Central Iranian playas: The first 3D near-surface model

Highlights

• Playas are valuable tools for studying the paleoclimate.

• Iran location makes it a suitable region for paleoclimate and human migration studies.

• We have studied the cores of wells drilled in the two large playas in the south and north of central part of Iran and created 3D models from near-surface structures.

• We proposed a scenario for the evolution of the playas.

• Based on 8-layer model of TP, central Iran has gone through four periods of severe climate change.

Abstract

Playas are a valuable tool for studying the climate of the past. In recent years, several studies have been conducted on Iranian playas, but there is a lack for subsurface models of playas. The aim of this research was to model the subsurface of playa to reconstruct the past climate. We employed the drilling cores of wells drilled in the two large playas in central part of Iran and created 3D models from near-surface structures. We obtained 3 models: an eight-layer model and a 4- layer model and a 2-layer models in 3 different parts. Based on the models, we have proposed a scenario for the evolution of central Iran playas. The 3D models show that the overlaying of the layers not only follows the morphology of the bedrock, but also that the mud layers sit exactly in the wet basins. In other words, layers of mud have formed in wetter-than-present surfaces. The 8-layers model suggest at least four periods of severe climate change had occurred in central Iran. Our 3D models are consistent with the reality of the climate change during Quaternary time scale in arid environment of central Iran.

Introduction

In Spanish, the word playa is used for beach and shore. However, in scientific community (e.g. Briere, 2000) it means a discharging intracontinental basin with a negative water balance (dry for 75% of the time and associated with evaporates). Playas are found throughout semiarid and arid regions around the world. There are about 50,000 playas on Earth (e.g. Rosen, 1994). They are important geomorphic features in the arid regions of the earth (e.g. Iran) since playas have an effective role in biodiversity of mammalian, avian, other vertebrate, invertebrate, and floral (Haukos and Smith, 1994). They are also important because of the economic value of some minerals rich in potassium and lithium (Reeves, 1978). It is also a good place to study paleoclimate of a region (e.g. Rosen, 1994).

Iran location between the monsoon rainfall regions of southwestern Asia, the high-pressure-dominated deserts of the Arabian Peninsula, and the winter rainfall zone of the eastern Mediterranean, makes it a suitable region for paleoclimate and human activities studies (e.g. Thomas et al., 1997; Walker and Fattahi, 2011; Vahdati Nasab et al., 2013; Khosravichenar et al., 2021). Numerous researchers have studied Iran's Playas. Iranian playas can be seen in satellite and aerial images in the form of flat areas in white, blue and cream colors (Fig. 1). Several researchers studied the geochemistry of Iran's different playas (e.g., Khademi et al. (1997), Farpoor et al. (2012), and Rahimpour-Bonab and Abdi (2012). Some other researchers try to explain depositional pattern of playa (Pourali et al., 2020), sand ramps developments in semi-arid regions of Iran (Sharifi Paichoon, 2019), deformation of salt diapirs (Roosta et al., 2019), and mega nebkha developments in Fahraj Basin, Kerman, Iran (Zahabnazouri et al., 2021). Environmental issues of the playas, as well as wind erosion and development of the playa surfaces, especially for the newly created surfaces such as the lake Urmia playa in the northwestern Iran, have been discussed, especially their impact in agricultural problems of the surrounding areas (e.g. Alizade Motaghi et al., 2020). The physical, chemical, and hydrogeochemistry of groundwater system of playas and semi-arid regions of Iran was another topic that has been considered by researchers (e.g. Lyons et al., 2020). Other topics include paleo climatological studies in the Quaternary of the central regions of Iran (Playas), each of which has recorded and reported climate change in different periods, and it has been argued that the current morphology of these areas is a result of these changes (e.g. Navidtalab and Moghim, 2020; Vaezi et al., 2018).

Iran location forms a natural bridge between central, southern, and southwestern Asia for human migration throughout the Pleistocene and Early Holocene (Khosravichenar et al., 2021; Vahdati Nasab et al., 2013). To better understand the geomorphic and paleoenvironmental background of former human dispersal, regional knowledge about such changes is necessary. In Mirak site, Semnan province, in the northern edge of the central desert of Iran, known as Dasht-e Kavir, Vahdati Nasab et al. (2013) studied late Pleistocene dispersal corridors across the Iranian plateau and introduced this

Site as a major Middle Paleolithic open-air site in the central part of Iran and they also proposed three migratory corridors (please see Fig. 10 of Vahdati Nasab et al., 2013) from the Caspian Sea to the southern part of Iran (root C runs between the southern foothills of the Alborz Mountains and northern edge of the central desert of Iran). Vahdati Nasab et al. (2019) also reported repeated human occupations during the late Pleistocene from this site. Vahdati Nasab and Hashemi (2016) studied Chah-e Jam site in the Damghan, Semnan province (northern edge of central Iran desert) and they concluded that these uninhabited lands (covered with playas, lakes and desert) were once occupied by hominin populations, and various factors such as climate change has emptied them of human populations.

Despite extensive research on Iran playas, there is a gap in the study of their subsurface 3D structures. Their subsurface model will help to understand the impact of climate change in their environment. As another result of studying subsurface structures of playas, we can have a better understanding of groundwater circulation systems in their adjacent settings. These studies (3D subsurface structures of playas) will also provide answers to the questions and challenges of other researchers who have worked on other issues related to playas. For example, if archaeological sites have been repeatedly occupied by human populations, it may be due to climate change related to the playa environments.

In this study, we present for the first time, 3D subsurface stratigraphy (up to 50 m) based on compact drilling networks in two Central Iranian playas (Torud Playa- TP and Khoor-va-Biabanak Playa -KBP). We use XRD, XRF, SEM-EDS, granulometry analysis, and Atterberg limits in our study to propose a scenario for their formation.