Environmental Factors Driving Carbonate Distribution in Marine Sediments in the Canary Current Upwelling System

Abstract

This study illustrates the complex interaction between environmental parameters and carbonate distribution in marine sediments along the Tarfaya–Boujdour coastline (26–28° N) of Northwest Africa. Analysis of 21 surface sediment samples and their associated bottom water properties (salinity, temperature, dissolved oxygen, nutrients) reveals CaCO₃ content ranging from 16.8 wt.% to 60.5 wt.%, with concentrations above 45 wt.% occurring in multiple stations, especially in nearshore deposits. Mineralogy indicates a general decrease in quartz, with an arithmetic mean and standard deviation of 52.5 wt.% ± 19.8 towards the open sea, and an increase in carbonate minerals (calcite ≤ 24%, aragonite ≤ 10%) with depth. Sediments are predominantly composed of fine sand (78–99%), poorly classified, with gravel content reaching 6.7% in energetic coastal stations. An inverse relationship between organic carbon (0.63–3.23 wt.%) and carbonates is observed in upwelling zones, correlated with nitrate concentrations exceeding 19 μmol/L. Hydrological gradients show temperatures from 12.41 °C (offshore) to 21.62 °C (inshore), salinity from 35.64 to 36.81 psu and dissolved oxygen from 2.06 to 4.21 mL/L. The weak correlation between carbonates and depth (r = 0.10) reflects the balance between three processes: biogenic production stimulated by upwelling, dilution by Saharan terrigenous inputs, and hydrodynamic sorting redistributing bioclasts. These results underline the need for models integrating hydrology, mineralogy and hydrodynamics to predict carbonate dynamics in desert margins under upwelling.

1. Introduction

Marine carbonates and the oceanic CO₂-carbonic acid system are increasingly recognized for their dual functions as both a source and a carbon sink in the global carbon cycle on different time scales. Due to their impact on atmospheric CO₂ dynamics and future climate change, the study of these components has become a focal point for scientific investigation and modeling efforts [1,2,3]. The preservation and dissolution of carbonates in marine sediments are generally controlled by the interplay of several factors, such as deep-sea saturation state (Ω), which governs mineral stability thresholds [4]; metabolic dissolution induced by the degradation of organic matter [5]; intrinsic mineralogical solubility [6], where aragonite dissolves more readily than magnesium-rich calcite, which is itself more soluble than magnesium-poor calcite, a solubility that increases with Mg content and pressure [7,8]. There are also other influencing factors, such as hydrodynamic particle sorting, with currents and waves removing fines, concentrating robust bioclasts and thus affecting the carbonate fraction shortly after deposition, as well as dilution by terrigenous inputs [9,10]. Today, these processes are disrupted by anthropogenic ocean acidification, which reduces carbonate saturation, elevates critical horizons (lysocline and carbonate compensation depth, CCD) and amplifies dissolution, even at shallow depths [11,12].

Michel et al. (2019) proposed a classification of carbonate factories on the continental shelf, and identified four distinct categories: the Biochemistry factory, the Photozoan-T factory, the Photo-C factory, and the Heterozoan-C factory [13]. The Biochemistry factory refers to carbonate production driven by microbial activity. The photozoan T factory, on the other hand, is powered by calcifiers that are dependent on temperature and light; the dominant taxa include coccolithophores, particularly Emiliania huxleyi, Gephyrocapsa oceanica, Calcidiscus leptoporus, and Helicosphaera spp., which frequently account for 40–60% of the export of particulate inorganic carbon in upwelling zones [2]. Furthermore, aragonite is produced by large-shelled pteropods, which include butterfly snails like Limacina spp.; however, they are susceptible to dissolving below saturation depths [14].

The photo-C factory is associated with photic environments but characterized by moderate primary productivity; it typically involves planktonic (Globigerina bulloides, G. ruber) and benthic foraminifera that contribute to CaCO₃ flux under photic conditions. Finally, the heterozoan-C factory consists of light-independent organisms, typically found in colder or more nutrient-rich environments, such as bryozoans and foraminifera.

