بازسازي تغييرات آب وهوايي هولوسن در زاگرس جنوبي: شواهد گرده شناسي و زغال در رسوبات درياچه پريشان

Reconstruction of climate change in southern Zagros Mountain: pollen and charcoal records from sediment of Parishan Lake

دگرگوني‌هاي اقليم بر زندگي انسان، چشم‌انداز و استراتژي‌هاي اقتصادي-اجتماعي تاثير ژرفي به‌جا مي‌نهد. بدين دليل انسان هميشه در پي دستيابي به پيشينه، شواهد و علل و آينده دگرگوني‌هاي اقليم بوده است. با توجه به كمبود مطالعات آب و هواشناسي ديرينه در ايران هدف اصلي اين پژوهش روشن كردن قسمت ناچيزي از گذشته تغييرات آب‌وهوايي زاگرس جنوبي در دوره هولوسن است. بدين منظور مغزه اي 5/8 متري از درياچه پريشان برداشت شده و روي آن مطالعات گرده شناسي انجام شد. با توجه به يافته هاي اين پژوهش، پوشش گياهي در يانگر درياس از نوع استپي خشك تا نيمه بياباني و تقريباً بدون درخت بوده است كه نشان دهنده آب‌وهواي سرد و خشك است. پوشش گياهي غالب در اين زمان اسفناجيان و درمنه بوده اند و تيره گندميان و پوشش گياهي درختي به شدت كاهش يافته اند. با شروع دوره بين يخبندان هولوسن تيره گندميان جايگزين تيره اسفناجيان شده‌و درختان بنه و بادام در منطقه شروع به رشد كرده‌اند. در هولوسن پيشين گياهان آبزي به شدت كاهش يافته و در كنار آن فراواني زغال افزايش بسيار زيادي داشته است. بر اساس اين شواهد آب‌وهواي گرم و خشك در هولوسن پيشين حاكم بوده است. شرايط موجود در هولوسن مياني سبب گسترش جنگل هاي بلوط شده است. جنگل هاي بلوط در هولوسن مياني تثبيت شده‌اند و تا عصر كنوني فراواني خود را حفظ كرده‌اند. بيشترين مقدار گياهان درختي در هولوسن مياني ظاهر شده و در كنار آن‌ تيره اسفناجيان به كمترين مقدار رسيده است كه تاييد كننده گرم و مرطوب بودن آب‌وهواي هولوسن مياني است. ويژگي كلي هولوسن پسين مشابه با هولوسن مياني است، ولي با توجه به كاهش بلوط و افزايش پسته-بادام رطوبت نيمه گرم سال در هولوسن پسين كمتر از هولوسن مياني شده است. اين شواهد نشان‌دهنده ي گرم‌تر بودن اين دوره نسبت به هولوسن مياني است.

Introduction Natural variability is an inherent feature of climate which occurs in both short and long time scales. Climate changes and variations have profound impacts on human life, landscape and socio-economic strategies. In this regards, human always sought to achieve the literature, evidence, reasons and future of the climate changes. Due to the scarcity of paleoclimatological researches in Iran, the main objective of this study is clarifying the some parts of climate changes during the Holocene in the southern Zagros Mountain. In this regard Parishan Lake in Kazeroon were selected. Methodology For reconstruction of paleoclimatology palynology proxy were used. Two samples have sent to Miami University (USA) for radiocarbon dating. In the spring of the 2012, a 8.5 meters core was recovered from Parishan Lake. The core was then sub-sampled in 10 centimeter intervals for palynological analysis. Lycopodium tablets were added to calculate the pollen concentrations. Samples were processed according to Moore et al. (1991) with a modi?cation. Pollen determination was performed using some pollen atlases such as (Moore et al., 1991; Demske et al., 2013), reference slides of paleoclimatology 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. Pollen of aquatic plants, Chenopodiaceae and Pinaceae trees were excluded from the total pollen sum. Pollen percentages were calculated in Polpal Excel and the pollen diagrams were created by Polpal Diagram software. Two AMS dating showed the core is belonging to the Holocene and Late-Pleistocene. Result and discussion The dominant vegetation at the late Pleistocene (Younger Dryas) is Chenopodiaceae and arboreal pollens are very low. The amount of Poaceae is very low too but the Artemisia rise during Younger Dryas. The non-arboreal pollen in this period covers 95% of total pollen. The amount of Centaura, Cousinia and apiaceae is high that show the cold climate; and amount of Chenopodiaceae, Artemisia, Ephedra and Cichorioideae is high that show the dry climate at this period. So the climate in Younger Dryas was cold and dry. Younger Dryas end at 10200 years ego. At the beginning of the Holocene interglacial period (10200 BP), Chenopodiaceae-Artemisia was replaced by Poaceae, and pistachio-Amygdalus species have grown in the region. Therefore, semi-arid steppe vegetation has changed to Amygdalus-Pistachio savanna that these changes in vegetation reflect the increase in rainfall. Available evidence represents that the climate in early Holocene was hot and dry. In middle Holocene that start from 7100 BP, climatic conditions led to development of Quercus forests and savanna vegetation has been changed to forest. Quercus forests have stabilized in middle Holocene and have retained their frequency to the present. The amount of Centaura, Cousinia and Apiaceae is low that show the warm climate; and amount of Chenopodiaceae, Artemisia, Ephedra and Cichorioideae is low that show the humid climate at this period. Maximum amount of arboreal species have appeared in middle Holocene while the Chenopodiaceae-Artemisia species have reached to their minimum in this era. This evidence reflects the existence of hot and humid climate in middle Holocene. General features of late Holocene are similar to middle Holocene except that a slight decline occurred in arboreal and Poaceae species and Chenopodiaceae have increased. It is represents that in late Holocene climate was drier than middle Holocene. Increase in Quercus and Pistachio-Amygdalus illustrate that late Holocene moisture in the warm half of the year was less than the mid-Holocene. These evidences indicate that this period was warmer than the mid-Holocene and reason of the dryness of warm half of the year can be searched in this factor. Conclusion According to the results of this research, vegetation of the late Pleistocene was arid to semi-arid steppe and almost without arboreal species. The common feature is that climate was cold and dry in late Pleistocene which is associated with increase in Chenopodiaceae-Artemisia and decline in Poaceae specially in arboreal species. At the Early Holocene semi-arid steppe vegetation has changed to Amygdalus- Pistachio savanna that these changes in vegetation reflect the increase in rainfall. The amount of charcoal at early Holocene was very high and aquatic plants very low. Available evidence represents that the climate in early Holocene was hot and dry. Maximum amount of arboreal species have appeared in middle Holocene while the Chenopodiaceae-Artemisia species have reached to their minimum in this era. This evidence reflects the existence of hot and humid climate in middle Holocene. General features of late Holocene are similar to middle Holocene except that late Holocene moisture in the warm half of the year was less than the mid-Holocene. These evidences indicate that this period was warmer than the mid-Holocene.