Discovering the Fine-Scale Morphology of the Gulf of Cádiz: An Underwater Imaging Analysis

Abstract

The dense and deep water flow that leaves the Mediterranean Sea to the Atlantic flows through the upper and middle slope of the Gulf of Cádiz as a powerful bottom stream that models and interacts with bathymetry. The detailed analysis of underwater images, obtained with a photogrammetric sled in the central area of the upper and middle slope of the Gulf of Cádiz, together with multibeam bathymetry and oceanographic and sediment types data, has allowed conducting a detailed study of the seafloor microtopography and the predominant oceanographic dynamics in the study area. Different fine-scale spatial bedforms were identified, such as ripples, dunes, burrows, mounds, obstacle marks, rock bottoms, and low-roughness bottoms using underwater images. Besides, a geostatistical study of the different video transects studied was carried out and allowed us to differentiate three types of bottoms depending on the processes that affect their microtopography.

1. Introduction

Bedforms are dynamic features originated by the interaction between a fluid and the bottom sediment. They are composed of siliciclastic and/or carbonate particles of the gravel, sand and mud textural type. The variability in the geometry and the spatial and temporal scale of bedforms is very wide, from a few centimetres to several km; and from seconds to centuries respectively [1].

The first studies of bedforms were carried out in the field of aeolian and river geomorphology, and later they expanded to the coastal zone and the marine environment. The initiation of morphological and sedimentary distribution studies in deep marine environments started in the 1970s [2,3]. The knowledge and understanding of morphological observations and the processes involved in sedimentary dynamics have been progressively improving accompanied by a significant advance in instrumentation designed for the acquisition of data from the seabed. The incorporation of high-resolution seafloor mapping by multibeam echosounder, side scan sonar, Remotely Operated Vehicles (ROVs), or Autonomous Underwater Vehicles (AUVs), as well as the improvement of hydrodynamic measurements with high-frequency current meters such as Acoustic Doppler Current Profiler (ADCP), have allowed advancing in the knowledge of the bedforms fluid dynamics [4,5], the relationship between the geometry of bedforms, and the flow [6,7] and the integration of bedforms in stability diagrams [8,9,10,11].

When the flow intensity exceeds a threshold value known as the critical condition for initiation of sediment motion, the sediment particles start to move, developing bedforms. Numerous works have analyzed the development of bedforms on flat bottoms and their relationship to near-bottom flow [12,13,14,15,16,17]. The main physical parameters that control the development of bedforms are the speed and depth of the flow, the density and viscosity of the fluid, the size and density of the particles, the supply of sediments, and the bottom roughness. The main oceanographic processes able to generate or maintain bedforms in different marine environments are (a) waves, tides, and wind-induced currents [18,19]; (b) thermohaline ocean currents, contour currents, and other specific bottom currents [20,21]; (c) density flows and turbidity currents [22,23]; and (d) internal waves and eddies [9,24]. In particular, in the deep environments of the continental slope, the processes that trigger the development of bedforms are related to bottom currents, turbidity currents, and associated oceanographic processes. However, despite the progress made in the study of bedforms in deep marine environments, the possible mechanisms responsible for generating bedforms and the associated sediment transport processes are still incomplete [25] due, mainly, to the difficulty of obtaining very-high resolution data at deep sea depths.

Increasingly, the use of ROTVs (Remotely Operated Towed Vehicles) or ROVs (Remotely Operated Vehicles) to obtain photographs and videos of the marine sea bottom is frequent. Since the beginning of this century, the use of photogrammetric techniques in a variety of marine scientific disciplines has been enhanced [26,27]. These techniques not only allow the accurate analysis of the seabed on a fine-scale, but they allow the analysis of vertical and at surfaces in a single model and they can be also applied to the study of marine habitats [9]. The use of photographs and videos of the seabed, combined with bathymetry and oceanographic and sediment type data, allows identifying these morphologies and inferring the orientation and speed of the bottom currents involved in their formation [28,29,30]. Some authors have constructed diagrams in which the bedforms are related to the velocity of the current and the type of substrate from data collected in depth waters or acquired under controlled laboratory conditions [9,10,11]. These diagrams are very valuable for assessing the intensity and variability of bottom currents that may have a significant influence on the location of submarine pipelines and other underwater facilities.

The Gulf of Cádiz is a natural laboratory to study bedforms generated by the Mediterranean overflow current, an energetic gravity flow that carries dense, saline Mediterranean Water [30]. A large number of studies have been devoted to the understanding of the dynamics of these large-scale bedforms and their interaction with the Mediterranean current [30,31,32,33,34,35,36]. However, not much effort has been devoted to the investigation of fine-scale bedforms, with only a few studies being carried out in the area [37,38].

