Papers by Noureddine Elmejdoub
World Academy of Science, Engineering and Technology, International Journal of Environmental and Ecological Engineering, 2016
<p>Continental carbonates constitute an interesting topic of study ... more <p>Continental carbonates constitute an interesting topic of study since they are important archives recording climate and paleoenvironmental changes. In Tunisia, calcretes are formed during the Pliocene-early Pleistocene (Villafranchian). They mainly occur in the center and on the coastal plain of Djeffara (Southern East) while their presence is more sporadic in the North of the country. Continental carbonates may form in the soil, groundwater, and palustrine and lacustrine environments. Four criteria are used to differentiate these different environments: host rock, components and micromorphological texture, subaerial exposure (seasonality), and flora and fauna.</p><p>The purpose of this study is to investigate the palaeoenvironmental significance of Tunisian calcretes for the Plio-Pleistocene. For this aim, well-developed carbonate beds were studied along a north-south transect that crosses the climatic boundaries between the latitudes 33 ° and 37 ° N: North (N36º.43.713 E10º.06.681’) Center (N35º.07.077 'E10º. 14.545 ') and South (N33º.28.898' E10º.20.597 ').</p><p>Based on the macroscopic, petrographic, and cathodoluminescence observations in association with scanning electron microscopy results, we classified thin sections extracted from the massive horizons in all sites into 8 facies types:</p><p>The Northern site is characterized by 1) pisolithic calcretes and 2) laminar calcretes showing several beta microfabrics such as rhizolith, peloids, intraclasts, coated grains, bioclast debris, cracks, alveolar septal, spherulite, and organic matter. Moreover, 3) pseudo microkarst carbonates were detected in Northern Tunisia by the presence of vertical layers, peloids, intraclasts, circum crack grain, pedotubules, and rhizoconcretions, and bioclasts.</p><p>At the central site, 4) laminar calcretes are characterized by planar layers. Both 5) massive brecciated calcretes and 6) mottled nodular brecciated calcretes are distinguished by the abundance of cracks. All these facies types’ shows similar pedogenic components such as peloids, coated grains, gastropod shell, alveolar septal, ooids, and bioclasts.</p><p>The southern site is characterized on the one hand by 7) laminar calcretes composed of peloids, nodules, coated grains, cracks, and on the other hand by 8) groundwater nodules showing a massive aspect with alpha microfabrics. In all sites, the nodular horizons are pedogenic.</p><p>Thus, our Plio-early Pleistocene calcretes formed in three main depositional environments: pedogenic, groundwater, and palustrine. The groundwater calcretes are formed under phreatic conditions while the Tunisian pedogenic calcretes and palustrine carbonates exhibit subaerial exposure characteristics (cracks). Except for southern groundwater calcretes, all other types of carbonates show biogenic traces. The palustrine carbonates developed above lacustrine mud. Whereas, pedogenic and groundwater calcretes, develop on different types of host rocks (siliceous and clayey). The difference between fabrics indicates that during the Pliocene-early Pleistocene calcrete form in the north in palustrine settings while it forms in groundwater context in the south. Furthermore, the transition between the different environments is controlled mainly by variation in the water table suggesting a variable but generally more humid climate during the Plio-Pleistocene in North Africa.</p>
Current understanding of sapropel formation in the Mediterranean suggests a combination of anoxia... more Current understanding of sapropel formation in the Mediterranean suggests a combination of anoxia and enhanced primary productivity. Enhanced anoxia should have resulted from freshwater input most likely triggered by the northward expansion of the African monsoon during the last interglacial when sapropels S5, S4 and S3 formed at ~125 ka, ~105 ka and ~83 ka respectively in various East Mediterranean sub-basins. We investigated coastal sediment successions in the Gulf of Sirt to test the hypothesis of a, besides the Nile, second source of freshwater. The coastal sediment succession in the Gulf is characterised by oolitic grainstones representing coastal backshore and foreshore depositional environments in onlap and in offlap position, respectively. The spatial distribution of these deposits indicates the maximum transgression shoreline situated today around 20 km inland marked by a broad dune belt parallel to the modern coastline. The oolitic sediments represent a period of elevated sea-surface temperature and lack of fluvial discharge lasting around 120 – 60 ka according to our optical dating results in conjunction with global sea-level data.