The Northwest African margin constitutes a distinctive natural setting where various environmental parameters interact to govern carbonate sedimentation. This area is characterized by one of the world’s most productive upwelling systems, the Canary Current (CC) upwelling system [15,16] with significant terrigenous inputs from the Sahara Desert [9,17], creating complex interactions that shape sediment composition and distribution. Previous research on the Northwest African margin has outlined general trends in sediment composition, highlighting the prevalence of biogenic sands and terrigenous inputs [10,18,19, 20]. Specifically, these studies have identified quartz of Saharan eolian origin as a dominant lithogenic component in shelf sediments, reflecting the strong influence of arid climate and wind transport [9,10,17]. Concurrently, the biogenic carbonate fraction is recognized as significant, primarily composed of calcifying organisms like foraminifera and coccolithophores, especially in areas influenced by upwelling-driven productivity [20,21,22].

Among the different environmental settings where carbonate production processes operate, the Northwest African margin stands out due to its unique combination of intense upwelling, terrigenous input, and varying biogenic activity. This region provides a particularly dynamic context for investigating how carbonate factories respond to both natural and anthropogenic forcing.

As an area subjected to intense coastal upwelling, the Canary Current (CC) upwells cold waters, rich in nutrients (NO₃⁻, PO₄³⁻, Si(OH)₄) and dissolved CO₂, low in oxygen and undersaturated in carbonates (Ω < 1) [23,24]. These waters profoundly modify the physicochemical properties of bottom waters and condition the diagenesis of surface sediments as follows: microbial degradation of organic matter releases interstitial CO₂, forms carbonic acid, and lowers pH, causing early dissolution of carbonates as early as Ω < 1 [6]. On the Northwest African margin, the main carbonate producers are coccolithophores (e.g., Emiliania huxleyi, Gephyrocapsa spp.), planktonic (Globigerina bulloides), and benthic (Bolivinella seminuda) foraminifera, as well as pteropods (family Limacinidae) and coastal bivalves, whose abundant calcareous shells and foraminiferal tests (hard external skeletons) contribute significantly to carbonate accumulation [5,25].

The study area is located in the extreme northwestern region of Africa (Tarfaya–Boujdour area), between latitudes 28° N and 26° N. This zone is characterized by an arid climate dominated by persistent northwest trade winds and the Saharan Air Layer (SAL). Coastal upwelling, driven by the interplay of wind-forced Ekman transport, the southward-flowing Canary Current (averaging speeds of 0.1–0.3 m/s) [26], and Earth’s rotation, plays a critical role in shaping regional oceanography [27,28,29]. The Canary Current’s velocity further modulates sediment transport and nutrient distribution, influencing marine productivity [16].

The morphology of the continental shelf features a shallow platform (<150 m depth) (Figure 1), extending up to 140 km offshore north of Cape Blanc. However, its width diminishes near Cape Ghir (around 25 km) and Cape Yubi (approximately 75 km northward) [30,31]. The study area acts as a dynamic turbulence channel, influenced by the interaction of the upwelling systems of Boujdour and Cape Juby (Tarfaya). These upwelling systems are particularly active in autumn and spring, driven by the intensification of trade winds and atmospheric pressure gradients. According to [32], Cape Juby (Tarfaya) has seasonal upwelling, strongly influenced by climatic factors, such as wind speed and direction. The Boujdour upwelling is typified by the development of filaments that carry nutrient-rich waters offshore [32,33].

This study combines mineralogical (XRD), geochemical (Total Organic Carbon (COT), CaCO₃ content), and granulometric data with physical and chemical bottom water data (temperature, salinity, dissolved oxygen, nutrients). Its main objective is to decipher the mechanisms governing the distribution of carbonates in marine sediments of this portion of the NW African margin of Canary Current upwelling system.

