Introduction

Coastal zones encompass a diverse range of activity sectors, with sediment transport holding a significant role within the environmental context. In the Mediterranean region, paleoclimatic records distinctly reveal that the Holocene era is characterized by millennial-scale climatic fluctuations. These variations are closely tied to shifts in temperature and fluctuations in rainfall, which are prominently evident during this period (e.g., Bond et al., 2001; Mayewski et al., 2004; Magny, 2004; Giraudi, 2005; Wanner et al., 2008; Magny et al., 2011; Giraudi et al., 2011; Arnaud et al., 2012; Benito et al., 2015; Al-Hurban et al., 2023). The African Mediterranean climate demonstrates a notable diversity in its climatic systems, ranging from humid equatorial conditions to arid seasonal tropical climates and subtropical climates. In fact, increasing temperatures across Africa are anticipated to elevate evaporation demands (Goulden et al., 2009).

Indeed, rising temperatures across Africa are expected to increase evaporation demands (Bedair et al., 2023; Terink et al., 2013). Furthermore, the regional climates in Africa have undergone significant changes since the last glacial maximum, approximately 21,000 years ago (Wanner et al., 2008; Armstrong et al., 2023). These transformations have been documented through pollen syntheses, illustrating notable shifts. Understanding the complex history of past climatic changes and their effects remains a challenge. Notably, exceptional records based on charcoal and pollen proxies have shed light on anthropogenic influences (Jaouadi et al., 2016; Marquer et al., 2008; Nash & Meadows, 2012; Schulz et al., 2002). In recent years, the Tunisian coast has garnered increased attention within the environmental context (Zaîbi et al., 2011, Zaibi et al., 2012, Zaïbi et al., 2016; Gargouri and Zouari, 2019; Gammoudi et al., 2021). Over the past two millennia, despite the expansion of Sebkha basins at the expense of agricultural lands, industrial zones, and urban areas in Tunisia, records have provided limited insights into the finer fluctuations of temperature and humidity.

According to Lakhdar et al., (2021), the southern coast of Tunisia is characterized by subtle tides that reach their peak in the Gulf of Gabes, renowned in the Mediterranean for its tide oscillations. The southeastern Tunisian coastal zone features the presence of Sebkhas or salt flats. Multiple studies have established that climatic changes during the Holocene period were numerous, although less pronounced in their variations compared to those during the preceding glacial cycle (Chairi, 2005; Jaouadi et al., 2016; Lakhdar et al., 2006; Schulz et al., 2002; Zaibi et al., 2012). However, the Holocene coastal deposits hold crucial insights into the evolution of successive paleoenvironments, regional hydrodynamic influences, tectonic activities, and hydro-isostatic responses of the littoral fringe to sea-level fluctuations (Zaîbi et al., 2011; Engel et al., 2021). In Sebkhas, the effects of wind deflation add another layer of complexity, at times quite vigorous and capable of altering the topographic landscape.

The research area, situated in the southeastern region of Tunisia, appears to have witnessed the establishment of an estuarine open lagoon environment around 7460 calendar years BP. Subsequent to this phase, several periods of sedimentation interruptions occurred, leading to the formation of sandy ridges, particularly around 5408 calendar years BP. These episodes have contributed to shifts in paleoenvironments, leading to the transformation of increasingly enclosed lagoons that eventually evolved into the present-day Sebkha environment (Zaîbi et al., 2011). The region has been the subject of extensive investigation (Zaîbi et al., 2011, Zaibi et al., 2012, Zaïbi et al., 2016; Gargouri and Zouari, 2019).

This study’s primary objective is to elucidate the sedimentary characteristics of the Holocene within the Sebkha core. The aim is to discern the origin of minerals and provide insights into the source and nature of organic matter. By examining variables such as carbonate percentage, chemical elements, organic content, and granulometric distribution, one can deduce variations in the climatic conditions of deposition environments. In order to reinforce and validate the connections between geochemical and mineralogical analyses, a variety of techniques, including principal component analysis (PCA), were utilized, encompassing processes such as X-ray diffraction (XRD), X-ray fluorescence (XRF), scanning electron microscopy (SEM/EDX), and infrared (IR) spectroscopy, among others.

In the present study, we examine the sources, composition, and evolution of organic matter (OM) in the recently deposited sediments of the Sebkha El-Guettiate using Rock–Eval 6 (RE) parameters and inorganic analysis to assess the types of inorganic and organic inputs in the core, to discuss the results of RE parameters and the geochemical proxy data, and to determine the source and state of organic matter preservation in the Sebkha environment. But, these are preliminary results that can be supplemented by a further micro-morphological assessment and more involvement in pedo-sedimentary events.

