Journal of Remote Sensing & GIS

Geochemical Variability and Spectral Analysis of a Core from the El-Guettiate Sebkha in the Mediterranean Saline System during the Holocene

Najia Bouabid^1,2*, Feyda Srarfi³ and Mohamed Ali Tagorti^{4 1}Department of Water Sciences and Techniques, University of Gabès, Zrig Eddakhlania, Tunisia
²Department of Geology, University of Gabès, Zrig Eddakhlania, Tunisia
³Department of Science of Tunis, El Manar University, Tunis, Tunisia
⁴Department of Geology, University of Tunis El Manar, Tunis, Tunisia
^*Correspondence: Najia Bouabid, Department of Geology, University of Gabès, Zrig Eddakhlania, Tunisia, Email:

Received: 02-Jan-2024, Manuscript No. JGRS-24-24537; Editor assigned: 05-Jan-2024, Pre QC No. JGRS-24-24537 (PQ); Reviewed: 19-Jan-2024, QC No. JGRS-24-24537; Revised: 26-Jan-2024, Manuscript No. JGRS-24-24537 (R); Published: 02-Feb-2024, DOI: 10.35248/2469-4134.24.13.328

Introduction

According to paleoclimatic research, the Quaternary, more precisely the Holocene, covers around 10,000 years BP. All these studies have opted for the Holocene because of its duration and apparent climatic stability. Today, however, the Holocene is considered a period characterized by rapid climatic variability. For this reason, numerous studies have been carried out over the last few decades to understand the Quaternary and present-day environments. However, many studies have focused on wetlands, which are considered a complex and vulnerable ecosystem in particular sebkhas and the evolution of Tunisian coastal systems during the Holocene [1].

Coastal marine ecosystems are generally considered to be the most vulnerable. Due to rapid industrialization, the amount of toxic chemicals and metals in coastal ecosystems has increased [2-4]. Climate change is affecting global water and air circulation. However, some regions react quickly, while others have to wait a while before changing their deposition record. North Africa is an area vulnerable to climate change and is the first region of the Mediterranean basin to be affected [5]. For both academic and practical reasons, saline systems have been a popular subject of study. They produce economically valuable brines and evaporated minerals [6].

In general, local tectonic, global eustatic and local climatic factors have driven Holocene sedimentation in southeast Tunisia [7]. Spectral analysis of the different parameters recorded from Holocene deposits, either buried in sebkhas or in outcrop, allows us to determine that these factors guarantee climatic cyclicity [8]. The aim of this research was to examine Holocene cyclostratigraphy on a core belonging to a semi-exoric system of the El-Guettiate sebkha, located on the Mediterranean coast.

Study area

The El-Guettiate sebkha, the subject of this work, is a coastal depression located in south-eastern Tunisia. Covering an area of 15 km², its boundaries are bounded by the Sfax plain to the north, the Mediterranean Sea to the east, El-Hicha lagoon to the south, and the BouSaïd plain to the west. The study area includes the Oum El Ghram and BouSaïd basins in south-eastern Tunisia. It has a semi-arid climate characterized by irregular rainfall with an annual average of around 200 mm (Institute national de Meteorology de Sfax). In geological terms, the region features a variety of formations ranging from the Lower Cretaceous to the Quaternary [9,10].

The Upper Cretaceous is characterized by a dominance of carbonates with intermittent clay intercalations, while the Lower Cretaceous strata comprise detrital and carbonate deposits. In addition, the alluvial marls and limestones along the coastal strip are part of the Mio-Pliocene series [11-15]. The sebkha is separated from the Mediterranean by a narrow cliff 2 km long and 100 m to 200 m wide, which collapses due to wave action.

A 2 km long spit of sand connects this cliff to the south. The sebkha communicates with the Mediterranean via a tidal channel 10 m wide and 7 m deep. The transport of Sebkha surface sediments brings out the deeper sediments through numerous tidal channels, which are influenced by both tidal and fluvial inputs. A system of schorres and slikkes characterizes the central part of the sebkha. The schorres are occupied by halophilic vegetation, mainly composed of saltwort. Elliptical troughs develop on the surface of the schorre. These troughs are shallow (10 cm to 30 cm), 0.5 m to 3 m wide and elliptical in shape (Figure 1) [16].

Figure 1: Location map of the study area.

