1180 Submarine structures made by leaking gases

European status and distribution

Submarine structures made by leaking gases have a restricted distribution in European waters due, in part, to their relationship to sources of shallow gas, which occur in the North Sea, a small portion of the Irish Sea and part of the Mediterranean Sea. The Annex I habitat is recognised to occur in Danish inshore waters as well as UK offshore waters.

UK status and distribution

Within UK waters, this habitat is mainly associated with large pockmarks formed through the expulsion of shallow gas. These pockmarks are commonly found in the Fladen and Witch Grounds in the northern North Sea as well as part of the Irish Sea. Areas of shallow gas are mainly found in UK offshore waters.

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Site accounts

• Braemar Pockmarks Extra-Regio
  The Braemar pockmarks are a series of crater-like depressions on the sea floor, two of which contain the Annex I habitat Submarine structures made by leaking gases. In this location, large blocks, pavements slabs and smaller fragments of methane derived authigenic carbonate have been deposited through a process of precipitation during the oxidation of methane gas. These carbonate structures provide a habitat for marine fauna usually associated with rocky reef, and very specific chemosynthetic organisms which feed off both methane (seeping from beneath the sea floor) and its by-product, hydrogen sulphide (Judd, 2001). Larger blocks of carbonate also provide shelter for fish species such as wolf-fish and cod. Close by the two pockmarks containing blocks of carbonate, to the south west, there is another block of carbonate which is not associated with a pockmark (Hartley, 2005).
• Croker Carbonate Slabs Extra-Regio
  The Croker Carbonate Slabs is an area in the mid-Irish Sea, approximately 30km west of Anglesey, where a total area of over 800 ha of the Annex I feature “submarine structures made by leaking gases” have been identified. The site lies in 70m water depth in the north descending down to approximately 100m at the south west corner. The seabed surface is composed of extensive areas of exposed methane-derived authigenic carbonate (MDAC). These MDAC structures range from ‘low relief’ (elevation of up to 20cm above the surrounding seabed) to ‘high relief’ (elevation over 20cm above the surrounding sediment, and often up to 2m). The seabed habitats created by these MDAC structures are distinctive, supporting a diverse range of marine species that are absent from the surrounding seabed characterised by coarse sediment. Areas of ‘high relief’ MDAC support a diverse range of soft corals, erect filter feeders, sponges, tube worms and anemones whilst the ‘low relief’ MDAC is colonised with scour-resistant hydroids and bryozoans. The surrounding sediment is highly mobile and consists of poorly sorted sand (from fine to coarse grained) with a large proportion of broken shell gravel and whole shells interspersed with rippled sand. A blue-grey clay was also commonly seen towards the southern end of the site.
• Scanner Pockmark Extra-Regio
  Scanner pockmark is a large seabed depression in the northern North Sea which contains large blocks of the Annex I habitat Submarine structures made by leaking gases. The blocks lie in the base of the pockmark and support fauna more typically associated with rocky reef. These carbonate structures are notably colonised by large numbers of anemones (Urticina felina and Metridium senile) and squat lobsters (Dando et al., 2001). These features also appear to support micro-organisms known as ‘chemosynthesizers’ which utilise the discharged methane and its by-product, hydrogen sulphide (Judd, 2001). The gutless nematode Astomonema southwardorum, which may have a symbiotic relationship with chemosynthetic bacteria, is unique to this site (Austen et al, 1993). Fish (hagfish, haddock, wolf-fish and small redfish) also appear to be using the pockmark depressions and the carbonate structures for shelter (Dando, 2001).

   

  Scanner Pockmark is situated approximately185km off the north east coast of Scotland near the centre of the Witch Ground Basin, in waters of approximately 150 m depth. The pockmark contains two deep areas, though overall is roughly oval in shape and measures approximately 900 m by 450 m across with a depth of around 22 m below the surrounding sea floor (Hovland and Judd, 2007). This site also contains the Scotia pockmark complex in the north, a composite feature composed of two deeper sections with active methane seeps (Dando, 2001). The volumes of these pockmarks (Scanner: approximately 1 million m3) are considerably greater than the normal pockmarks in the area.