48. HYDROGEN- AND CARBON-ISOTOPE COMPOSITIONS OF METHANE FROM DEEP SEA DRILLING PROJECT SITE 618, ORCA BASIN1

• Main Authors: Roger A. Burke, William M. Sackett, James M. Brooks
• Other Authors: The Pennsylvania State University CiteSeerX Archives
• Format: Text
• Language: English
• Subjects: Orca
• Online Access: http://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.632.3764; http://www.deepseadrilling.org/96/volume/dsdp96_48.pdf

Hydrogen- and carbon-isotope compositions of methane (δ D-CH4 and δ13C-CH4) and pore water hydrogen-isotope compositions (δD-H2O) in several samples from the uppermost 100 m of Orca Basin sediments show little variation with depth and have mean values of- 184,-73.5, and 7‰, respectively. The δD-CH4 is typical of that generally found in deep-sea sediments and suggests that the methane was produced biologically almost entirely via the CO2 reduction pathway. Production of small amounts of methane (-15 % of the total) from acetate dissimilation cannot be com-pletely ruled out, however. The pore water is more enriched in deuterium than present day Gulf of Mexico deep water, suggesting that the brine found at the bottom of Orca Basin was formed at a time when appreciably more isotopically light water was tied up in continental ice sheets than is at present. The δ13C-CH4 values reported here are similar to pre-viously determined brine δ13C-CH4, but substantially more enriched in 13C than previously determined pore water δ13C-CH4. Isotopic evidence and the decrease in interstitial salinity with depth suggest that most of the methane production within the Orca Basin system occurs in sediments deeper than about 5 m sub-bottom. Small deposits of isotopically anomalous methane occurring in near-surface sediments may be the result of spatially variable inputs of slumped mate-rial to the sediments underlying the anoxic, hypersaline brine.