Materials and Methods

A 100 cm core was produced in the El-Guettiate sebkha during field expeditions. Sampling was carried out in the laboratory every 0.5 cm to obtain 200 cm samples. We then measured MS magnetic susceptibility, CaCO₃, chemical element concentrations, SEM and mineralogical analysis was carried out by X-ray diffraction (XRD) analysis with a Type “PANalytical” Pro, at the Laboratory de Mineral Resources et Environment of the Faculty des Sciences de Tunis. The mineralogical proportions of the minerals were determined using X’PERT HighScore Plus software.

We based our present work on the dating of Zaïbi et al. as our core (100 cm) was compared in length and proximity [17]. Tephrochronological dating revealed a sedimentation rate of around 0.32 mm per during the Upper Holocene at Sebkha El- Guettiate, so we can designate an age-depth model to complete the temporal spectral analysis of this sebkha.

We use spectral analysis and Principal Component Analysis (PCA) to process the data obtained. The PAST program was used to perform the spectral analysis.

Results and Discussion

Sediment mineralogy, geochemistry and magnetics were used to infer lithofacies and depositional environments

According to the mineralogical analysis of Sebkha El-Guettiate, the diffractogrammes of SG core sediments studied at a depth of 60 cm identified silicates, carbonates and evaporites. These results are characterized by the presence of mineral percentages. The majority of Quartz peaks were found to be between 76% (SG-160) and 98% (SG-40). However, analysis of the carbonate minerals (calcite, dolomite, magnesium and Ankerite) shows significant peaks at the middle beginning of cores SG-80, SG-100, SG-120, SG-140 and SG-160. In addition, evaporites (gypsum and halite) (SG40 and SG-180) were the most dominant X-ray diffraction peaks (Figure 2).

Figure 2: X-ray diffractogrammes of the whole rock core.

Images of the Sebkha sediments were examined by scanning electron microscopy after mineralogical compositions had been determined. According to the results of the SEM images, representations of quartz minerals are generally present throughout the samples. In addition, clay types such as illite were observed, and a pyrite mineral (FeS₂) in automorphic form was also observed (sample SG-150). But almost all samples contain rhombohedra crystals of carbonate minerals, such as calcite and dolomite. However, almost all samples are represented as cubic crystals, as evaporite minerals (gypsum and halite) can surround quartz crystals. Gypsum is present as white crystalline lamellae in samples from SG-100 to SG-200 (Figure 3).

Figure 3: Scanning Electron Microscope (SEM) images of sediment samples from the El-Guettiate sebkha. Note: Quartz; Calcite; Halite; Gypsum; Dolomite; Clay minerals.

Figure 4, shows Ca, K and Na concentrations (mg/kg) throughout the sebkha El-Guettiate core. With the exception of depth intervals 0-5 cm, 15-35 cm, 50-70 cm and the lower level, the sediments show high sodium and low calcium concentrations. High Ca concentrations in the lower and upper centimetres are most often followed by an increasing trend in halite concentration (0-5 cm and 6-30 cm depth). Potassium shows an increasing trend in the 5 cm to 20 cm, 22 cm to 28 cm and 65 cm to 80 cm interval and the rest of the core indicates low concentration variability.

Figure 4: Evolution of magnetic susceptibility, carbonate percentage and geochemical elements along the core of El-Guettiate sebkha. Note: Sodium(Na)mg/kg; CaCO₃(%).

The percentage of CaCO₃ in the core varies from 6% to 34%. However, the first 10 cm have high carbonate content, with an average of 18%. The second interval extends to 40 cm and is characterized by an increase in the carbonate percentage. The first and second intervals are repeated until the end of the sebkha core to indicate the establishment of revealing conditions at the top of the core. We note that the variation in carbonate percentage is probably due to solar activity. Consequently, high values indicate an arid climate and low values indicate a humid climate.

The magnetic susceptibility of the sediment samples varies from 13 10-6 SI to 45 10-6 SI along the length of the sebkha core. Indeed, the greatest depths were recorded at 19 cm to 79 cm, while the lowest were recorded at 0-18 and 80 to the end of the core (Figure 4).

Holocene cyclicity

Na and Ca: The main factors involved in climate change in the El-Guettiate sebkha (south-eastern Tunisia) were identified using spectral analysis. PAST software reveals important climatic factors by processing chemical element percentage values. Calcite and gypsum, the symbols of precipitated evaporite minerals, were present. Plagioclase feldspar, the main source of Na, decomposes easily by hydrolysis when sufficient moisture is available, releasing water.

The individualization of a cycle extends 1300 years, according to sodium-based spectral analysis. On the other hand, the individualization of two cycles extends over approximately 1000 years and 1700 years, according to calcium-based spectral analysis. Numerous other studies have established a cycle of around 500 years [18].