The main aim of this paper was to document the relation between bottom currents and type of sediments in the formation and spatial evolution of bedforms in the Gulf of Cadiz using submarine video images.

2. Regional Settings

The study area is located at the NE of the Gulf of Cádiz continental slope, between 200 and 800 m depth (Figure 1). The Gulf of Cádiz is located at the SW of the Iberian Peninsula, in the transition zone between the African and Eurasian lithospheric plates, and in the westernmost part of the Mediterranean Alpine orogen, represented by the arch of Gibraltar (Figure 1a). The Gulf of Cádiz continental slope is made up of three domains: the upper, middle, and lower slope, depending on the gradient and morphological characteristics (Figure 1b) [39,40]. The upper slope extends between 120 and 400 m depth with a slope of 1.3°. The middle slope runs between 400 and 1200 m depth, with an average slope of 0.4°. The lower slope extends to 2000 m depth with an average slope of 0.8°.

The upper slope is crossed by turbidite channels with ENE-WSW direction, lengths ranging between 200–32,000 m and widths between 103–1080 m (Figure 1c) [42]. The middle slope is characterized by the presence of a Contourite Depositional System (CDS) [32] developed after the opening of the Strait of Gibraltar (SoG), at the end of the Messinian. Specifically, the study area is in the ridges and channels sector proposed by Hernández-Molina et al. [43] for the middle slope. The main physiographic features of the middle slope include the presence of two diapiric ridges: the Guadalquivir diapiric ridge and the Cádiz diapiric ridge, both with NE-SW direction and height between 100–200 m (Figure 1c) [44]. Besides, three large contourite channels with sinuous morphology and one smaller channel are observed in this area (Figure 1c). The Gusano channel with E-W direction, and the Huelva and Cádiz channels, with SE-NW direction. They have lengths ranging from 10 to 150 km, widths between 1.5–12 km, and reach depths of incision between 10–350 m [33]. The Gusano and Huelva channels cut through the Guadalquivir diapiric ridge. The Tofiño channel is the smallest of the contourite channels in the study area, located north of the Cádiz diapiric ridge with a NW-SE direction. All these morphological elements are located around contourite deposits, although other erosive morphological types such as moats, marginal valleys, and depressions associated with the base of mud volcanoes and diapiris stand out [33]. The mud volcanoes that occur in the study area are Tarsis, Pipoca, Anastasya, and Gazul (Figure 1c) [45,46].

The oceanographic circulation in the Gulf of Cádiz is controlled by the two-layer water exchange between the Atlantic and Mediterranean basins through the SoG, with Upper Eastern North Atlantic Central Water (ENACW; 35.6–36.1 psu and 11–17 °C) flowing above the dense MOW (36.1–36.9 psu and 13 °C) [30,47,48]. Both layers are separated by a sharp haline and density interface that sits at about 200 m at Spartel Sill (western SoG) and plunges westwards. The MOW current flows vigorously as a westward gravity current following the upper and middle slope (between 400 and 1200 m depth). Typical MOW current velocities exceed 1 m/s and decrease westward. Interaction with the rough topography past Spartel steer the partition of the main MOW vein into a number of individual branches (M1—densest to M5—lighter) (Figure 1c).

When the MOW current hits the Cadiz diapiric ridge, the upper branch M5 wades between the ridge and the shelf break to form the Mediterranean upper core (UC; [49]). M5 carries about 0.6 Sv at 0.3 m/s. M4 fills the intervening valleys south of the Cadiz diapiric ridge and along the northern flank of the Huelva channel about 0.4 m/s, transporting 0.3 Sv of a denser MOW variety. The deeper branches (M1 to M3) flow northwestwards through the Cadiz channel carrying about two thirds of the bulk MOW transport (1.7 Sv) [36].

Our study area is influenced by the shallower NACW, with associated current velocities of about 0.2 m/s, and by the upper (M5 and M4) MOW branches (Figure 1c), characterized by currents ranging from 0.3 to 0.4 m/s [37]. The density interface between both water bodies is located between 400 and 500 m in depth [34].

3. Materials and Methods

3.1. Data Acquisition

The direct observation data analyzed in this study were obtained during three oceanographic expeditions carried out as part of the research project ISUNEPCA (2015–2017). High-resolution underwater images were collected using HORUS ROTV to characterize bedforms and sedimentary substrate types. Specifically, we worked with 49 video transects (Figure 1c) that were captured at 0.5 m above the seafloor, at a speed of 0.6–0.7 Kt during transects of 10–20 min intervals. The mean distances explored in these transects were between 80 and 380 m.