2. Materials and Methods

2.1. Field Surveys

Two oceanographic surveys were conducted as part of the oceanographic and environmental monitoring program of the INRH (National Institute of Fisheries Research) and the EAF-Nansen program (2017–2023). The EAF-Nansen Program is a joint initiative between the Food and Agriculture Organization of the United Nations (FAO) and Norway, with scientific support from the Norwegian Institute of Marine Research (IMR).

During these missions, multiple transects perpendicular to the coastline were carried out, allowing for the collection of approximately 21 surface sediment samples (

Table 1. Location of stations, depths, and types of collected samples.

Station	Longitude	Latitude	Depth (m)	Collected Samples
1	12°45′54.0” W	28°15′56.9” N	101	Sediments and water
2	13°06′53.3” W	28°19′46.9” N	500
3	12°55′39.0” W	27°57′51.0” N	29
4	13°03′01.0” W	28° 00′35.9” N	44
5	13°09′54.7” W	28° 07′02.6” N	101
6	13°14′15.0” W	28°11′07.8” N	510
7	13°15′15.0” W	27°37′08.4” N	20
8	13°24′26.5” W	27°53′32.4” N	550
9	13°22′25.7” W	27°20′18.6” N	20
10	13°27′34.2” W	27°16′40.8” N	40
11	13°26′39.0” W	27°11′01.7” N	18
12	13°29′46.2” W	27°11′13.2” N	50
13	13°38′04.2” W	27°14′17.5” N	100
14	13°42′14.4” W	27°15′35.3” N	559
15	13°29′58.9” W	27° 01′ 35.4” N	20
16	13°35′42.0” W	27°05′33.0” N	72
17	13°36′37.8” W	26°43′21.7” N	18
18	14°09′55.1” W	26°45′42.1” N	520
19	14°14′43.1” W	26°26′21.1” N	20
20	14°29′30.1” W	26°11′08.5” N	16
21	15°04′02.3” W	26°11′42.0” N	513

), using a Van Veen grab. Sampling was conducted aboard the research vessels Al Hassan Al Marrakchi (December 2021) and Dr. Fridtjof Nansen (March 2020, mission 2020401). The stations were located between 26.2° N and 28.3° N latitude, effectively covering the Tarfaya–Boujdour area, with depths ranging from 18 m (coastal stations) to 725 m (continental slope).

2.2. Analysis of Bottom Water Physicochemical Parameters

The physicochemical properties of bottom water, including salinity, temperature, and dissolved oxygen, were investigated using vertical profile data obtained from a CTD SBE 911 probe equipped with an oxygen sensor.

Bottom water samples were also collected using Niskin bottles attached to the CTD-Rosette system and analyzed for nitrate (NO₃⁻), phosphate (PO₄³⁻), and silicic acid (Si(OH)₄). Nutrient concentrations were measured using a SEAL QuAAtro39 continuous segmented flow analyzer aboard the R/V Dr. Fridtjof Nansen. Analytical precision, based on triplicate analyses, was ±0.05 µM for nitrate, ±0.01 µM for phosphate, and ±0.1 µM for silicic acid. Detection limits were 0.02 µM, 0.005 µM, and 0.05 µM, respectively.

Dissolved oxygen in bottom water was also measured by Winkler titrations to validate sensor data from the CTD.

2.3. Geochemical Analysis

2.3.1. Mineralogical Composition (XRD)

The mineralogical composition of the bulk sediment samples was analyzed using X-ray powder diffraction (XRD). A Bruker D8 ADVANCE ECO diffractometer (Bruker AXS GmbH, Karlsruhe, Germany), equipped with a LynxEye detector and Ni-filtered Cu Kα radiation (λ = 1.5404 Å) was employed for this analysis. Scans were conducted over 2θ angles ranging from 10° to 70° at a scan rate, using a step size of 0.067°. Mineral identification was carried out with the High Score software version 3.0.

