RRS Charles Darwin Cruise 101C Leg 1, 05 Jun-13 Jul 1996. TOBI surveys of the continental slope west of Shetland

Abstract

This was the first of two cruises, the overall objective of which was to undertake an integrated baseline environmental survey of the continental slope west of Shetland. The major objective of this cruise was to obtain TOBI (30 kHz) and 100 kHz sidescan sonar imagery of the designated survey area west of Shetland. All the objectives of the cruise were achieved in full or exceeded. Over 14000 km2 of seafloor was imaged with the TOBI system during 25 days of survey and 650 line km of 100 kHz data were obtained in 3.5 days of survey. Twelve sample stations were occupied during trials of the multicorer, box corer and Day grab. As anticipated, the mapped sediment facies variations are strongly related to waterdepth. Iceberg ploughmarks dominate the seafloor structure to depths of about 500 m. At mid slope depths, sediment bedforms and erosion due to current activity can be seen. At greater depths, less energetic depositional conditions prevail. Highlights of the sonar survey include the discovery of a field of barchan sand-dunes at a waterdepth of 300 m, a small sediment slide at 900-1000 m waterdepth, and an extensive sandy contourite sheet between the 850 and 1000 m depth contours between 61° and 61°20’N. The second cruise of the two cruises, mainly devoted to seabed sampling, is described in Southampton Oceanography Centre Cruise Report No. 7.

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