Heeschen, Katja U; Hohnberg, Hans-Jürgen; Haeckel, Matthias; Abegg, Friedrich; Drews, Manuela; Bohrmann, Gerhard (2007): Hydrocarbon concentrations from pressurized cores in the northern Gulf of Mexico. PANGAEA, https://doi.org/10.1594/PANGAEA.772821, Supplement to: Heeschen, KU et al. (2007): In situ hydrocarbon concentrations from pressurized cores in surface sediments, Northern Gulf of Mexico. Marine Chemistry, 107(4), 498-515, https://doi.org/10.1016/j.marchem.2007.08.008
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Two newly developed coring devices, the Multi-Autoclave-Corer and the Dynamic Autoclave Piston Corer were deployed in shallow gas hydrate-bearing sediments in the northern Gulf of Mexico during research cruise SO174 (Oct-Nov 2003). For the first time, they enable the retrieval of near-surface sediment cores under ambient pressure. This enables the determination of in situ methane concentrations and amounts of gas hydrate in sediment depths where bottom water temperature and pressure changes most strongly influence gas/hydrate relationships. At seep sites of GC185 (Bush Hill) and the newly discovered sites at GC415, we determined the volume of low-weight hydrocarbons (C1 through C5) from nine pressurized cores via controlled degassing. The resulting in situ methane concentrations vary by two orders of magnitudes between 0.031 and 0.985 mol kg**-1 pore water below the zone of sulfate depletion. This includes dissolved, free, and hydrate-bound CH4. Combined with results from conventional cores, this establishes a variability of methane concentrations in close proximity to seep sites of five orders of magnitude. In total four out of nine pressure cores had CH4 concentrations above equilibrium with gas hydrates. Two of them contain gas hydrate volumes of 15% (GC185) and 18% (GC415) of pore space. The measurements prove that the highest methane concentrations are not necessarily related to the highest advection rates. Brine advection inhibits gas hydrate stability a few centimeters below the sediment surface at the depth of anaerobic oxidation of methane and thus inhibits the storage of enhanced methane volumes. Here, computerized tomography (CT) of the pressure cores detected small amounts of free gas. This finding has major implications for methane distribution, possible consumption, and escape into the bottom water in fluid flow systems related to halokinesis.
Median Latitude: 27.644483 * Median Longitude: -91.319076 * South-bound Latitude: 27.469690 * West-bound Longitude: -91.508030 * North-bound Latitude: 27.782800 * East-bound Longitude: -90.980910
Date/Time Start: 2003-10-15T21:14:00 * Date/Time End: 2003-11-08T17:44:00
SO174/1_118 * Latitude: 27.469690 * Longitude: -91.507150 * Date/Time: 2003-10-21T23:21:00 * Elevation: -552.0 m * Location: Bush Hill * Campaign: SO174/1 (OTEGA II) * Basis: Sonne * Method/Device: Multi autoclave corer (MAC)
SO174/1_63 * Latitude: 27.542160 * Longitude: -90.993250 * Date/Time: 2003-10-15T21:14:00 * Elevation: -1049.0 m * Campaign: SO174/1 (OTEGA II) * Basis: Sonne * Method/Device: Multi autoclave corer (MAC)
Datasets listed in this publication series
- Heeschen, KU; Hohnberg, H-J; Haeckel, M et al. (2007): (Table 1) Analysis of pore water (PW) and hydrocarbon gases (HC) on sediment samples after degassing, and the recovery pressure (pressure) on SO174 samples. https://doi.org/10.1594/PANGAEA.772735
- Heeschen, KU; Hohnberg, H-J; Haeckel, M et al. (2007): (Table 2) Total gas volume released upon degassing and its hydrocarbon composition. https://doi.org/10.1594/PANGAEA.772736
- Heeschen, KU; Hohnberg, H-J; Haeckel, M et al. (2007): (Table 3) Molalities of C1-iC5 in pore water below the AOM zone. https://doi.org/10.1594/PANGAEA.772753
- Heeschen, KU; Hohnberg, H-J; Haeckel, M et al. (2007): (Table 4) In situ methane concentrations (Cm), amount of dissolved methane and gas hydrate ratios in pressure cores assuming an occurrence of methane below the AOM reaction zone. https://doi.org/10.1594/PANGAEA.772754
- Heeschen, KU; Hohnberg, H-J; Haeckel, M et al. (2007): (Table 5) Averaged hydrocarbon compositions of gas hydrates sampled at GC185 and GC415 and the standard deviation encountered in the samples. https://doi.org/10.1594/PANGAEA.772816