CLIMATIC SIGNIFICANCE OF GRAIN SIZE PARAMETERS OF THE WET AEOLIAN SEDIMENTARY FILLING OF SEBKHA MHABEUL, SOUTHEAST TUNISIA: CONTRIBUTION OF THE GENETIC APPROACH ELHOUCINE ESSEFI*, NAJOUA GHARSALLI AND CHOKRI YAICH Department of Earth Sciences, National Engineering School of Sfax, University of Sfax, Tunisia. [EE, NG, CY] RU: Sedimentary Dynamics and Environment (DSE) (Code 03/UR/10-03), National Engineering School of Sfax, University of Sfax, Tunisia [EE, NG, CY] [*For Correspondence: E-mail:
[email protected]] ABSTRACT This paper aims to investigate the climatic signals within the wet aeolian sedimentary deposits of sebkha Mhabeul. The descriptive grain size distribution (mean, mode, median) was not enough to better visualize the climatic variability. The here proposed genetic approach highlighted the individualization of the four climatic stages through the primary and secondary modes and the ratio between them. Humid climatic phases are associated with higher values of secondary mode. Arid climatic stages are characterized by higher values of the primary mode. As convolution of the two previous parameters, the ratio increases due to arid climatic phases. During the arid periods of WP and Medieval Climatic Anomaly (MCA) values of the ratio are high whereas during the wet period of the Little Ice Age (LIA) values are lower. In doing so, the genetic approach is proposed as better tool to the investigation of the sedimentary environments than the descriptive one. Keywords: Climatic stages, grain size distribution, genetic approach, sebkha Mhabeul, Southeast Tunisia. INTRODUCTION Saline systems in Tunisia may record the climatic variability during the Holocene. These climatic changes are inferred through paleontological (e.g., Zaibi et al., 2011), geochemical (Pomel et al., 2006), mineralogical (Lakhdar, et al., 2006) and sedimentological (Essefi et al., 2013a,b,c) proxies. Nonetheless, the sedimentary proxy seems more relevant since the geochemical climatic signal can be diluted in the homogeneity imposed by the water table reaching the sebkha surface (Essefi et al., 2012c; Essefi, 2013) and by the fact that
typical fossils are not always present in saline systems, especially in endorheic ones (Essefi et al., 2012c). In this way, the study of sedimentary systems and processes proves vital to the understanding of present and past earth surface processes. Particle size, for example, is used as a proxy for climate change and sea-level fluctuations (Prins et al., 2000; Stuut et al., 2002a,b). This work describes the evolution of the grain size distribution during the last millennium along a 37 cm core from sebkha Mhabeul. It was given special care to the
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contribution of the genetic approach of the grain size analysis as a proxy for changes of depositional processes and the detection of the aeolian fraction. STUDY AREA The 4 Km2 sebkha Mhabeul is located on the northern edge of the coastal Djeffara plain (Fig. 1). This 40 Km wide plain is covered with marine and lacustrine Quaternary deposits and bordered to the south by an escarpment of Mesozoic limestones (Stengele and Smykatz-Kloss, 1995). The region is characterized by an arid climate imposing real desert conditions, with considerable variation of temperature and precipitation. The wet winter is marked by low temperatures (1 to 2 °C) of continental character while on the other hand the dry period of the summer is characterized by excessive temperatures reaching sometimes 40 °C in the shade (Tagorti et al., 2013). The total rainfall average is about 100 mm/year with significant monthly variations. The indexes of evaporation are at their highest level in July and August reflecting a noticeable correlation with the temperature variation (Tagorti et al., 2013). Depending on the season, the wind sector changes and sometimes blows with an instantaneous speed easily reaching 10 m/s. The winds of the wet season are generally from the west to east while those of the dry season are from the south to south-south-west. The latter are frequently accompanied with sirocco, especially during June, July and August. The sebkha of Mhabeul is disjointed from the eastern larger sebkha Melah by an accumulation of Upper Pleistocene sediments, which avoids any connection of the former to the sea. Its sedimentation mode is mainly controlled by precipitation and runoff from small hydrographic networks (Schulz et al., 1995, 2002). This weak hydrography is due to a less contrasted topography (Fig. 1b,c). Nonetheless, at these lands, where wind is always blowing, the aeolian sedimentation could by no means be excluded. The site was previously studied by Marquer et al. (2008), who inferred the climatic variability during the last millennia. This study was useful in the correlation with other playas in Tunisia, such as the sebkhas of Dkhila and Souassi in eastern Tunisia (Essefi, 2009; Essefi et al., 2013b,c).
