Pollen Recorded Humidity Changes In The Northern Margin Of The Qinghai-tibetan Plateau Over The Past 300 Years And Comparison With Tree-ring Records

The Qinghai-Tibetan Plateau(QTP)is a droven force and an amplifier of global climate change.The northern part of QTP is climatically influenced by the westerlies and the summer monsoon systems in the western and eastern part of Qilian Mountains respectively,and regional climate change has unique complexities of the transition zone between the two climate systems.Therefore,it is an ideal area for studying the interaction between monsoon and westerlies.With global warming,the spatial heterogeneity of humidity changes is strengthened.The past 300 years is a critical interval bridging the gap between meteorological records and geological records,and it also includes the transition period from the cold Little Ice Age(LIA)to the Current Warming Period(CWP).Here we use pollen record from core TE18A of Tian'E Lake(66 cm,128 samples in total with an average temporal resolution of?2 years)and core GH17B of Gahai Lake(42 cm,42 samples in total with an average temporal resolution of?8.5 years),with chronologies based on ²¹⁰Pb and ¹³⁷Cs,and compared with other climate records,mainly tree ring records,to discuss the spatial differences of humidity changes over the past 300 years.In the extremely arid area of the western Qilian Mountains and the surrounding dryland areas,where forest is absent,it is unclear how humidity conditions have changed over the past several centuries.We have obtained a high-resolution pollen record spanning the last 300 years from the sediments of Tian'E Lake in the western Qilian Mountains,which helps fill the spatial gap in recent climate records from the region.The results show that the pollen assemblages are dominated by Artemisia and Amaranthaceae(=Chenopodiaceae),and therefore the A/C ratio(Artemisia/Chenopodiaceae)can be used to reconstruct changes in humidity conditions.Based on the pollen A/C ratio,we found two relatively wet periods of?1740-1750 and 1840-1980,and two dry intervals of?1750-1840 and 1980-2018.This pattern of variation is similar to that reconstructed from nearby humidity records based on tree-ring width adjacent to Tian'E Lake area and with instrumental records from meteorological stations over the past several decades.However,forested mountain areas reflect local climate and may experience a different pattern of humidity variations than extensive dryland areas on a longer timescale.The pollen data of GH17B core in Gahai Lake,located in the east of Qinghai Lake,show that grassland vegetation is dominant during the past 300 years,and pollen Poaceae/Artemisia(P/A)ratio can indicate the humidity conditions.Based on the pollen P/A ratio,two dry intervals are identified for:1650-1750s and 1970-2010s;relatively wet periods for 1750-1890s and fluctuating but still humid in 1890-1970s.In the past 300 years,there are obvious spatial differences in the variation of humidity in the northern QTP recorded by pollen and tree ring.The western Qilian Mountains and Qaidam Basin experience common wet conditions during the periods1760s,1850-1920s,and dry conditions during 1770-1830s,and a trend of decreasing moisture conditions from the1920s onward.In the northeastern margin of the QTP,drought events in 1920s are universal.During periods of rapid temperature rise,the climate in the northern QTP tends to be warmer and drier except for the high-altitude areas on the southern slope of the middle Qilian Mountains.The drought is strongest at the end of the 20th century and the beginning of the 21st century.For the transition zone between the westerlies and the monsoon region,the humidity changes over the past 300 years are relatively complicated.Pollen records of Tian'E and Gahai Lake show that humidity conditions are not only different from the climate change pattern in the westerly zone,but also different from that in the monsoon zone.During the past hundred years,during the period when the temperature in the northern hemisphere has risen sharply,the climate in the westerly region is warm and humid,and the climate in the monsoon region and the monsoon marginal region is warm and dry.These spatiotemporal discrepancies among different records may also reflect the interaction of the monsoon and the westerlies.