Preface The expedition ARK XIX/3 with the German icebreaking RV ...

A. 4.4.6 Biological and geological processes shaping the Porcupine Bank northwest
margin
Wheeler, A.
The above results highlight many of the significant findings that have been made
concerning the environmental dynamics on the Porcupine Bank margin and the
influences and controls that these exert on biological and geological processes. Prior
to these studies, the scientific community had a limited understanding of the nature of
the seabed in this area. The area had been mapped extensively with seismic lines,
regional side-scan sonar and multibeam echosounder but these data sets only reveal
the morphology and location of interesting structures and tell us little about the reality
on the seafloor. An exception to this is side-scan sonar backscatter that reveals areas
of high backscatter suggesting a hard rough seabed possibly as a result of either
extensive erosion and/or biological growth. It was therefore from this basis that the
dives and deck operations were planned in a successful attempt to understand the
meaning and significance of these key features on the Bank.
Two major conclusions can be drawn from these studies. Firstly, that the detailed
morphology and seabed features imaged on the Porcupine Bank by "Victor 6000"
reveal a story of seabed erosion. This erosion story has not only been active over
geological timescales but is also ongoing under contemporary conditions. The high
current speeds encountered by "Victor 6000" are testament to this.
Secondly, these high current speed are probably at the upper tolerance for Lophelia at
present times. Numerous, often isolated, although impressive communities of
Lophelia, as well as other suspension feeding megafauna, were encountered and
documented. However, it was obvious that there is not rigorous growth of Lophelia at
present on the Bank although exposed seabed remains and the exposed internal
fabric of the carbonate mounds suggests that this was not always the case. It is
suggested here that in the geological past, under more favourable conditions, this
Bank was teeming with life, similar to the lower "Belgica mounds" at present imaged
earlier in the cruise, but in contemporary times, the carbonate mounds are largely in a
dormant phase. One notable exception to this is the "Twin mounds" that do show
signed of recent rigorous growth of deepwater coral although this mound has been
heavily trawled. The targeting of this mound by fishermen is probably not coincidental
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