Reconstructing the Holocene and Late-Pleistocene Climate Changes of the Central Zagros Using Palynological Evidences of the Hashilan Wetland

Document Type : Research Article

Authors

• Reza Safaeirad ¹
• Ghasem Azizi ¹
• Hosein Mahammadi ¹
• Hamid Alizadeh Lahijani ²

¹ University of Tehran

² national research center of Oceanography and meteorogical science

10.22067/geo.v3i3.38429

Abstract

1. Introduction
The Zagros Mountains present a great potential for paleo-environmental studies for the reconstruction climates of the late-Pleistocene and Holocene. Iranian Zagros Mountains form the northeastern boundary of the Fertile Crescent and Separating the Mesopotamian lowlands from the Iranian Plateau. It is postulated that Near East was a place for the domestication of plants and animals, and the question has been raised as to how far changes in climate and vegetation have stimulated the evolution of food production. So investigation of paleo-environment of the Zagros Mountains as a part of the Near East is very important. There are some major climatic systems over the Zagros Mountains which are very sensitive to the abrupt climate change. Change in the positions and intensities of these systems during the Quaternary could have influenced the paleo-environment of the region. In order to reconstruct these changes, evidence of Quaternary climate change in Zagros is necessary. Previous paleo-environmental researches have revealed general future of the paleo-climate of the Zagros Mountains during the late-Pleistocene and Holocene but yet more researches are needed to reveal details of past climates of Zagros. Hashilan Wetland located in central Zagros, containing 12 m of sediment dating back into Pleniglacial time, is a suitable site for better understanding both paleo-climate and palaeo-environment of the not only Zagros region but also Near East during the last 40000 years. Here we used pollen analysis for reconstructing the past vegetation and paleoclimate of Hashilan Wetland located in Kermanshah province.
2. Study Area
Hashilan Wetland is located in the vast plain of Hashilan in the Kermanshah province, Central Zagros. Formation of this type of wetland in such areas is very notable. The wetland has a special form, so that there are many small islands (approximately 110 islands) on the surface of one side of the plain and strakes of water exist among these islands. Main input of the wetland is Sabzali spring in the north part of the plain. The plain is surrounded in both west and north by Khurrin Mountains (2500 m) and in south by Vais Mountains (1800). The study area is located in steppe region of the Zagros and oak trees do not exist in the catchment of the wetland. Asteraceae, Poaceae and Cyperaceae are the most common plants of the wetland. In the modern climate of the area more precipitation occurs in the cold seasons and summers are dry and hot.
3. Material and Methods
In the spring of the 2012, a 12 meters long sediment core was retrieved from Hashilan Wetland using Russian corer and here we performed pollen analysis on its 5 uppermost. In order to pollen analysis, the core was sub-sampled in 20, 10 and sometimes 5 centimeter intervals. Samples were processed according to Moore et al (1991) with a modification. Lycopodium tablets were added to calculate the pollen concentrations (Stockmarr 1971). The samples were treated with a sequence of 10% KOH (to break up sediment and remove humic acids), 10% HCl acid (to remove carbonates), 48% HF acid (to remove silicates: sand, silt, clay), acetolysis (to remove some organic matter, cleaning and staining the surface of the pollen grains), and tertiary butanol. Finally the sample were stored in silicon oil and mounted on slides for examination using a light microscope at 400× magnification. Pollen determination was performed using some pollen atlases such as (Moore et al., 1991; Demske et al., 2013), pollen reference slides of palaeo-climatology laboratory of geography faculty of university of Tehran, and pollen and spore websites of Arizona and Australia. More than 300 total land pollen grains were counted for each spectrum except for the spectra of 185, 195, 225, 235 and 245 cm which had exceptionally low pollen concentrations. Pollen of aquatic plants, riparian and pine trees were excluded from the total pollen sum. Pollen percentages were calculated in Polpal Excel and the pollen diagrams were created by Polpal Diagram (PP Diag.) software. Three radiocarbon dates were obtained at the Poznan Radiocarbon Laboratory from 80, 755 and 1193 cm depth of the core which respectively those ages are 2100 ± 25 BP, 31500 ± 300 BP and 39500 ± 700 BP. These three AMS dating showed the core is belonging to the Holocene and Late-Pleistocene.
4. Results and Discussion
After the experimental stages and counting the pollen grains, the pollen data was displayed in the form of pollen diagrams. The pollen diagrams were zoned using constrained cluster analysis by sum-of-squares (CONISS) as an option within Polpal software. The pollen diagrams were zoned into 8 pollen assemblage zones (PAZs) and these PAZs were interpreted from viewpoint of palaeo-climatology. H-1 to H-5 PAZs belong to the late Pleistocene and H6 to H8 PAZs belong to the Holocene. Late-Pleistocene represents treeless vegetation and is marked by steppe plants such as Artemisia, Chenopodiaceae, and Poaceae. The Late-Pleistocene has divided into 5 pollen assemblage zone (PAZ) based on changes in frequency of these major steppe plants. With the onset of the Holocene steppe vegetation has changed into pistachio-oak savanna and Poaceae increased abruptly at the expense of Chenopodiaceous and Artemisia. Furthermore, Aquatic plants have increased and riparian plane trees have appeared. Drought tolerant shrubs of Amygdalus have been a component of the vegetation cover in the early Holocene. In this time, the wetland has been desiccated sometimes. In spite of these changes the oak trees could not expand and a limiting factor prevented the expansion of these trees. Gradually, frequency of the oak trees has increased and ultimately in the mid-Holocene open oak woodland has expanded and its frequency remained constant until now.
5. Conclusion
Hashilan Wetland has a great potential for high-resolution paleonological investigation for the reconstruction of the vegetation and climate change during the Late-Pleistocene and Holocene. A pollen record prepared from the Hashilan Wetland shows some major changes in vegetation and climate. The general patterns of vegetation change are consistent with previous studies from western Iran and southeastern turkey (van Zeist and Bottema 1977; Wick et al., 2003). The late-Pleistocene Artemisia- Chenopodiaceae steppes imply a cool, dry climate. Based on some minor changes in the frequency of these steppe plants, PAZs of H1, H3 and H5 are drier than H2 and H4. The palynological evidences of vegetation change in the early Holocene such as: (I) development of drought tolerant pistachio trees and Amygdalus shrubs, (II) limitation of oak growth, (III) growth of riparian trees along water bodies, and (IV) desiccation of the wetland imply a long dry season in the warm period of the year which due to significant increase in temperature the precipitation-evaporation ratio (P-E ratio) has became strongly negative during this dry season. So we concluded that in the early Holocene precipitation has been limited to cold season of the year and duration of the hot, dry season has been longer than today. Gradually by development of precipitation duration, the hot, dry season has become shorter. These changes in seasonality led to rise in precipitation-evaporation ratio and climate amelioration. This new condition allowed oak trees to expand and oak woodland increased at the expense of pistachio-oak savanna in the mid-Holocene. As we see in the pollen diagram (figure 2) constant frequency of oak woodland since the mid-Holocene until present indicates that modern climatic regime of the central Zagros has been established from the mid-Holocene. Paleontological evidences reveal that human interferences have affected the environment and vegetation during last millennia.
More dating are required to high-resolution analysis of the whole core in order to reveal details of changes in vegetation and climate during both late-Pleistocene and Holocene.

Keywords

• Palaeo-climatology
• Paleonology
• Late-Pleistocene
• Holocene
• Central Zagros
• Hashilan Wetland

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