Cold seeps and benthic habitat on the Pacific margin of Canada

Cold seeps on the Pacific margin of Canada occur in 130 m water depth in an actively trawled region of Hecate Strait. The seep sites are located within a NW/SE orientated linear pockmark chain. Within the largest of the pockmarks, actively venting authigenic carbonate chimneys (mounds) occur, and the chimneys are surrounded by a wide halo of shells and shell debris. The faunal assemblage found at the cold seeps is unique from that of the surrounding soft substrate habitat and is not typical of other cold seep communities. All taxa are relatively common species found on the British Columbia continental shelf, although some, such as the Oregon triton (Fusitriton oregonensis) were in unusually high abundance for a habitat dominated by soft substrate. These taxa were apparently attracted by the presence of hard substrate in an otherwise soft seabed environment, especially those sessile organisms, such as giant plumose anemones (Metridium giganteum) and encrusting sponges that require hard substrate for attachment. No direct evidence could be found that linked the presence of any epifauna to the presence of hydrocarbons; however the presence of the gutless clam Solemya reidi and high abundance of Oregon triton strongly implies a relationship to the cold seeps. The significant preservation of shell debris that surrounds the carbonate chimneys suggests that the chemistry of the near-seabed water has been altered, due to the presence of the hydrocarbon seepage. The exposure of the carbonate chimneys is a result of discontinuous expulsion of thermogenic gas and continued deepening of the pockmarks.