Effects of core retrieval and degassing on the carbon isotope composition of methane gas hydrate- and free gas-bearing sediments from the Blake Ridge

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doi: 10.2973/odp.proc.sr.164.209.2000
Author(s): Wallace, Paul J.; Dickens, Gerald R.; Paull, Charles K.; Ussler, William, III
Ocean Drilling Program, Leg 164, Shipboard Scientific Party, College Station, TX
Author Affiliation(s): Primary:
Texas A&M University, Ocean Drilling Program, College Station, TX, United States
Other:
University of Tokyo, Japan
Ocean Drilling Program, United States
U. S. Geological Survey, United States
University of Texas at Arlington, United States
University of Michigan, United States
Agder College, Norway
University of Bristol, United Kingdom
McGill University, Canada
Osaka Kyoiku University, Japan
Woods Hole Oceanographic Institution, United States
Lamont-Doherty Earth Observatory, United States
Osservatorio Geofisico-Sperimentale, Italy
La Trobe University, Australia
GEOMAR, Federal Republic of Germany
Hokkaido University, Japan
Université Pierre et Marie Curie, France
Georgia Institute of Technology, United States
Geological Survey of Japan, Japan
CREGU BP 23, France
Federal Institute for Geosciences, Federal Republic of Germany
Naval Research Laboratory, United States
James Cook University, Australia
University of North Carolina at Chapel Hill, United States
Volume Title: Proceedings of the Ocean Drilling Program; volume 164; scientific results; gas hydrate sampling on the Blake Ridge and Carolina Rise; covering Leg 164 of the cruises of the drilling vessel JOIDES Resolution, Halifax, Nova Scotia, to Miami, Florida, sites 991-997, 31 October-19 December 1995
Volume Author(s): Paull, Charles K.; Matsumoto, Ryo; Wallace, Paul J.; Black, Nancy R.; Borowski, Walter S.; Collett, Timothy S.; Damuth, John E.; Dickens, Gerald R.; Egeberg, Per Kristian; Goodman, Kim; Hesse, Reinhard F.; Hiroki, Yoshihisa; Holbrook, W. Steven; Hoskins, Hartley; Ladd, John; Lodolo, Emanuele; Lorenson, Thomas D.; Musgrave, Robert J.; Naehr, Thomas H.; Okada, Hisatake; Pierre, Catherine; Ruppel, Carolyn D.; Satoh, Mikio; Thiéry, Régis; Watanabe, Yoshio; Wehner, Hermann; Winters, William J.; Wood, Warren T.; Miller, Christine M.; Reigel, Ruth
Source: Proceedings of the Ocean Drilling Program; volume 164; scientific results; gas hydrate sampling on the Blake Ridge and Carolina Rise; covering Leg 164 of the cruises of the drilling vessel JOIDES Resolution, Halifax, Nova Scotia, to Miami, Florida, sites 991-997, 31 October-19 December 1995, Charles K. Paull, Ryo Matsumoto, Paul J. Wallace, Nancy R. Black, Walter S. Borowski, Timothy S. Collett, John E. Damuth, Gerald R. Dickens, Per Kristian Egeberg, Kim Goodman, Reinhard F. Hesse, Yoshihisa Hiroki, W. Steven Holbrook, Hartley Hoskins, John Ladd, Emanuele Lodolo, Thomas D. Lorenson, Robert J. Musgrave, Thomas H. Naehr, Hisatake Okada, Catherine Pierre, Carolyn D. Ruppel, Mikio Satoh, Régis Thiéry, Yoshio Watanabe, Hermann Wehner, William J. Winters, Warren T. Wood, Christine M. Miller and Ruth Reigel; Ocean Drilling Program, Leg 164, Shipboard Scientific Party, College Station, TX. Proceedings of the Ocean Drilling Program, Scientific Results, Vol.164, p.101-112. Publisher: Texas A & M University, Ocean Drilling Program, College Station, TX, United States. ISSN: 0884-5891
Note: In English. 9 refs.; illus., incl. 3 tables
Summary: 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 δ13C values from PCS cores at Sites 995 and 997 are relatively constant at -65 per mil to 62 per mil 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 δ13C 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 δ13C 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 δ13C 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 per mil-2 per mil 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.
Year of Publication: 2000
Research Program: ODP Ocean Drilling Program
Key Words: 02 Geochemistry; 07 Marine Geology and Oceanography; Aliphatic hydrocarbons; Alkanes; Atlantic Ocean; Blake-Bahama Outer Ridge; C-13/C-12; Carbon; Cores; Gas hydrates; Hydrates; Hydrocarbons; Isotope ratios; Isotopes; Leg 164; Marine geology; Marine sediments; Methane; North Atlantic; Northwest Atlantic; ODP Site 994; ODP Site 995; ODP Site 996; ODP Site 997; Ocean Drilling Program; Organic compounds; Pore pressure; Sediments; Stable isotopes
Coordinates: N314708 N314708 W0753245 W0753245
N314812 N314812 W0753118 W0753118
N322938 N322938 W0761127 W0761127
N315035 N315035 W0752807 W0752807
Record ID: 2001025805
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