Mediterranean ‘raised beaches’ were subject to Quaternary research since the early years of the 2... more Mediterranean ‘raised beaches’ were subject to Quaternary research since the early years of the 20th century. The uniqueness of a warm-loving molluscs fauna immigrating into the Mediterranean made the coastline a prime interest for studying Quaternary sea-level changes. Today, we have a detailed picture of this historically important coastline characterised by tectonically dormant coastal zone alternating with zones that are subject to tectonic deformation. As part of the Word Atlas of last interglacial shorelines (WALIS) database we compiled 10 MIS 5e proxies for the eastern Mediterranean area available at http://doi.org/10.5281/zenodo.4454553 (Mauz, 2020). These datapoints are sea-level indicators of variable quality situated between 0±1 m and 12±10 m resulting in a reconstructed MIS 5e palaeo-sea level situated between 0±2 m and 13±10 m.
Quaternary Science Reviews, 2012
The response of shorelines to climate change is controlled by fall and rise of the sea level and ... more The response of shorelines to climate change is controlled by fall and rise of the sea level and by the alteration of the coastal environment due to changing fluvial discharge and biological activity. In the Mediterranean this response is complicated by the geographic proximity of the North Atlantic and the African Monsoon climate systems, by a time and space specific interaction of eustatic and water-load components of sea level and by the mid-latitudinal time lag between orbital forcing and terrestrial response. Here, six Mediterranean coastal records are presented which contribute to our understanding of how mid-latitudinal coasts respond to orbital forcing. The sediment sequences show sharp switches between siliciclastic-and carbonate-dominated nearshore environments where carbonate-rich sediments are composed of oolitic grainstones. From modern analogues it is deduced that the oolitic sediments represent a period of relatively high annual sea-surface temperature and lack of fluvial discharge. The warm-arid period was recorded at w114 ka on the southeast Iberian coast, at w113 ka on the Levant coast, at w110 ka on the coast west of the Nile delta and at w83 ka on the north Saharan coast. It lasted 10e20 ka in east (Levant coast) and west (Iberian coast) and lasted 40 ka or more in the central-south of the east Mediterranean. Timing and duration of the coastal proxy allow inferring instantaneous and dominant response to external forcing in the east and west and delayed and prolonged response due to dominant regional forcing in the centre of the East Mediterranean. A 9 m eustatic sea-level highstand during MIS 5e is suggested with a start of the subsequent sea-level fall at w118 ka while evidence for multiple MIS 5e highstand and a highstand during MIS 5a remain elusive.
Journal of Sedimentary Research, 2014
ABSTRACT The Trab El Makhadha beach-ridge plain presents a progradational shoreline feature on th... more ABSTRACT The Trab El Makhadha beach-ridge plain presents a progradational shoreline feature on the Gulf of Gabes (Tunisia). Diachronic analysis coupled with a morpho-sedimentary approach was applied to elucidate the genesis mode of the beach-ridge plain, its sedimentary sources, and the climatic conditions responsible for the prograding trends in the Gulf of Gabes. Shapes, dimensions, sedimentologic compositions, and other textural parameters of the sandy ridges have been analyzed. Therein, we found that the Trab El Makhdha Coast is made up of two distinctive categories of eolian beach-ridge complex (progradational sequences), implying a significant change in climatic and hydrological conditions during the deposition of each ridge. In the Holocene sequence, about 5600 14C years old, the predominance of detrital material suggests an important contribution to sediment supply from continental input. The preservation of the eolian beach ridge and its adjacent marine sediment has been favored by the postglacial hydro-isostatic rebound. Today, the paucity of rainfall is reflected by the decrease in input of terrigenous material. Therefore, the modern sequence, which has been emerging since 1987, is made mainly of bioclastic sand supplied by subaqueous shoals due to a rapid rise in sea level. The observed decrease of sea level around 2100 14C yr B.P. largely contributed to the formation of the swale. The formation of the beach-ridge plain shows that since the middle Holocene highstand the coastal segment of Trab El Makhadha has been governed by equilibrium of sediment abundance.
Quaternary Research
To understand past and future sea-level variability, it is important to know if during an intergl... more To understand past and future sea-level variability, it is important to know if during an interglacial the eustatic sea level is constant or oscillates by several meters around an average value. Several field sites within and outside the tropics have been interpreted to suggest such oscillations during Marine Oxygen Isotope Stage (MIS) 5e (129–116 ka). Here, we present our analysis of one such non-tropical site, Hergla, where a facies succession indicates two foreshore deposits above each other, previously interpreted as MIS 5e sea-level highstand amplified by a second rise. Our study, based on field, microfacies, and optical age Bayesian statistics shows a sea-level rise forming the upper foreshore strata that coincided with the global sea-level rise of the MIS 5a interstadial. The site does therefore not provide evidence for the MIS 5e double peak. We conclude from our analysis that the facies-based proxy is insensitive to small-scale sea-level oscillation. Likewise, uncertainties...