Bathymetric data taken from INDEMARES [50] and ISUNEPCA projects (Figure 1c) were acquired using Kongsberg EM-710 (70–10 kHz and 400 soundings per swath) multibeam echosounder system (MBES) operating in high-density mode and using pitch and yaw stabilization. CARIS HIPS & SIPS 9.0 data processing software was used to produce a 15 × 15 m bathymetric grid model.

Interpolated fields of near-bottom velocity observations taken from Sánchez-Leal et al. [36] were used to evaluate the interaction of oceanographic processes with bedforms and substrates. Sánchez-Leal et al. [36] averaged 4339 LADCP observations taken a maximum of 15 m from the bottom. Both the near-bottom and water column profile data were interpolated on a 0.025 × 0.025 × 25 m grid (Figure 2a) using Data-Interpolating Variational Analysis (DIVA) [51].

The sedimentary distribution model is taken from Lozano et al. [52] (Figure 2b). This model was created from backscatter data and 46 sediment samples that were collected using box-corers and Shipek grabs. The classification of sedimentary textures was based on the ternary diagram of Folk [53], which considers the percentage by weight of the gravel, sand, and finer (silt+clay) fractions, but using the simplified classification proposed by Long [54] for habitat mapping.

3.2. Direct Observation Data Analysis

The video transect was studied from three different approaches to extract the maximum possible information: first, a descriptive analysis was performed in which the videos were viewed and the qualitative characteristics of each transect were noted; secondly, a metric analysis was performed at 10 representative frames of each video transect. To correct the deformation of the frames due to the angle of acquisition of the images, which was 45°, a mesh with 7.5 × 7.5 cm cells was made based on the frontal conical projection; finally, for the construction of the 3D models, frames corresponding to 3 s of the video were taken, representative of the complete transect, using the software Blender 2.79b. These frames were exported to AgisoftPhotoScan 1.4.5, which uses the “Structure from Motion” (SfM) technique to perform a photogrammetric reconstruction of the filmed surface. The software allows generating a dense point cloud in .las format and the corresponding orthophoto in .tif format (Figure 3). With ArcGIS 10.6.1 software, the .las files of the different photogrammetric reconstructions were processed to create digital terrain models (DTMs). For this goal, we used the interpolation method by the inverse of the weighted distance (IDW) with different cell size, depending on the characteristics of the point cloud (Figure 3). The MDTs generated were scaled using sled lasers and used to perform quantitative surface measurements and generate 3D models.

3.3. Geostatistic Analysis

The dataset used and the information obtained in the analysis phases was integrated into ArcGIS to create a spatial database that allowed execute a geostatistical analysis of video transect variables. The “Grouping Analysis” tool was used to find a solution in which all the entities within the specified groups are as similar, or as different, as possible between them.

For this study, those variables that provide significant differences to the video transects were used: (1) sediment type; (2) identified bedforms; (3) large-scale geomorphological type on which the transects are located; (4) bottom current speed; and (5) depth. Grouping analysis was performed without spatial restriction since video transects can have the same characteristics even if they do not share borders or spatial proximity. The default output of the Grouping Analysis tool is a new feature class that contains the fields that are used in the analysis plus a new one that identifies which group each video transect belongs to.

4. Results

The study was conducted on two scales: (1) a larger spatial scale, taking into account the morphological, oceanographic, and sedimentary characteristics of the place where each of the videos was taken; and (2) a smaller scale, in which the bedforms were identified and classified. Finally, with all information obtained, a statistical grouping analysis was carried out.

4.1. Large-Scale Spatial Analysis

First, four major geomorphological zones (turbidite channel, contourite channel, diapiric ridge, and contourite deposit) were defined, and the video stations were classified based on them (Figure 2a).

4.1.1. Turbidite Channels Zone

The turbidite channels are located in the east of the study area perpendicular to the upper and middle slope. In this zone, 11 video transects are located between 220 and 480 m depth (Figure 2a).

The transects made in the upper slope, between 220 and 330 m depth (T1, T3, T5, T7 and T10), are located on mud and muddy sand sediments with current speeds between 0.09 and 0.17 m/s (Figure 2a,b). On the other hand, the transects located in the middle slope (T6, T8, T11, T14, T15, and T24) are between 370 and 480 m depth, over mud and sandy mud sediments and with current speeds between 0.23 and 0.3 m/s (Figure 2a,b).

4.1.2. Contourite Channels Zone

A total of nine video transects were made in the zone of contourite channels, eight of which were in the large channels (Cádiz, Gusano, and Huelva channels) and one in the Tofiño channel (Figure 2a).