Miller indices are used in this study to identify atomic lattice plane families. Each triplet (hkl) represents a group of planes that are evenly spaced and parallel. Reciprocal spacing rules relate the distance between these planes (known as interplanar spacing) to the Miller indices, which may be determined using Bragg’s law:

nλ = 2d_hkl sinθ

λ is the X-ray wavelength, θ is the Bragg angle, and n is the diffraction order. Bragg defines constructive interference as when the path difference between waves scattered by adjacent planes equals an integer multiple of the wavelength.

Using the Miller index and Bragg’s law, you can confirm the presence of distinct mineralogical phases on the basis of XRD peak positions.

2.3.2. Organic Matter Content Determination (LOI)

Organic matter content (wt.%) was determined using the Loss on Ignition (LOI) protocol of Nelson and Sommers (1996) [34]. Each sample is dried, weighed, and heated to 550 °C for 4 h to burn the organic matter. LOI of the studied sediment samples is calculated by comparing the sample weight before and after ignition at 550 °C, where its calculation represents the OM content in the sediment samples.

OM content (wt.%) can be calculated as follows:

$$\%OM={\displaystyle \frac{Beforeignitionweight\left(\mathrm{g}\right)-Afterignitionweight\left(\mathrm{g}\right)}{Beforeignitionweight\left(\mathrm{g}\right)}}\times 100$$

2.3.3. Carbonate Content Determination

The Bernard calcimeter method determines carbonate content through a straightforward chemical reaction: carbonates react with hydrochloric acid (HCl) to release carbon dioxide (CO₂). First, a finely ground sample (particle size < 2 mm) is placed in a sealed reactor containing a 10% HCl solution. The CO₂ released during the reaction increases the internal pressure, which is accurately measured using a calibrated manometer. This pressure value is then converted into a calcium carbonate (CaCO₃) percentage using standardized calculations (AFNOR standard) [35].

2.4. Granulometric Analysis

A Mastersizer 3000 laser diffraction analyzer (Technology platform, Chouaib Doukkali University, El Jadida, Morocco) was used to quantify particle sizes. The particles have a refractive index of 1.555 and range in size from 0.375 to 3500 μm. Mud-rich samples are suspended in distilled water for 24 h prior to testing. Ultrasound is utilized to guarantee that the fine particles are evenly distributed throughout the measurement. The raw weights for each grain-size class were converted to weight percent for each sub-sample and classified into three sizes: mud (silt and clay), sand, and gravel. The median and the percentage of gravel, sand, and mud were plotted using GRADISTAT (version 4.0).

2.5. Map Generation

The study area map was generated using Surfer with data from the GEBCO_2024 Grid and Ocean Data View (ODV) version 5.6.4 [36]. Maps showing the distribution of carbonates, sediment granulometry, and hydrological parameters were generated using Geographic Information System (QGIS) Version 3.28 [37].

2.6. Statistical Analyses

Principal component analysis (PCA) was performed using R statistical software (version R-4.4.1). We applied Spearman’s rank correlation to assess the strength and direction of monotonic relationships between all study variables, irrespective of their distribution or linearity. Spearman’s coefficient (ρ) ranges from −1 (perfect negative association) to +1 (perfect positive association), with 0 indicating no monotonic relationship.

The resulting correlation matrix is color-coded by sign (blue = positive, red = negative) and annotated for significance, where statistical significance is declared at p < 0.05—values ≥ 0.05 are denoted “ns” (not significant). For ease of interpretation, we classify the absolute magnitude of ρ according to widely used thresholds—very weak (<0.20), weak (0.20–0.39), moderate (0.40–0.59), strong (0.60–0.79), and very strong (≥0.80) [38].

3. Results

3.1. Bottom Water Properties

The distribution of bottom water properties, including temperature, salinity, dissolved oxygen (DO), and nutrient concentrations (nitrates and phosphates), is illustrated in Figure 2.

Temperature in the bottom waters showed significant variability, ranging from 12.41 °C in colder offshore zones to 21.62 °C in coastal areas. The highest temperature was specifically recorded in the Boujdour region, which could indicate a local coastal influence or particular hydrographic conditions in this area.