This paper aims at outlining shallow marine carbonates that can be used to infer coastal and hint... more This paper aims at outlining shallow marine carbonates that can be used to infer coastal and hinterland climate for the time period of deposition.
Quaternary Geochronology, 2012
In this study thermally transferred (TT) OSL and post-IR elevated temperature IRSL (290 C) (pIRIR... more In this study thermally transferred (TT) OSL and post-IR elevated temperature IRSL (290 C) (pIRIR 290) dating are applied to deposits covering coastal terraces on the Cap Bon peninsula, Tunisia. Both methods perform well under standard performance tests; dose recovery tests using a modern analogue show that doses relevant to our study can be recovered accurately. Residual signals in the modern analogue for both signals are very small (w2 Gy). For the younger (<250 ka) deposits reasonable good agreement of the ages is observed between both methods, and in addition with standard quartz OSL dating. Systematic discrepancy in ages is found for the older sediments. The TT-OSL underestimate (compared to the pIRIR 290 ages) is most likely due to the short lifetime of the TT-OSL trap; we estimate a lifetime of w0.7 Ma at 19 C (mean ambient air temperature of the study area). This is the first time this lifetime has been derived from geological data and it is within the range of previously published laboratory estimates. This result suggests that TT-OSL is not likely to provide a significant extension of the age range beyond that available from other methods. Our preferred pIRIR 290 ages suggest that the geological setting on Cap Bon is not as simple as previously suggested.
Mid-Holocene sea-level emergence was recorded along a mid-latitude coastal site in the Gulf of Ga... more Mid-Holocene sea-level emergence was recorded along a mid-latitude coastal site in the Gulf of Gabes, Tunisia, in beach rock deposits. However, the post-emergence sea-level curve and associated hydro-sedimentary processes have rarely been documented. This study presents new records of sea-level changes along the Tunisian coast since the mid-Holocene. Each phase of the sea-level curve was evidenced and supported by lithostratigraphic, morphological and archaeologic markers. Post-emergence sea-level fall due to postglacial isostatic rebound was recognized in regressive sequences which vary depending on available accommodation space, from a small pocket beach to a well developed dune system. Sea-level stabilization following isostatic rebound has promoted the end phase deposits of the regressive sequences. However, this stability was interrupted during the late Holocene by a high-energy storm event that accumulated a transgressive boulder unit at 2 m above present. Shape and disposition of the boulder ridge behind a prograding modern shoreline suggests that sea level has not changed during the late Holocene. The existence of a Roman fish pond on the upper foreshore confirms stability of sea level during the last two millennia. The combination of lithostratigraphic, morphological and archaeologic markers represents a tool for improving the accuracy of dating of erosional and depositional periods in mid-coastal sections.
To understand past and future sea-level variability, it is important to know if during an intergl... more To understand past and future sea-level variability, it is important to know if during an interglacial the eustatic sea level is constant or oscillates by several meters around an average value. Several field sites within and outside the tropics have been interpreted to suggest such oscillations during Marine Oxygen Isotope Stage (MIS) 5e (129–116 ka). Here, we present our analysis of one such non-tropical site, Hergla, where a facies succession indicates two foreshore deposits above each other, previously interpreted as MIS 5e sea-level highstand amplified by a second rise. Our study, based on field, microfacies, and optical age Bayesian statistics shows a sea-level rise forming the upper foreshore strata that coincided with the global sea-level rise of the MIS 5a interstadial. The site does therefore not provide evidence for the MIS 5e double peak. We conclude from our analysis that the facies-based proxy is insensitive to small-scale sea-level oscillation. Likewise, uncertainties associated with age estimates are too large to robustly infer a short-term sea-level change.
The continental shelf of the Gulf of Gabes is outlined, during the MIS 5c and MIS 5a onshore high... more The continental shelf of the Gulf of Gabes is outlined, during the MIS 5c and MIS 5a onshore highstands, by the genesis of forced regressive beach ridges situated respectively at –19 m b.s.l/100 ka and –8 m b.s.l/80 ka. This area, considered as a stable domain since at least the last 130 ka (Bouaziz et al. 2003), is a particular zone for the reconstruction of the late quaternary sea-level changes in the region. Shuttle Radar Topography Mission (SRTM) data and field observations are highlighted to deduce interaction between hydrodynamic factors and antecedent topography. Variations in geomorphology were attributed to geological inheritance. Petrography and sedimentary facies of the submerged coastal ridges reveal that the palaeocoastal morphology was more agitated than today and the fluvial discharges are consistent. Actual morphologic trend deduced from different environment coasts (sandy coasts, sea cliffs and tidal flat) is marked by accumulation of marine sands and progradation.
