Abstract
Continental shelves along tropical semi-arid margins remain sparsely mapped at high resolution, limiting process-based interpretations of bedforms dynamics and reducing quality of baseline information required for marine spatial planning and offshore development. In this study we present an integrated geomorphological and sedimentological baseline for two middle- to outer-shelf sectors of the Ceará Basin, on Northeast Brazil. We combined multibeam echosounder bathymetry and backscatter, side-scan sonar imagery, and sediment sampling to map bedform morphology and sediment textures in detail across more than 12 km2 in total. High-resolution mapping reveals strong spatial contrasts in seabed morphology and acoustic texture across the two areas. The Western Area comprises a heterogeneous mosaic of geomorphic elements, including a channel, clustered circular depressions, and a terrace-like bench, associated with marked variability in backscatter and grain size, from granule-grade sediment on the bench to sandier substrates in depressions and channel-floor domains. The Eastern Area is dominated by sand-wave and organized into a crest–trough system, with geomorphometric patterns and side-scan sonar textures pointing to spatial variability in seabed roughness and bedform expression. Together, these results highlight how inherited shelf morphology and present-day hydrodynamic reworking interact on a sediment-limited shelf to produce contrasting bedform–sediment domains over short along-shelf distances. The mapped bedform-sediment domains provide a robust physical baseline to guide future process studies and to inform monitoring and risk-aware planning for infrastructure and marine spatial management on the Brazilian semi-arid continental shelf.
1 Introduction
Continental shelves play a crucial role in global energy supply (; ) and in conserving marine ecosystems (; ). In recent years, shelf bedforms have been extensively studied due to the increasing demand for reliable spatial data to support marine spatial planning approaches (; ; ). Despite significant advancements in seafloor mapping, only around 25% of the shallow-water zones have been mapped at high resolution (; ). This limited coverage constrains our ability to characterize sediment-transport processes and to evaluate the seabed impacts of offshore activities, particularly along tropical and semi-arid margins, increasing uncertainties in sustainable shelf management.
Shelf-associated bedforms and sedimentary deposits result from complex sediment-hydrodynamic systems, primarily controlled by fluid flow properties, seabed topography, sediment grain size, and transport mechanisms (van Dijk et al., 2021). These features reflect distinct morphosedimentary regimes, ranging from supply-dominated settings with active progradation and terrigenous sediments accumulation to sediment-starved or accommodation-dominated sectors, favoring carbonate sedimentation and benthic habitat development (; ; ). Bedforms also provide indirect constraints on near-bed conditions, including current- and wave-driven dynamics (Wilckens et al., 2023; ), and on terrain controls such as slope and ruggedness (; Taylor et al., 2024). Over time, several bedform classification schemes have been developed to better understand these features, most commonly based on their orientation, wavelength and height (). While such classifications provide a useful framework for bedform analysis, their application to coarse-resolution datasets may oversimplify sediment properties, inadequately resolve heterogeneous areas, and overlook bedform variability. Such limitations can bias environmental interpretations, thereby affecting ecological assessments and submarine engineering planning (Testa and Bosence, 1999; ).
High-resolution geophysical data are essential for accurately identifying bedform morphologies and, consequently, for managing offshore infrastructure and assessing seabed environmental impacts (; ; ). The improved mapping and integration capabilities provided by modern geophysical techniques, such as Side Scan Sonar (SSS) imagery and multibeam echosounder (MBES) bathymetry and backscatter, enable a more comprehensive characterization of seabed morphology than legacy coarse-resolution datasets. These advances in observation techniques provide a quantitative basis for interpreting sediment distribution and bedform organization (; Smith et al., 2023), sediment mobilization by waves and currents (; ), and spatial patterns in hydrodynamic forcing (Smith et al., 2018; ). Although Southern Hemisphere nations have increasingly adopted modern mapping technologies through industry-led and publicly funded initiatives, applications remain disproportionately concentrated in Northern Hemisphere regions ().
In response to growing energy demand, offshore wind farms () and oil and gas exploration (Ysaccis et al., 2023a) have been planned along the Brazilian Equatorial Margin (BEM). Recent exploration success in the Guyana and Tano basins, analogues to the BEM basins, has heightened expectations for significant hydrocarbons discoveries, positioning the BEM as a new frontier for offshore exploration in Brazil (; Ysaccis et al., 2023b; ). This interest has stimulated new high-resolution surveys in the BEM, producing baseline datasets that support more robust assessments of seabed characteristics and, where available, benthic habitats. Within the BEM, sediment supply can be broadly described by two end-member systems: (i) the Amazon System, strongly influenced by the interannual dynamics of the Amazon River Plume (), and (ii) the Semi-Arid Coast System, characterized by very low riverine input and higher water transparency (). In this context, achieving consistent high-resolution coverage across both systems remains logistically challenging, as the BEM shelf extends more than 300,000 km² and survey effort is necessarily uneven. Consequently, detailed bedform studies have concentrated on the Amazon sector, often in the context of benthic habitats mapping (; ; ; Vale et al., 2022; ), whereas bedforms along the Brazilian Semi-Arid Coast (SAC) remain comparatively poorly described, limiting ecosystem management and offshore development planning (; ).
In this study we map and quantify the occurrence and distribution of shallow water bedforms in two sectors of the SAC continental shelf by integrating newly acquired high-resolution geophysical datasets and sediment samples. This integration of data provides a description and quantitative baseline of the distribution and sediment composition of mid-sized bedforms in the Ceará Basin. Specifically, it supports the understanding of how shelf morphology and hydrodynamic forcing shape bedform evolution and present-day activity on a broad, sediment-limited tropical shelf.