The salinity distribution in bottom waters revealed a typical coast-to-offshore gradient, with values varying between 35.64 and 36.81 psu. Maximum salinities were also observed in the coastal zone of Boujdour (Figure 2b), suggesting evaporation processes or a specific mixing of coastal water masses.

Regarding dissolved oxygen (DO) (Figure 2c), concentrations exhibited a wide range. Values dropped as low as 2.06 mL/L in certain areas, indicating conditions of low oxygenation. Conversely, the highest concentrations reached 4.72 mL/L, generally in shallower and potentially better-ventilated bottom waters. This variability highlights the presence of oxygenation gradients, with zones tending towards hypoxia, characteristic of continental margins where oxygen consumption is high due to organic matter degradation and ventilation is limited.

For nitrates (NO₃⁻) (Figure 2e), concentrations showed maximum values reaching 17.56 µmol/L. These high concentrations were observed primarily in the deep offshore waters of Tarfaya and northern Boujdour, areas often associated with intense organic matter remineralization under low oxygen conditions.

Phosphate (PO₄³⁻) values (Figure 2f) were generally low, ranging from 0.03 µmol/L to 1.10 µmol/L. A notable feature is the co-occurrence of low phosphate concentrations with high nitrate concentrations offshore Tarfaya, which may suggest preferential utilization or phosphate limitation in the local biogeochemical cycle. Conversely, in the Laâyoune region (Figure 2e,f), we recorded simultaneous minimum concentrations for both nitrates and phosphates, potentially indicating a zone of high biological consumption or low input of these two essential nutrients.

3.2. Sediment Granulometric Distribution

The granulometric analysis of superficial sediments in the studied area between Tarfaya and Boujdour (Figure 3) reveals the predominance of fine sand in a marine environment [39], characterized by poorly sorted sediments. A clear dominance of the sandy fraction (Figure 3b), with percentages ranging from 78 wt.% to 99 wt.%, while the muddy fraction is generally below 28 wt.% (Figure 3c), and the gravel fraction remains very low (wt.%) (Figure 3a). The sediments, classified as slightly gravelly muddy sand, exhibit mean grain sizes ranging from 123.2 to 420.1 μm, predominantly within the fine sand category. The highest percentage of gravel (6.7 wt.%) is observed at station 3 (Tarfaya zone) (Figure 1), where the sediment is classified as gravelly sand (Figure 3a). The modal distributions are complex, and the variable skewness indicates dynamic coastal sedimentary processes influenced by marine currents and the regional hydrodynamic conditions typical of the narrow continental shelf of the NW African margin.

3.3. The Distribution of Carbonates and Organic Carbon

The carbonate content in the studied sediments ranges between 16.8 and 60.5 wt.%, with an average of approximately 32 wt.%. Higher concentrations (>45%) are observed in shallow stations within the Tarfaya and Laayoune regions and offshore Cap Boujdor (Figure 4a). When CaCO₃ shows low content, organic carbon (Corg) and NO₃⁻ show high concentrations, as illustrated in Figure 2e and Figure 4a,b.

Organic carbon values vary between 0.63% and 3.23%, with a mean value of 1.7%, indicating significant variability in productivity conditions and organic matter preservation influenced by coastal upwelling occurring in the Tarfaya and Boujdor regions (see Figure 1).

3.4. Mineralogical Assemblage

Figure 5a–c illustrate the general trend of mineral assemblages across stations from the inner shelf (0–30 m depth) and middle shelf (30–100 m depth) to the outer shelf (>100 m depth).

