Year: 1985
Type: book-chapter
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.
Authors: A.W.A. Jeffrey, R.C. Pflaum, T.J. McDonald, J.M. Brooks, K.A. Kvenvolden
Institutions: United States Geological Survey, Texas A&M University
Language: English
Subfield: Environmental Chemistry
Field: Environmental Science
Domain: Physical Sciences
Open Access status: bronze
Funder: Office of Naval Research
OpenAlex ID: W2496841345