2 Material and methods
2.1 Study site
The study sites comprise two survey areas (hereafter Western Area, WA, and Eastern Area, EA) on the middle-to-outer continental shelf of the Ceará Basin, northeastern Brazil (Figure 1). The Ceará Basin shelf extends approximately 63 km from the coast to the shelf break and exhibits a natural morphosedimentary zonation, comprising an inner shelf (< 20 m), middle shelf (20–40 m); and outer shelf (> 40 m to shelf break) (). The middle shelf is characterized by multiple breaks in slope that form a stepped morphology of benches and terraces, interpreted as palaeocoastlines associated with past sea-level stillstands (). On the outer shelf, the seabed is locally incised by canyon-head notches linked to submarine canyons that cut into the adjacent continental slope, while incised valleys occur elsewhere across the shelf as expressions of palaeodrainage pathways (Silva Filho et al., 2007; ; ).
Figure 1
The sedimentary regime of the Ceará Basin shelf is strongly influenced by the region’s semi-arid climate, which is controlled by the Intertropical Convergence Zone (ITCZ). This region is characterized by strong seasonality and two well-defined periods: a rainy season from December to May (positive water balance), and a dry season (negative water balance) with strong trade winds (4 to 11 m s-1) from June to November (Soares et al., 2021; ). This climatic setting limits riverine sediment inputs and results in a mixed carbonate-siliciclastic system, with modern carbonates and relict siliciclastic grains (Ximenes Neto et al., 2018a; ). Modern siliciclastic supply is mainly confined to the shoreface and shoaling zone and is strongly controlled by waves and longshore currents, in a setting influenced by the North Brazil Current (NBC), alongside a semidiurnal, mesotidal regime (; ). These processes favor seabed reworking, bedform development, and winnowing of fines, leaving coarser and better-sorted sands and shell debris in higher-energy settings (; Ximenes Neto et al., 2025).
Beyond its sedimentary interest, this portion of the Ceará Basin also hosts significant benthic assemblages, including macroalgae, sponges, and reef-building corals, that remain relatively understudied despite its ecological significance (; ). The coexistence of these habitats with expanding offshore infrastructure, including long-standing submarine cables and oil platforms (), and the projected installation of offshore wind farms (), has spurred more robust environmental oversight. Ongoing land-use transformations across these shore-to-offshore environments have raised concerns about anthropogenic pressures and highlighted the need for high-resolution seabed mapping as baseline information to properly support effective environmental assessment and marine spatial planning in the region.
2.2 Geophysical data collection and processing
The study areas were surveyed utilizing an integrated geophysical approach combining MBES and SSS systems. MBES bathymetry and backscatter established the foundational framework for high-resolution submarine topography and 3D characterization of seafloor morphology, complemented by SSS imagery that delineated fine-scale topographic features and sediment distribution patterns. MBES data were acquired using an R2Sonic 2022, and SSS data were acquired using a Klein MA-X View 600 (key acquisition parameters in Supplementary Table 1). Sound velocity was set at 1500 m s-1 during acquisition and subsequently corrected using site-specific sound velocity profiles (SVPs). Navigation and positioning were conducted using an Applanix POS-MV system integrated with a DGPS receiver, resulting in a horizontal positioning uncertainty of less than 1 m. MBES data were collected along straight-line transects at a uniform speed of 5 knots with real-time motion compensation (heave, pitch, roll, and yaw) from an inertial measurement unit (IMU). A patch test was conducted to determine angular offsets, which were applied during processing.
Bathymetry and backscatter were processed in SonarWiz8 (Chesapeake Technology, New York, NY, USA), including data cleaning and gridding to a final 0.5 m resolution. Key terrain parameters, including aspect, slope and roughness, were computed from the bathymetry grid to provide complementary descriptors of seabed morphology beyond depth alone. Slope highlights gradients and geomorphic boundaries, aspect captures seafloor orientation relative to directional hydrodynamic forcing, and roughness quantifies fine-scale irregularity and heterogeneity (Wilson et al., 2007; ). Each of these parameters supports the delineation and characterization of bedforms and may provide valuable inputs for characterizing spatial variability in seafloor complexity and its potential ecological implications (Walbridge et al., 2018; ). Backscatter patterns were identified and interpreted based on the intensity, texture, and heterogeneity or homogeneity of the backscatter mosaics. These patterns are commonly associated with substrate characteristics: harder or rougher seafloor materials generally produce stronger backscatter signals, while softer or smoother substrates give weaker returns (; ). To aid the delineation of seafloor facies, Backscatter Strength (BS) was analyzed alongside SSS texture and sediment samples. SSS data were processed and mosaicked in SonarWiz8 at 0.08 m pixel size.
To ground-truth acoustic interpretations, 12 sediment samples were collected using a Van Veen grab sampler, each representing the upper 0–10 cm of the seabed. Grain size classification was delineated into three textural categories, according to the Wentworth (1922) scale, expressed in both millimeter (mm) and phi (Φ) units: gravel-grade (> 2 mm;< -1 Φ); sand-grade (2–0.063 mm; −1 to 4 Φ); fine-grained/mud-grade (< 0.063; > 4 Φ). Grain-size distributions were measured using sieving, and summary statistics (sorting, skewness, kurtosis) were calculated using the method.