Journal of Sedimentary Research, Mar 18, 2014
The Trab El Makhadha beach-ridge plain presents a progradational shoreline feature on the Gulf of... more The Trab El Makhadha beach-ridge plain presents a progradational shoreline feature on the Gulf of Gabes (Tunisia). Diachronic analysis coupled with a morpho-sedimentary approach was applied to elucidate the genesis mode of the beach-ridge plain, its sedimentary sources, and the climatic conditions responsible for the prograding trends in the Gulf of Gabes. Shapes, dimensions, sedimentologic compositions, and other textural parameters of the sandy ridges have been analyzed. Therein, we found that the Trab El Makhdha Coast is made up of two distinctive categories of eolian beach-ridge complex (progradational sequences), implying a significant change in climatic and hydrological conditions during the deposition of each ridge. In the Holocene sequence, about 5600 14C years old, the predominance of detrital material suggests an important contribution to sediment supply from continental input. The preservation of the eolian beach ridge and its adjacent marine sediment has been favored by the postglacial hydro-isostatic rebound. Today, the paucity of rainfall is reflected by the decrease in input of terrigenous material. Therefore, the modern sequence, which has been emerging since 1987, is made mainly of bioclastic sand supplied by subaqueous shoals due to a rapid rise in sea level. The observed decrease of sea level around 2100 14C yr B.P. largely contributed to the formation of the swale. The formation of the beach-ridge plain shows that since the middle Holocene highstand the coastal segment of Trab El Makhadha has been governed by equilibrium of sediment abundance.
Uploads
Papers by Noureddine Elmejdoub
metals, major cations, total organic carbon and dissolved oxygen) were determined on forty samples collected from the surface and the bottom of the lagoon water. Our results reveal that the southwestern part of the lagoon shows the highest levels of toxicity approaching critical thresholds and exceeding the thresholds toxicity in some points. The southeast and northeast parts of the lagoon are characterized by relatively high Total Organic Carbon (TOC) content of about 5.81 mg.l-1. This high TOC content is related to the agriculture organic matter and the presence of shellfish areas. Hence, this lagoon experiences mineral
and organic pollution of anthropogenic origin essentially. This pollution affects water quality of the lagoon and can be transmitted to the Mediterranean Sea by water exchange through the channel.
The coastal sediment successionin the Gulf is characterised by oolitic grainstones representing coastal backshore and foreshore depositional environments in onlap and in offlap position, respectively. The spatial distribution of these deposits indicates the maximum transgression shoreline situated today around 20 km inland marked by a broad dune belt situated today inland and parallel to the modern coastline. The oolitic sediments represent a period of elevated sea-surface temperature and lack of fluvial discharge lasting around 120 – 60 ka according to our optical dating results in conjunction with global sea-level data.
While there is compelling evidence from the Levante coast for the dominant role of the Nile discharge in generating conditions for sapropels to form, we cannot confirm a similar role of the Sahabi and Kufrah rivers. These rivers should have flown through the Saharan desert during the wet period of the last interglacial and should have supplied a large lake situated in the coastal zone of the Gulf which, in turn, should have drained into the East Mediterranean Sea.But our reconstruction of the last interglacial environment in the coastal zone of the Gulf does not suggest the existence of a lake or rivers reaching the sea.
Along the Eastern coasts of the Cap Bon, the petrographic contrast within the Last Interglacial deposits brought evidence for significant changes in climatic conditions. The establishment of wetter conditions at the beginning of MIS 5e contributes to sediment supply from continental runoff, and then to predominance of siliciclastic material; whereas the change of these conditions at the end of the MIS 5e was associated with a strong decrease in humidity and development of carbonate sedimentation.
The Western coasts of Cap Bon show the predominance of siliciclastic sedimentation during the whole Last Interglacial. This was probably related to the influence of the Western Mediterranean surface water circulation. The quartz-rich sediment found in this area might be provided by influence of the Algerian Current which supplied terrigeneous material available in the North-African littoral zone.
The post-Last Interglacial sedimentation consists of coastal siliciclastic dunes installation after MIS 5e highstand. The implementation of these aeolianites was advantaged by the availability of material supplied by the Algerian Current and the north-westerly winds.
The revised chronostratigraphic framework of marine Pleistocene deposits in the Cap Bon peninsula, available as well in all Tunisian coasts, permits to identify the influence of climatic events on sedimentary imprints.