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Introduction and sampling
The Nyegga region is located on the edge of the Norwegian continental slope and the northern flank of the Storegga Slide, on the border between two large oil/gas prone sedimentary basins - the Møre Basin to the south and the Vøring Basin to the north (Bunz et al., 2003). Seepage features like pockmarks are widespread at the seabed in Nyegga. Gas hydrate samples have been recovered in 2006 by HERMES program (Ivanov et al, 2007). However only Paull et al (2007) assessed the methane release from Storegga slide and the southwest of Nyegga, no more geochemical study on seepage fluids in Nyegga has been done.
In July and August 2007, a coring cruise was conducted to investigate the geochemistry and geology of gas hydrate/seepage fluids in the southeast of Nyegga by R/V G.O.SARs. Six gravity cores of up to 3.5 m long were collected in 639 to 740 m water depth. There are 4 cores taken inside or on the flanks of pockmarks. Two cores are from a gas mound. Pore waters were extracted from unsplit/split cores by Rhizon samplers. But no gas hydrate samples were recovered.
Results and discussion
Totally 41 pore waters were obtained. The sulfate concentrations ranged from 27.6 to 0.4 mM, and dissolved inorganic carbon (DIC) concentrations from 3.0 to 18.9 mM. Both the sulfate profiles and the DIC profiles (4 of 6) are linear within the sulfate reduction zone, indicating that the microbially mediated anaerobic methane oxidation is the predominant sulfate-consuming process. Therefore the upward methane flux is ongoing in these sediments. The other two cores are from the inside of pockmarks. One pockmark is active with fluid seepage on the flank but inactive inside.
Sulfate gradients measured in the southeast of Nyegga indicate that methane fluxes are 15 to 49 mmol/m2/yr, which are high in comparison with other known methane hydrate sites (D'Hondt, et al., 2004; Borowski et al., 2000). These confirm that conditions in the southeast of Nyegga are suitable for methane accumulation and gas hydrate formation.
Conclusions
1. The geochemistry of pore fluids confirms that gas hydrates do exist in the southeast of Nyegga.
2. The methane seepage is localized, even within one pockmark.
References
1. Paull, C.K., Ussler III, W., and Holbrook, W.S. (2007), Geophys. Res. Lett., 34, doi:10.1029/2006GL028331
2. IVanov, M., et al. (2007), Eos, 88(19), 3pp
3. Bunz, S., Mienert, J., and Berndt, C. (2003), Earth Planet. Sci. Lett., 209, 291-307
4. D'Hondt, S.L., et al. (2004), Science, 306, 2216-2221
5. Borowski, W.S., et al. (2000), Proc. ODP, Sci. Results, 204.
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