The Cádiz contourite channel area hosts four video transects. Two of them are located on a dune field at its northwestern end, between 569 and 654 m depth (T9 and T13), over sand and muddy sand sediments and with current velocities around 0.32 m/s (Figure 2a,b). The other transects (T2 and T4) are located on the northeastern flank of the channel, between 390 and 430 m depth, over muddy sediments and with current speeds of around 0.23 m/s (Figure 2a,b). In the Huelva channel, another two video transects were made between 600 and 682 m depth (T29 and T39). The T39 transect is located on the north flank of the channel, over muddy sand sediments and with current speeds of 0.23 m/s, while the T29 transect is located inside the channel at the eastern end, on sandy sediments and with current speeds of 0.37 m/s (Figure 2a,b). In the Gusano channel, two transects were made between 550 and 566 m depth (T32 and T35). Both transects were found at the eastern end of the channel, on sandy sediments and associated with flow rates between 0.27 and 0.29 m/s (Figure 2a,b). At the northeast end of the Tofiño channel, a video transect was made at 470 m depth (T31), on muddy sands and associated with flow rates around 0.34 m/s (Figure 2a,b).

4.1.3. Diapiric Ridges Zone

In this zone, a total of 10 video transects were made: four in the Guadalquivir ridge and six in the Cádiz ridge (Figure 2a).

The video transects made in the Guadalquivir ridge (T37, T41, T42, and T44) are distributed between 483 and 524 m depth. Transects T37 and T41 are located on the southeastern flank of the ridge, on sandy mud and with current speeds between 0.15 and 0.18 m/s (Figure 2a,b). Transects T44 and T42 are located on the northwestern flank of the ridge, over marginal valleys. In this flank the muddy bottoms predominate and the current speeds are slightly lower, between 0.14 and 0.15 m/s (Figure 2a,b). In the Cádiz ridge, transects were made between 463 and 784 m depth (T16, T17, T18, T21, T22, and T25). Transects T16 and T17, on the southeastern flank of the ridge, are located on sandy mud, and the current speeds in this zone range between 0.25 and 0.29 m/s (Figure 2a,b). T21 transect is located at the northeast end of the diapiric ridge, on sands and with current speeds values around 0.27 m/s (Figure 2a,b). Transects T18, T22, and T25 are located on the northwestern flank of the diapiric ridge. Current speeds increase to the south, from 0.19 m/s recorded in transect T25, to 0.32 m/s of transect T18. In the same way, the bottom sediment size increased from mud to muddy sands (Figure 2a,b).

4.1.4. Contourite Deposits Zone

Contourite deposits of the study area are mainly in the middle slope, interrupted continuously by diapiric ridges, contourite channels, and mud volcanoes. In total, 17 video transects were made on these deposits, 13 on the western side of the ridges and four on the north (Figure 2a).

A total of five video transects (T43, T45, T47, T48, and T49) between 498 and 607 m depth were made in the contourite deposits to the west of Guadalquivir ridge. The sedimentary deposits are composed mainly muddy or sandy mud sediments, and the current speed ranges between 0.06 and 0.16 m/s (Figure 2a,b). To the west of the Cádiz ridge, eight video transects (T19, T23, T26, T27, T28, T34, T36, and T38) were made between 487 and 624 m depth. The bottom is mainly covered by sandy mud, and the current speed ranges between 0.13 and 0.25 m/s, except in transects T19 and T23, in which speeds of 0.30 m/s are reached (Figure 2a,b). Video transects made to the north of the diapiric ridges (T30, T33, T40, and T46) are between 440 and 485 m depth. They are located on muddy sand and sandy mud sediments and the current speed is slightly higher than that of the other contourite deposits, between 0.24 and 0.34 m/s (Figure 2a,b).

4.2. Fine-Scale Spatial Analysis

The fine-scale study of the Gulf of Cádiz seabed has allowed us to identify, describe, and classify up to six types of bedforms: (1) ripples; (2) dunes; (3) obstacle marks; (4) burrows and mounds; (5) plane beds; and (6) rocky bottoms.

4.2.1. Ripples

Ripples identified in the study area were studied following the classification proposed by Simon and Richardson [55], which allowed differentiating five types of ripples (Figure 4): (a) straight; (b) ladder-back (c) sinuous; (d) disorganized sinuous; and (e) linguoid. The analysis focused on its morphological characteristics, but morphometric, sedimentary, and oceanographic features have also been taken into account.