2.3 First-order wave-mobility analysis
To provide a first-order estimate of seabed mobility, significant wave height (Hs) and peak wave period (Tp) were extracted from two global wave reanalysis products: WAVERYS (), by the Copernicus Marine Environment Monitoring Service (CMEMS), and ERA5 (), by the European Centre for Medium-Range Weather Forecasts (ECMWF). Wave parameters were extracted from the nearest grid cell to the WA and EA centroids (Supplementary Tables 2, S3). Near-bed orbital velocity (ub) was calculated using linear wave theory (Soulsby and Whitehouse, 1997):
where ub represents the peak orbital velocity amplitude at the seabed, h is water depth and k is the wave number (2π/L, where L is wavelength). As the wavelength was not directly available from the reanalysis products, k was obtained iteratively from the linear dispersion relation:
where g is the gravitational acceleration. Calculations were performed for representative depths of 35 m in the EA and 45 m in the WA, and for the shallowest and deepest mapped depths of each area (Supplementary Table 4). Critical velocities for incipient sediment motion (ucr) were estimated for the observed grain-size classes (Supplementary Table 5) using a Shields-type approach (Shields, 1936):
where τcr is the critical bed shear stress, ρ is seawater density, and fw is the wave-friction factor. The critical bed shear stress was calculated as:
where θcr is the critical Shields parameter estimated using the Soulsby–Whitehouse threshold formulation (Soulsby and Whitehouse, 1997), ρs is sediment density, and D50 is median grain size. The ucr expression was obtained by equating τcr to the maximum wave-induced bed shear stress:
with fw estimated following Swart (1974). The ucr calculation was solved iteratively because fw depends on the orbital excursion at threshold. Threshold exceedance was calculated as the percentage of wave records for which estimated ub exceeded ucr for each sediment class (Supplementary Figures 1, S2). Full product metadata, grid-cell information, and threshold values are provided in Supplementary Tables 2-S5.
For WAVERYS, wave-direction statistics were also extracted and summarized to characterize the regional directional wave-exposure context during energetic conditions. Direction was interpreted as wave-from direction, and wave-to direction was calculated as the opposite azimuth:
Directional distributions were summarized as Hs-binned wave roses for the full record and for site-specific energetic-event subsets exceeding the P90, P95, and P99 Hs thresholds.
3 Results
3.1 Distribution and morphology of seafloor bedforms
3.1.1 Western area
The Digital Bathymetric Model (DBM) for the WA reveals depths ranging from 42.5 to 52.8 m across a mapped area of 4.9 km² on the outer shelf (Figure 2a). The most prominent features include: (i) a channel, approximately perpendicular to the local coastline, in the northwest portion of WA; (ii) an elevated bench-shaped surface in the east–southeast, and (iii) a clustered field of circular depressions around 48 m water depth. Depths across the WA range by 10.3 m, with the deepest values restricted to the northwestern channel feature (Figure 2b.i), whereas the shallowest depths coincide with the elevated bench-shaped surface east–southeast (Figure 2c.i). The observed channel exhibits a SW–NE orientation, reaches a maximum water depth of 52.8 m, and extends for approximately 1.5 km, with apparent continuity beyond the mapped area. The channel displays V-shaped cross-sections, with asymmetric margins and widths ranging from 100 to 500 m (Figure 2b.ii). Slope and roughness attain their highest values along the channel margins (Figure 3). In the east–southeast WA, the DBM indicates a bench-shaped surface characterized by relatively flat bathymetry at approximately 42 m, locally bounded by steeper breaks in slope expressed as sharp contour inflections. The associated scarp edges rise by up to 3 m above the adjacent seabed, forming a clear step between the flat surface and surrounding lower-elevation domains (Figure 2c.ii). This bench-to-basin transition is expressed by subtle gradients in slope and a roughness break at the scarp edges (Figure 3). Circular depressions with diameters ranging from 10 to 100 m are prevalent particularly in the southwestern portion of WA (Figure 2d.i). These features form discrete negative-relief steps of approximately 2 m and occur mostly as a clustered field rather than isolated structures, producing locally rugged bathymetry within the broader depression-rich sector (Figure 2d.ii). The depressions also generate minor but discernible variations in slope and roughness gradients (Figure 3). Overall, morphometric patterns indicate a gradual transition between elevated and deeper zones, yielding a continuous but irregular seabed surface at the scale of the mapped WA.
Figure 2
Figure 3
3.1.2 Eastern area
The depths in the EA area range from 30.7 m to 36.8 m, covering a total area of 7.2 km2 across the mapped middle-shelf (Figure 4a). The most outstanding features within this area are the strip-like positive relief ridges interpreted as sand waves, based on their morphology and medium-sand grain size, as examined in detail in Section 3.3.2. Three sand waves are identified in the southern and western portions of the EA, at depths of approximately 30–35 m, forming elongated strips trending SW–NE (Figure 4b.i). The sand waves vary approximately from 1.0 to 1.4 km in length, 250 to 350 m in width, and 2 to 5 m in height. Wavelengths range from 1.0 to 1.5 km, defining a crest–trough system that partitions the southern–western EA into alternating positive- and negative-relief elements (Figure 4b.ii). Within the mapped EA tile, the submarine sand waves footprints occupy approximately 1.1 km², corresponding to 15% of the mapped area. Slope and roughness gradients show systematic flank asymmetry, with higher values concentrated along the northwest side of the sand waves, whereas southeast flanks exhibit lower gradients and smoother terrain (Figure 5). All three sand waves display similar tidal scours trough patterns, and subtle superimposed dunes are observed within trough areas. Outside the sand-wave domain, much of the EA is characterized by a relatively flat plain at depths of 35 m (Figure 4c.i). In northeastern EA, isolated depressions form discrete negative-relief features, each defining steps of approximately 1 m relative to the surrounding seabed (Figure 4c.ii). Morphometric variables reveal minor localized variations in slope and roughness along depression rims within the otherwise low relief plain (Figure 5).
