are thermally produced by low-temperature chemical diagenesis of indigenous organic matter.