AHERMATYPIC CORAL BANKS OFF MID-NORWAY - EVIDENCE FOR A LINK WITH SEEPAGE OF LIGHT-HYDROCARBONS

Citation
M. Hovland et al., AHERMATYPIC CORAL BANKS OFF MID-NORWAY - EVIDENCE FOR A LINK WITH SEEPAGE OF LIGHT-HYDROCARBONS, Palaios, 13(2), 1998, pp. 189-200
Citations number
48
Categorie Soggetti
Geology,Paleontology
Journal title
ISSN journal
08831351
Volume
13
Issue
2
Year of publication
1998
Pages
189 - 200
Database
ISI
SICI code
0883-1351(1998)13:2<189:ACBOM->2.0.ZU;2-A
Abstract
Large (up to 31-meter high) coral banks (or bioherms) occur on the con tinental shelf off mid-Norway at water depths between 220 and 310 mete rs. They are built up by the cold-water, ahermatypic, scleractinian co ral Lophelia pertusa (L.). A 3-km-wide and 200-km-long traverse was ma pped geophysically across a large part of the mid-Norway shelf: A tota l of 57 suspected individual banks were found. Although they occur in local clusters of up to 9 banks per Km(2), the mean density along the whole transect is only 0.09 suspected banks per Km(2), with the highes t regional density (1.2 banks per km(2)) occurring above subcropping p resumed Paleocene bedrock. A detailed investigation employing an ROV ( remotely operated vehicle) was conducted of a cluster consisting of 9 individual banks. Based on geophysical, visual, geochemical, radiocarb on, and other analyses, we conclude that at least some of the coral ba nks have been forming at the same locality for over 8,000 years, and t hat there is a strong correlation between coral-bank occurrence and re latively high values of light hydrocarbons (methane, ethane, propane, and n-butane) in near-surface sediments. To explain, the structure and distribution of these coral banks, we propose a model where they form as a consequence of local fertilization that results from focused hyd rocarbon micro-seepage of deep thermogenic hydrocarbons migrating to t he surface along inclined, permeable sedimentary strata. A direct coro llary of this model is that if and when the source of local fertilizat ion is shut off; the bioherms die out. This possibly could be the reas on why extinct bioherms are more common, than live ones in some areas of the ocean.