Study area

The study area, with a rectangular shape, is situated along the coastal fringes of Southeast Tunisia, specifically within the Gulf of Gabes in the southwestern Mediterranean region. Tunisia, showcasing a wide bioclimatic range, covers a total area of 518 km2 within coordinates of 34°13′48′′N and 10°03′06′′E (Fig. 1). This region is characterized by an arid climate and is positioned approximately 15 km to the south of Skhira city.

Fig. 1
figure 1

Location of the Sebkha of El-Guettiate within the framework of a Mediterranean and Atlantic connection, limit of the catchment area of the study area, and location of the core taken from SG

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The Skhira coastal region within the central part of the Gulf of Gabes displays a distinctive morphology that varies proportionally. Progressing from north to south, the coastline is marked by a commanding cliff ranging from 1 to 15 m in height. This cliff is followed by a paralic system consisting of two maritime marshes, namely El-Guettiate and Dreïaa. These marshes are sheltered by a cliff and a narrow sandy rim (Gargouri, 2011).

In contrast, the northern section of the Sebkha is adjacent to the Mediterranean, separated by a narrow strip of land. This strip occasionally unveils an elongated cliff, stretching over 2 km in length and spanning 100 to 200 m in width. This northern cliff transitions to the south, transforming into a barrier beach system reminiscent of sand spits that extend for 2 km (Zaîbi et al., 2011). The connection between the Mediterranean Sea and the Sebkha occurs primarily through a tidal channel on the eastern side. This channel measures 10 m in width and reaches depths of up to 7 m. The Sebkha El-Guettiate serves as an outlet for the Bou Saïd and El Ghram wadis, which collectively have a catchment area estimated at 378.2 km2 (Fig. 1).

Methods and materials

During the field expedition, a sediment core measuring 100 cm in length (SG: 34°12′14.45ʺN; 10°04′28.27ʺE) was collected from the Sebkha El-Guettiate, as depicted in Fig. 1. This core was sampled at intervals of 0.5 cm, yielding a total of 200 samples. Subsequently, eleven samples were carefully selected, maintaining a 10-cm interval between each, for the purpose of conducting geochemical and mineralogical analyses.

The examination of the mineralogical composition of the fine fraction was carried out through X-ray diffraction (XRD) analysis.

This analysis involved utilizing raw sample powders to identify phases. The XRD analysis took place at the Laboratory of Nanomaterials and Systems for Renewable Energies within the Energy Research and Technology Center (CRTEn) in Borj Cedria, Tunisia. This component encompassed a comprehensive mineralogical analysis of the entire rock and clays using the X-ray diffraction method. Its purpose was to determine the mineralogical composition of the sediment samples that were collected.

X-ray diffractograms were generated using the X’Pert High Score Plus software. The software then compared the resulting diffractograms with the nearest available reference files to discern the mineralogical composition. In summary, the field expedition yielded a sediment core from Sebkha El-Guettiate, and subsequent analyses involved careful selection of samples for geochemical and mineralogical investigations. X-ray diffraction (XRD) analysis was conducted on fine fraction samples to identify mineral phases.

Based on the mineralogy of the whole rock, some samples were subjected to SEM (scanning electron microscope) at the Tunisian Company of Petroleum Activities (ETAP) observations to determine the texture of the figurative elements. Again, the infrared (IR) technique complements X-ray diffraction analysis in the study of poorly crystallized materials. This method of analysis is non-destructive for the sample. The measurements were carried out by transmission. The device used is a dispersive infrared spectrometer of the PerkinElmer brand. The pellets were made from an intimate mixture of 1 mg ground sample and with 100 mg of potassium bromide (KBr) under a pressure of 4.5 110 Pa. To determine the percentages of CaCO3, the Denard Calcimeter is used. But, major and minor chemical elements were measured by XRF spectrometer, and the concentrations of potassium (K) and sodium (Na) were measured by flame photometer for 200 samples at the Valorization of Useful Materials Laboratory (LVMU) of the National Research Center in Materials Science (CNRSM), Borj Cedria, Tunisia. However, the organic matter analyses used the Rock–Eval 6 at the Tunisian Company of Petroleum Activities (ETAP). For statistical data processing, the statistical software XLSTAT 2023 was used.

