Pollen Evidence For Vegetation And Climate Changes In Wudalianchi Crater Lake Since The Last Glacial

Abrupt climate change has more significant impact on humans compared with the gradual change of climate,which prompts scientists to find out the regular of abrupt climate change.Northeratern China is an ideal area for the research of paleovegetation and paleoclimate due to its unique location in East Asian Monsoon region and its large forest coverage.However,current research on this area since the last glacial still has some shortcomings as follows:1)The long-term records of vegetation and climate based on high-resolution and high-accuracy in this region are extremely scarce,which hinders the understanding of spatiotemporal characteristics and driving mechanism of climate change in this region;2)The precipitation pattern during Younger Dryas(YD)in this region revealed by different records is quite different,and whether the climate was humid or drought is still inconclusive;3)Quantitative reconstructions of Holocene precipitation in this region show different pattern.In this dissertation,vegetation and climate evolution in northeastern China since the last glacial(48-0 ka BP)has been reconstructed,based on the reliable chronological framework established by fourty-two AMS14C dates together with physical and chemical analysis(including magnetic susceptibility,TOC,TN and C/N),pollen analysis and pollen-based quantitative reconstructions on the sediment of Wudalianchi Crater Lake in Heilongjiang Province.In view of the existing disputes in northeastern China,vegetation and climate evolution during the last glacial,YD and the Holocene are discussed respectively,and possible mechanisms are also discussed by the comparison of regional records.The results and understandings are as follows:(1)Sedimentary environment proxies(magnetic susceptibility,TOC,TN and C/N ratio),pollen assemblages,and principal component analysis results based on pollen percentage all show that vegetation and climate went through significant changes on orbital and millennial-scale in northeastern China during the last glacial(48-11.7 ka BP).On the orbital scale,vegetation surrounding the lake were dominated by weakly swampy meadow during MIS3 period.During late MIS3 period,herbaceous type of swampy reached the lowest value,indicating a highest lake-level of the last glacial period.During MIS2,herbaceous type of swampy expanded significantly,indicating a significant drop of lake level.On the millennial scale,Alnus and PC2 rapidly responded to climate change.Five significant cold events(44.3 ka BP,41.3 ka BP,36.1 ka BP,27-28 ka BP,and 15 ka BP)were identified,corresponding to Heinrich 1,Heinrich 3,Heinrich 4,Dansgaard-Oeschger 12 stadial and Heinrich 5 recorded in Greenland ice core.Meanwhile,D/O1-2,D/O5-8 and D/O11-12 events are also identified by PC2.PC1 is more sensitive to humidity,indicating that during the H4,D/O12 stadial and H5,the lake level rose significantly,precipitation increased,and the climate was wet.There was no significant change of lake level during H1 and H3 event By comparison,it's found that pollen record in Wudalianchi Crater Lake was consistent with that in other parts of China on the orbital-scale climate change,and all of them were controlled by Summer July insolation.On the millennial scale,lake-level rose during H4,D/O12 stadial and H5 were likely influenced by regional Northeast Monsoon.There was no significant lake-level rise during H1 and H3 possibly due to a longer distance of Okhotsk water vapor transport to this region,resulting from the large ice volume in the northern hemisphere at this time.We also find that H events recorded by pollen records in high latitudes are more prominent,while there is no obvious response in pollen records from low latitude.It is preliminarily speculated that the propagation of H events signal in the East Asian monsoon region may be ongoing from high latitudes to low latitudes.(2)Pollen assemblages and quantitative reconstruction of TC2 core showed that YD occurred during 12.7-11.7 ka BP,and vegetation were dominated by broad-leaved Betula and Alnus with extremely low herbaceous taxas,suggesting significant cold and wet climate.Quantitative reconstruction of Tann(mean annual temperature)decreased about 3-5? during YD period,and the Pann(mean annual precipitation)increased 300 mm approximately,suggesting cold and wet climate.The pollen records of Wudalianchi are consistent with other pollen records in Northeast China,indicating that vegetation in northeastern China responded to climate change simultaneously and rapidly.By comparing,we find that climate was cold and dry in the monsoon marginal region of West northeastern China,while the East northeastern China was mainly cold and wet.It is speculated that the precipitation increase during YD was the same as that of the last glacial during MIS3 period,which was affected by Northeast monsoon.(3)Pollen-based quantitative reconstructions show that the Holocene temperature maximum occurred at 9.0-7.0 ka BP and the precipitation maximum occurred at 5.5-3.1 ka BP in Wudalianchi Crater Lake,northeastern China.By comparing with other paleoclimate records,it is found that the temperature change in Wudalianchi area is basically consistent with that in northeastern China and in the global scale.In contrast,Holocene precipitation maximum in Wudalianchi area occurred much later than other sites in northeastern China.There is no single factor to explain the spatiotemporal complexity of precipitation pattern,indicating that Holocene monsoon precipitation may be driven by multiple factors such as ice cover and sea surface temperature.