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| Format: | Dataset Open Access |
| Language: | en |
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PANGAEA
2000
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| Online Access: | https://doi.org/10.1594/PANGAEA.803742 |
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| _version_ | 1867171811064545280 |
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| author | Wallace, Paul J Dickens, Gerald Roy Paull, Charles K Ussler, William III |
| author_facet | Wallace, Paul J Dickens, Gerald Roy Paull, Charles K Ussler, William III |
| collection | Datos científicos de ciencias marinas y ambientales |
| contents | 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 d13C 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 d13C 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 d13C 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 d13C 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. |
| format | Dataset Open Access |
| id | pangaea_https___doi_org_10_1594_PANGAEA_803742 |
| institution | PANGAEA |
| language | en |
| publishDate | 2000 |
| publisher | PANGAEA |
| record_format | pangaea |
| spellingShingle | (Table 1) Concentration and isotopic composition of methane in PCS gas samples from ODP Leg 164 sites Wallace, Paul J Dickens, Gerald Roy Paull, Charles K Ussler, William III 164-994C; 164-995A; 164-995B; 164-996A; 164-996D; 164-997A; 164-997B; DEPTH, sediment/rock; DRILL; Drilling/drill rig; DSDP/ODP/IODP sample designation; Elevation of event; Event label; Joides Resolution; Latitude of event; Leg164; Longitude of event; Mass spectrometer, Finnigan, MAT 252; Methane; Methane, absolute volume; Nicaraguan Rise, North Atlantic Ocean; North Atlantic Ocean; Ocean Drilling Program; ODP; Sample code/label; Sample ID; Sample volume; δ13C, methane 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 d13C 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 d13C 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 d13C 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 d13C 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. |
| title | (Table 1) Concentration and isotopic composition of methane in PCS gas samples from ODP Leg 164 sites |
| topic | 164-994C; 164-995A; 164-995B; 164-996A; 164-996D; 164-997A; 164-997B; DEPTH, sediment/rock; DRILL; Drilling/drill rig; DSDP/ODP/IODP sample designation; Elevation of event; Event label; Joides Resolution; Latitude of event; Leg164; Longitude of event; Mass spectrometer, Finnigan, MAT 252; Methane; Methane, absolute volume; Nicaraguan Rise, North Atlantic Ocean; North Atlantic Ocean; Ocean Drilling Program; ODP; Sample code/label; Sample ID; Sample volume; δ13C, methane |
| url | https://doi.org/10.1594/PANGAEA.803742 |