Results

Sedimentology and mineralogy studies

In the process of examining and comprehending the arrangement of essential sediment layers within the Sebkha El-Guettiate (depicted in Fig. 2), it is observed that the proportion of finer sediment particles (< 63 µm) fluctuates between 19 and 69.8%. Furthermore, the computation of the proportion attributed to coarser particles, with diameters surpassing 63 µm, ranges from 8 to 28%. Notably, the particle size analysis of this core reveals a considerable variance in the proportions of sandy and silty components. However, the examination of the mineral composition within Sebkha El-Guettiate reveals distinctive diffractograms in the sediments of the SG core, extracted from depths of 20 cm and 80 cm. These diffractograms unveil a specific arrangement of minerals, including silicates (quartz, feldspar, and phyllosilicates), carbonates (calcite, dolomite, magnesium, and Ankerite), and evaporites (gypsum and halite), as illustrated in Fig. 3. Notably, these findings are characterized by the proportional presence of various minerals, as outlined in Table 1. These percentages accentuate the prevalence of Quartz peaks, which are distributed across 56% (SG-160) and 98% (SG-40).

Fig. 2
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Grain size evolution (lower fraction percentage of 0.063 mm and upper fraction percentage of 0.063 mm) of the Sebkha El-Guettiate

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Fig. 3
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X-ray diffractograms of the whole rock (samples SG-40 and SG-160) of the Sebkha

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Table 1 Percentages of the mineralogical compositions of the El-Guettiate Sebkha core (Qz: quartz; Felds: feldspar; Ca: calcite; Ank: ankerite; D: dolomite; Gyp: gypsum; H: halite; Ka: kaolinite; I: illite)
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Nevertheless, the analysis of carbonate minerals (calcite, dolomite, magnesium, and Ankerite) discloses substantial peaks mainly in the initial and middle sections of the core, particularly in SG-80, SG-100, SG-120, SG-140, and SG-160. Furthermore, there is an evident dominance of X-ray diffraction peaks corresponding to evaporite minerals (gypsum and halite) in SG-40 and SG-180. Conversely, minerals such as illite, phyllosilicates, and feldspar exhibit relatively lower abundance within this Sebkha core.

Scanning electron microscope

After determining the mineralogical compositions, the Sebkha sediments were identically examined using scanning electron microscopy images with energy-dispersive X-ray. Following results, SEM/EDX images generally show representations of quartz minerals (Fig. 4a), which were granted in all samples. In addition, types of clay were observed like illite. A pyrite mineral (FeS2) in automorphic form was also observed (sample SG-150) (Fig. 4f). However, carbonate minerals (calcite and dolomite) exist in almost all samples in the form of rhombohedral crystals (Fig. 4a,e). However, in evaporite minerals (gypsum and halite), halite can surround quartz crystals, which is also represented in almost all samples in cubic crystal form (Fig. 4c). While gypsum is found in the samples (SG-100 to SG-200), it is shown as white lamellae crystals (Fig. 4d).

Fig. 4
figure 4

Scanning electron microscope (SEM/EDX) images of sediment samples from the El-Guettiate Sebkha: (a) quartz, (b) calcite, (c) halite, (d) gypsum, (e) dolomite, and (f) clay minerals

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The most common quartz grains are subrounded with the microtextural characteristics of aeolian grains: fresh curved cracks and percussion shapes (Ahmady-Birgani et al., 2023). Conchoidal fractures have sharp edges. The grains in this sample recorded a complex sedimentary history with alternating aeolian and aqueous environments (Zaîbi et al., 2011).

The oval mineral in Fig. 4b is a quartz primary mineral, which has been rounded and banded by carbonate; similar results are found by Lakhdar et al. (2006) in Sebkha Boujemmel (SE Tunisia), in Sebkha Mhabeul (SE Tunisia) by Gammoudi et al., (2021), and in Lake Urmia in Iran by Ahmady-Birgani et al. (2023). This shape shows that the ooid cortex is formed by superimposed carbonate crystals that may correspond to dissolution traces and microbial activity (Martín et al., 2020; Shen et al., 2022) and seem to be formed in a subtidal environment under moderated energy reflecting the fluctuating climatic conditions.

Kaolinite is a clay mineral, which probably has an inherited and detrital origin. The kaolinite amounts vary between 0 and 1%. Generally, it occurs in hot and humid intertropical regions in the oxidative environment (Molén, 2024), and it is considered as the alteration product of feldspars or illites (Gallela, 2010; Gammoudi et al., 2021). In fact, the coarser particles of clay aggregates in Fig. 4f have most probably aggregated in saline conditions; the same phenomena were registered in Korba lagoon (NE Tunisia) by Bouden and Chaabani (2004)and in Bizerte lagoon (north of Tunisia) by Srarfi et al., (2010).

The infrared spectrometer

Infrared spectrometry (IRTF) is an additional analytical technique employed in laboratories alongside chemical and mineralogical analyses. This method facilitates the examination of various aspects, including the chemical composition of vibrating ions, molecular group vibration patterns, and the identification of absorption bands’ intensity and position, aiding in the interpretation of functional groups (Trabelsi, 2017). Typically, the wave number in such studies ranges from 4000 to 400 cm−1. The FTIR (Fourier-transform infrared) results of the SG core samples (SG-20, SG-40, SG-60, SG-80, SG-100, SG-120, SG-140, SG-160, SG-180, and SG-200) are presented in Fig. 5.