Figure 4
Figure 5
3.2 Seafloor sediment distribution patterns
3.2.1 Western area
The MBES backscatter mosaics in WA revealed acoustic reflectivity patterns that are closely associated with the main morphological elements (Figure 6a). Backscatter values in the processed mosaic range from -67 to -28 dB and are interpreted here relatively within the mosaic. The northwestern channel displays alternating bands of relatively higher and lower backscatter aligned with the channel thalweg and coincident with the deepest bathymetry. In contrast, the east–southeast bench is characterized by extensive zones of relatively higher backscatter interspersed with mottled textures, indicating a heterogeneous seabed, consistent with harder and/or coarser substrate conditions. The southwestern depression field exhibits comparatively uniform, lower backscatter patches, suggesting smoother seabed conditions and finer-grained sediment accumulation within circular depressions and along channel-floor segments. Overall, backscatter patterns indicate a systematic relationship between morphology and seabed texture across WA, with higher backscatter (> −50 dB) generally associated with harder/rougher seabed conditions and lower backscatter (< −60 dB) with softer/smoother conditions within the processed mosaic.
Figure 6
High-resolution SSS imagery enabled identification of fine-scale geomorphic units, consistent with the MBES backscatter patterns, including sand ripples (Figure 6b), biodetrital/granular patches (Figure 6c), and anthropogenic targets (Figure 6d). Homogeneous low-return seabed occurs mainly along the western to northwestern channel-floor domain and within parts of the circular depression field, where the SSS response is smooth and laterally continuous, associated with relatively finer/sandier substrates and locally developed linear bedforms. In contrast, high-return, irregular-granular patches occur locally along rougher sectors, including parts of the bench/scarp transition and localized areas around depression margins, consistent with coarser bioclastic material, ranging from granules to coarse sand. Ripple textures are observed locally across the WA, including the western–southwestern sector and transitional areas toward the east–southeast bench domain. SSS imagery also delineates linear, high-contrast anthropogenic features, including drag marks/scours and exposed pipeline segments, which occur throughout WA but are more concentrated in the eastern sector.
3.2.2 Eastern area
In the EA, MBES backscatter mosaics reveal fewer distinct acoustic textures than in WA, but a similar morphology–substrate relationship, with relatively higher backscatter concentrated on positive-relief features and more uniform returns across the surrounding plain (Figure 7a). Backscatter values in the processed EA mosaic range from approximately -54 to -26 dB and are interpreted here relatively within the mosaic. Sand-wave crests exhibit consistently higher backscatter (> −37 dB), consistent with coarser/rougher substrates such as coarse sand and shell debris. The central dune corridor displays banded backscatter patterns with intermediate values (−44 to −41 dB), suggesting along-feature alternations between relatively coarser and finer substrates across crests, flanks, and troughs. In the southeastern sand wave zone, high−backscatter crests are separated by lower−backscatter troughs. The northern plain and southwestern depressions exhibit predominantly lower backscatter (< −45 dB), consistent with smoother seabed conditions; faint lineations locally suggest biogenic structures or anthropogenic disturbance (e.g., trawl marks).
Figure 7
High-resolution SSS imagery resolves seabed texture at the scale of sand-wave crests, flanks and troughs and provides the basis for mapping discrete seafloor units. Areas surrounding positive-relief features show higher returns and irregular textures, consistent with relatively coarser sediments, whereas flatter sectors display a homogeneous low-return response consistent with smoother finer-grained substrates (Figure 7b). Superimposed dune textures occur near sand-wave troughs and on dune flanks, and in the southwestern sector, where linear to sinuous patterns suggest active sand dunes (Figure 7c). Anthropogenic features are also evident, including exposed pipeline segments concentrated in the northeastern sector and associated drag marks; similar linear disturbances occur more widely across the area (Figure 7d).
3.3 Characterization of seabed sediments
3.3.1 Western area
Surficial sediment sampling points were selected to capture the main backscatter domains in WA (Figure 6a). Five samples were collected across the bench–depressions–channel system (Table 1). The bench interior is dominated by granule-grade sediment (WA01), transitioning downslope to coarse sand (WA03) and locally medium sand (WA04) along the bench flanks. A scarp-adjacent depression is characterized by coarse sand (WA02), whereas medium sand occurs within the channel-floor/thalweg domain (WA05). These grain-size contrasts are consistent with the acoustic interpretation, with relatively higher backscatter over granular/rougher substrates on the bench and lower returns over sandier sectors. Overall, despite the limited set of samples, the granulometric data indicates grain-size variability among the bench interior, bench slopes, depressions, and the channel-floor domain, suggesting a conceptual hierarchy of sedimentary environments within WA.
Table 1
| Sample point | X coordinate | Y coordinate | Mean grain size | Sorting (Std. deviation) | Skewness | Kurtosis | Associated geomorphological feature |
|---|---|---|---|---|---|---|---|
| WA01 | 501227 | 9674105 | Granule | Very well sorted | Very positive | Very platykurtic | Bench (Interior) |
| WA02 | 501591 | 9673816 | Coarse sand | Poorly sorted | Approximately symmetric | Mesokurtic | Bench (Flank) |
| WA03 | 501297 | 9673778 | Coarse sand | Poorly sorted | Approximately symmetric | Mesokurtic | Bench (Interior) |
| WA04 | 501422 | 9673862 | Medium sand | Poorly sorted | Approximately symmetric | Mesokurtic | Bench (Flank) |
| WA05 | 500531 | 9674183 | Medium sand | Poorly sorted | Negative | Mesokurtic | Channel |
Granulometric characterization of Western Area.