X-ray powder diffraction patterns of sediment reveal that the samples are principally composed of quartz (SiO₂) (2θ = 26.6°and 20.8°). The peaks of secondary phases are attributed to the carbonate minerals, the principal one is calcite CaCO₃; (Ca_1−xMg_x) CO₃ with the characteristic peaks at 2θ = 29.3°, 35.94°, 39.91°,43.71°, 49.11°, 39.38°, 43.12°, 47.47°, 48.47°, and 23.01° indicating corresponding indices (104), (110), (113), (202), (116), (113), (202), (018), (116), and (012), respectively [40], the second one is Aragonite (CaCO₃). Minor peaks related to clay minerals such as Kaolinite (Al₂Si₂O₅(OH)₄), illite (K_xAl₂(Si_4−xAl_x)O₁₀(OH)₂) and a weak characteristic peak of montmorillonite (Na,K)_x(Al_2−xR_x)Si₄O₁₀(OH)₂ clay was observed at 2θ = 5.8 °and 19.8° assigned to the (001) and (020) plan reflection [41]. Furthermore, the Feldspar, Halite (NaCl), and Dolomite (CaMg(CO₃)₂) (2θ = 30.9° and 41°) are also identified.

The mineralogical distribution shows significant variations with depth. The percentage of quartz decreases with depth (from 69 wt.% to 52 wt.%) (Figure 5a–c), reflecting depletion of terrigenous inputs. In contrast, the carbonate mineral content increases with depth, where the recorded percentage of calcite reached 24 wt.% in the outer shelf compared to the inner shelf, indicating a growing contribution from carbonate producing organisms.

A significant increase in aragonite passes from 5 wt.% to 10 wt.% suggesting better preservation at greater depths.

4. Discussion

4.1. Interaction Between Sedimentological and Hydrological Parameters

Complex interactions between physical processes, biogeochemical cycles, and external inputs are revealed by the correlations between sedimentological and hydrological indices found in the study area. The thermohaline gradient caused by coastal upwelling is seen in the strong negative correlation between depth and temperature (r = −0.94) and salinity (r = −0.95) (Figure 6). By bringing cold, nutrient-rich, and less salinized waters to the surface, this process encourages high primary productivity and planktonic organisms to produce carbonate (calcite and aragonite) [32,42]. However, the predominance of calcite, which is less soluble than aragonite, reduces the amount of carbonate that dissolves partially at depth [23,43].

This interplay of strong biogenic carbonate production associated with upwelling—which counteracts dissolution [6]—and dilution by quartz (up to 70% in coastal zones) (Figure 5a) from Saharan dust inputs—which artificially lowers carbonate proportions [9]—coupled with hydrodynamic processes, explains the weak correlation between CaCO₃ and depth (r = 0.10).

A slightly positive correlation is observed between CaCO₃ and gravel-sized sediments (r = 0.50). Coarse fractions generally have a higher carbonate content due to the presence of bioclastic debris, including shell fragments and skeletal remains of benthic organisms, as has been demonstrated in other shelf environments influenced by upwelling. For example, Milliman (1993) showed that bioclastic debris is the main source of carbonates in continental shelf deposits [44]. Similar trends have been observed along the Atlantic margin of Northwest African, where upwelling induced productivity enhances the contribution of biogenic carbonate remains to shelf sediments, further underscoring the significant role of bioclastic inputs in these coarse fractions [10,32].

Additionally, carbonates and aragonite have a positive correlation (r = 0.34), suggesting that aragonitic creatures such as mollusks and corals have contributed to the mixture [45].

Mud and organic carbon (Corg) have a positive connection (mud: r = 0.69), suggesting that oxidation is limited by quick burial in fine grained sediments. Organic matter leaching in high energy situations is seen in its negative association with sand (sand: r = −0.66) [46]. The remineralization of organic matter and the disintegration of siliceous tests (such as diatoms), which are characteristic of hypoxic zones, leads to an accumulation of nutrients (PO₄³⁻, Si(OH)₄) with increasing water column depth (bathymetry) (r = 0.51–0.64). This increase is a consequence of sustained nutrient release from organic matter decomposition combined with limited biological uptake and reduced vertical mixing in deeper, oxygen-depleted waters [46].

Finally, the diagenetic processes where interstitial brine evaporation prevents carbonate preservation are highlighted by the negative correlation between halite and CaCO₃ (r = −0.62) [45].

