Climate-Human interaction over the Iranian Plateau during the Upper Pleistocene-Holocene: A review

During the past two decades an array of studies have shed light on potential links between the evolution of Hominids as well as Human dispersal out of Africa and episodes of abrupt climate change. Although archaeological evidences suggest that anatomically modified humans appeared in Africa between 200 and 150 ky ago, the timing of the early humans migration out of Africa remained unclear. Strategically located between the Arabian Peninsula and the Eurasia, the Iranian Plateau played a key role with respect to early hominin’s migration route out of Africa. However, due to the scarcity of the Pleistocene-Holocene high resolution paleoclimate records with robust chronology for this region, the impact of climate variability on early human migration and settlements in this region has not fully explored. The climate of the Iranian Plateau is mainly controlled by the interactions between three major synoptic systems: the mid-latitude Westerlies, the Siberian Anticyclone (SA) and the Indian Ocean Summer Monsoon (IOSM). Changes in the intensity and latitudinal position of these synoptic systems is governed by both internal and external climate forcings which ultimately control precipitation and atmospheric dust content across this region. In recent years a number of paleoclimate reconstruction for the Iranian Plateau have emerged, which helped us to better understand the climate human interaction during the Pleistocene-Holocene. Upper Pleistocene paleoclimate records from north, central-northwest and northwest Iran indicate a substantial correlation between different terrestrial climate archives (lake sediment, peat and loess deposit) across the region. The independent proxies from these archives revealed wet climate condition prevailed over the Iranian Plateau during the marine isotope stages MIS5a,b, MIS5c, MIS5e, MIS6b, MIS6d-e and most likely during the MIS3-4 and MIS7a. Comparison between these climatic episodes and chronologically constrained archaeological evidences suggest the occupation of the Southern Caucasus, Zagros and Near East regions by early humans coincides with the upper Pleistocene wet periods and when the appearance of the wet periods became less frequent, most of the early human settlements/occupations appeared in coastal regions of the Near East. A 13600-yr high-resolution multi-proxy paleoclimate reconstruction for the Holocene based on the peat record from Neor Lake, NW Iran, revealed several episodes of abrupt climate change with intensive dust input to the region. This record also suggest changes in solar insolation (and possibly irradiance) most likely played a key role in controlling water availability and aeolian input to the studied region. Comparison between climate reconstructions and regional historical records show nearly all episodes of dry condition with enhanced flux of aeolian input to the Neor Lake corresponded with episodes of drought and famine in the region as well as with transitions in major Mesopotamian and Iranian civilizations. These findings indicate that the West Asian ancient human societies were likely highly susceptible to abrupt climate variability during the current interglacial period. Climate reconstructions and pollen assemblage from Almalou Crater Lake in NW Iran indicated several wet episodes during the past 2400 years which coincides with the enhancement of anthropogenic pollens. This may suggest the expansion of agricultural activities during the wet periods. The total organic carbon (TOC) and 13CTOC values are positively correlated with the variation in lithogenic elements. A significant drop in δ13CTOC values centered at ~80 yr BP at Almalou reflects post-industrial anthropogenic input of carbon to the environment. The same pattern of TOC and 13CTOC values were seen in a 4600-yr long peat record from Sar Bijan region in SW Iran indicating the role of climate fluctuation on biomass production over the Iranian Plateau. A similar drop in δ13CTOC values centered at ~100 yr BP is also seen in Sar Bijan peat record.