Giant carbonate mud mounds in the southern Rockall Trough

Seismic profiles, sidescan sonar data, bottom photography and sampling proved for the first time that there are giant carbonate mounds (bioherms) in the Rockall Trough. The Pelagia Mounds, on the northern Porcupine Bank, are relatively isolated whereas most of the Logachev Mounds, on the southeast Rockall Bank, form a field of closely spaced, contiguous mounds. They are all found on the upper slope at depths between about 500 and 1200 m and as a result of this study about 500 have been mapped. They have a variety of shapes and the larger ones are very steep-sided, up to 350 m high and 2 km wide at the base. Sediment samples show that the mounds consist of pale coloured muds, mainly aragonite, usually with live and/or dead cold-water corals at the seafloor and with buried dead corals. Coral thickets appear to be detected as a brush-like signature on high resolution profiles and as a speckled pattern on high resolution sidescan sonar records. Shelly sands are found on the seabed between the mounds. Currents in the mound areas are strong enough to transport sands along slope as medium-sized sand waves and to prevent pelagic deposition, thus providing surfaces for initial coral settlement by the winnowing of fines to leave glacial dropstones and exposed rocks. These observations show that the poleward-directed upper slope current of the eastern North Atlantic extends further south than hitherto known and that there is a southwesterly directed current on Rockall Bank. There are waves on the Logachev Mounds, with a wavelength of about 20–30 m, that are thought to be moulded in carbonate muds by across-slope directed internal tidal currents and/or cascading currents. Mounds develop above erosional surfaces seen on seismic data. The huge amount of carbonate mud seems to be produced by the rapid growth and breakdown of cold-water corals living in a very favourable environment. Intermediate nepheloid layers formed upstream by the strong currents may provide the food source for the deep water suspension feeders. Conditions for giant mound growth require an ideal balance between current speed and sedimentation rate, among other factors. No evidence for the suggested link to methane seepage has been found so far.