(a)

Straight ripples are two-dimensional bedforms with continuous lateral ridges, with a straight plan view and asymmetrical profile. They have an average value of the wavelength of 8.17 cm, with values between 5 and 10.8 cm, and their height ranges between 0.5 and 4 cm, with an average value of 2.5 cm. They are associated with flow rates between 0.2 and 0.3 m/s and develop on sand and muddy sand sediments. These ripples have been identified in the contourite channels zone, arranged perpendicular to the dune leeside (Figure 4a).

(b)

Ladder-back ripples are composed of a main set of straight ripples perpendicular to the dune leeside, and a secondary set of overlapping ripples with an angle of 90° over the former (Figure 4b). The main set has continuous lateral crests, with a linear plan view and asymmetrical profile. They have an average wavelength value of 7.7 cm and a height of 1 cm. The secondary set consists of laterally discontinuous straight ripples, with a linear plan view and symmetrical profile. They have wavelengths around 7 cm and heights of 0.5 cm. These ladder-back ripples are associated with sandy sediments and flow rates around 0.30 m/s. As well as the straight ripples, ladder-back ripples have been identified in the contourite channels, associated with some dune leeside (Figure 4b).

(c)

Sinuous ripples are three-dimensional bedforms that have continuous lateral crests and transverse to the flow, with a wavy plan view and asymmetrical transversal profile. The crestlines are generally out of phase, with a wavelength ranging between 7 and 9.7 cm and an average value of 8 cm. They have heights between 1 and 3.5 cm, with an average value of 1.7 cm. They are associated with flow rates between 0.29 and 0.34 m/s and develop on sandy and muddy sand sediments. In leeside, they usually have large accumulations of biogenic material. This type of ripple was identified mainly at the edges of the contourite channels, although they are also observed in the depressions of the west flank of the diapiric ridges (Figure 4c).

(d)

Disorganized sinuous ripples are 3-dimensional bedforms that have discontinuous lateral ridges and transverse to the flow, with a wavy plan view and asymmetric profile. The crest lines are out phase, with wavelengths between 6 and 16 cm, and an average value of 11.5 cm. The height varies between 0.7 and 1.6 cm, and the average value is 1 cm. These ripples are associated with flow rates between 0.23 and 0.34 cm/s and develop over sand sediments. These ripples were identified in the contourite channels and the contourite deposits with greater current speeds (Figure 4d).

(e)

Linguoid ripples are 3-dimensional bedforms that have discontinuous and transverse crest to the current flow, with a moon plan view and asymmetric profile. The different lines of crests are out phase, with an average wavelength value of 10.4 cm, and ranging between 5 and 20 cm. The height reached by these ripples varies between 0.5 and 5 cm, the average value being 2.7 cm. They are associated with flow rates between 0.23 and 0.31 m/s and develop on sand and sandy mud sediments. In the lee side, they have accumulations of biogenic sediment of greater grain size. Linguoid ripples, like the previous ones, were identified in the contourite channels and the zones of higher velocity of the bottom current (Figure 4e).

4.2.2. Dunes

According to Ashley [56], the four dune fields identified in the contourite channels of the study area contain large two-dimensional dunes. They have continuous ridges laterally and transverse to the flow of the current, with a linear plan view and asymmetric profile. The average wavelength is 60 m with maximum and minimum of 130 and 50 m respectively, and they reach heights of up to 0.5 m. These bedforms are associated with flow rates between 0.23 and 0.34 m/s and develop in sandy bottoms (Figure 5).

On the dunes, the pattern of ripples varies depending on its surface position (Figure 6a). In the stoss side linguoid, disorganized sinuous ripples occur (Figure 6b), while in the leeside there is a flat surface with a high slope that constitutes the dune front (Figure 6c). On the trough of the dunes, there are straight ripples perpendicular to the dune or ladder-back ripples (Figure 6d). Immediately after the straight ripples, a stoss side from the next dune, sinuous ripples appear with large accumulations of biogenic material (Figure 6e), which pass into linguoid and disorganised sinuous ripples as we move away from the dune front.

4.2.3. Obstacle Marks

The interaction between the bottom current and the obstacles present in the seabed, such as rocks or benthic organisms, generates different types of marks around and leeward of the obstacle. Taking into account the sedimentary characteristics of these marks, we differentiated two types in the contourite channels zone: (a) sandy and (b) coarse sediment.

(a)

Sandy marks consist of elongated mounds that are generated leeward from the obstacle. These mounds reach up to 0.5 m in length and are associated with obstacles up to 10 cm high. Around the obstacle, a small erosive groove of between 0.5 and 1 cm is generated (Figure 7a,b). These marks are related to sandy sediments and flow rates around 0.23 m/s.