3.3.2 Eastern area
Sediment samples from EA indicate a largely uniform medium-sand substrate across all seven stations (Table 2), including locations on sand-wave crests/flanks, inter-wave plains, and a local inter-wave depression and its margins (Figure 7a). Although the dominant grain-size class is consistent across these fine-scale domains, subtle differences in grain-size distribution shape (skewness and kurtosis) occur between sand-wave and inter-wave settings. These results indicate that EA is more uniform in mean grain size than WA and are consistent with a sand-wave field developed on broadly homogeneous medium sand, with fine-scale domain variability expressed primarily through changes in distribution shape rather than shifts in mean grain size.
Table 2
| Sample point | X coordinate | Y coordinate | Mean grain size | Sorting (Std. deviation) | Skewness | Kurtosis | Associated geomorphological feature |
|---|---|---|---|---|---|---|---|
| EA01 | 518627 | 9652799 | Medium sand | Poorly sorted | Negative | Leptokurtic | Inter-wave plain |
| EA02 | 519058 | 9653028 | Medium sand | Moderately sorted | Approximately symmetric | Platykurtic | Inter-wave plain (depression) |
| EA03 | 519229 | 9652300 | Medium sand | Poorly sorted | Negative | Leptokurtic | Sand-wave (crest) |
| EA04 | 518931 | 9652483 | Medium sand | Moderately sorted | Approximately symmetric | Leptokurtic | Inter-wave plain |
| EA05 | 518162 | 9652572 | Medium sand | Moderately sorted | Approximately symmetric | Mesokurtic | Inter-wave plain (depression) |
| EA06 | 518062 | 9653389 | Medium sand | Moderately sorted | Approximately symmetric | Leptokurtic | Sand-wave (crest) |
| EA07 | 518903 | 9652718 | Medium sand | Moderately sorted | Approximately symmetric | Mesokurtic | Inter-wave plain |
Granulometric characterization of Eastern Area.
4 Discussion
4.1 Bedforms patterns on a continental shelf with limited sediment supply
The WA and EA sectors of the Ceará Basin shelf display contrasting morphologies and sediment textures, reflecting the combined influence of limited sediment supply, hydrodynamic forcing, and inherited shelf morphology (Figure 8). Although both areas occur on the same continental shelf and are subject to similar regional oceanographic conditions, their bedform assemblages differ markedly. This contrast suggests spatial variability in sediment availability and in the frequency with which hydrodynamic forcing exceeds thresholds for sediment mobilization.
Figure 8
The semi-arid climate of northeastern Brazil results in intermittent river discharge, such that modern siliciclastic input to the shelf is spatially restricted and episodic (). Consequently, the sediment budget of the Ceará shelf is largely controlled by relict Pleistocene deposits and ongoing biogenic carbonate production, consistent with the mixed carbonate–siliciclastic character documented for this region and other semi-arid shelves (Testa and Bosence, 1999; ; ). In the WA, circular depressions characterized by lower backscatter and relatively finer-grained seabed suggest preferential trapping of sand within sinkhole-like negative-relief features. These morphologies may reflect drowned palaeokarst (doline-like) morphologies formed during last-glacial lowstands when parts of carbonate shelves were subaerially exposed (Ximenes Neto et al., 2018a), and subsequently modified by marine reworking, as documented elsewhere on the Brazilian shelf (, ). Pockmarks can produce similar clustered circular depressions in multibeam data; however, no seep-related indicators are evident in available surface data, and robust assessment would require subsurface information that is not available here (). In contrast, the eastern WA bench exhibits consistently higher backscatter and a granule-rich sediment cover, suggesting a rough terrace surface with limited fine-sediment cover and locally exposed hard-bottom substrate (Ximenes Neto et al., 2018a; ). The geomorphic partitioning of grain size in WA, with granule- to coarse-sand sediment on the bench and poorly sorted coarse–medium sand along adjacent flanks and the channel floor, is consistent with winnowing and coarse-lag development on elevated surfaces, coupled with localized redistribution and trapping of sand-sized material in adjacent lower-relief domains. These observations are in agreement with previous descriptions of the Ceará shelf, where relict siliciclastic sediments are reworked and mixed with modern carbonate fragments rather than buried by sustained terrigenous input (). The EA shows a simpler morphosedimentary organization dominated by extensive sand-wave fields. Grain-size analyses indicate a uniform medium-sand substrate across sand wave crests and inter-wave areas, with only subtle variations in kurtosis and skewness. This homogeneity is compatible with a common sediment source and limited modern replenishment, whereas the presence of sand wave and superimposed dunes relief indicates ongoing sediment reworking of existing sediment bodies. Comparable sand-wave fields on sediment-limited shelves have been interpreted as products of reworking of lowstand sand sheets or palaeo-deltaic deposits, where present-day processes maintain and reshape existing sediment bodies rather than generating new supply (; ). In this context, the EA sand waves are most plausibly interpreted as relict-derived features that remain morphodynamically active under modern hydrodynamic conditions (; Simarro et al., 2015; Ximenes Neto et al., 2018a).
