The origin of deep-water, coral-topped mounds in the northern Rockall Trough, Northeast Atlantic

Mounds associated with the cold water coral <i>Lophelia pertusa</i> are widespread in the North Atlantic, although the factors controlling their distribution are not well understood. Here we examine a group of small, coral-topped mounds (the Darwin mounds) which occur at 1000 m water depth in the northern Rockall Trough, northwest of the UK. Individual mounds are up to 75 m in diameter and 5 m high, although some \textquoteleftmound-like\textquoteright targets seen on sidescan sonar have little or even negative relief. Some mounds are associated with \textquotelefttail-like\textquoteright features, imaged as elongate patches of moderate backscatter up to 500 m long, elongated parallel to prevailing bottom currents. High-resolution sidescan images and seabed photographs show hundreds of coral colonies, each a metre or so across, on each individual mound. Many other organisms, mainly suspension feeders, occur in association with the coral. Piston cores from the mounds contain predominantly quartz sand with only scattered coral fragments, showing that bioclastic material is not a major contributor to mound building. A field of seabed pockmarks occurs immediately south of the Darwin mounds. On sidescan sonar data, pockmarks are low relief, circular depressions, typically around 50 m in diameter. The seafloor around the pockmarks consists of uniform, heavily-burrowed, muddy sediments and no specific biological communities, nor any sedimentological or photographic evidence for active seepage, were observed. The distribution of mounds and pockmarks suggests a gradual transition from mounds in the north to pockmarks in the south. This, combined with the lack of bioclastic material in the mound sediments, suggests that both mounds and pockmarks are created by fluid escape from below the seafloor. Mounds occur where fluids carry subsurface sand to the surface, where it forms mounds because bottom currents are not strong enough to disperse it. Pockmarks form where muddy material is eroded by fluid escape but dispersed by bottom currents. Despite the origin of mounds through fluid escape, we suggest that it is the elevated mound topography, rather than any fluid escape, that is advantageous to the corals. This is supported: (1) by the wide variety of suspension-feeding organisms that occur on the mounds, since all of these are unlikely to have a specialised seepage-related lifestyle, and (2) because corals and their associated community do not occur around pockmarks, where seepage has also occurred but elevated topography is absent.