3. MATERIALS AND METHODOLOGY A 37 cm core was taken in sebkha Mhabeul, (Fig. 1). According to the age-depth model based on the tephrochronology (Marquer et al., 2008), the mean rate of sedimentation during the last millennium is about 0.33 mm/year. Accordingly, the 37 cm core of the sebkha Mabheul covers the last 1100 yr (Fig. 2) (Essefi et al., 2014). This time interval should encompass different climatic stages (Essefi et al., 2013b,c): the Warming Present (WP), the Late Little Ice Age (Late LIA) (Wiles et al., 2008), Early Little Ice Age (ELIA), and the Medieval Climate Anomalies (MCA). In the laboratory, grain size distribution and magnetic properties were evaluated by FRITSCH laser granulometer and Bartington MS2B probe, respectively. Granulometric data were investigated in terms of genetic grain size distribution. Magnetic properties are efficient tool for environmental and paleoenvironmental investigations (Essefi et al., 2015a,b). The low frequency Magnetic Susceptibility (MS) was measured by a Bartington MS2B probe (in the laboratory of Sedimentary Dynamics and Environment, National Engineering School of Sfax), at frequencies of 0.47 kHz. Samples were packed into 10 cm3 cylindrical plastic (perspex) pots for MS analysis. Within lakes and playas, the magnetic susceptibility may have plaeoclimatic significance (Deotare et al., 2004). During humid climate phases, high-level lake or groundwater rise, the sedimentation is governed by paramagnetic aeolian sediment coming from far regions (Deotare et al., 2004). The vicinities of the playa are consolidated and aeolian sedimentation rich with diamagnetic minerals is limited (Essefi et al., 2013a). On the other hand, during arid climate phases, the geochemical sedimentation of diamagnetic minerals (Deotare et al., 2004) enhanced by a fall of playa level and groundwater, which in turn enhanced the erodability of sediment rich in evaporites within the watershed (Essefi et al., 2013a). Descriptive Grain Size Analysis The deposits of Tunisian saline systems are basically clayey. The fine grain size needs a sophisticated technology. Fritsch apparatus allows
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the study of the grains ranging between 0.01 µm and 2 mm. Then, it produces a graph containing the cumulative and the frequency curves. Fritsch apparatus is equipped with sophisticated software (MaScontrol) providing the calculation of different statistical parameters. In addition to numerous graphs and other options, MaScontrol also offers its user statistical values, which may contain useful information for evaluation of
particle size distribution such as the mean, the mode, and the median. The plots of the mean, mode, and median start from 80 µm, 80 µm and 60 µm respectively (Fig. 2), in order to better visualise the variability. In spite of the use of this specific scale, the plot does not show an obvious variability of the climate. In this way, we call for the genetic approach.
