Isotopic Analysis of Gas from the Cariaco Trench Sediments

  • Graeme L. Lyon
Part of the Marine Science book series (MR, volume 3)


The 198 meter core of Quaternary sediments obtained from Hole 147, Leg 15 of the Deep Sea Drilling Project was laid down under anoxic conditions in the Cariaco Trench below 882 meters of water off the coast of Venezuela. Pockets of gas from core depths between 45 and 180 meters were found to be 86–99% methane. The carbon dioxide content of the ten pockets sampled reached a maximum at about 130 meters.

Isotopic analysis of the methane showed δ13CPDB values between -59 and -76°/∘∘. This is consistent with a biological origin due to the anaerobic reduction of organic matter in the sediments. The D/H ratios of the methane in all samples were similar, with a mean δDSMOW value of -178°/∘∘. In carbon dioxide δ13CPDB and δ18OPDB both fell in the range of +0.5°/∘∘ to -4.5°/∘∘, and both values approached zero on the PDB scale as depth increased.

It might be expected that the methane would have had time to reach isotopic equilibrium with the carbon dioxide and interstitial water. The isotopic fractionations between CH4 and CO2 for 13C/12C and between CH4 and H2O for D/H were compared with equilibrium fractionations calculated from thermodynamic data. The trench temperature of 17°C differs from thermodynamic equilibrium temperatures by about 40°C.


Government Printing Interstitial Water Hydrogen Isotope Isotopic Equilibrium Core Depth 


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Copyright information

© Plenum Press, New York 1974

Authors and Affiliations

  • Graeme L. Lyon
    • 1
  1. 1.Department of Scientific and Industrial ResearchInstitute of Nuclear SciencesLower HuttNew Zealand

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