Fig. 5
figure 5

Infrared spectra of Holocene sediments of the Sebkha El-Guettiate along the SG core

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Within these results, several distinct bands can be observed in succession.

  • Notably, small and delicate absorption bands spanning 3226–3533 cm−1 and 3620–3712 cm−1 are prevalent across nearly all samples. These bands correspond to elongation vibrations of the OH groups of adsorbed water (Felhi, 2010).

  • Existing research (Madéjova and Komadel, 2001; Madejova et al., 2002; Viscarra Rossel et al., 2006; Janik March–April, 2007; Truche, 2010) suggests that bands at 3500 cm−1, 3575 cm−1, 3620 cm−1, 3694 cm−1, and 3700 cm−1 can be attributed to the elongation of the OH band in kaolinite. The absorption band at 3620 cm−1 is distinctive for kaolinite, setting it apart from illite and smectite (Truche, 2010).

  • Additionally, the 3500 cm−1 band (observed in SG-40 and SG-60) is relevant to OH group vibrations in water molecules and OH deformation vibrations in kaolinite and gypsum (Yusuf, 2023).

  • A minor peak absorption band, visible within the range of 2300–2500 cm−1 (SG-160, SG-180, and SG-200), indicates the vibration of the CO molecule in the presence of calcite (Truche, 2010). Additionally, a low-intensity wavenumber range spanning 2830–2950 cm−1 is observed in the SG-60, SG-80, and SG-160 samples, and these are associated with aliphatic CH3-CH2 groups (Lakhdar, 2009; Kanbar et al. 2021). According to Sivakumar et al., (2012), this band signifies the vibration of the OH band in organic carbon.

  • A distinctive band at 1870–1992 cm−1 is indicative of the stretching vibrations of Si–OH groups, pointing to the presence of quartz (Sivakumar et al., 2012).

  • An intense band ranging from 1420 to 1795 cm−1, prominent in the SG-80, SG-100, SG-120, SG-140, SG-160, SG-180, and SG-200 samples, signifies group elongation vibrations attributed to the presence of carbonates (Saikia et al., 2003; Sivakumar et al., 2012).

  • A flattened and moderately noticeable absorption band around 1015–1120 cm−1 is evident in all samples and is linked to Si–O bands in kaolinite (Kanbar et al., 2021; Saikia et al., 2003; Sivakumar et al., 2012). Fine absorption band ranging from 990 to 1080 cm−1 corresponds to the vibration of the Si–O group in silicates (Kanbar et al., 2021; Trabelsi, 2017).

  • A band situated at 850–880 cm−1 displays Al–OH deformation vibration in the presence of calcite (Saikia et al., 2003).

  • An evident absorption band in all samples, found within the range of 770–794 cm−1, is characteristic of the valence vibrations associated with Si–O band stretching in quartz and silicates (Saikia et al., 2003; Sivakumar et al., 2012).

  • A band located at 517–533 cm−1 corresponds to the deformation vibrations of the Al–O–Si group in the presence of kaolinite (Saikia et al., 2003; Sivakumar et al., 2012; Trabelsi, 2017).

  • A consistent band at 453–464 cm−1 is observed across all samples and is attributed to Si–O bands in feldspars (Sivakumar et al., 2012).

In conclusion, the FTIR technique proves to be a fundamental method for the mineral analysis of quartz, kaolinite, calcite, feldspar, and organic carbon.

Geochemical study

Bernard calcimetry

The variation in the calcium carbonate (CaCO3) contents of the samples analyzed along the core studied varies between 11 and 26%, allowing for modification that the CaCO3 contents are low and classified as little limestone. This classification of the calcium carbonate contents of the Sebkha El-Guettiate is probably due to the rainfall of calcite and/or dolomite. Indeed, the high value of CaCO3 corresponds to a depth of 60–90 cm at the core. The results of the calcimetric methods will be testified later by X-ray fluorescence spectrometry by assembling the results with those of the mineralogical study Table 2.