Fig.1. (a) Location of Sebkha Mhabeul in southeast Tunisia and core site: (b) N-S topographic profile; (c) E-W topographic profile
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Fig. 2. Mean, mode, and median of the grain size distribution, magnetic susceptibility and cyclostratigraphy along the core of sebkha Mhabeul Genetic Grain Size Analysis More significance has long been attributed to the shapes of the cumulative grain-size curves rather than distribution curves of sediments (e.g., Cailleux and Tricart, 1962; Bridge, 1981). However, in the details of their form, grain-size distribution curves are more telling than cumulative curves about constituent sub-
populations of particles (Allen and Haslett, 2006). Wet aeolian sediment is distinguished as a mix of aeolian and aqueous particles based on modes of the grain size distribution (Sun et al., 2002; Allen and Haslett, 2006; Manté et al., 2007; Essefi, 2009). Sun et al. (2002) considered the grain size fraction centered around 6 μm as fine aeolian component and the fraction centered around 60 μm as the coarse aeolian component. In contrast,
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the coarse aqueous component is centered around 380 μm and the fine aqueous fraction is centered around 1 μm. According to Allen and Haslett (2006), the shape of a grain size distribution curve displays various ‘features’ (F) characteristic of the dispersed sediment, classified according to their frequencies. The primary (M) and secondary (m) modes have the highest frequencies. Shoulder-like segments (S) are of a lower strength than the primary and secondary modes. Particular features are absent (A) from some samples, and unrealized/unidentified (occluded, O) in others. In addition to the traditional sand/silt/clay subdivisions used in the literature, Manté et al. (2007) coined the term colloids as the geochemical fraction between 0.063 µm and 1 µm. Based on the method of grain size distribution features of Allen and Haslett (2006), the descriptive classification of Flemming (2000), and the reference curves of Cailleux and Tricart (1962), Essefi (2009) distinguished three types of aeolian sediments. First, the aeolian sand transported by strong wind has a distinctive mode at 500 µm and two shoulders at 250 µm and 1600 µm, falling within the Flemming (2000) classification as sand. Second, the slightly silty aeolian sand (Flemming, 2000) could be transported by a moderate wind with a mode at 315 µm and two shoulders at 200 µm and 800 µm. Third, the silty aeolian sand (Flemming, 2000) could be transported by calmer/weaker wind, with the mode at 160 µm and the two shoulders at 250 µm and 1000 µm. Thus, the fractions centered around 6, 60, 160, 250, 315, 500, 1000, and 1600 µm belong to the aeolian component and the fraction centered around 1 and 280 µm belong to the aqueous component. Sometimes, the fraction is centered around a value next to the primary modes, so we use the pre-modifier “translated” in order to show the slight modification. For instance, if a fraction is centered at a shoulder of 2 µm instead of 1.5 µm, it is called a translated shoulder (Essefi et al., 2013a). In this work, we can say that the mode around 110 µm is the translated of 160 µm with ratio between them equal to 1.45. In addition, the mode around 4 µm is the translated of 6 µm with ratio between them equal to 1.5. The primary and secondary modes were found on the cumulative curves. Their evolution and the ratio between them were plotted along the core (Fig. 7).
RESULTS Magnetic Properties and Descriptive Grain Size Distribution In the uppermost part of the core (3.5 cm), the stage Warming Present (WP) stretches from present to 1918, i.e. ≈95 yr; the establishment of modern conditions is characterised by stable conditions. Added to a small salt crust, this period is dominated by a clayey sedimentation rich with organic matter. Geophysically, we notice a downward increase of values of the magnetic susceptibility. Sedimentologically, the mean, the mode and the median of the grain size distribution are stable. Second, the Little Ice Age (LIA) (Wiles et al., 2008); it stretches between the 95yr and 613yr. Along the depth between 3.5 cm and 22 cm, the clayey sedimentation makes up the twofold and threefold laminates. This period itself is divided into different sedimentary and magnetic cycles. Geophysically, the first cycle (3.5 to 14), the Late LIA is dominated by a downward decrease of values of the magnetic susceptibility. Sedimentologically, the mean, the mode and the median of the grain distribution tend toward a slight increase. Third, from ca. 400yr to 613yr, the Early Little Ice Age (ELIA), shows moderate values of the magnetic susceptibility. Along this period, we did not record values lower than 30 x 10-6. The climate is relatively wet, but stable. Fourth, the Medieval Climatic Anomaly (MCA) stretching from 613 yr to 1100 is unstable and dry. The magnetic susceptibility shows a tendency toward the decrease. The magnetic susceptibility may reach values lower than 30 x 10-6. Genetic Grain Size Distribution Warming Present (WP) The WP period is dominated by a sandy sedimentation. The top of the cycle (Fig. 3; 0-1 cm) shows an obvious coarsening; it is characterized by a primary mode (M: ca 110 µm) as indication of the coarse eolian sedimentation. The secondary mode (m: 4.2 µm) as indication of the fine aeolian sedimentation. The bottom of the cycle (Fig. 3; 2-3 cm) shows an obvious fining; it is characterized by a primary mode (M: ca 103 µm) as indication of the coarse eolian
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sedimentation. The secondary mode (m: 3.8 µm) as indication of the fine aeolian sedimentation. The upward coarsening is due to an increasing dryness during the WP.