4.2. Carbonate Distribution: Biogenic Production, Dissolution, and Environmental Controls

The spatial distribution of carbonates (CaCO₃) in the study area is a direct reflection of a dynamic equilibrium between biogenic production, dissolution processes, external terrigenous inputs, and local hydro-environmental variables, with each of these drivers shaping the observed CaCO₃ patterns in the sediments.

Biogenic production and its influence on carbonates: The Canary Current’s coastal upwelling systems deliver cold, nutrient rich (PO₄³⁻, Si(OH)₄) waters [24]. According to [22] these upwelled waters are initially characterized by lower pH (~7.85–7.95) and aragonite saturation state (ΩAr ~1.5–2.5) due to elevated CO₂ solubility at low temperatures, they critically promote high primary productivity [22,23,47]. This leads to significant biogenic carbonate production by calcifying organisms, including coccolithophores (e.g., Emiliania huxleyi) and planktonic/benthic foraminifera (e.g., Globigerina bulloides) [5,42]. This constant biogenic input is a primary control, contributing to higher CaCO₃ concentrations in areas influenced by strong upwelling and elevated biological productivity [6,10,23].

Dissolution processes and their role in carbonate preservation: Our sampling sites (≤602 m) lie well above regional lysoclines (calcite: ~5000 m; aragonite: shallower), indicating that water column dissolution has a limited direct impact on the observed CaCO₃ distribution [48]. However, dissolution at the sediment–water interface and within porewaters significantly influences CaCO₃ preservation. Depth-dependent dissolution, driven by increasing hydrostatic pressure and CO₂ produced by microbial respiration, can decrease porewater pH and promote CaCO₃ solubility [6,23]. The aerobic oxidation of organic matter (OM) in oxic marine environments produces CO₂, which combines with water to form carbonic acid (H₂CO₃), leading to carbonate dissolution and localized acidification (CaCO₃ + CO₂ + H₂O → Ca²⁺ + 2HCO₃⁻) [6]; this reaction is dominant in shallow, well-oxygenated sediments and mixed water columns, influencing carbonate dissolution patterns in coastal zones [45]. Conversely, anaerobic processes like sulfate reduction (producing H₂S) and methanogenesis (producing CH₄) dominate organic matter degradation in anoxic environments (such as deep muds) [49,50]. These processes produce less CO₂ and actively preserve carbonate minerals (Calcite and Aragonite) by maintaining a higher porewater pH. Therefore, the spatial variability in redox conditions (oxic vs. anoxic) directly influences the balance between dissolution and preservation of CaCO₃ [49,50].

External Terrigenous Inputs as a Dilution Factor: Mineralogical analysis reveals quartz (69%) dominance, primarily originating from Saharan eolian inputs [51]. This high terrigenous input acts as a significant diluent, thereby reducing the relative proportion of biogenic CaCO₃ and strongly influencing its spatial distribution. This effect is particularly pronounced in areas heavily impacted by eolian dust, leading to lower observed CaCO₃ percentages despite potential biogenic production. Calcite (19–24%) and aragonite (5–10%) proportions consistently reflect the biogenic contribution, aligning with previous Northwest African margin studies [10,52]. The distribution of clay minerals also supports this origin, with illite and kaolinite dominating in coastal stations, while source studies confirm that illite, chlorite, and kaolinite originate from the erosion of the Anti-Atlas (onshore) and are transported by climatic and marine processes [53]. In the outer shelf, montmorillonite is more abundant, with its stability in cation-rich upwelling environments linked to pH and alkalinity changes resulting from the decomposition of organic matter [46,54].