Table 2 Correlation matrix of organic matter parameters with core depth
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X-ray fluorescence spectrometry

The results of the X-ray fluorescence technique are represented in the curves which are specific to the evolution of the percentages of the chemical elements (SiO2, CaO, MgO, Na2, O, Al2 O3, Fe2O3, MnO, TiO2, SO3, and K2O) as a function of core depth (Fig. 6). These results show that the variations of the contents of the elements CaO, MgO, Na2O, Al2O3, Fe2O3, MnO, TiO2, SO3, and K2O are absolutely comparable to the core depth, which also become in the opposite direction corresponding to the SiO2. According to geochemical and mineralogical analysis, this study is very rich in silica variation: between 81.4% (SG-130) and 88.43% (SG-50), but the percentages of other elements [for Na2O between 0.16% (SG-50) and 0.54% (SG-90) and for K2O between 0.31% (SG-170) and 0.7% (SG-110)]; finally, the Fe2O3 levels vary between 0.14% (SG-50) and 0.61% (SG-90). CaO levels vary between 3.16% (SG-10) and 6.6% (SG-110); those in MgO vary between 0.22% (SG-30) and 0.54% (SG-90), and for Al2O3, they vary between 1.21% (SG-190) and 2.3% (SG-110) for the core Fig. 7.

Fig. 6
figure 6

Vertical variation of the chemical elements of the core samples studied

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Fig. 7
figure 7

Principal component analysis of mineralogical parameters (Qz: quartz; Felds: feldspar; Ca: calcite; Ank: ankerite; D: dolomite; Gyp: gypsum; H: halite; Ka: kaolinite; I: illite) and parameters of the mineral elements of the Sebkha El-Guettiate core

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Organo-geochemical study

For this part, Rock–Eval 6 is used based on the estimation of the total organic carbon content (TOC) of the sediments. The data presented in Table 3 shows the results of the following parameters: TOC, HI (hydrogen index), IO (oxygen index), Tmax (maximum temperature), S1 (quantity of free hydrocarbon compounds), S2 (amount of potential hydrocarbon compounds), and S3 (oxygenated compounds). However, this technique admits the resolution of the parameters of the geochemical analysis (Lafargue et al., 1998). The maximum temperature values of the core samples vary between 355 °C (SG-120) and 440 °C (SG-100). Table 3 shows the values of the percentages of total organic carbon which vary between 0.1 and 0.32%; the values of S1 vary between 0.11 mg of HC/g of sediment (SG-140) and 0.22 mg of HC/g of sediment (SG-60); the values of S2 correspond to 0.16 mg of HC/g of sediment (SG-160) to 1.02 mg of HC/g of sediment (SG-1); S3 modifies only 0.48 mg of HC/g of sediment in the SG-20 sample to 1.38 mg of HC/g respectively in the SG-60 sample. The variations in the hydrogen indexes (HI) are characterized by a higher value for the SG-160 sample of 206 mg HC/g of TOC and 331 mg of HC/g of TOC for the SG-1 sample. The IH versus IO diagram (Fig. 8), leading to the elemental composition of a large number of samples without minerals (Disnar et al., 2013), shows that the majority of samples from the core of Sebkha El-Guettiate particularly decrease in IH as depth increases in the sediment core. It may correspond to the mineralization of organic matter and the continental origin of the organic stock.

Table 3 Results of organic matter parameters and SG core depth by Rock–Eval technique 6
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Fig. 8
figure 8

Factorial plane of organic matter parameters with core depth

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Indeed, at Rock–Eval, the maximum temperature (Tmax) is characterized by a slight variation from 355 to 440 °C, the highest peak being 440 °C for the SG-100 sample.

Statistical analysis of results (ACP)

We used the multivariate method of principal component analysis (PCA) to complete the factorial relations (F1 × F2) of the matrix of correlations between the parameters of the mineralogical compositions and the elements of the minerals and the parameters of organic matter of the carrot SG (Fig. 7).

The eigenvalues characterize that the first factorial plane, composed of the axes F1 and F2, explains 58.30% of the total inertia. Axis I formulates 41.28% of the core depth variance, which is positively correlated with quartz and calcium carbonate. Then, we explained a good positive correlation between CaO and MgO (R2 = 0.79), between TiO2 and Al2O3 (R2 = 0.76), between Na2O and Al2O3 (R2 = 0.98), between Al2O3 and MgO (R2 = 0.88), and between Al2O3 and K2O (R2 = 0.87) (Table 3). The graphs resulting from the factorial analysis make it possible to determine parameters of organic matter express 83.13% of the variance (Fig. 8 and Table 2), highlighting the design of homogeneous group statistics.

According to the factor (F1 × F2), it has been shown a good positive correlation between S1 and S2 (R2 = 0.755), between Tmax and S1 (R2 = 0.842), between COT and S1 (R2 = 0.871), between Tmax and S2 (R2 = 0.746), between COT and S2 (R2 = 0.980), between IH and S2 (R2 352 = 0.861), between COT and Tmax (R2 = 0.857), and between IH and COT (R2 = 0.792), but we observe a correlation negative all the parameters with the core length depth (Table 2).