Late Little Ice Age (Late LIA) The Second stage is located between 3.5 cm and 14 cm; it is called the Late Little Ice Age (Late LIA) (Wiles et al., 2008); it is characterized by a primary mode (M: ca 110 µm) as indication of the coarse aeolian sedimentation, a translated secondary mode (m: ca 4 µm) as indication of the fine aeolian sedimentation, and an occluded to absent fraction (O, A: ca 20 µm) (Fig. 4). The Late Little Ice Age (Late LIA) is expressed by an upward increase of the fine aeolian fraction (Fig. 4). On the other hand, the coarse aeolian fraction shows a slight decrease. The increase of the fine aeolian fraction is an indicator of a decreasing aridity. Wet conditions are marked by weak amounts of the coarse fraction. This availability of the dust in the atmosphere is controlled by the aridity. The more the climate is arid the more the coarse aeolian dust is available (REF). Early Little Ice Age The third stage is located between 14 cm and 22 cm; it is called the Early Little Ice Age (Early LIA); it is characterized by a primary mode (M: ca 115 µm) as indication of the coarse aeolian sedimentation, a translated secondary mode (m: ca 3 µm) as indication of the fine aeolian sedimentation, and an occluded to absent fraction (O, A: ca 20 µm) (Fig. 4). The Early LIA is expressed by an upward increase of the fine aeolian fraction (Fig. 5). On the other hand, the coarse aeolian fraction shows a slight decrease. Medieval Climatic Anomaly (MCA)
Fig. 3. Genetic analysis of the grain size distribution during the Warming Present (WP): the interplay of primary and secondary modes
The fourth stage is located between 22 cm and 37 cm; it is called the Medieval Climatic Anomaly; it is characterized by a primary mode (M: ca 117 µm) as indication of the coarse aeolian sedimentation, a translated secondary mode (m: ca 3.5 µm) as indication of the fine aeolian sedimentation, and an occluded to absent fraction (O, A: ca 20 µm) (Fig. 4). The MCA is expressed by an upward decrease of the fine aeolian fraction (Fig. 6). On the other hand, the coarse aeolian fraction shows a slight increase.
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Fig. 4. Grain size distribution of the (Late LIA) along the core from sebkha Mhabeul: variability of diameter and the frequency of the primary and secondary modes
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Fig. 5. Genetic analysis of the grain size distribution of the Early Little Ice Age along the core from sebkha Mhabeul DISCUSSION Along the core carried out at sebkha Mhabeul, variability of the magnetic susceptibility and grain parameters are linked to climatic variation. The Warming Present (WP) is marked by a downward increase of values of the magnetic susceptibility as indication of an increasing wetting and a decrease of the diamagnetic minerals coming from the in situ precipitation within the playa and aeolian sedimentation of the watershed. The Late LIA is dominated by a downward decrease of values of the magnetic susceptibility as indication of an increasing dryness and an increase of the diamagnetic minerals coming from the in situ precipitation within the playa and aeolian sedimentation of the watershed. Ferromagnetic minerals are quite limited due to a limited rainfall. In spite of a tendency toward an increase of values of MS, the second cycle may be also classified as
wet, comparably with the dry period taking place at end of WP. This Late LIA is a transition period between the dry period taking place at the end of WP and the wet conditions taking place ELIA. The Medieval Climatic Anomaly (MCA) stretching from 613 yr to 1100 is unstable and dry. The magnetic susceptibility shows a tendency toward the decrease. The magnetic susceptibility may reach values lower than 30 x 10-6. In spite the use of specific scales for the grain parameters, they do not show an obvious variability. As a matter of fact, Flemming (2000) pointed out the limit of the descriptive grain size distribution in inferring the radical changes in sedimentary processes. In addition, Allen and Haslett (2006) used the genetic grain size distribution to infer climatic conditions and variation of depositional environment.
