HR: 11:10h
AN: OS32A-04    [Abstracts]
TI: Pockmarks with Methane Derived Authigenic Carbonate in the Norwegian Trench, Northern North Sea
AU: * Forsberg, C
EM: cff@ngi.no
AF: Norwegian Geotechnical Inst., P.O. Box 3930, Ullevaal Stadion, OSLO, N-0806 Norway
AU: Hovland, M
EM: mhovland@statoil.com
AF: STATOIL, Forus, Stavanger, N-4035 Norway
AU: Planke, S
EM: planke@vbpr.no
AF: Volcanic Basin Petroleum Research AS, Forskningsparken, Gaustadalleen 21, OSLO, N-0349 Norway
AU: Rike, A
EM: Anne.Gunn.Rike@ngi.no
AF: Norwegian Geotechnical Inst., P.O. Box 3930, Ullevaal Stadion, OSLO, N-0806 Norway
AU: Strout, J
EM: james.strout@ngi.no
AF: Norwegian Geotechnical Inst., P.O. Box 3930, Ullevaal Stadion, OSLO, N-0806 Norway
AU: Svano, G
EM: gesv@statoil.com
AF: STATOIL, Forus, Stavanger, N-4035 Norway
AU: Svensen, H
EM: henrik.svensen@fys.uio.no
AF: Physics of Geological Processes, Univ. Oslo, P.O.Box 1048, Blindern, OSLO, N-0316 Norway
AU: Tjelta, T
EM: ttjelta@statoil.com
AF: STATOIL, Forus, Stavanger, N-4035 Norway
AB: Pockmarks are ubiquitous in large parts of the Norwegian Trench with some areas containing more than 25 pockmarks per km². While the presence of these pockmarks has been known for a couple of decades, the timing and mechanism of their formation is not fully understood. A detailed ROV survey of a 0.25 km² area containing one clustered and four normal pockmarks was conducted in March 2005. The main aim of this survey was to detect and sample any recent or on-going seep activity within the pockmarks and to provide high quality seafloor data for further investigations. Within our study area, carbonate rock and soft sediment samples were recovered. Furthermore, the area was mapped in high resolution by the use of ROV-mounted multibeam echosounder and sub-bottom profiler (Innomar parametric "pinger"). The high-resolution bathymetry image reveals that the clustered pockmark consists of a more than 10 m deep central depression with a diameter of about 100 m, surrounded by seven small pockmarks (5 m deep, 30 m in diameter). The four normal pockmarks are circular, with a typical depth of 5 m and diameter of 50 m. In addition, a number of very small unit pockmarks have been detected. These have diameters of about 1 m, and occur in clusters within the clustered pockmark and in long strings intersecting two of the normal pockmarks. The Innomar records reveal a high amplitude reflection at about 4 m depth indicating the presence of gas or a change in lithology. A carbonate rock was sampled from the central part of the clustered pockmark. The rock has a typical seep carbonate structure, which is documented by petrographical and x-ray diffraction analyses showing that the sample consists of fibrous and micritic aragonite, clay, a minor component of calcite and pyrite. The carbonate sample has a seep carbonate isotope signature (δ ¹³C=-56.4; δ ¹⁸O=4.0) and therefore a composition characteristic of methane derived authigenic carbonate (MDAC). Finally, genetic analyses give a signature suggesting anaerobic methane oxidation. The results therefore imply methane seep activity in the pockmark. However, no present activity (e.g., bubbles) could be observed on the video record. The current activity level will be investigated during a cruise in October this year, during which cone penetration testing, extensive geological, geochemical and geotechnical sampling, and installation of multi-level piezometers for long term measurements of pore pressure and temperature will be performed.
DE: 3002 Continental shelf and slope processes (4219)
DE: 3004 Gas and hydrate systems
DE: 3025 Marine seismics (0935, 7294)
DE: 3045 Seafloor morphology, geology, and geophysics
DE: 3050 Ocean observatories and experiments
SC: Ocean Sciences [OS]
MN: Fall Meeting 2005