10. EFFECTS OF CORE RETRIEVAL AND DEGASSING ON THE CARBON ISOTOPE COMPOSITION OF METHANE IN GAS HYDRATE- AND FREE GAS-BEARING SEDIMENTS
FROM THE BLAKE RIDGE¹

Paul J. Wallace,² Gerald R. Dickens,³ Charles K. Paull,^{4, 5} and William Ussler III⁴

ABSTRACT

Cores with sediment and gas from the gas hydrate reservoir on the Blake Ridge were recovered at near in situ pressure using the pressure core sampler (PCS) on Leg 164. Stepwise degassing of these cores and subsequent analyses of gas samples provide a unique data set concerning the in situ abundance and composition of gases in marine sediments. Carbon isotope analyses were conducted on CH₄ from 90 of the gas samples to compare the isotopic composition of in situ CH₄ with the isotopic composition of gas in sediments recovered by conventional coring procedures. Weighted averages of CH₄ ¹³C values from PCS cores at Sites 995 and 997 are relatively constant at -65 to 62 PDB between 300 m below seafloor (mbsf) and the bottom of the holes (700-750 mbsf). These values indicate a microbial origin for the methane. The weighted averages of CH₄ ¹³C values for PCS gas samples are comparable to those for gas recovered from voids in standard advanced piston (APC) and extended core barrel (XCB) cores recovered from similar depths. This demonstrates that ¹³C of CH₄ in gas hydrate- and free gas-bearing sediments is not fractionated during degassing of normal cores, even though as much as 99.8% of the original CH₄ in the sediments can be lost during core recovery. However, during degassing of a core inside of the PCS, anomalous methane ¹³C values are frequently observed for the first degassing step, which involves release of CH₄-poor air that is trapped inside the PCS during deployment. Experiments on degassing of CH₄-saturated water in a simple, sediment-free system analogous to the PCS demonstrates that carbon isotope fractionation of 1.5-2 between CH₄ gas and dissolved CH₄ can occur after large pressure drops if the system is not allowed to re-equilibrate. This fractionation effect is probably amplified during gas release from PCS core samples because gas must escape through fine-grained sediment. Kinetic fractionation of carbon isotopes likely accounts for some of the nonsystematic methane carbon isotopic variations that are observed during degassing of PCS cores.

¹Paull, C.K., Matsumoto, R., Wallace, P.J., and Dillon, W.P. (Eds.), 2000. Proc. ODP, Sci. Results, 164: College Station, TX (Ocean Drilling Program).
²Ocean Drilling Program, Texas A&M University, College Station, TX 77845, U.S.A. wallace@odpemail.tamu.edu  
³School of Earth Sciences, James Cook University, Townsville, QLD 4811 Australia.
⁴Department of Geology, University of North Carolina, Chapel Hill, NC 27599-3315, U.S.A.
⁵Present address: Monterey Bay Aquarium Research Institute, 7700 Sandholdt Road, Moss Landing, CA 95039-0628, U.S.A.

Date of initial receipt: 23 April 1998
Date of acceptance: 12 January 1999
Ms 164SR-209