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Fig. 6. Genetic analysis of the grain size distribution of the Medieval Climatic Anomaly along the core from sebkha Mhabeul The primary and secondary modes identified on the cumulative curves and the ratios between them were plotted along the core (Fig. 7). The scale of
the plot influences the visualisation of the results. For instance, the plot of the secondary mode and the ratio adopted logarithmic scales allows the
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visualisation of oscillations. The plot of the primary mode adopted a linear scale relatively diluting the climatic signal. During the WP, the plot of the secondary mode shows a significant downward increase. During this period, the primary mode does not show such a significant increase. Subsequently, the ratio M/m downward decreases due to wetter climatic conditions. In the Late LIA, the values of secondary mode continue their increase with a significant oscillation. And, values of the primary mode remain relatively stable. However oscillating, the ratio has a downward decrease tendency. The climate tends toward more humid conditions. In the Early LIA, values of the secondary mode shows an obvious tendency toward increasing the grain size
diameter; the secondary mode reaches its highest values. However increasing, values of the primary mode were not enough to counterbalance the secondary ones resulting hence in lowest values of the ratios. During this period, the ratio reaches its lowest value due to optimum wet conditions. During the MCA, the secondary mode decreases as indication of a decrease of the fine aeolian fraction due to drier climatic conditions and stronger aeolian activity expressed by the stability of values of the primary mode. The genetic approach has the merit of obviously visualizing the variability of the climatic record trough the comparison between the fine aeolian fraction, which is basically indicating wetter condition, and coarser one, which indicates dry episodes.
Fig. 7. Evolution of primary and secondary modes and the ratio between them along the core of sebkha Mhabeul: arid phases marked by high values of the ratio
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CONCLUSION Values of the magnetic susceptibility show that the core encompasses four climatic stages: the Warming Present (WP), the Late Little Ice Age (Late LIA), Early Little Ice Age (ELIA), and the Medieval Climate Anomalies (MCA). The descriptive grain size distribution (mean, mode, median) was not enough to better visualize the climatic variability. The genetic approach highlighted the individualization of the four climatic stages. Humid climatic phases are associated with high values of secondary mode and the ration. The genetic approach has the merit of obviously visualizing the variability of the climatic record trough the comparison between the fine aeolian fraction, which is basically indicating wetter condition, and coarser one, which indicates dry episodes. The genetic approach is better for the investigation of the sedimentary environments since it shows an obvious variability along the core. COMPETING INTERESTS Authors have declared that no competing interests exist. REFERENCES Allen, J.R.L., Haslett, S.K., 2006. Granulometric characterization and evaluation of annually banded mid-Holocene estuarine silts, Welsh Severn Estuary (UK): coastal change, sea level and climate. Quaternary Science Reviews 25, 1418–1446. Bridge, J. S., 1981. Hydraulic interpretation of grain-size distributions using a physical model for bedload transport. Journal of Sedimentary Petrology, 51, 1109-1124. Cailleux, A., Tricart, J., 1962. Le modèle glaciaire et nival, SEDES, Paris, 508 pages. Le modelé des régions périglaciaires, SEDES, Paris, 512 pages. Essefi, E., 2009. Multidisciplinary study of Sidi El Hani Saline Environment: the History and the Climatic Variability. Master thesis, Faculty of sciences of Sfax, University of Sfax. Essefi, E., 2013. Wet aeolian sedimentology and sequence stratigraphy in Eastern Tunisia: implications for wet aeolian sedimentology and
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