Sedimentary geochemistry as an indicator of late Quaternary climate changes in Jazmurian playa

Climate, as the average temperature or precipitation over a long period of time, has been constantl  fluctuating and changing, and there is no simple relationship between climate change and human biological and cultural reactions.Understanding the environmental characteristics of the past is important to understand climate changes.Climate changes are reflected in the physical and chemical characteristics of sediments, which can be used to understand sedimentary processes and ancient sedimentary environments.In the meantime, the sediments of playa lakes are sensitive indicators of local climates And due to the influx of large volumes of sediments during floods and as a result of the high rate of sedimentation, it is possible to record climatic and environmental changes with high resolution. Jazmurian playa in the southeast of Iran consists of sand, mud and salt flats. Over time, under the influence of water and wind processes, Jazmurian has left various deposits in deltaic, lake, playa and wind environments.

Jazmourian plateau between latitudes 58˚-40' to 59 ˚-14' and latitudes 27˚,10' to 27 ˚, 41' in southeastern Iran and west of Sistan and Baluchestan province and east of Kerman province. It is located at an altitude of about 360-500 meters above sea level. The catchment area is about 69,600 square kilometers, of which about 34,160 square kilometers are mountainous areas, 32,440 square kilometers are plains and foothills, and the remaining 3,000 kilometers are swamps and salt marshes. For this study, intact sediment cores were used to reconstruct the Paleo climate. Five sedimentary cores with a maximum depth of 7 m and a total depth of about 30 m were collected and investigated from the sediments under of different areas of Jazmourian Playa using a hand-held core. Sedimentary facies were identified and distinguished based on sediment texture and composition, composition of destructive sediments and organic matter, color, presence of plant and shell residues, and other macroscopic components. According to sedimentological studies and changes in sedimentary facies, eight separable sedimentary facies belonging to river, playa, lake, delta and wind environments were identified. To estimate the time of climatic events in the region, the results of the survey of Vaezi et al. (2019) were used.

In order to reconstruct the environmental and climatic conditions of Jazmurian playa by using the geochemistry of sediments, 5 sediment cores were taken from the inner part of the playa. Coring was done by manual auger to a depth of about 7 meters. In taking the cores, the changes of sedimentary facies were recorded based on macroscopic evidence such as the type of layering surfaces, the geometric shape of sediments, sedimentary structures, animal remains and contents, plant remains, the presence of evaporite minerals, the relative degree of hardness, color and its changes. Reconstruction of the spatial distribution of different sedimentary environments and their displacement and conversion to each other was done based on the data of sedimentary cores in different areas and finally the stratigraphic column of the studied cores was drawn. Half of the collected samples were taken to the laboratory for preparation and chemical analysis, and the other half was archived at 4°C. A total of 19 sediment samples from a core (comprehensive core) were sent to the laboratory of the Geological and Mineral Exploration Organization of the country for chemical analysis. Varian 735-ES model ICP-OES (ICP Optical Emission Spectroscopy) device was used for elemental analysis and the results of the anomaly level of 54 other elements in terms of ppm were identified by the ICP device. In the studied cores, the relative changes of elements in different time periods are very high, this situation indicates the presence of severe changes in the sedimentary sequence of Jazmurian playa bed. This study provides a breakdown of environmental and climatic changes in the southeastern region of Iran during at least the last 20 thousand years. In this research, climatic and environmental events have been identified in the sedimentary sequence taken from Jazmurian playa according to elemental anomalies and 8 elemental ratios. The elemental ratios of Mn/Al and K/Al as an indicator of chemical weathering changes in the watershed in the period of 20 thousand years are changing between 0.0154 to 0.0135 and 0.226 to 0.252, respectively. An increase in these ratios indicates the existence of favorable conditions for increasing the amount of chemical weathering in the watershed and a corresponding increase in humidity and temperature, and its decrease indicates the presence of dry conditions in the basin.

Sedimentological investigations, statistical parameters of sedimentology and sedimentary facies of the cores taken from Jazmurian playa led to the identification of five sedimentary environments including wind, fluvial, deltaic, lake and playa sediments in Jazmurian basin. The sedimentation rate in Jazmurian playa is different in different depths, this rate reaches 0.01 mm per year in the surface parts. The average sedimentation rate in Jazmurian playa is about 0.4 mm per year. V/Cr, Mn/Al, K/Al, Fe/Ca, Ti/K, Ca/Mn, Ca/Sr, Fe/Al element ratios to reconstruct the climatic and environmental conditions, the amount of debris (clay sediments and wind deposits) The input to the basin, the severity of weathering and erosion in the catchment basin and the fluctuations of the lake water level indicate the climatic condition of Jazmurian region in two time periods. The time period of the late Pleistocene when suitable humid conditions prevailed. During this period, the amount of chemical weathering in the catchment area and sediment load to the playa has been significantly high. Since the beginning of the Holocene, climatic and environmental fluctuations have increased in the basin, and until the beginning of the middle Holocene, there has been a gradual decrease in humidity, with the beginning of the middle Holocene, this process has accelerated, and wind-blown sands have entered the northern and western parts of Jazmurian playa. From the beginning of the middle Holocene until now, according to the investigation of sedimentary facies and elemental ratios, at least two dry periods in 8200 and 4200 years ago can be identified and traced.