(b)

Coarse sediment marks are small depressions generated around and leeward from the obstacle. They are generally associated with obstacles of more than 25 cm in height that generate marks of more than 2 m in length (Figure 7c,d). These marks are related to flow rates around 0.32 m/s and sandy and mixed sediments.

4.2.4. Burrows and Mounds

In turbidite channels and contourite deposits, sea bottoms with high bioturbation rates were identified as burrows and associated mounds. These burrows were classified according to their opening diameter: small (<1 cm); medium (1–3 cm); and large (>3 cm) (Figure 8).

Small burrows are mainly on muddy sand bottoms and have average densities of 19 burrows/m², with values between 9–30 burrows/m² (Figure 8a). They are associated with flow rates between 0.15–0.30 m/s, with an average value of 0.23 m/s.

Medium burrows are associated with muddy and sandy mud bottoms with average densities of 35 burrows/m² with a range greater than that of small ones, between 7–70 burrows/m² (Figure 8b,d). The flow rates associated with these burrows range from 0.08–0.32 m/s, with an average value of 0.19 m/s.

Large burrows are associated with muddy bottoms, and they occur with densities between 18–45 burrows/m² with an average value of 28 burrows/m² (Figure 8c,d). These burrows are associated with the lowest flow rates, between 0.06–0.20 m/s, with an average value of 0.16 m/s.

4.2.5. Plane Beds

Plane beds are characterized by not presenting well developed sedimentary structures. Two types can be distinguished depending on their sandy or muddy-sand texture (Figure 9).

Sandy plane beds have a very low roughness (Figure 9a), and are associated with flow rates around 0.30 m/s. They were identified primarily in the contourite channels. On the other hand, muddy-sand plan beds showed greater roughness, and small burrows were observed in them (Figure 9b). Muddy-sand plane beds are associated with lower flow rates, between 0.10–0.25 m/s and have been identified in the contourite deposits and the vicinity of the diapiric ridges.

4.2.6. Rocky Bottoms

These are bottoms consisting mainly of mixed sediment in which the rocky substrate emerges (Figure 10). They are associated with high flow rates, between 0.30–0.36 m/s, and have large accumulations of biogenic material and benthic organisms associated with rocky outcrops. This type of bottom has been identified primarily in diapiric ridges and in parts of the contourite channels near the ridges.

4.3. Grouping Analysis

The results obtained by applying the Grouping Analysis tool to the variables specified in Section 4.3 allowed us to differentiate five groups among all the analyzed video transects (Figure 11a).

Group 1 (G1) and group 5 (G5) were composed of 5 and 11 video transects respectively. Transects that conform to them were in the areas with the lowest values of current speed between 0.06–0.20 m/s, and the finest sediment, mud type (mud, sandy mud, and muddy sand). Both groups presented burrows as main bedforms, but differed in their distribution by the study area, since the G1 video transects were distributed by the shallowest area of the turbidite channels, between 222–331 m depth, and those of the G5 were located on contourite deposits between 498 and 607 m depth (Figure 11b).

Group 3 (G3) and group 4 (G4) were composed of eight and five video transects respectively. It presented intermediate values of current speed around 0.25 m/s, and sediment type, predominantly sandy mud and muddy sand. The most abundant bedforms in these transects were low roughness bottoms. The video transects of the G3 were distributed along the deepest turbidite channel area, between 368–490 m depth, while the G4 transects had a high geographical dispersion, being at depths between 391–784 m (Figure 11b).

Group 2 (G2) was composed of 10 video transects. It presented the highest values of current speed between 0.22–0.36 m/s, and sediment type, predominantly sand and mixed sediments. The video transects of the G2 were found in the contourite channels and the diapiric ridges at depths between 391–784 m. Dunes, ripples, and rock bottoms were the main bedforms in the G2 (Figure 11b).

5. Discussion

The spatial distribution of the bedforms identified in the upper and middle slope of the Gulf of Cádiz reflects a balance between three factors: bathymetry, type of sediments observed at the bottom, and the current speed. This balance shapes the seafloor microtopography.

Bedforms Spatial Distribution and Involved Processes

Although often represented as a flat surface on a large-scale, the seabed has a significant topography on a fine spatial scale. Taking into account the results obtained in the grouping analysis, three types of microtopographies can be inferred according to the bedforms and the processes that originate them: biogenic, physical, or mixed.