The hydrogen index (IH) diagram as a function of the oxygen index (IO) (Fig. 8), providing information on the origin of the organic matter (Koegel-Knabner and Rumpel (2018), indicates all the samples of the core had heterogeneous organic matter with both marine and terrestrial origins. It is characterized by a variation of 355 to 440 °C of the maximum temperatures of peak 2. Organic matter (OM) is richer in humic matter which begins to mineralize to become lignified OM.

Discussion

Deposit sedimentology and the environment

The investigation of the sedimentary environment leads us to the conclusion that a significant portion of the sands within Sebkha El-Guettiate originates from a marine-like setting and common quartz grains are subrounded with characteristics of aeolian grains like fresh curved cracks and percussion shapes (Ahmady-Birgani et al., 2023).

Although multiple conchoidal fractures can also form in aeolian or aqueous environments, the impact of wind is more likely, as the high energy required to create such fractures in the water is not compatible with the characteristics of frequent precipitation and the abundance of euhedral NaCl crystals (Battiau-Queney et al., 2023). On the other hand, strong onshore winds can generate a temporarily high level of wind energy, especially described by Ouaja et al., (2013) as responsible for Aeolian transport in golf of Gabes.

The predominant sedimentary makeup in the studied region consists of the sandy fraction, which is notably enriched in carbon. The surface of the Sebkha is notably abundant in the salt crust (composed of gypsum and halite), which indicates a relationship with the Mediterranean Sea, particularly during instances of Sebkha flooding. This communication is particularly prominent in this specific area. According to Chen and Hur (2015), the sedimentary layers contain organic matter that primarily comprises humic substances or organic residues complexed through less soluble chemical processes. However, within deposits, the presence of organic matter becomes significant for bacterial activity (Pacton et al., 2007). Within this Sebkha, the gypsum crystals are observed to break down into sandy and silty levels during the process of evaporation. As stated by Pomel et al., (2008), during rainy periods, the coexistence of a gypsum and organic matter layer, fostering the presence of bacteria, signifies an alternating pattern between dry and flood conditions.

Contribution of chemical elements and mineral source

The examination of Sebkha sediments involves the utilization of mineralogical composition, as well as FTIR, SEM/EDX, and XRD analyses. The outcomes of the mineralogical assessments of sediment samples are intricately tied to the distinctive nature of various mineral constituents within the overall rock structure, as outlined in Table 1. This investigation encompasses a range of minerals including dolomite, calcite, gypsum, quartz, illite, ankerite, halite, and feldspar.

The variations in the composition of non-clay minerals offer insights into the distinguishing features of the sediments. Notably, calcite is prevalent across the majority of core sediments, accounting for proportions that range from 1 to 14%. The distribution of these percentages suggests a potential origin through chemical precipitation, biochemical processes, or interactions involving organisms with limestone formations. Quartz exhibits notably high proportions relative to other minerals in all sampled sediments, ranging from 56 to 98%. As indicated by Gargouri (2011), surface sediments from the distant regions of Sebkha El-Guettiate and the mouths of wadis (wadi El-Guettiate, wadi Aouien, and wadi Bou Said) originate from the quaternary emergence of the Gabes golf course.

Furthermore, the occurrence of dolomite is relatively low, with percentages ranging from 1 to 8% along the core scales (Table 1). The origins of this mineral are attributed to either direct precipitation from seawater or certain marine substances such as algae and foraminifera (as cited in Arrim, 1996; Cappellen, 2003).

However, the presence of clay minerals, specifically the kaolinite fraction < 0.2 µm, constitutes a minor percentage ranging from 0 to 2% in certain sediment samples.

Upon analyzing the correlation matrix table, we observe several notable trends. Feldspar exhibits negative correlations with quartz (R2 =  − 0.57). This could be attributed to the mineral’s continental detritus origin (Chairi, 2004).

The carbonate levels within the core sediments reveal a prevalence of calcite in comparison to dolomite. First, it could be a result of the erosion of adjacent carbonate-rich land formations. Alternatively, it might stem from a chemical process involving the direct precipitation of CaCO3. Comparable findings have been reported in the Tunisian South East (as mentioned by Gargouri, 2011; Bouaziz et al., 2015; Lakhdar et al., 2021) and the Tunisian Sahel (Tagorti, 1990).

The mineralogical analysis results find support in the X-ray fluorescence spectrometer analyses performed on the same core samples, revealing a SiO2 content ranging from 81.39 to 88.84%.

The geochemical composition results, characterized by generally low CaCO3 content, strongly suggest a chemical origin related to evaporation, indicating a humid climate. The carbonate contents might be a result of siliceous dissolution, accompanied by the accumulation of magnesium, which contributes to progressive dolomitization.