Several studies of the North Atlantic and Mediterranean regions indicate that the 1000 years and 1300 years cycles have a probable link with ocean circulation and solar activity (Figures 4 and 5) [19- 22].

Figure 5: Spectral analysis based on the Na and Ca contents.

SI and CaCO₃: Spectral analysis of the data for the various parameters can highlight the cyclicity clearly identified along the core. However, the spectral analysis of magnetic susceptibility shows that the individualization of a cycle extends over about 1000 years.

Zaibi dating, 2011 used to estimate the age-depth model of the El-Guettiate sebkha. In contrast, spectral analysis of carbonate percentage data reveals the individualization of two cycles spanning 1600 years and 860 years.

The second harmonic of the 1500 years cycle of oceanic thermohaline circulation is barely attributed to the 700 years cycle [23]. This cycle may also indicate a close link between the Indian Ocean and African rainfall. It may also be the result of a coupling between the Indian Ocean and African precipitation, or it may be the result of the coupled ocean-atmosphere variability that is inherent in the tropical monsoon system [24]. With regard to the 1700 years cycle highlighted a frequency of 1660 years using sediment colour as an indicator of carbonate content (Figure 6) [25].

Figure 6: Spectral analysis based on the magnetic susceptibility and CaCO₃ percentage.

Principal component analysis applied to the sediment samples highlighted the various parameters (magnetic susceptibility, carbonate percentage, calcium, sodium and potassium content). Projection of the variables onto the factorial plane (1 × 2) shows that the main axis accounts for more than 51.64% of the total variance.

Figure 7, shows that the results for these sebkha El-Guettiate parameters prove that Magnetic Susceptibility (MS) is anti-correlated with sodium level, calcium level and carbonate percentage. The PCA demonstrated that fluctuations in carbonate content can explain the climatic variability represented by magnetic susceptibility as an indicator of climate change. During wet periods, the hydrographic network brings sediments from the vicinity of the continent, which increases the rate of magnetic matter and causes the dissolution of CaCO₃ and mineral salts. In arid periods, MS values decrease due to the deposition of diamagnetic materials such as carbonate and quartz, but the percentage of CaCO₃ and the rate of mineral halite increase considerably (Figure 7) [26].

Figure 7: Application of principal component analysis to the sediment samples highlighting the various parameters like Magnetic Susceptibility (MS), Carbonate (CaCO₃) percentage, Calcium (Ca), Sodium (Na) and Potassium (K) content.

Conclusion

The study of the El-Guettiate sebkha allows us to visualize the presence of a wet soil by examining mineralogical data, carbonate levels, geochemical elements (Na, K and Ca) and magnetic susceptibility. According to the mineral parameter evaluation study, evaporites (halite, gypsum), silicates (quartz and feldspar) and carbonates (calcite, dolomite and magnesian) constitute the nonclay minerals. As a general rule, variation in carbonate (CaCO₃) content indicates that high levels of CaCO₃ indicate a dry climate, while low levels indicate carbonate dissolution in a humid climate.

Calcium data and carbonate percentages showed double cycles, from 1600 years to 860 years and from 1700 years to 889 years, respectively. But potassium data showed no significant cycles. However, the 1000 year and 1300 year cycle was highlighted by spectral analysis of magnetic susceptibility and sodium data.

Consequently, Holocene cyclostratigraphy is the result of combined solar activity, oceanographic and atmospheric conditions.

Acknowledgments

Dr. Ahmed Hichem Hamzaoui, Bachir Slimi, and Ramzi Ben Massoud for their help and assistance in the geochemical field and mineralogical analysis at the Borj Cedria Technopole Center, and to Mr. Mohamed Lassoued director of CRDA Sfax, Dr. Mabrouk Boughdiri chef Department de faculty of sciences of Bizerte University of Carthage Tunis and Dr. Elhoucine Essefi for his contribution to the missions sampling.

Author Contributions Statement

The authors significantly contributed to preparing, writing, and revising the manuscript. Najia Bouabid contributed to data interpretation and wrote the major manuscript, sampled, data collection and analyses. Feyda Srarfi contributed to writing and revising the manuscript. Mohamed Ali TAGORTI revised the manuscript. All authors read and approved the final manuscript.

Declarations

Conflict of interest

The authors declare no competing interests.

Funding

There is no funding for this work.

Data availability

The datasets used and analysed during the current study are available from the authors on reasonable request.

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