Fig 1.
General bathymetric map of the Canary Islands Seamount Province (CISP).
Note the seamounts, islands and other relevant physiographic features as well as the regional surface current system. The Canary Basin, the Canary Ridge, the Canary Channel and its diapir belt extending northwards are indicated. Numbers attached to seamounts and islands represent individual oldest age estimate where available, after Van den Bogaard [28]. AC: Azores Current. CC: Canary Current. CUC: Canary Upwelling Current. NEC: North EquatorialCurrent. SV: Selvagens. L: Lanzarote. F: Fuerteventura. GC: Gran Canaria. TF: Tenerife. LP: La Palma. EH: El Hierro. Bathymetry in meters from GEBCO.
Table 1.
Multibeam bathymetry and sub-bottom profiling systems mounted onboard the three vessels used to survey the Concepcion Bank.
Fig 2.
Bathymetry of Concepcion Bank and surrounding deep seafloor.
(A) Bathymetric map from multibeam data (bright colours) and GEBCO (dull colours). The dashed lines delimit the three SWNW oriented parallel depth sectors described in section 4.4. (B) Bathymetric cross section along a SE-NW direction. Significant changes in slope are indicated. See location in A. Vertical exaggeration is 14:1. Summit location is indicated by a black triangle with a label showing its depth in meters.
Fig 3.
Depth and slope analysis of Concepcion Bank.
(A) Depth and slope histograms. (B) Depth vs. slope plot and depth and slope histograms of the summit plateau. Dashed lines in the depth vs. slope plot indicate depths where slope changes markedly, thus helping identifying four depth ranges with distinct slope gradients and trends (158–500 m increasing, 500–750 m fairly constant, 750–1,000 m slightly decreasing, beyond 1,000 m increasing). (C) Depth vs. slope plot and depth and slope histograms of the bank flanks.
Fig 4.
Relationship between the depth of the summit plateau slope break (SPB) and the depth of the flank foot (FF) at Concepcion Bank.
(A) Polar histogram showing the SPB depth (dashed line) vs. FF depth (continuous line). (B) Correlation plot between SPB depth and FF depth per geographic sector (NE to NW). (C) Above: Height difference between FF depth and SPB depth (crosses) and the mean value of this difference (dashed line). Below: SPB and FF depths around the bank.
Table 2.
Slope and depth values of the summit plateau, flanks and surrounding deep seafloor of Concepcion Bank according to the depth profiles in Fig 5.
Fig 5.
Bathymetric sections across Concepcion Bank and relationship between depth of the flank foot and slope of the summit plateau.
(A) Radial depth sections from the geometric centre of Concepción Bank outwards. Note the pronounced basal height difference between the west and northwest sectors (dark blue and magenta) and the rest of the bank. Vertical exaggeration is 5:1. (B) Shaded relief image shows the location of the depth sections in A. (C) Relationship between flank foot (FF) depth and average slope of the summit plateau (ASSP) for each depth section. Colours as in A and B. Also see Table 2.
Fig 6.
Images and plots showing how the depth of summit plateau slope break (SPB) and the depth of flank foot (FF) have been calculated.
(A) Digital Terrain Model (DTM) of the along slope depth second derivative (red is negative, blue is positive). (B) Detail of the second derivative DEM across the edge of the summit plateau and foot of slope, showing a section normal to SPB and FF. An inflection point (IP) marks where the second derivative changes from convex (red) to concave (blue). (C) Depth of the section. Vertical exaggeration is 3:1. (D) First derivative (slope) along the section. (E) Second derivative (profile curvature) along the section. Dashed lines in B, C, D and E point SPB, IP and FF.
Fig 7.
Geomorphologic features of the Concepcion Bank.
The summit plateau slope break (SPB) and the flank foot (FF) separate the main morphologic domains of the bank: (i) the summit plateau, (ii) the flanks, and (iii) the surrounding abyssal seafloor.
Fig 8.
Distribution and properties of sediment waves at the Concepcion Bank summit plateau.
(A) Shaded relief image showing the general distribution of sediment waves. (B) Details of the sediment wave field on the SW quadrant of the summit plateau, with colours representing different average crest directions and white arrows showing the interpreted wave migration directions. X-X’ and Z-Z’ lines locate sub-bottom profiles shown in Fig 9. Scattered isolated positive features aligned approximately NS, close to the western boundary of the sediment wave field, are mounds that are imaged in Figs 10, 11 and 12. (C) Polar diagram of the directions of sediment wave crests, with colours representing different crest directions according to B and length of sectors representing direction frequencies. (D) Crest direction vs. average crest depth plot. (E) Average crest depth frequency histogram. (F) Depth profiles across a sediment wave train. The brown solid line shows depth according to the Digital Elevation Model (DEM) of Concepcion Bank (left vertical scale), and the black solid line shows depth variation after subtracting the slope local trend along profile (detrended, right vertical axis). See location in B. Vertical exaggeration is 5:1 for the brown line and 15:1 for the black line.
