Clay mineralogy of the sediments in Sabzevar playa, NE Iran; implication for the late quaternary climate changes

The Quaternary period was characterized by drastic changes in climatic conditions worldwide (Roy et al.,۲۰۰۶) so that the escalation of eolian processes, desert formation and drying of lakes are attributed to thisperiod (Koutavas et al., ۲۰۰۲). Playa is a region with negative water balance for more than half a year aswell as capillary properties close to the surface and sediments (Rosen, ۱۹۹۴). In some cases, playa sedimentsare the only evidence of past environmental conditions in arid and semi-arid regions (Davis, ۱۹۹۴). Clayminerals are one of main proxies for paleoenvironment and paleoclimate studies (Thiry, ۲۰۰۰). Also,changes in clay minerals in sediments are often indicative of climate changes (Birks et al, ۲۰۱۲). The analysisof clay plains in playas and dried pluvial lake provides a good criterion for determining the origin ofsediments and climate change (Oliveira et al, ۲۰۰۲); (Battarbee, ۱۹۹۹). Hence, some researchers (Bentz,۲۰۱۷), (Jones, ۱۹۸۳), (Srodon, ۱۹۹۷), (Tateo et al., ۲۰۰۰), (Kadir et al., ۲۰۱۶ DeVogel et al., ۲۰۰۴) haveconducted valuable studies on late Quaternary climate changes using clay minerals. The present studyaims to reconstruct the late Quaternary climate based on clay minerals obtained from Sabzevar Playa innortheastern Iran. The Sabzevar playa is categorized as a typical playa in central Iran (Kearey, ۲۰۰۹), wherelocated in the eastern part of the Great Kavir basin. Total surface area of this playa is about ۲۶۴۸ KM۲between latitude ۳۵°۵۵'۰۰''–۳۶°۲۵'۰۰'' N and longitude ۵۶°۱۵'۰۰''–۵۷°۴۵'۰۰'' E (Figure. ۱). The topographicalelevation values of the study area vary from ۷۵۰ to ۹۰۰ meters above sea level (a.s.l). Playa sediments areevaporative and clastic, producing in a region with geological features belonging to the Quaternary period(Survey of Geology of Iran, ۲۰۰۵). The playa has been surrounded by ophiolites, ultrabasic rocks andpelagic limestones. This area has a semi-arid climate with annual precipitation of ۱۵۰–۲۰۰ mm and annualtemperature of ۱۶–۱۷ °C (Sabzevar synoptic station) in the period of ۱۹۵۰–۲۰۰۰. The geologicalinvestigation of the region was conducted using four geological maps of Abbas-Abad, Davarzan, Bashtinand Sabzevar (۱:۱۰۰,۰۰۰ in scale). The characteristics of sampling profiles were recorded using GPS in ۷areas at different geomorphic surfaces of the playa during the field observation. All profiles were excavatedby a hand auger, manufactured by the German company Windas, with ۵cm diameter and ۱۰۰cm depth(figure ۲). Additionally, to identify the type and formation of fibrous clay minerals, ۷ samples of intact drysediments were selected from different surfaces and depths. These samples were examined by Scanningelectron microscope (SEM) at the central laboratory of the Ferdowsi University of Mashhad (SPUTTERCOATER SC۷۶۲۰). The sample were also analyzed for major elements using EDS. The clay minerals weredetermined by X-ray diffraction analysis (XRD) in three conditions; intact, saturated with ethylene glycoland heated at ۵۵۰°C for ۴ houres (Moore and Reynolda, ۱۹۸۹). Before XRD analysis the carbonates andorganic matters of the samples were removed by leaching for ۱۲ hours (with ۳۰ minutes of an ultrasonicbath) with ۱۰% acetic acid and ۱۰% H۲O۲. This step of the process was repeated several times until allcarbonates and organic matters were removed. The results of X-Ray diffractograms and Scanning ElectronMicroscope (SEM) images (Figure. ۳) of clay minerals derived from four different depths in Sabzevar playa indicate the existence of authigenic and allogeneic clay minerals including Palygorskite, Sepiolite, Smectite,Illite, Montmorillonite, Chlorite and Kaolinite. Physical erosion and weathering of metamorphic andsedimentary rocks as well as mafic and ultramafic igneous rocks around Sabzevar playas are the source ofmany allogeneic clay minerals; however, some other clay minerals such as smectite and Palygorskite andSepiolite fibres, are observed as both authigenic and allogeneic, due to the weak drainage and evaporativesalineenvironments in the playa. In fact those fibre clay minerals are characteristic of saline-alkaline regions(Kovda and Samiolova., ۱۹۶۹; Szaolcs, ۱۹۸۹), which are formed due to the evaporation and concentrationof the brine rich in magnesium and alkaline carbonates (Hardie and Eugster, ۱۹۷۰). It can be argued thatdue to the dry climate, pH> ۸ in the area and weathering of the surrounding rocks, magnesium and silicondioxide-rich Illite has been transformed into Palygorskite. The EDX of SEM images at two depths of ۹۰-۱۰۰cm and ۳۰-۴۰ cm in profiles reveal the authigenic formation of Palygorskite and Sepiolite clay mineralsunder the dry and warm climate under suitable geochemical conditions. Smectite is characteristic of salinealkalineenvironments and warm and humid conditions. Also, it has an authigenic origin in poor drainageconditions where evaporation is progressive (Szaolcs, ۱۹۸۹). Vermiculite is also formed during thetransformation of Illite to Smectite. The presence of authigenic Smectite and Vermiculite at depths of ۶۰ to۷۰ cm in the profiles reflects more humid conditions compared to the present time. Kaolinite is generallyformed by physical erosions with high hydrolysis in warm and humid climates (Moore and Reynolds, ۱۹۸۹;Perederij, ۲۰۰۱). However, in arid and semi-arid regions, due to unfavorable climati conditions, this mineralis not formed. At a depth of ۶۰ to ۷۰ cm in the studied profiles, Kaolinite is found in high quantity. Given thepresence of some authigenic Smectite minerals in this depth, it can be inferred that they have been formedin more humid conditions or moved around the playa by water currents and deposited in the playas Illitesmostly appear at depths of ۶۰ to ۷۰ cm (Table ۱), which reflects severe physical erosion and hot and humidclimates. Today, due to the arid and semi-arid climate of the region, the conditions are not fertile forphysical erosion of igneous rocks and Illite formation.