Carbonate Mounds and Cold Water Corals at the NE Atlantic Margin
December 3, 2009
Tjeerd C.E.van Weering (and coworkers)
Royal NIOZ, Texel, the Netherlands
Kilometres long and wide, single as well as clustered arbonate mounds occur on the SE (Porcupine) and SW Rockall Trough (SWRT) margin in distinct depth zones between 600 and 1000 m water depth, characterised by bottom water temperatures between 6.2 and 9.8 °C and a salinity range of 35.1-35.4. Hardgrounds of various dimensions are found within the mounds and crop out at their flanks.
The SWRT margin mounds are several km in diameter, can be up to 380 m high and occur mostly as elongated clusters up to 10 km in length. The SWRT mounds are covered on their tops and upper flanks with living colonies of the framework building cold water corals Lophelia pertusa and Madrepora oculata and to a lesser extent Stylaster sp and associated benthic fauna. Recently a range of other coral covered mound structures of various shapes and sizes were noticed on the western Rockall Bank margins.
The SERT margin mounds usually occur as isolated single mounds with a maximum diameter at the sea bed of 1-2 km and a maximum height of 50-100 m and are covered with isolated colonies of cold water corals.
Mound development shows strong contrasts between and within SERT and SWRT, however a number of cores from both regions reflect continuous sediment accumulation over the last ~11000 years, in the time slice before 11 ka BP several hiatuses disturb the sediment record.
Near bed hydrodynamic conditions were measured by long term deployment (up to one year) of seabed observatories. Observed (strong) near bed currents are suggested to strongly affect hardground formation and to control carbonate mound shaping. Strong, (but not too strong) near-bed currents, and downward transport of (fresh) organic particles by internal tidal waves are shown to strongly enhance the growth and development of the cold water coral colonies and associated biota on the tops and upper flanks of mounds. thus emphasizing an oceanographic control on mound formation.