Wallace, Paul J.; Dickens, Gerald R.; Paull, Charles K.; Ussler, William, III (2000): Effects of core retrieval and degassing on the carbon isotope composition of methane gas hydrate- and free gas-bearing sediments from the Blake Ridge. Texas A & M University, Ocean Drilling Program, College Station, TX, United States, In: 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, Thiery, Regis, Watanabe, Yoshio, Wehner, Hermann, Winters, William J., Wood, Warren T., Miller, Christine M. (editor), Reigel, Ruth (editor), 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, 164, 101-112, georefid:2001-025805

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 CH (sub 4) from 90 of the gas samples to compare the isotopic composition of in situ CH (sub 4) with the isotopic composition of gas in sediments recovered by conventional coring procedures. Weighted averages of CH (sub 4) delta (super 13) C 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 CH (sub 4) delta (super 13) C 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 delta (super 13) C of CH (sub 4) 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 CH (sub 4) in the sediments can be lost during core recovery. However, during degassing of a core inside of the PCS, anomalous methane delta (super 13) C values are frequently observed for the first degassing step, which involves release of CH (sub 4) -poor air that is trapped inside the PCS during deployment. Experiments on degassing of CH (sub 4) -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 CH (sub 4) gas and dissolved CH (sub 4) 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.
Coverage:
West: -76.1127 East: -75.2807 North: 32.2938 South: 31.4708
Relations:
Expedition: 164
Site: 164-994
Site: 164-995
Site: 164-996
Site: 164-997
Data access:
Provider: SEDIS Publication Catalogue
Data set link: http://sedis.iodp.org/pub-catalogue/index.php?id=10.2973/odp.proc.sr.164.209.2000 (c.f. for more detailed metadata)
This metadata in ISO19139 XML format