Geochemical analyses also reveal a negative correlation between CaO (from calcite) and SiO2 (from quartz) (Table 4). Sediments in the core displaying higher levels of Al2O3, SiO2, and CaO may be linked to the dissolution of evaporites and the precipitation of clay minerals (Gallela, 2010), which is indicated by a positive correlation (Table 4). Interestingly, the vertical variations in hydroxides Na2O, Al2O3, CaO, K2O, TiO2, MgO, MnO, SO3, and Fe2O3 within the core sediments follow a parallel trend, while SiO2 levels exhibit an opposing pattern (Fig. 6). This progression suggests the presence of phyllosilicates (Bouden and Chaabani, 2004; Gammoudi et al., 2021).

Table 4 Correlation matrix of mineralogical compositions and mineral element parameters of Sebkha El-Guettiate core (Qz: quartz; Felds: feldspar; Ca: calcite; Ank: ankerite; D: dolomite; Gyp: gypsum; H: halite; Ka: kaolinite; I: illite)
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The positive correlation (R2 = 0.79) observed between MgO and CaO signifies the presence of dolomite, a mineral closely associated with feldspar (Warren, 2000). At an oceanic level, the constancy in Mg levels, combined with variations in seabed diffusion, leads to the replacement of Ca levels within seawater. Consequently, this mineralogical phenomenon contributes to the dominance of marine carbonate and evaporite precipitates.

Additionally, Table 4 reveals a significant positive correlation between Al2O3 and TiO2 (R2 = 0.81). The element Ti is especially present within phyllosilicates as detrital minerals (Dabard, 1990; Condie et al., 1992; as referenced in Gammoudi et al., 2021).

Figure 6 demonstrates robust positive correlations among various chemical elements in core sediments, including K2O/Al2O3, K2O/Fe2O3, TiO2/Al2O3, and Al2O3/SiO2. These elements are generally attributed to illite, feldspars, and kaolinite. The TiO2/Al2O3 binary diagram, in particular, displays a positive correlation (R2 = 0.67) for the core samples. The correlation between K2O/Al2O3 also exhibits a strong positive trend (R2 = 0.76) within core sediments, highlighting the abundance of aluminous components. Similarly, the correlation between K2O/Fe2O3 and Al2O3/SiO2 demonstrates substantial positive correlations (R2 = 0.57 and R2 = 0.59 respectively) within core sediments. As indicated by Gallela (2010), these associations point to the presence of various minerals, involving iron clay minerals, oxides, and iron hydroxides (Srarfi et al., 2019). The relationship between K2O and Fe2O3 is primarily influenced by the presence of illite and kaolinite alterations, as well as the prevalence of feldspars (Bouden and Chaabani, 2004; Gallela, 2010).

Notably, all samples exhibit richness in Al2O3 and SiO2, indicative of the presence of clays. Generally, the larger quartz grains observed along the core represent the dominant Aeolian sediment constituents. The clays of the Sebkha are inherited, and therefore, their composition reflects the procession of clay minerals of the watershed soils. They are characterized by an enrichment of kaolinite and illite. The study of the clays shows the similarity of the procession in the upper levels with that of the sedimentary column, which underlines the constancy of the terrigenous feeding. The same results are found in Korba lagoon in the NE of Tunisia (Bouden and Chaabani, 2009).

Certainly, the investigation into organic matter involves determining the total organic carbon (TOC) content of core samples, which exhibits a range from 0.1 to 0.32%. These values are lower than COT registered in other similar systems in Tunisia (Tunis lagoon from 1 up to 1.69%, Korba from 0.48 up to 5.21% (Bouden and Chaabani, 2004), Moknine from 0.3 to 1.7% (Chairi et al., 2010), Sidi El Hani and Mejerda mouth from 1.13 to 2.63% (Triki et al., 2022)). This is in relation to the silty and sandy sediment nature and core location in a sea pass. The low values are due to OM mineralization due to environmental conditions, along with Tmax values below 440 °C (Table 3). Tmax values above 300 °C indicate that the organic matter in the sebkhet El-Guettiate is not residual and highly altered. The origin of OM under these conditions is difficult to determine (Triki et al., 2022), but Rock–Eval analysis suggests that it is richer in lignin in the subsurface and humic compounds. Likewise, the Tmax suggests that much of the OM in the Sebkha may be humic or fulvic acids since temperatures are between 300 and 400 °C (Disnar et al., 2013). Land-based OM in sediments contains abundant humic compounds to which lignin can also adhere. The high content of S3 attest to the presence of terrigenous plants OM (Chairi, 2005) source of the fragments of lignin. These humic compounds are a likely source of refractory carbon for the anaerobic bacteria metabolism, especially in the presence of Mn oxides (Table 3) (Triki et al., 2022).

