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Seabed Surveys

In 1996, 20,000 square kilometres of seabed lying to the west of the Shetland Isles was mapped and sampled in a survey commissioned by the Atlantic Frontier Environmental Network (AFEN). A further survey was carried out in 1998. The 1996 survey covered an area the size of Wales and included all the acreage of the UK Atlantic Margin which had been licensed for oil & gas exploration before 1995. In 1998 a further 10,000 square kilometres of seabed covered by the 17th oil licensing round to the north and west of Scotland was surveyed (see map). The same strategy was adopted for both surveys. In spite of fascinating early results from the "Lightning" and "Porcupine" cruises described by Wyville Thomson in his book The Depths of the Sea (1874), the area has been comparatively poorly studied scientifically. It is known that these waters and their seabed are complex and variable with a much greater range of water depths than found in the North Sea, and in the deeper areas (> 600m) seawater temperatures stay below zero all year round.

The aim of the AFEN surveys was to give wide area views of the shape and texture of the seabed, the animals living there and the physical processes moulding them, both of today and over the last 10-15,000 years since the last Ice Age. These overviews were needed to form the basis for environmental management decisions being taken by individual oil companies, and to inform discussions with government and a range of environmental, fisheries and other interested groups.

The surveys were a wholly collaborative effort being conceived and designed by a range of government, academic (mainly from the Southampton Oceanography Centre) and industry scientists and representatives. For further information, see Geotek website.

Fieldwork for both surveys was undertaken in 2 stages, using two NERC research vessels. In 1996, RV "Charles Darwin" carried out the first stage mapping of the seabed topography (shape) using side-scan sonar. Sonar reflected sound waves image the seabed using two frequencies - 30KHz in deep water and 100KHz in waters of less than 200m. The results of the mapping were used to ensure that the broad range of seabed habitats identified were targeted in the second stage of the surveys when sea floor samples were collected by grab and corer from over 200 stations (in 1996) for a range of biological, physical and chemical analyses. In addition to these samples, the seabed was extensively photographed to show small seabed features and the more widely dispersed surface animals.


1996/1998 Survey Maps

Click on the image below to view the full map:

1996/1998 Survey Maps

Results - Seabed Topography

Side-scan sonar images showed a sea floor diverse in features and texture. Most notably, the whole seabed between 200-500m depth is covered by iceberg ploughmarks dating from the latter part of the last Ice Age, caused by icebergs gouging and scarring the sediments during the time when the sea-level was around 100m less than it is today.

In waters of less than 200m the sediments were tide swept coarse sands and gravels. In deeper waters between 500-850m a range of sand and mud sediments were found with ripples or waves. Between 850-1000m the sediments are fine sands with a smooth, uniform topography while the bottom of the Faroes-Shetland Channel is composed of mud with some gravel. Glacial drop stones, boulders deposited on the seabed from ice sheets or icebergs, are common over the area.

A notable discovery was a new submarine "landslide" - named the AFEN Slide and shown on the right - these slides occurred at the end of the last Ice Age when large amounts of sediment were being washed into the area as the ice sheets and glaciers melted. Studies of seabed stability by the British Geological Survey and others indicate that the seabed is now stable.

In the 1998 survey an area on the south flank of the Wyville-Thomson Ridge was found to be covered with numerous small carbonate mounds (named the "Darwin Mounds") while an adjacent area of seabed was pockmarked. Both features may have formed as a result of fluid escape from the seabed.

In an area immediately to the south of the Wyville-Thomson Ridge, in water depths of around 1,000m, a field of over 100 small seabed mounds was discovered. The mounds are up to 100m across and a few metres high. This mound field has been named the "Darwin Mounds". Uniquely for such features, these mounds have "tails" which are evident on sidescan sonar images, but do not seem to be visible at the seabed.

Seabed

Results - Seabed Biology

Starfish

The seabed fauna of the Atlantic Margin includes the deep-water coral Lophelia pertusa. Lophelia occurs extensively on the west side of the Faroe-Shetland Channel, but has only been found as small isolated patches in the West of Shetland waters. No large colonies of this coral were found or seen during the AFEN surveys. However, small Lophelia colonies appear to grow on the Darwin Mounds.

Analyses of the animals collected in grab and core samples taken throughout the Atlantic Margin area have recorded over 1,000 different species, mostly worms, starfish, shellfish, crabs and shrimps. The major patterns in animal distributions appear to be:

  • Shelf (down to 300m), with species typically found at shelf depths in the northern North Sea.
  • Upper Slope (300-500m), a distinctive assemblage of species but with affinities to the shelf depth fauna.
  • Mid (500-900m) and Lower (900+m) Slopes: the water masses to the north and south of the Wyville-Thomson Ridge have distinctly different characteristics, resulting in a marked divide in the faunal communities found in these two areas:
    • North of the Wyville-Thomson Ridge: in cold waters, arctic species dominate with diversity decreasing with depth.
    • South of the Wyville-Thomson Ridge: in the comparatively warmer waters, Atlantic deep-water species predominate with diversity increasing gradually with depth.

These patterns of animal distribution are the result of the interaction of a range of environmental and biological factors. Over large scales, water depth, sediment type and water temperature seem to be the most important factors in the Atlantic Margin.

The first stage of biological analysis was completed, that of identifying broad patterns of animal distribution. Because the fauna of the area has been relatively poorly studied in the past, putting correct names to all the species was not possible immediately. AFEN has donated its sample collections to the National Museums for Scotland in Edinburgh and in 1999 and 2000 provided a total of 16 bursaries to fund further taxonomic research. Several species new to science were anticipated and are materialising. Most of the new species described from the AFEN survey can be expected to be found more widely in similar conditions along the Atlantic Margin.

The Future

These projects have greatly enhanced understanding of the Atlantic Frontier area by providing integrated maps which show how the sea floor processes are operating. This information is being used to improve environmental protection. The 100% coverage gives operators a first look at the region for pipeline route planning, and other seabed operations. The analysis of samples and data will continue in the near to medium term and AFEN is actively encouraging full publication of the results in peer reviewed journals to ensure the information is publicly available. Biological material is being deposited in the collections of the National Museums of Scotland for long term curation and to be available for loan to specialists.

As part of its wider commitment to developing greater understanding of the environment, AFEN also supported more general research programmes of relevance to the Atlantic Margin such as the benthic (seabed) boundary layer (BENBO) programme, funded by the Natural Environment Research Council (NERC), and a wider Research Council, government and industry funded programme called Managing Impacts in the Marine Environment (MIME 1997 - 2000). These programmes provided resources for independent academic experts to examine subjects including:

  • The distribution and biology of cold water coral around the British Isles
  • The chemistry and biology of the Atlantic Margin seabed and the likely fate of sediment inputs
  • The geochemical effects of depositing drilling wastes on the seabed
  • The development of more sophisticated methods of analysing the content of discharged chemicals and effluent.
  • The benthic ecological importance of widely fluctuating seawater temperatures

Further studies to the immediate north and west of the AFEN survey, west of the Shetland Isles, were undertaken in 1999 and 2000, using funding from the Department of Trade and Industry.


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