Characteristics Of Vegetation Succession And Climate Change In The Monsoon Margin Region Since Mid-Holocene Recorded By The Pollen From Gouchi Lake

The mid-Holocene is a period of special concern for the study of global climate change,during which the overall climate is warmer than the present.It is a geological similar type for the prediction of future climate warming scenarios.At the same time,the mid-Holocene is unstable under the warm climate background:it experiences several abrupt climate events with obvious characteristics.And it had an important impact on social development.In this paper,core GC15B derived from Gouchi Lake located in the monsoon margin region was selected as the study object.The age-depth relationship was established by AMS¹⁴C dating and pollen analysis was carried out on259 samples of lake sediments.Meanwhile,combined with Principal Components Analysis?PCA?and other means to reconstruct the vegetation succession process and climate change sequence since the mid-Holocene?8130 cal a BP?in the Gouchi area with high resolution.In addition,by comparing and analyzing the related records,the history of monsoon evolution and its driving mechanism of the monsoon marginal area since the mid-Holocene was discussed with a preliminary discussion.The results showed that the most of herbaceous pollen content in Gouchi sediments was more than 90%and less than 10%pollen from trees since the mid-Holocene.This showed that the vegetation types in the study area were mainly grassland and the climate was generally dry.But the moist herb pollen content was relatively high in some periods and there was a clear wet period.According to the changes of pollen assemblage,the overall environmental characteristics could be divided into three periods:?1?8130-5320 cal a BP:The pollen concentration was the highest in the study section.Artemisia was the main species and its content was more than 60%.The pollen contents of wet plant pollen and fern spores reached the maximum of the core.The pollen concentration of trees was the highest in the study period.This indicated that the vegetation in the study area was mainly grassland landscape with Artemisia.In addition,the wet plants such as Polygonum and Cyperus were crowded around the lake.The pollen concentration and the A/C reached the highest in the study core on average.The PCA axis1 was obviously negative.The climate was the most humid and the East Asian summer monsoon?EASM?was at the strongest period.?2?5320-1490 cal a BP:The concentration of pollen decreased.In the palynological assemblage,in addition to the main pollen of Artemisia,the content of wet plant pollen significantly decreased and the pollen content of Artemisia plants increased.This indicated that the vegetation in the study area was still the Artemisia-dominated steppe environment.The area of wet herbaceous plants decreased and the area of drought-tolerant Chenopodiaceae plants increased;The A/C in this core decreased on average and the score of the PCA axis1 was gradually positive.All results displayed that the climate tended to be drought.Particularly after2800 cal a BP,the pollen content of drought-tolerant Chenopodiaceae rose sharply.The score of PCA axis1 was significantly positive.And in the meantime,the drought trend was obviously aggravated.The EASM was weakened rapidly.?3?1490 cal a BP to present:the pollen content of Chenopodiaceae in the pollen assemblage rose to the highest in the study period.This exhibited that the dry steppe area in the study area expanded significantly;The pollen concentration reached the lowest in the study period and the A/C ratio continued to decline.The climate reached the most arid period in the study stage,meanwhile,EASM was at the weakest.The regional climate history and EASM evolution since the mid-Holocene broadly follows changes of the Northern Hemisphere summer insolation?NSHI?.The overall wet period?8130-5320cal a BP?corresponding to high NHSI values.EASM was at strong period at the time and then there's a continuous reduction process in summer insolation.Specifically,since 2800 cal a BP,in combination with the influence of ENSO,EASM declined rapidly and the climate became further arid.Abrupt climate change events were also clearly recorded in Gouchi record.The environmental information reflected by pollen showed that six climatic?cold and dry?drought events in this study:7200 cal a BP,5500 cal a BP,4200 cal a BP,2800 cal a BP,1400 cal a BP and 500 cal a BP had existed in the monsoon margin region of northern China since the mid-Holocene.This was consistent with domestic and foreign studies,such as the cold events recorded by ice-rafted debris in the North Atlantic and the Greenland ice core.