The first bottom type (F1) showed a biogenic microtopography (Figure 12), with a large number of burrows and mounds resulting from the excavation and foraging of different crustaceans. These bottoms were distributed through two zones: the shallowest part of the turbidite channels zone, to the NE of the study area; and in the contourite deposits that are located west of the diapiric ridges. The main difference between these two zones is their hydrodynamic context. The shallowest part of the turbidite channels zone is under the influence of the ENACW current [36], which most of the time flows towards the SoG, but in which, according to Lobo [57], gravitational processes take place that interfere with the continuous flow of the ENACW [31]. On the other hand, the contourite deposits zone remains sheltered from the MOW current by the diapiric ridges. All these factors can favor the presence of muddy sediments, which facilitate the formation of burrows. Burrows identified in these bottoms are of medium and large size developed on muddy and muddy-sand sediments, and the associated flow rate was the lowest measured in the study area, with an average value of 0.14 m/s.

The low flow rates measured in the F1 bottoms suggest that low energy conditions prevail. This situation permits the settling of the finest particles [20] (Figure 13) and allows the edification of fine deposits that are conducive to the establishment of different organisms, whose activity disturbs the sedimentary structure and configures different ichnological patterns on the seabed [58]. Through bioturbation, organisms build structures such as burrows and mound that are observed in these bottoms types and modify sediment properties such as porosity and grain size. All this influences the processes of consolidation and erosion of sediment and ultimately leads to the modification of existing physical sedimentary structures [59].

From the observations made in the video transects, it can be established that species such as Nephrops norvegicus, Squat lobster, Goneplaxromboides, and Monadeuscouchii, among others, are responsible for the construction of burrows (Figure 8b,d). These organisms dig the burrows transporting the subsurface sediment to the bottom surface and mounds give a rough and unconsolidated appearance to the seabed. In addition, these constructions favor the vertical displacement of particles between the water-sediment interface and the interior of the sediment, which further alters the properties of the sediment [58]. In the turbidite channels, where gravitational flows can eventually occur [57], biogenic sedimentary structures can be buried by sediment transported in these flows.

The second bottom type (F2) presents a physical microtopography characterized by a large number of dunes, ripples, obstacle marks, and rock bottoms that are the result of the interaction of currents with the seabed (Figure 12). These bottoms are widely distributed throughout the study area, occupying the contourite channels and the zone of contourite deposits north of the diapiric ridges. F2 bottoms are composed of sand and rock with coarse sediment. These are related to the highest flow near-bottom velocities, exceeding 0.3 m/s and associated with the MOW M5 and M4 veins that sweep across these bottoms past the Cadiz diapiric ridge [36]. M5 flows through the zone of contourite deposits north of the diapiric ridges, reaching speeds of up to 0.34 m/s, while the M4 core crosses Cádiz ridge through the intermediate valleys and is channelled through the Huelva channel at speeds up to 0.37 m/s.

These strong current velocities allow the uninterrupted sediment transports that restrain the long-lasting formation of bioturbation features [37,59]. Therefore, in these bottoms, sand and coarse sediments predominate in which, depending on the textural characteristics of the grain (size, density, and shape) and the characteristics of the current (speed, depth, temperature), they can originate different bedforms [9] (Figure 13).

Disorganized sinuous and linguoid ripples are observed on sandy bottoms, while in the dune fields, sinuous, straight and ladder-back ripples are also identified. According to some authors, there is a transition from two-dimensional to three-dimensional ripples controlled by increased flow speed [9,20,60]; however, in other works this transition is related to the persistence of the flow [61]. If we take into account that the formation of ripples originates from current vortex that creates some irregularities in the surface of the sediment, and that the crests of the ripples represent a point of flow separation that generates turbulent vortex [62], we can consider that both the flow speed and its persistence control this transition from two-dimensional to three-dimensional ripples. In this way, in sandy bottoms with flow rates between 0.19 and 0.45 m/s, straight ripples would be generated first, which, with the persistence of the flow, would begin to grow in height, thus generating greater microturbulence after its point of separation of flow, and as a consequence the continuity of the ridges would be broken, giving way to the three-dimensional ripples.

As in other studies [13,61], a relation between the size of the ripples (wavelength and height) and the characteristics of the flow was not found, but there seems to be a direct relation between the wavelength and the grain size. However, in bottoms with grain size greater than 0.7 mm in diameter, no ripples were formed.

Regarding the dunes, different studies have related them with current speeds between 0.3–0.75 m/s and with average grain sizes greater than 0.15 mm [9,63]. Its size seems to be related with the current velocity. Dunes can be originated by turbiditic currents, bottom currents, or internal waves [64,65]. In the study area, the dune fields were in the contourite channels, so they were mainly associated with the bottom currents. While it is true that the velocities observed in the dune fields were somewhat low (0.23–0.34 m/s), the interaction of the currents with the bathymetry may trigger the formation and or amplification of internal waves [66,67] across the MOW-ENACW interface as well as other turbulent processes that may module the formation and migration of the dunes.

