Furu Mienis

On the influence of cold-water coral mound size on flow hydrodynamics, and vice versa

Using a combination of in situ observations and idealistic 2-D nonhydrostatic numericalsimulations, the relation between cold-water coral (CWC) mound size and hydrodynamics is explored forthe Rockall Bank area in the North Atlantic Ocean. It is shown that currents generated by topographicallytrapped tidal waves in this area cause large isopycnal depressions resulting from an internal hydrauliccontrol above CWC mounds. The oxygen concentration distribution is used as a tracer to visualize the flowbehavior and the turbulent mixing above the mounds. By comparing two CWC mounds of different sizesand located close to each other, it is shown that the resulting mixing is highly dependent on the size of themound. The effects of the hydraulic control for mixing, nutrient availability, and ecosystem functioning arealso discussed.

Internal wave turbulence at a biologically rich Mid-Atlantic seamount

The turbulence regime near the crest of a biologically rich seamount of the Mid-Atlantic Ridge southwest of the Azores was registered in high spatial and temporal resolution. Internal tides and their higher harmonics dominate the internal wave motions, producing considerable shear-induced turbulent mixing in layers of 10–50 m thickness. This interior mixing of about 100 times open-ocean interior values is observed both at a high-resolution temperature sensor mooring-site at the crest, 770 m water depth being nearly 400 m below the top of the seamount, and a CTD-yoyo site at the slope off the crest 400 m horizontally away, 880 m water depth. Only at the mooring site, additionally two times higher turbulence is observed near the bottom, associated with highly non-linear wave breaking. The highest abundance of epifauna, notably sponges, are observed just below the crest and 100 m down the eastern slope (700–800 m) in a cross-ridge video-camera transect. This sponge belt is located in a water layer of depressed oxygen levels (saturation 63±2%) with a local minimum centered around 700 m. Turbulent mixing supplies oxygen to this region from above and below and is expected to mix nutrients away from this biodegraded layer towards the depth of highest abundance of macrofauna.

Hydrodynamic controls on carbonate mound development: long term in situ seabed BOBO-lander observations and CTD-casts at the SW Rockall Trough Margin and in the Gulf of Cadiz

Long term records obtained by in situ seabed observatories and repeated CTD casts show the presence of a high energetic environment in and around two cold water coral mound provinces at the SW RT margin and in the Gulf of Cadiz. In both areas cold water corals are present on the mounds, but the areas differ strongly in geological setting, as well as in sedimentology and hydrography. Measurements of near bed hydrodynamics as recorded with freefalling seabed observatories and water column observations show that currents in mound areas have a major influence on the presence of living cold water corals and on the shape of carbonate mounds. While at the Southwest Rockall Trough margin a dense cover of live corals has been demonstrated, only isolated living colonies and coral debris covered with mud occur on the mounds in the Gulf of Cadiz, indicating that watermass properties and dynamics on the Southwest Rockall Trough are at the moment most favourable for coral growth.

Imprint of Holocene Climate Variability on Cold‐Water Coral Reef Growth at the SW Rockall Trough Margin, NE Atlantic

U-Th ages and temperatures derived from Li/Mg have been measured on coral fragments of Lophelia pertusa and Madrepora oculata collected from two sediment cores, which were taken from cold-water coral (CWC) mounds at 700-790 m water depth at the SW Rockall Trough margin. Our data, combined with previous published data, have allowed us to first estimate the occurrence of CWC at the SW Rockall Trough margin during the Holocene and, second, to better constrain the environmental conditions driving variability in CWC growth. CWC abundance is marked by a pronounced increase in the mid-Holocene (similar to 6 ka) and is modulated by millennial-scale variability throughout the late-Holocene. The mid-Holocene proliferation of CWC coincides with lowest IRD abundances and a major reorganization of the circulation at thermocline depth in the Rockall Trough, marked by the progressive replacement of the fresh-cold Sub-Arctic Intermediate Water (SAIW) by the saltier and nutrient-rich Eastern North Atlantic Water (ENAW). This event must have established a modern-like winter mixed layer and thermocline structure, generating suitable conditions for enhanced surface productivity, downslope transport of food particles, bottom current acceleration at mound depth and thus CWC growth. Several short time intervals of decreased CWC occurrences closely match prominent increases in North Atlantic drift ice and storminess in Northern Europe. We, therefore, propose that high detrital supply and/or changes in the vertical density gradient associated with millennial-scale ice-rafted detritus (IRD) events are the likely controlling factors for CWC growth and subsequent mound formation on the SW Rockall Trough margin.