Study On Paleoecology Reconstruction Derived From Pollen Records Of Fanjingshan Mountain

Compared with other geological periods,the climatic conditions of Holocene are closer to the present and the future.The regularity and suddenness of Holocene climate change are likely to happen again in the future.The areas under mutual control of multiple atmospheric circulations are sensitive to climate change and are key sites for studying the evolution of climate.East Asia is a typical monsoon climate zone.A comprehensive understanding of climate change since Holocene in East Asian monsoon?EAM?region is of great significance for revealing the driving mechanism of global climate change and predicting future climate change.Southwest China is an important area for studying the characteristics of Holocene climate change in EAM region.To this end,Fanjingshan Mountain,which is influenced by southwest monsoon from the Indian Ocean,southeast monsoon from the Pacific Ocean and winter monsoon from East Asia,was chosen as the research area in this paper.Based on the basic theories of response of vegetation to climate change and chronology,through the collection of sedimentary cores,9 samples of organic samples from different layers of JL15 core of Jiulongchi Wetland in Fanjingshan was selected for accurate AMS¹⁴C dating.Based on accurate age-depth model,using sediment loss,pollen,total carbon and nitrogen content,and other environmental proxies,the evolution process and history of the climate since Holocene in Fanjingshan area was reconstructed to explore the Holocene climate evolution mechanism and the response to global climate change events in the area.The main conclusions are as follows:?1?The paleoecological evolution process of Fanjingshan since Holocene is mainly divided into five stages,which have undergone alternating changes from cold-warm-cold,dry-wet-dry,and the sedimentary environment of Jiulongchi has also changed from ice bucket lake with much water into a wet marsh and later turned into a dry marsh.Since the Holocene,Fanjingshan mainly experienced cold-dry period?11.9¹0.7 cal ka BP?,transition period?10.7⁸.0 cal ka BP?,warm and wet period?8.0⁴.6 cal ka BP?,warm-dry period?4.6².6 cal ka BP?and cold-dry period?2.6⁰ cal ka BP?.During the period of 11.9⁸.0 cal ka BP,the temperature in the Jiulongchi area gradually changed from cold to warm,and during the warm period of 8.0⁴.6 cal ka BP,it enjoyed the suitable climate for Fanjingshan.Subsequently,from 4.6 cal ka BP,the temperature gradually changed from warm to cold.In the early Holocene,it was warm and humid,and the medium-term became warmer and more humid.In the late Holocene,the climate gradually changed from warm to cold and gradually dried,but there was a tendency to warm.It is consistent with the Holocene climate change trend studied by the predecessors,reflecting the reconstructed Fanjingshan Holocene climate and the regional climate change pattern reflects the characteristics of global climate change.?2?The vegetation type experienced a change from evergreen deciduous broad-leaved mixed forest-deciduous evergreen broad-leaved mixed forest-evergreen-green broad-leaved forest-deciduous evergreen broad-leaved mixed forest,and gradually formed the modern vegetation landscape of Jiulongchi.The vertical band spectrum distribution of Fanjingshan since the Holocene had moved upward and downward corresponding to climate change.Compared with the elevation of modern vegetation distribution,the vertical band spectrum of Fanjingshan moved to higher altitudes in the early and mid-Holocene,and moved most in the Holocene suitable period?8⁴.6 cal ka BP?.In late Holocene,it moves to low-altitude areas,the variation range of which is about 100 m,and the variation characteristics are in good agreement with the Holocene climate change trend.The vegetation belt in eastern China showed the law of zonal northward migration during the Holocene period,while the western region showed the characteristics of westward migration,vertical migration and horizontal migration.Compared with other pollen records in southwestern area,the vegetation in the region has a vertical migration to high altitudes during the Holocene warm period.However,the migration of vertical band spectrum of alpine vegetation during the Holocene period requires more paleoecological records to prove.?3?Core JC15 of Fanjingshan recorded 5 short and rapid cooling events since the Holocene,which occurred at 2.6,3.4,4.2,9.4,and 10.0 cal ka BP,respectively.Core JL15 responded earlier to the 4.2 ka BP cooing event and no obvious response to the 8.2 ka BP dry and cold event.A comparative study with climate records in adjacent areas found that the driving mechanism of Holocene climate change in Southwest China is mainly related to the advance and retreat of the monsoon,the north-south movement of ITCZ,and the underlying surface.A comparative study with the climate records of adjacent areas found that the area near the JL15 hole of the Fanjingshan Jiulongchi was less affected by the southwest monsoon in the early Holocene,and the winter monsoon had a greater impact.In the middle of the Holocene,the southwest monsoon turned weaker and stronger.When the temperature rises,the precipitation first increases and then decreases;in the late Holocene,the southwest monsoon weakens again.Compared with the records of Yunnan Lugu Lake,Erhai Lake and Guizhou Caohai Nanxun Core,the four regions responded to climate change events by 2³ times,but the response time was different,the length and time were different,reflecting the uniqueness of Fanjingshan landform of its“ecological island”effect.