Distribution and orientation of the ripples across the surface of the dunes highlight hydrodynamic variations. The sequence of ripples observed, from straight within the dune trough to sinuous ripples and then disorganized sinuous or linguoids towards the crest of the dune, represents an increase in height above the seabed and a longer exposure time to the current flow as the dune migrated. In general, the orientation of the ripples indicates that the general direction of the flow is perpendicular to the dune crest. However, the straight ripples that were identified within the dunes appeared perpendicular to them, which indicate that as in the case of ripples, the dune crest assumes a point of flow separation. Wynn et al. [29] presented a similar pattern of ripples variation through barjanoid dunes, also based on the study of submarine images.

The presence of ladder-back ripples in some of the dune trough is somewhat poorly documented to date. In coastal environments, these ripples are related to a combination between oscillations and currents or the refraction of the waves if it affects the coast obliquely [68]. However, in deep environments it is not clear yet what the origin of these sedimentary structures may be. Stow et al. [37] suggest that they may be related to tidal currents, which are of great importance in the Gulf of Cádiz [69].

The bottoms covered by mixed sediment in which rocky substrate emerges and obstacle marks are observed, at the SE end of the Tofiño channel, suggest current velocities ranging from 0.3 to 1.0 m/s [9,70]. These very high velocities prevent the deposition of the hemipelagic sediment allowing authigenic carbonated (slabs and crusts) associated with diapirism in the zone to emerge [71]. Local acceleration as the current flows around one of these rocks or organisms anchored to the bottom would allow the formation of the identified obstacle marks [20].

Therefore, we can affirm that the MOW current is persistent and energetic enough to originate three-dimensional ripples throughout the F2 bottoms and together with related hydrodynamic processes (internal waves, and benthic boundary layer turbulence) keep the dune fields active. Dunes would migrate along the contourite channels and produce energetic zoning that allows the succession of ripples from two-dimensional to three-dimensional. In the Tofiño channel, either because of its narrow channel or due to its location between the Cádiz diapiric ridge, the flow increases its speed, giving rise to bedforms that require more energy to originate.

The last bottom type (F3) presents a mixed microtopography in which bedforms of both biological and physical origin were observed (Figure 12). These bottoms were the most abundant in the study area, distributed by the middle slope of the turbidite channels zone, in the diapiric ridges, and on the edge of the contourite channels. Bedforms that were identified were medium and small burrows with their associated mounds and plane bed. F3 bottoms are covered by muddy sand sediments and are related to average flow rates of 0.25 m/s. They are characterized by complex hydrodynamics associated with the confluence of ENACW and MOW currents and their interaction with the structural highs. On the one hand, the zone of turbidite channels is bathed by the interface between the two existing water masses in the study area, ENACW and MOW [36], in which internal waves and other turbulent processes take place that can shape the seabed [72]. Additionally, in these bottoms there are gravitational processes that can interfere with the bottom currents [42,57]. On the other hand, the interaction of the MOW with diapiric ridge and mud volcanoes generate deposition of windward sediments of these structures and turbulent leeward flows that produce erosion [46].

In F3 bottoms, the flow of the current resuspends the finest fraction of the sediment, causing muddy-sand bottoms in which some burrows are identified (Figure 13). The resulting average grain size prevents large and high-density burrows from developing [73]. The interaction between the current flow and the mounds associated with the construction of the burrows would generate the muddy-sands plan beds that have been identified. According to Fries et al. [74], these low-roughness bottoms would represent an intermediate state between the bottoms with biogenic sedimentary structures and the formation of ripples. Whether or not ripples are formed depends on the density and height of the mounds, such that, the greater the number of mounds and larger, the greater the turbulence that would be generated by the interaction with the current and more likely the ripples formation.

6. Conclusions

The integrated interpretation of submarine images, multibeam bathymetry, and sedimentary and oceanographic data allowed us to study and analyze in detail the microtopography of the study area.

The bedforms identified are the result of biological, sedimentary, and physical processes that interact with each other to shape the seabed. A classification of the seabed was proposed that allows the establishment of the predominant energy conditions in the different bottom types. When bottom current speed is high, bedforms with a physical origin predominate, while, under low flow rates, sedimentary structures of biogenic origin are predominant. With intermediate flow rates, biogenic and physical sedimentary structures coexist on the seabed.

Improving underwater photogrammetry techniques and recording of short and medium-term variability of the near-bottom currents system remains a key objective for future work.