Hydrodynamic forcing on the Ceará shelf is primarily driven by the NBC, trade-wind-induced longshore flows, and tides. Observations and modeling indicate that bottom currents intensify seasonally, reaching near-bottom magnitudes of up to 0.35 ms-1, in the rainy season, and 0.50 ms-1, in the dry season (Silva Dias et al., 2018). WAVERYS first-order wave-orbital estimates further indicate that energetic wave conditions can produce near-bed velocities capable of mobilizing sand-sized sediment in both areas, with P99 ub values at representative depths reaching 0.36 ms−1, in the WA, and 0.39 ms−1, in the EA (Supplementary Figure 1). Together, these current magnitudes and wave-orbital estimates indicate that hydrodynamic forcing can exceed critical thresholds for incipient motion of medium to coarse sand during energetic conditions (Testa and Bosence, 1999; Vital, 2014), supporting the development and maintenance of dunes and sand waves on mixed carbonate–siliciclastic shelves. In the WA, the SW–NE-oriented channel and its asymmetric morphology are consistent with alongshore currents that locally scour the substrate and redistribute sediment along channel margins. Elevated slopes and roughness along bench scarps and channel flanks further suggest intermittent sediment transport and erosion rather than continuous deposition, a pattern typical of sediment-starved outer shelves influenced by episodic high-energy events (; ). Sediment threshold-exceedance estimates at representative WA depths also indicate that mobility potential is spatially and texturally partitioned. Medium sand, observed in the channel-floor/thalweg domain and locally along bench flanks, shows the highest threshold exceedance, reaching 18–22%, under WAVERYS. Coarse sand, associated with bench-slope and scarp-adjacent depression domains, shows lower but still episodic exceedance of 11–18%, whereas granule-grade sediment from the bench interior is rarely mobilized, with exceedance restricted to 1–3% (Supplementary Figure 2). This textural gradient supports a WA seabed characterized by localized reworking and winnowing of sand-sized fractions along the channel, flanks, and scarps, while coarser bench-top substrates and inherited morphology are more likely to persist between high-energy events. In the EA, sand-wave orientation (SW–NE) and flank asymmetry (steeper northwestern sides) are compatible with prevailing northwestward alongshore flow (Teixeira and MaChado, 2013; Silva Dias et al., 2018) and with combined wave–current flows acting on a shallow sandy seabed (Ximenes Neto et al., 2018b; ). Superimposed dunes observed on sand-wave crests and in inter-wave troughs indicate bedform development and sediment mobility across multiple spatial scales (; ). The near-orthogonal orientation of these superimposed bedforms relative to the main sand-wave field suggests that they may record an additional hydrodynamic component, rather than only the flow responsible for maintaining the primary sand waves. This geometry is consistent with a secondary or event-modulated forcing component (; ), such as along-crest secondary flows (; ), storm-wave-driven oscillatory motion (; Wu et al., 2023), or bidirectional tidal reworking (; ). WAVERYS wave-direction results indicate that energetic waves in the EA are mainly from the E–ENE and propagate toward W–WSW (Supplementary Figure 4), providing a plausible oblique wave-related forcing component relative to the alongshore flow inferred from the primary sand-wave asymmetry. However, future work should include local current measurements or repeated bathymetry surveys to resolve the formative mechanism of these superimposed large dunes. The broadly homogeneous medium-sand substrate is consistent with wave-assisted mobilization during energetic conditions, with WAVERYS threshold-exceedance estimates for medium sand at representative EA depth reaching 26.3–31.8% (Supplementary Figure 2), suggesting recurrent event-driven reworking rather than continuous fair-weather migration. Comparable ridge–dune hierarchies on other sediment-limited shelves have been associated with episodic mobilization during high-energy events, rather than persistent migration under fair-weather conditions (; ).
Inherited morphology also exerts a strong control on bedform organization, particularly in the WA. The channel negative relief is compatible with a drowned platform incised-valley/palaeochannel template developed during low sea-level stands, when large sectors of the northeastern Brazilian shelf were subaerially exposed and fluvial valleys were excavated. Such features can be preserved through Holocene transgression where sedimentation rates are low and reworking dominates, limiting burial and maintaining inherited morphology (Vital et al., 2010b; ). Comparable shelf-crossing incised valleys on the SAC, such as the Coreaú incised valley system in the Ceará Basin (Ximenes Neto et al., 2021), the Apodi–Mossoró (Vital et al., 2010a) and Açu valleys () in the Potiguar Basin, show similar valley-scale geometries and illustrate how valley planform and segment changes may be guided by antecedent topography and inherited structural fabrics (Tavares et al., 2022). In these analogues, post-transgressive hydrodynamic forcing, such as winds, tides, waves, and along-shelf currents, contributes to rework and redistribute sediments within and along valley margins, providing a plausible mechanism for the asymmetric morphology observed on the WA channel, particularly where flows become locally channelized within valley corridors (). The WA flat-bench surface bounded by sharp scarps may correspond to a terrace/erosional ravinement surface formed during lowstand–transgressive reworking on a sediment-limited outer shelf (; ). The spatial coincidence between higher backscatter patches and bench–scarp morphology suggests that present-day sediment texture is strongly influenced by seafloor relief, promoting coarse lag accumulation and only patchy fine-sediment veneers on the bench, with finer material preferentially trapped in adjacent lows under ongoing hydrodynamic reworking (). In the EA, sand-wave dimensions, spacing, and relief are comparable to sand-bank systems described on other continental shelves; however, the absence of clear indicators of sustained lateral migration in the mapped morphology suggests a system dominated by reworking and morphological persistence rather than rapid wave translation (; ).
Taken together, the WA–EA comparison highlights how sediment limitation, hydrodynamic forcing, and antecedent morphology interact to generate distinct bedform fields on the same shelf. The WA comprises a heterogeneous mosaic of inherited surfaces, localized sediment traps, and hydrodynamically controlled sediment textures, whereas the EA hosts more laterally continuous sand-wave and superimposed dunes fields developed on a relatively homogeneous substrate. The transition from deeper outer-shelf settings in the WA (~45 m) to the shallower EA (~35 m) suggests a depth-related modulation of near-bed hydrodynamic forcing, affecting the frequency with which wave-orbital motions and combined wave–current flows exceed transport thresholds. Under WAVERYS, median wave-induced ub is approximately 1.6 times higher in the EA than in the WA at representative depths (Supplementary Figure 3), consistent with stronger estimated near-bed wave forcing expected over the shallower shelf. This supports more frequent wave-assisted reworking in the shallower sand-wave field, thereby favoring its development and maintenance. In deeper or morphologically sheltered sectors of the WA, inherited features and coarser substrates are more likely to be preserved between high-energy events, while finer-grained sediments may accumulate or be reworked locally. While the total mobilization potential may be higher under combined wave-current conditions than estimated from wave-only orbital velocities, this should be quantified in future work using local coupled wave-current hydrodynamic and sediment-transport modeling. These results illustrate that sediment-limited shelves can simultaneously host stable inherited structures and active bedforms, emphasizing the importance of local morphology when interpreting bedform dynamics and seabed evolution.
