Climate Change Characteristics In The Asian Westerlies Dominated Area Recorded By Geochemical Proxies During Late Quaternary

Northwestern China is located in the arid central and eastern parts of the mid-latitude Eurasian continent.It was home to the“Silk Road”connecting Eastern and Western cultures and trade.The climate in this region is mainly affected by westerly circulation?referred to as“Westerlies dominated Zone”?.It is dry and with very little precipitation year round,with strong evaporation and fragile ecosystems.It is particularly sensitive to changes in climatic factors such as precipitation and temperature.Therefore,the region is an ideal place for studying the climate change and environmental evolution of the past.This region is also an important source of global dust,so climate change in the region has a significant impact on global surface material circulation and marine biogeochemical processes.Since the end of the last century,some researchers in China have proposed that northwestern China is controlled by the westerly circulation system,as opposed to the Asian monsoon.Subsequently,the comprehensive multi-proxy records prove that at the Holocene scale,the humidity evolution pattern of the westerly dominated region has a distinct out-of-phase relationship with the Asian monsoon region whereas at a shorter millennial scales,the humidity evolution of the westerly dominated region is"anti-phase"with the Asian monsoon region.In recent years,observational data has shown that the core area of Westerlies dominated zone,which is completely affected by the westerlies,has a different climate change pattern with the whole Westerlies dominated,and the climate change in the core area of the Westerlies dominated area is more reflective of the evolution characteristics of Westerlies.Due to climate proxy and chronological uncertainties,the following problems require further study:1.Quantitative reconstruction of Holocene summer precipitation of the core area of Westerlies dominated zone;2.The spatial evolution characteristics of the Westerlies during the Holocene,including the evidence and its possible driving mechanism;3.Limnological,environmental,and climate history characteristics in the Westerlies dominated zone since the Late Quaternary.This research was supported by the National Natural Science Foundation of China by a key project entitled“The Model and Mechanism of Climate Change in the Last Glacial/Interglacial in the Arid Region of Northwest China”awarded to Fahu Chen.We selected different type of archives spanning different time scales in the region.These archives include:a Holocene loess section named Lujiaowan?LJW10?in the northern slope of Tienshan in Xinjiang,a Holocene sediment core from Jiang Co?JC-2014-1?in the central part of the Qinghai-Tibetan Plateau,and a long drill core from Bosten Lake in Xinjiang?BST12B?.We used high-resolution K-feldspar infrared post-infrared luminescence?pIRIR?dating data to establish the chronology of the LJW10 Holocene loess profile..Total organic carbon isotope ratios of loess were used to quantitatively reconstruct Holocene summer precipitation.Bulk sediment AMS¹⁴C dating was used to establish the chronology of the Jiang Co sedimentary core?JC-2014-1?in the Qinghai-Tibetan Plateau.Hydrogen isotope ratios of the leaf waxes in the lake sediments were used to track the spatial evolution of the westerlies during the Holocene.K-feldspar pIRIR dating and AMS¹⁴C dating of bulk sediments and shells established the chronology of the long drill core from Bosten lake?BST12B?.A muti-proxy approach including grain size,color,XRF elements scanning data and loss ignition,was used to assess the lake evolution history and climate change characteristics of the region since the Late Quaternary.The main conclusions of this thesis are:1.Quantitative reconstruction of summer precipitation in XinjiangBy investigating the relationships between bulk surface soil carbon isotope ratios and climatic factors on the northern slope of the Tienshan Mountains in Xinjiang,we found a strong linear correlation exists between carbon isotope ratios and summer precipitation in this area.Based on this relationship,we established a correlation equation between soil carbon isotope ratio and summer precipitation,and then applied this equation to the LJW10 loess section that located on the northern slope of the Tienshan Mountains in the core area of Westerlies dominated zone,to quantitatively reconstruct summer precipitation changes during the Holocene.Results show that during the early to middle Holocene?12-6 ka?,summer precipitation amount was very little and without significant changes.Since the mid-Holocene,summer precipitation has been continuously increasing,and late Holocene precipitation is the greatest.This is in an"anti-phase"relationship with precipitation records in areas affected by the Indian monsoon.The continuous increase of summer precipitation since the Holocene is mainly caused by the CGT?circumglobal teleconnection?driven by northern hemisphere summer solar radiation changes,corresponding to a positive to negative phase change of the AO/NAO since the Middle Holocene,which also has increased the water vapor that was sent to the westerlies region,and increased the summer precipitation.2.The spatial evolution of the Westerlies during the HoloceneBy comparing the relationship between the summer westerly index?calculated from observational data?and summer precipitation isotopes,we found that the stronger?weakened?the summer westerly is,the more negative?positive?summer precipitation isotopes are in the westerlies affected region.Holocene lake sediments from Jiang Co in the central of Qinghai-Tibet Plateau show that the hydrogen isotopic ratios of leaf waxes in the northern part and central of the plateau started to decrease in the mid-Holocene?6 ka?.A global climate model TRACE21 was used to simulate the intensity variations of the westerlies during the Holocene.It shows that the intensity of the westerlies increased sharply since the mid-Holocene,and the southern boundary of the westerly circulation zone subsequently expanded to the south.3.Climate and environmental evolution of the Westerlies dominated area since the last interglacial periodIn this study,we used GDGTs to reconstruct the temperature changes since the Last Interglacial.Additionally,a multi-proxy approach,including grain-size,XRF-derived elemental data,color and loss on ignition,was used to complement these analyses We found that during the Last Interglacial,there were 3 relatively high lake level periods,which occurred 125.7-114.7 ka,98.7-95.1 ka,77.9-66.8 ka,respectively.The timing of these 3 high lake level periods correspond to the 3 humid period recorded by the Kansu loess section in Xinjiang,which is also located in the core area of Westerlies dominated zone.These high lake level periods are“out-of phase”with summer insolation?65°in the northern hemisphere?and the stalagmite oxygen isotope records from the Asian monsoon region.In addition,we also found that the lake level of Bosten Lake fluctuated sharply during the LGM.Although the LGM lake level was much lower than that of the last six millennia,it was still wetter than from 66-25 ka,when the lake was completely dry.Accordingly,we can confirm that the“out-of phase”relationship evident in the Westerlies dominated region and Asian monsoon dominated region during the Holocene also existed in the Last Interglacial.We speculate that the“out-of phase”phenomenon which occurred during both the Holocene and Last Interglacial is mainly driven by the phase change of the NAO?north Atlantic oscillation?and CGT?circumglobal teleconnection?...