Ninth international conference on - Marum

78
Abstracts of posters
δ 13 C of Kukuy K-2 mud volcano was rather large (from -60 to -55 ‰) and contained much ethane. Goloustnoye
Flare, where gas hydrate discovered in 2007, also showed high concentration of ethane and similar isotopic
compositions of methane and ethane to those in Kukuy K-2, respectively. In the profiles of methane and ethane
δ 13 C hydrate gas seemed to be several permil smaller than the gas in dissolved pore water, whereas Hachikubo et
al. (2007) reported undetectable fractionation in δ 13 C at the formation of methane and ethane hydrates. We
conclude that the hydrate gas of Kukuy K-2 and Goloustnoye Flare is mainly microbial origin and partly
contains thermogenic methane and ethane. A new diagram of ethane δ 13 C and δD shows regional characteristics
and provides information on how these hydrocarbons accumulate in the lake bottom sediment.
Isotopic composition of gas hydrates obtained from offshore Sakhalin,
the Sea of Okhotsk
A. Hachikubo 1 , A. Krylov 1,2 , H. Sakagami 1 , H. Minami 1 , Y. Nunokawa 1 , H. Shoji 1 , Y. K. Jin 3 , A. Obzhirov 4
1 Kitami Institute of Technology, Kitami 090-8507, Japan
2 Present address: All-Russia Research Institute for Geology and Mineral Resources
of the Ocean, St.Petersburg 190121, Russia
3 Korea Polar Research Institute, KORDI, Yeonsu-gu, Incheon 406-840, Korea
4 V.I. Ilíchev Pacific Oceanological Institute FEB RAS, Vladivostok 690041, Russia
Gas hydrates are crystalline clathrate compounds composed of water and gas molecules that are stable at low
temperature, high partial pressure of each gas component, and high gas concentration. New hydrate-bearing
seepage structures offshore Sakhalin, the Sea of Okhotsk, have been investigated from 2003 to 2006 within the
framework of the CHAOS project (hydroCarbon Hydrate Accumulations in the Okhotsk Sea). We retrieved
hydrate-bearing sediments by using a gravity corer on October 2003, May 2005 and May 2006 (CHAOS1,
CHAOS2 and CHAOS3, respectively). Whiticar et al. (1986) proposed a genetic classification diagram for
natural gas using methane isotopes. In the diagram, large and small δ 13 C values of methane indicate thermogenic
and microbial origins, respectively, and δD of methane also provides information on methyl-type fermentation or
CO2 reduction in the microbial origin. Isotopic compositions of carbon and hydrogen were measured by using a
continuous flow-isotope ratio mass spectrometer (CF-IRMS, DELTA plus XP; Thermo Finnigan). Methane δ 13 C
and δD were in the range -65 to -62 ‰ and -205 to -195 ‰, respectively. These results indicate a microbial
origin produced by CO2 reduction according to Whiticar's diagram. Regional characteristics of isotopic
composition were also found between each seepage structures. In the depth profile of isotopic composition
between hydrate gas and the gas in dissolved pore water seemed to be the same in the case methane δ 13 C,
whereas less than 10 ‰ difference between them was found in methane δD. Hachikubo et al. (2007) reported
that δD of hydrate-bound molecules of methane hydrate was several ‰ lower than that of residual gas molecules
in the formation processes, while there was no difference in the case of δ 13 C. Ethane concentration in the gas
samples was 30-150ppm for hydrate-bearing sediments and 10-100ppm for non-hydrate sediments, and
depended on each seepage structure. We discussed the formation process of gas hydrate in the shallow sediment
cores according to the experimental results of isotopic fractionation at the formation of gas hydrates.
The origin of the Nyegga Pockmark Field on the Norwegian continental margin
B. O. Hjelstuen 1 , H. Haflidason 1
1 Department of Earth Science, University of Bergen, Allegt 41, 5007 Bergen, Norway
TOPAS high-resolution seismic profiles and EM1002 bathymetric data, collected during a R/V G.O. Sars cruise
in 2007, confirm that slope deposits in the Nyegga area on the south Vøring Plateau, Norwegian margin, are
intensively perforated by pockmarks. More than 200 seabed depressions have been identified. The features have
diameters of up to 200 m, maximum depths of c. 10 m, and they are situated in an area where gas hydrates and
pre-Pliocene polygonal faults are found. The largest, and most complex, pockmarks are found along the rim of
the hydrate reservoir. Shallow cores indicate that some of the pockmarks are active at present. The pockmarks
have evolved within a sedimentary package that is characterized by glacimarine and hemipelagic well-layered
deposits. Glacigenic debris flow units, deposited during shelf edge glaciations, interfinger these sediments on the
upper slope. The well-layered sediments are delivered to the continental slope by icebergs, suspension fall-out in
front of an ice margin and from meltwater plumes released during the disintegration of the Fennoscandian Ice
Sheet. The sediment rates may have been as high as 35 m/ky during deposition of these sediments. Our findings
suggest a relation between the Nyegga Pockmark Field and the observed gas hydrate reservoir. However, we
note that the pockmarks only are found in a very limited area of the 3000 km 2 large South Vøring Plateau gas
hydrate region. We ascribe this to local variations in sediment characteristics.