4.2 Ecological and management implications of bedform–sediment patterns on the SAC shelf
The mapped bedform and sediment domains on the SAC shelf provide a physically based framework for interpreting potential habitat heterogeneity and for supporting marine spatial planning (MSP), monitoring, and offshore development. On sediment-limited shelves, seabed morphology and substrate type are widely recognized as first-order controls on benthic habitat distribution, particularly where biological data coverage is sparse or uneven (; ). In this context, the EA and WA domains represent distinct physical templates that may influence the spatial organization of benthic communities, while also constraining the suitability and risk associated with human activities. Because direct biological observations are not available for the study sites, ecological implications are presented as hypotheses based on physical proxies.
Rhodolith beds, Halimeda banks, and other mesophotic bioclastic deposits are common on the middle to outer shelf of northeastern Brazil and across the Western South Atlantic (; ). These biogenic substrates are known to support structurally complex habitats and diverse assemblages of fishes and benthic invertebrates, and they have been increasingly recognized as conservation-relevant features. In the WA, the relatively higher-backscatter bench and SSS-identified granular patches near channel margins are consistent with rougher or semi-consolidated seabed conditions and coarse sediment, which may provide attachment surfaces for coralline algae and sessile invertebrates (Tosi et al., 2017; ). In contrast, adjacent lower-backscatter depressions and channel-floor domains, where finer sediments accumulate, may favor communities associated with softer substrates and detritus-based trophic pathways (). This juxtaposition of coarse and fine seabed conditions over short spatial scales is consistent with habitat heterogeneity commonly reported from mixed carbonate–siliciclastic shelves, where turbulence over coarse patches inhibits fine-sediment deposition and maintains substrate contrasts (; ). In the EA, sand-wave crests exhibit higher backscatter and pronounced bedform relief, whereas the flanks and inter-wave areas exhibit smoother morphologies and a more intermediate acoustic character. Although grain-size analyses indicate broadly uniform medium sand across these domains, differences in bedform expression and roughness imply variability in near-bed flow and substrate stability. Sand-wave crests may experience more frequent reworking and limit long-term colonization by sessile organisms, whereas more stable flanks and inter-wave plains may be more favorable for benthic colonization (; ). However, in the absence of direct biological observations, such interpretations remain tentative and should be tested through targeted ecological surveys.
Habitats associated with mesophotic bioclastic deposits, including rhodolith beds, have been identified as priority conservation targets due to their ecological significance and sensitivity to physical disturbance (; ; ). On the Western South Atlantic shelf, these habitats can function as biodiversity reservoirs and potential refugia (; ). Accurate spatial delineation of hardgrounds and coarse bioclastic domains, such as those inferred for parts of the WA, can therefore inform zoning schemes that balance conservation with economic activities. MSP strategies may consider these domains as candidates for conservation or restricted-use zones, particularly where sediment replenishment is limited and recovery from disturbance may be slow.
High-resolution mapping of bedforms and acoustic facies also provides a critical baseline for MSP and offshore infrastructure planning on the SAC shelf. Sand waves, ripples, and associated bedforms can undergo episodic reworking during energetic events, leading to burial or exposure of seabed infrastructure (). In the EA, sand-wave orientation, spacing, and flank asymmetry suggest preferential sediment-transport pathways that should be considered when routing pipelines, cables, or other installations. Conversely, in the WA, channel and circular depressions may function as sediment traps or zones of focused scouring, implying potential risks of differential burial or exposure depending on event-driven sediment redistribution (). Incorporating such geomorphic information into MSP frameworks can reduce infrastructure vulnerability and maintenance costs.
The sediment-limited, mixed carbonate–siliciclastic nature of the SAC shelf implies that modern processes primarily rework ancient substrates, and that physical disturbances may persist over long timescales due to limited sediment supply (; ; ; ). Environmental impact assessments for activities such as bottom trawling, dredging, or offshore extraction should explicitly account for the vulnerability of hardgrounds, biogenic substrates, and morphologically stable bedforms. Because the sediment samples used in this study were targeted at representative acoustic and geomorphological domains, rather than arranged as a dense and spatially regular sampling grid, the sedimentological results should be interpreted as point-based constraints on mapped domains rather than continuous sediment-distribution surfaces. Long-term monitoring programs would benefit from integrating repeated MBES and SSS surveys with denser and more systematic sediment sampling, biological observations, and sediment-trap deployments. Such an approach would support future spatial prediction of sediment variability while allowing changes in bedform morphology, sediment texture, and associated benthic communities to be tracked through time, as well as helping to quantify sediment fluxes, seasonal variability, and the relative contributions of carbonate-bioclastic production and modern continental siliciclastic input to seabed sediment distribution.