Fig 9.
Sub-bottom seismic reflection profiles across bedforms on the summit plateau of Concepcion Bank.
(A) Profile across subparallel scarps north of the sediment wave field (X-X’ in Fig 8B). These scarps mark the change in average slope indicated in Fig 2B, from 1.6° to 2.7°. Vertical exaggeration is 15:1 (Z-Z’ in Fig 8B). (B) Profile across the sediment waves interspersed with fresh-looking mounds. Vertical exaggeration is 20:1.
Fig 10.
Depth-wise distribution and roughness analysis of the fresh-looking mounds on Concepcion Bank summit plateau.
(A) Shaded relief of the Bank indicating area illustrated in B (color code for seafloor orientation identical to cross sections in Fig 5). (B) Seafloor orientation map of the SW quadrant of the summit plateau with white dots highlighting the location of the fresh-looking mound forming a belt of up to 4 km width, close to the 600 m isobath. Colour code in A. Location in A and Fig 7. (C) DEM detail showing scattered mounds amidst the sediment wave field. (D) Mound roughness vs. depth. Each mound is represented by a circle where diameter is proportional to mound area. Roughness has been quantified using the Vector Ruggedness Measure (VRM; Hobson [63]).
Fig 11.
Fresh-looking mounds on the SW quadrant of the Concepcion Bank summit plateau.
(A) Shaded relief image coloured by depth of mounds amidst and often on top of sediment wave crests. See location in Fig 8. (B), (C) and (D) 500 kHz side scan sonar images of three mounds. Note the higher backscatter of the rough mound tops.
Fig 12.
Backscatter images of Concepción Bank derived from multibeam bathymetry.
High and low relative backscatter in black and white, respectively (See legend in A). (A) A general backscatter mosaic of the Bank and surrounding seafloor. Note the high backscatter of the area occupied by the sediment wave field, locally with mounds on top of the waves, on the SW part of the summit plateau (see Figs 7 and 8). The flanks of the bank mostly show high backscatter, which we mainly attribute to their steepness and presumed rocky nature. (B) Backscatter facies of the subdued mounds. Note the mottled facies to the right of the image where isolated mounds can be distinguished. A sharp backscatter shift to the left of the image marks the limit between high-density subdued mounds and an area devoid of mounds to the west. (C) Rocky outcrops on the Bank's summit; the shallowest peak is indicated by a black triangle. (D) Sediment waves with no mounds to the northwest of the rocky outcrop in C. (E) Three volcanic cones surrounded by low backscatter sediments and high backscatter moats. Small circle between the two northern cones indicate the location of pictures shown in Fig 15.
Fig 13.
Depth profiles of rim scarps extended in a clockwise direction from N.
Rim scarps are located on Fig 7. Vertical exaggeration is 20:1.
Fig 14.
Plan curvature and characterization of submarine canyons cutting through the Concepcion Bank flanks.
(A) Plan curvature map of the bank. Red colours correspond to positive plan curvature values (convex shapes) and blue to negative ones (concave shapes) highlighting crests and incisions amongst other positive and negative reliefs. (B) Detail of the plan curvature map in the northwest flank of the Bank where a submarine canyon cut into a flank-attached bulge. Coloured as in A. (C) Along-axis profiles of the three longest submarine canyons crossing the bulge attached to the northwest flank of the bank. Note differences in axial gradient between the upper and lower stretches. Vertical exaggeration is 5:1.
Fig 15.
ROV seabed pictures from the northeastern part of the study area, where winding bedforms and volcanic cones co-occur (see Fig 7).
(A) rocky outcrop, likely a basalt, amidst a muddy flat seafloor at 1,434 m depth. (B) Oblique view of the same rocky outcrop showing a horseshoe-shape resembling a remnant of the wall of a drained lava lobe with a collapsed upper crust. (C) 30 cm height vertical tubular shape possibly made of authigenic carbonates, surrounded by a flat seabed with scattered rock fragments at 1,409 m depth. (D) Flat seafloor with fragmented crusts on a muddy substrate at 1,418 m depth. Location of pictures is shown in Fig 12E.
Fig 16.
Sketch illustrating the hypothesis of breaking internal waves as the main mechanism explaining the presence of the sediment wave field and the associated depth-restricted narrow belt of cold-water coral mounds on the SW sector of Conception Bank summit plateau. The bathymetric section is the same as in Fig 2B. See location in the shaded-relief DTM included in this figure.