Nevertheless, some general trends, especially low TOC and HI values (206–331), are related to intense oxidation for the permanently emerged core (Chairi et al., 2010). The IH versus IO diagram (Fig. 8), leading to the elemental composition of a large number of samples without minerals (Disnar et al., 2013), shows that the majority of samples from the core of Sebkha El-Guettiate particularly decrease in IH as depth increases in the sediment core. According to the two diagrams (Fig. 9), there are two separate stocks of OM, probably originating from different sources.

Fig. 9
figure 9

Position of samples in the IH/OI and IH/Tmax diagram and MO types based on Espitalié et al., (1977)

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The infrared spectra obtained from the core sediments of El-Guettiate Sebkha (Fig. 5) display prominent OH deformation vibration absorption bands, CO bands, and aliphatic structures CH3-CH2 indicating IO oxygenated compounds (C-H, COOH…).

Chairi, in 2018, was found in Moknine Sebkha (a very similar environment to Sebkha El-Guettiate) at the edge of the Sebkha, where the facies is sandy or silty, the origin of the organic matter is mixed. The OM is continental in early digenesis in relation to the arid climate about 150 years ago. This outcome aligns with the mineralogical components observed.

We can compare the geochemical, granulometric, and mineralogical results of the sediments of the Sebkha El-Guettiate during the Holocene with other studies: Sebkha Mhabeul South – Eastern Tunisia, Sebkha khor Al-Adaid Qatar, Sebkha Kadim Alaa’ab Iraq, and Salton Sea – California (Table 5). We found that Tunisian Sebkhas had a similar composition of major elements as well as minerals. They denote similar climatic and sedimentation environments. Therefore, some singular characteristics should be mentioned. In Sebkha Kadim Alaa’ab Iraq, high evaporation rates are observed as water evaporates from shallow basins, leading to salt crystallization (Mohammad & Awadh, 2023). Microbial mats are present on the surface of Sebkha khor Al-Adaid Qatar, despite the fact that evaporitic minerals frequently accumulate below the surface. High salinity is demonstrated by high evaporation rates (Al Disi et al., 2023). In the Salton Sea – California, salt minerals with acicular or prismatic properties are more likely to be disruptive, increasing salt uplift (Buck et al., 2011). Sebkha El-Guettiate has lower concentrations of evaporitic minerals and salts (Na, Ca, Mg, and K), which may indicate less favorable conditions for high evaporation, probably due to the continuous communication with the sea. The clay components are greater than those found in Sebkha El-Guettiate. In fact, the core is taken in the middle of the depression in low-energy sedimentation. El-Guettiate’s sandy core is influenced by Mediterranean Sea dynamism.

Table 5 Comparative geochemical, grain size, and mineralogy of sediments in Sebkha El-Guettiate
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Conclusion

The El-Guettiate Sebkha, situated in southeastern Tunisia, is recognized for its deposition of sandy and loamy fractions. An assessment of mineralogical parameters reveals that non-clay minerals encompass evaporites (halite and gypsum), silicates (quartz and feldspar), and carbonates (calcite, dolomite, magnesium, and ankerite). Additionally, clay minerals, predominantly kaolinite and illite, are present. Remarkably high quartz content percentages, ranging from 56 to 98%, are observed with an obvious Aeolian contribution.

The compositional variations of non-clay minerals allow for the identification of exposed calcite, which is abundant in the majority of core sediments, with proportions ranging from 1 to 14%. Calcite’s presence suggests a chemical origin via precipitation and/or biochemical processes involving the interaction of organisms with limestone formations. Generally, fluctuations in carbonate content (CaCO3) indicate that elevated CaCO3 levels correspond to dry climates that favor evaporite accumulation. Conversely, lower contents point toward carbonate dissolution in humid climates.

Geochemical analyses highlight the dominance of the Si element, primarily sourced from quartz. The major elements (Mg, Ca, Na, Cl, and K) predominantly stem from minerals like dolomite, halite, gypsum, phyllosilicates, and potassium feldspar. Infrared spectrometry corroborates the mineralogical findings, confirming the presence of both aliphatic (CH2, CH3) and aromatic (C=O) structures. Investigating sedimentary organic matter offers insights into organic components resulting from aquatic production. El-Guettiate core has relatively low TOC values (ca. 0.1–0.32%). No significant difference is apparent in terms of OM content among the depth. Such features are thought to reproduce permanent emersion and dryness of the Sebkha borders, which results in extensive oxidative degradation of the OM.

This underscores the diverse origins contributing to Sebkha sediment formation, often influenced by saline systems.

However, this study remains preliminary, which needs to be followed by further study and future research that will provide finer microscopic analyses and include a more detailed assessment of sand morphoscopy to complement a more integrated micro-morphological assessment and more involved in pedo-sedimentary events of the study area.