At a broader scale, the SAC shelf forms part of the Western South Atlantic continental margin, where mesophotic reefs and rhodolith beds are widespread and increasingly incorporated into national and regional management frameworks. Policies addressing marine protected area designation and offshore licensing should explicitly recognize the distribution of geomorphological and sedimentary domains that underpin these ecosystems. Conceptual models describing stages of bioclastic deposit development emphasize the dual role of mesophotic biota as both sediment producers and ecological refuges; mapping these stages using geophysical proxies may help prioritize areas that are particularly vulnerable to mechanical disturbance or environmental change.
In summary, EA and WA bedform–sediment patterns illustrate how sediment limitation, hydrodynamic forcing, and inherited morphology interact to produce distinct physical habitat templates on the SAC shelf. While direct ecological data are required to confirm biological responses, the physical patterns documented here provide a robust baseline for interpreting habitat heterogeneity and for supporting MSP, monitoring, conservation, and infrastructure planning in semi-arid continental shelf settings.
5 Conclusion
This study provides a high-resolution geomorphological and sedimentological baseline for two outer- to middle-shelf sectors (EA and WA) of the Ceará Basin on the semi-arid continental shelf of northeastern Brazil. By integrating multibeam bathymetry, backscatter, side-scan sonar imagery, and sediment sampling, we demonstrate how sediment limitation, hydrodynamic forcing, and inherited shelf morphology interact to generate contrasting bedform fields within the same regional oceanographic setting.
The WA is characterized by a heterogeneous assemblage of inherited and morphologically controlled features, associated with pronounced contrasts in acoustic response and sediment texture. These features reflect the preservation of relict surfaces, hardgrounds, and localized fine-sediment accumulation. In this sector, antecedent morphology exerts a first-order control on present-day sediment distribution, resulting in a mosaic of relatively stable seabed domains where modern sediment reworking is spatially constrained. In contrast, the EA exhibits a simpler morphosedimentary organization dominated by extensive sand-wave fields developed over a relatively homogeneous medium-sand substrate, representing a more hydrodynamically responsive sector of the shelf, where wave–current interactions promote bedform maintenance and modification across multiple spatial scales. The comparison between WA and EA highlights that sediment-limited shelves are not uniformly inactive or relict-dominated but instead can host a spectrum of seabed states ranging from stable inherited morphologies to potentially reworked bedform fields. Depth-related changes in hydrodynamic exposure, combined with local antecedent topography, appear to define thresholds between these contrasting regimes on the Ceará shelf.
Beyond their geomorphological significance, the mapped bedform–sediment domains provide a physically grounded framework for interpreting potential habitat heterogeneity and supporting MSP, environmental monitoring, and offshore infrastructure management on the semi-arid continental shelf. In sediment-limited settings, disturbances to hardgrounds, coarse substrates, or others morphologically stable features may persist over long timescales, underscoring the importance of accurate seabed mapping as a baseline for impact assessment and adaptive management. Overall, this study demonstrates the value of integrated, high-resolution geophysical data for resolving the spatial complexity of sediment-limited continental shelves. The Ceará Basin shelf emerges as a system where inherited morphology and modern processes coexist, offering important insights for both process-based interpretations of shelf evolution and applied management in semi-arid marine environments.
Statements
Data availability statement
The data analysed in this study were obtained from PETROBRAS and are subject to data-use and distribution restrictions. The original contributions presented in the study are included in the article and/or Supplementary Material. Requests to access these datasets should be directed to the corresponding author.
Author contributions
LL: Conceptualization, Data curation, Formal analysis, Investigation, Methodology, Software, Visualization, Validation, Writing – original draft, Writing – review & editing. NM: Conceptualization, Funding acquisition, Project administration, Resources, Supervision, Writing – review & editing.
Funding
The author(s) declared that financial support was received for this work and/or its publication. This study was financed in part by the Coordenação de Aperfeiçoamento de Pessoal de Nível Superior – Brasil (CAPES; Finance Code 001). It was conducted within the framework of the project “Study of the Geotechnical Behaviour of Calcareous Soils and the Shallow Carbonate Section of the Brazilian Coast for Offshore Wind Farms,” developed through a partnership between the Federal University of Ceará (UFC) and PETROBRAS under the National Electric Energy Agency (ANEEL) R&D program (PD-00553-0069/2021). NMA acknowledges the support of the Brazilian National Council for Scientific and Technological Development (CNPq) (Process 313128/2025-1).
Acknowledgments
We are grateful to PETROBRAS for providing the MBES and SSS data.
Conflict of interest
The author(s) declared that this work was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.
The authors declare that this study received funding from Petróleo Brasileiro S.A. (PETROBRAS). The funder had the following involvement in the study: data collection.
Generative AI statement
The author(s) declared that generative AI was not used in the creation of this manuscript.
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Supplementary material
The Supplementary Material for this article can be found online at: https://www.frontiersin.org/articles/10.3389/fmars.2026.1841106/full#supplementary-material
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Summary
Keywords
bedform mapping, Brazilian Equatorial margin, bathymetry, submarine geomorphology, side-scan sonar
Citation
Leite LHJ and Maia de Almeida N (2026) Seafloor morphology and sedimentology in the Brazilian semi-arid continental shelf: a high-resolution geophysical baseline from the Ceará Basin. Front. Mar. Sci. 13:1841106. doi: 10.3389/fmars.2026.1841106
Received
27 March 2026
Revised
03 June 2026
Accepted
08 June 2026
Published
06 July 2026
Volume
13 - 2026
Edited by
Arthur C. Trembanis, University of Delaware, United States
Reviewed by
Fermin Palma, National Scientific and Technical Research Council (CONICET), Argentina
Ruiqing Liu, East China Normal University, Shanghai, China
Updates
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© 2026 Leite and Maia de Almeida.
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*Correspondence: Narelle Maia de Almeida, narelle@ufc.br
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