Ahermatypic coral banks off mid-Norway; evidence for a link with seepage of light hydrocarbons

Abstract

Large (up to 31-meter high) coral banks (or bioherms) occur on the continental shelf off mid-Norway at water depths between 220 and 310 meters. They are built up by the cold-water, ahermatypic, scleractinian coral Lophelia pertusa (L.). A 3-km-wide and 200-km-long traverse was mapped geophysically across a large part of the mid-Norway shelf. A total 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 highest regional density (1.2 banks per km 2 ) occurring above subcropping presumed 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, radiocarbon, and other analyses, we conclude that at least some of the coral banks have been forming at the same locality for over 8,000 years, and that there is a strong correlation between coral-bank occurrence and relatively 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 hydrocarbon micro-seepage of deep thermogenic hydrocarbons migrating to the surface along inclined, permeable sedimentary strata. A direct corollary of this model is that if and when the source of local fertilization is shut off, the bioherms die out. This possibly could be the reason why extinct bioherms are more common than live ones in some areas of the ocean.