عنوان مقاله [English]
نویسندگان [English]چکیده [English]
Numerous palaeoclimatologists have studied the climate of different regions of Iran during Quaternary based on palynology of lake sediments . In this study the paleoclimatic phases of northwest of Iran have been discussed . We used pollen analysis for reconstructing the past vegetation and palaeoclimate of Lake Neor area located in the northwest of Iran . In this research a high resolution pollen study has been done for reconstruction the climate of study region during the late - glacial to early Holocene .
In this study we used samples with interval distance equal 5 cm from the 3 meters of lower part of the core in which the samples cover 500 cm to 800 cm hight of the core. The pollen extraction followed the chemical treatment methods based on Moor et al. ( 1991 ) . The chemical treatment carried out in Institut Méditerranéen d’Ecologie et de Paléoécologie ( IMEP, UMR 6116 CNRS ) . Pollen determinations were performed by using the IMEP pollen reference slides collection and pollen atlases of Reille ( 1992 , 1995 , 1998 ) , Beug ( 2004 ) and pollen morphological descriptions by van Zeist and Bottema ( 1977 ) . Then during microscopic analysis we identified and counted the pollen grains by magnification equal 500X by using microscope Laica model . The pollen diagram drawn by the aide of TILIA and TGVIEW ( Grimm , 2004 , 2005 ) . At the first we analyzed the samples by interval distance equal 10 cm . After finding some important events we also analyzed the samples by interval distance equal 5 cm . Therefore in this study the vegetation dynamic studied by relatively high resolution for understanding more about some events which extracted from the pollen diagram .
Results and Discussion
This research showed the vegetation change from ~ 12800 to ~ 7800 years B . P . It represented the transition stage of the late-glacial to early Holocene in Neor Lake . The zonation was based on arboreal vegetation change . We detected the last warm phase of late - glacial ( A1 zone ) , the last cold phase of late-glacial or Younger Dryas ( A2 zone ) and early Holocene ( B1 , B2 , & B3 zones ) by interpretation of pollen diagram of Lake Neor . Chenopodiaceae , Artemisia , and Poaceae were dominant herbal vegetation cover during late-glacial . While Ephedra , Quercus , and Betula was dominant arboreal pollen at the same time . The tree pollen was absent from the area during Younger Dryas . It implied colder and dryer climatic condition during Younger Dryas . After ~ 11800 years B . P . the climate became warmer and early Holocene detected at the same time by increasing tree cover . According to dating result , B1 zone was from ~ 11600 to ~ 9300 years B . P . This period indicated a dominant herbal vegetation cover of Chenopodiaceae, Poaceae, and Artemisia and also tree cover of Quercus, Ephedra, and Betula. The dominant arboreal and non-arboreal pollen during early Holocene (B2& B3 zones) were respectively Quercus, Betula, Ephedra, and Poaceae, Artemisia, and Chenopodiaceae.
As a result, the pollen analysis as well as dating results revealed some minor differences in vegetational and environmental changes in Lake Neor area compared with other sites in the north-west of Iran. The different changes are probably related to high elevation of Neor region about 2500 m a.s.l.
The climatic indicators for detecting the wet and dry phases during the early Holocene consist of Artemisia to Chenopodiaceae ratio (A/C), Poaceae to Artemisia ratio (P/A), and Arboreal Pollen to Non-Arboreal Pollen ratio (AP/NAP). A dry phase detected during B1 zone ~ 9400 years B.P. This dry phase detected by the minimum amount of AP/NAP and A/C indicator.
It also seems that there were two wet phases at B1 zone ~9600 years B.P and at B2 zone~8200 years B.P. respectively. The decrease of Charcoal and increase of Typha latifolia and Myriophyllum is evident during wet phases. The interpretation of pollen diagram of Lake Neor suggested that there were the forest-steppe vegetation cover with Artemisia dominated in the Neor region about 8000-9000 yr B.P during the wet phase.