Local Hydro-environmental Variables (Hydrodynamics) and Sediment Sorting: Hydrodynamic forces, including strong bottom currents and wave action, play a crucial role in redistributing sediments. This leads to spatial sorting, where finer particles are preferentially transported offshore, while denser bioclastic debris (e.g., mollusk shells) accumulates at high-energy coastal stations (such as stations 3 and 4 in Figure 1) [25]. Thus, the spatial patterns of CaCO₃ concentrations are also shaped by these processes, resulting in higher concentrations of coarser carbonate fragments in more energetic coastal settings where fine terrigenous sediments are winnowed away. A positive, albeit not statistically significant, correlation between CaCO₃ and gravel-sized material (r = 0.38) supports the presence of coarse bioclastic debris in these coarser sediment fractions. Furthermore, carbonates and aragonite exhibit a positive, though not statistically significant, correlation (r = 0.34), suggesting a contribution from aragonitic organisms such as mollusks and corals. While turbulence-enhanced CO₂ degassing can potentially favor carbonate precipitation, these dynamics are significantly counteracted by dilution from aeolian quartz (70%), which depresses the relative proportion of CaCO₃ in coastal deposits [9].

The weak correlation between CaCO₃ content and water column depth (r = 0.10) is a key finding that underscores the complexity of carbonate cycling in these upwelling-desert margin systems [10,22]. This low correlation provides evidence that simple water-column depth-dependent dissolution (which is minimal as lysoclines are not reached) is not the primary factor controlling the overall spatial distribution of CaCO₃ [55]. Instead, the observed patterns are a complex fingerprint reflecting the interplay and relative dominance of the previously mentioned processes in different environments. Notably, organic carbon (Corg) shows a strong positive correlation with Mud (r = 0.68) and a strong negative correlation with Sand (r = −0.66). This pattern is consistent with organic matter preferentially accumulating and being better preserved in fine grained, low energy muddy sediments, while it is subject to more efficient removal or less efficient preservation in high energy sandy environments [5,56]. Furthermore, silicate (Si(OH)₄) and phosphate (PO₄³⁻) show a moderate to strong positive, though nonsignificant, correlation with increasing water column depth (r = 0.64 for Si(OH)₄; r = 0.51 for PO₄³⁻), indicating an accumulation of these nutrients in deeper, oxygen-depleted waters due to remineralization and limited scavenging, a process consistent with observed high Dissolved Inorganic Carbon (DIC) values in South Atlantic Central Water (SACW) upwelled waters [22,57]. Early diagenetic processes, such as halite formation in pore spaces, further influence carbonate stability; however, the correlation between halite and CaCO₃ is weak and nonsignificant (r = 0.09) [43,46].

The study area aligns with the “heterozoan-C” carbonate factory model, typical of temperate, nutrient-rich margins influenced by upwelling, which is characterized by the dominance of light-independent heterotrophic organisms, the mixing of biogenic carbonates with terrigenous sands, which thereby limits photozoan production, together with high hydrodynamic energy [13,56,57].

5. Conclusions

This study elucidated the mechanisms governing carbonate distribution in marine sediments of the Tarfaya–Boujdour margin (26–28° N) along the northwest African coast, which is a region subject to intense coastal upwelling and Saharan eolian influx. Integrated analysis of sedimentological, mineralogical, and hydrological properties reveals a dynamic equilibrium between three key processes: biogenic production stimulated by upwelling of nutrient-rich waters, dissolution induced by acidification of CO₂-enriched upwelled waters, and dilution by terrigenous inputs dominated by Saharan quartz. The weak correlation between carbonate content and depth reflects this complex compensation, where carbonate signals are modulated by antagonistic factors.

The spatial patterns observed confirm the margin’s suitability as a “heterozoan-C” carbonate factory, characterized by heterotrophic assemblages, a biogenic-terrigenous sedimentary mixture and high hydrodynamic energy limiting the accumulation of fine particles. Climate projections suggest that the intensification of Saharan upwellings and aeolian flows could accentuate carbonate dissolution and dilution, threatening the preservation of these sedimentary archives in a scenario of accelerated ocean acidification.

These results highlight the necessity of developing coupled models integrating local hydrology, current dynamics and terrigenous fluxes to anticipate the response of desert margin carbonate systems to global change. Future studies focusing on early diagenetic processes and interactions at the water–sediment interface would be essential to refine predictions of carbonate preservation in such dynamic environments.