The hydrogeochemistry of shallow groundwater from Lut Desert, Iran: The hottest place on Earth
Introduction
The Lut Desert (Dasht-e-Lut) in southeast Iran is one of the largest deserts in Iran, receiving < 30 mm of precipitation annually. Its bioclimate is classified has ‘tropical hyperdesertic,’ the hottest and driest bioclimate in the world in the Global Bioclimatic Classification (Djamali et al., 2011). The region has been referred to as the thermal pole of the Earth and is recognized as one of the hottest, if not the hottest, locations on planet Earth with surface temperatures as high as 70.7 °C recorded via satellite in the mid 2000's (Mildrexler et al., 2006). A recent measurement during the sampling campaign in Yalan dunes (March 4, 2017) returned skin temperature as high as 78.2 °C (Trescher, 2017). The mean annual precipitation (7-year average, 2003–2010) recorded at the nearest meteorological station, Shahdad synoptic weather station, located at the west flank of the Lut desert with the altitude of 400 m.a.s.l, does not exceed 30 mm. Precipitation is highest from January through March and the dry season lasts from April to November. Mean annual temperature at the station is 27.5 °C and the mean minimum and maximum daily air temperatures of the warmest and coldest months of the year are −2.6 °C and 50.4 °C respectively (Fig. S1). Remote sensing analysis from 2002 to 2003 report average daily summer temperatures as high as 45.3–55.7 C, with surface soil temperatures as high as 66.5 to 71.1 (Azizi et al., 2007). The Lut Desert has a total area of ~51,800 km2, and consists of sand dunes, ergs, mega-yardangs (kaluts), desert pavement, as well as large flat areas of salty soils and playas (Alavipanah et al., 2007; Ehsani, 2011). A semi-permanent river (Rud-e Shur) flows from NW of the area and passes the edge of Kaluts and ends in the central depression (Chaleh Markazi).
The Lut Desert is within the Lut Block, whose basement consists of pre-Jurassic metamorphic rocks (pelitic schist) as well as Jurassic sediments intruded by younger (Jurassic to Tertiary) granitic plutons and covered by Tertiary mafic to felsic volcanics (Arjmandzadeh and Santos, 2014; Tirrul et al., 1983; Yazdi et al., 2014). The region is active tectonically with many active faults (Boshrabadi et al., 2018).
A large portion of the region is too harsh for any plants, and has been referred to as aphytic (Mobayen, 1976) and devoid of life. However, recent work has described some of the extremophile microbial life existing in the soils and their abilities to withstand high temperatures, UV light, and even radiation (Mazkour et al., 2017; Mohseni et al., 2014; Shirsalimian et al., 2017). Although previous studies have described the unusual desert geomorphology (Yazdi et al., 2014), there has been little attempt to understand other surficial and shallow subsurface processes occurring there. In addition, the recent expeditions discovered a shallow subsurface water system in at least a portion of this extremely hot place (Stone, 2016; Trescher, 2017). As a first attempt to better understand the biodiversity and describe the physical environment of the Lut Desert, and as part of a large scientific program (Adaptation and Function of Lut Desert Biodiversity (AFLDB) project), we present the first geochemical data on these waters, speculate on their origin and evolution, and compare them to what is known about other hyper-arid environments in Israel, China, Chile, Australia and the western United States.
Section snippets
Methods
The sampling coordinates and depths are given in Table 1, and sampling locations shown in Fig. 1. Surface water or shallow groundwater samples (~1 m depth) were taken by hand in pre-cleaned 120 ml widemouth LDPE bottles. In some cases, overlying sediment/soil was removed by shovel prior to sampling the water. The water samples were filtered through 0.4 μm membrane filters using precleaned polycarbonate filtration system into a series of smaller precleaned LDPE bottles and stored in the dark at
Results and Discussion
The water geochemistry data and soil chemical parameters are shown in Table 2, Table 3, respectively. A number of general observations of these data can be made. All the waters are Na–Cl, but they are unlike seawater derived brines in that Ca2+ > Mg2+ and HCO3− > SO42− (Fig. 2). The K concentrations are uniformly low relative to Na. Unlike some hypersaline waters in extremely hyper arid environments, the Li, Mg, and Br concentrations are relatively low (Long et al., 1992; Lyons and Welch, 1997;
Conclusions
We present, to our knowledge, the first geochemical analysis of water from one of the hottest locations on the Earth, the Lut Desert, Iran. The data indicate that the salinity of these waters is produced by both evaporative processes and dissolution of previously deposited salts, and thus wetting and drying are very important factors in controlling brine geochemical evolution. Thermodynamic calculations indicate that waters are essentially in equilibrium with gypsum and are also highly
Acknowledgements
We are grateful to Ms Devin Smith for helping with water isotope measurements. The material for this contribution was available under the Adaptation and Function of Lut Desert Biodiversity (AFLDB) project, coordinated by the SAEEDI Institute for Advanced Studies (SIAS), University of Kashan and University of Tehran. The field project was partially funded by the LIA-HAOMA CNRS, France. We thank Mr. Bahman Izadi and the logistic team for their great effort during the field work. This study was
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