Cold-water Coral Mounds are found around the margins of the Atlantic Ocean, and adjacent seas such as the Mediterranean and Gulf of Mexico, from Norway and North Carolina in the north to Angola and Argentina in the south, at depths of between 200 and 1000 m. These Coral Mounds are grouped into provinces, numbering from a few tens to many hundreds of individual Mounds, spread along isobaths (control lines) in generally linear arrangements, sometimes merging to form continuous reefs that stretch for hundreds of kilometres. The shape of individual Mounds varies considerably, from elongate or v-shaped to circular or oval, apparently in response to local current conditions, with Mounds ranging from a few metres in height to over 300 m, and extending laterally for tens to hundreds of metres. These Mounds are constructed principally by the Cold-water Coral Lophelia pertusa, with other Corals and Shellfish making a contribution, as well as trapped sediments, both carbonate and siliclastic, and form slowly over hundreds of thousands of years, and are thought to represent a significant sink for atmospheric carbon.
In a paper published in the journal Marine Geology in February 2019, Dierk Hebbeln, Maren Bender and Stefanie Gaide of the Center for Marine Environmental Sciences at the University of Bremen, Jürgen Titschack, also of the Center for Marine Environmental Sciences at the University of Bremen, and of the Marine Research Department at Senckenberg am Meer, Thomas Vandorpe of the Flanders Marine Institute, David Van Rooij of the Renard Centre of Marine Geology at Ghent University, and Paul Wintersteller and Claudia Wienberg, again of the Center for Marine Environmental Sciences at the University of Bremen, describe a new Cold-water Coral Mound Province on the northwest coast of Morocco.
Coral Mounds were first observed in this area in 2002 during an expedition by the Research Vessel Belgica, and were the revisited in 2014 by the Research Vessel Maria S. Merian, with the objective of mapping their extent, using a KONGSBERG EM122 multibeam echosounder system. A total area of 1440 km² was mapped using ESRI ArcGIS™ v.10.
Hebbeln et al. found a total of 3463 Coral Mounds within the 1440 km² study area, giving an average density of 2.4 mounds per km². These mounds were found at depths of between 565 and 1155 m, though most were concentrated in two bands, between 890 m and 980 m and between 720 and 870 m, with the shallower band being home to about 59% of the mounds and the deeper band to about 24%. These bands were found on areas where the continental slope had an inclination of about 2-3°, while the area between the bands had an inclination of less than 1°. About 4% of the mounds were found in the area between the two bands, another 4% were found deeper than the deeper band, with about 10% found in waters shallower than the shallower band.
(A) Bathymetric map of the Atlantic Moroccan Coral Mound Province (AMCP) off northwest Morocco, indicated as site 1 in the overview map shown in (B). In the northeast part of the area (grey shaded), Coral Mounds are present within the El Arraiche mud volcano province restricted to water depths of 500-700 m. In water depths deeper than 700 m, coral mounds are mostly arranged along two slope-parallel belts, which follow distinct depth levels (shallow belt: 720-870 m, orange dotted line; deep belt: 890-980, red dotted line). In addition, several large mud volcanoes (MV), one mud diapir (MD), and two fault lines belonging to the South Western Iberian Margin (SWIM) fault system (white dashed lines) are indicated. (B) Overview map of the northwest African margin showing known Coral Mound Provinces (red/orange boxes). (1) Atlantic Moroccan Coral Mound Province (red box - this study), (2) West and East Melilla Coral Mound Provinces in the southern Alboran Sea, (3) Eugen Seibold Coral Mound Province north of the Agadir Canyon, (4) Giant Mauritanian Coral Mound Province. Hebbeln et al. (2019).
The Coral Mounds range in length from 24 to 2075 m, in width from 12 m to 584 m, and in height from 4 m to 50 m (though it was possible to measure the height of only a limited number of Mounds). This means that the individual mounds cover areas of between 300 m² and 950 000 m², though most were towards the smaller end of this range, with only 20 exceeding 120 000 m². The majority of the mounds are elongate in shape, extending downslope rather than along the contour lines. There was no clear correlation between the depth, size and orientation of the Coral Mounds.
An acoustic bathymetric study of part of the area revealed that in an area where 166 Coral Mounds visible above the seafloor were present, there were a total of 615 buried Mounds. If this can be accurately extrapolated to the whole province it would imply that in addition to the 3463 visible Mounds, there should be about 12 500 buried Mounds, suggesting that there are about 16 000 Coral Mounds on the northwest coast of Morocco in total.
(A) Still photograph showing the surface of a Coral Mound of the Atlantic Moroccan Coral Mound Province taken by the Remotely Operated Vehicle Cherokee operating from the Research Vessel Pelagia. The surface of this Mound is covered by dead/fossil Coral framework. (B) PARASOUND sub-bottom profile showing exposed and buried Coral Mounds of the Atlantic Moroccan Coral Mound Province. The Mounds root on multiple horizons pointing to several Mound formation periods during the past. The small box highlights the reflection pattern associated with selected Coral Mounds. Hebbeln et al. (2019).
The approximately 140 m offset between bands of Coral Mounds has been observed before off the coasts of Ireland (where the gap averages about 200 m) and Mauritania (where it is about 150 m). This pattern, with gaps of 140-200 m, is remarkably similar to the estimated difference in sea level during Pleistocene glaciations and warm periods. However the Corals are not thought to have grown at both these times, with Coral growth in the cooler waters off the coast of Ireland thought to have been restricted to warmer interglacial periods, while that in the warmer waters of Morocco and Mauritania is thought to have taken place during periods of glaciation.
Hebbeln et al. estimate that the Coral Mounds, exposed and buried, have an average volume of 84 000 m³, so that the ~16 000 Mounds have a total volume of 1344 km³, including a high proportion of carbonate produced by the Corals, potentially a total of 700 million tons of carbonate, which in turn equates to 84 million tons of carbon, making the Mounds an important carbon sink. If the Coral Mounds have accumulated in the last 900 000 years, with that accumulation going on only during periods of glaciation during that time (about 450 000 years), then the mounds would have accumulated carbonate at a rate of 1550 tons per year across the Atlantic Moroccan Coral Mound Province during periods of active accumulation. This equates to a carbon sink taking 1200 grams of carbon per metre squared every year, which when compared by the rates achieved by boreal peatlands (20 carbon per metre squared every year) or boreal forests (44 carbon per metre squared every year), suggests that the Mounds play an important role in global atmospheric carbon budgets.
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