Reconstruction Of Monsoonal Variations On The Northwestern Chinese Loess Plateau Since The Late Last Glacial Period

The East Asian monsoon plays a key role in the global atmospheric circulation, anddirectly controls and drives the climate variations of East Asia as well. The completeloess-paleosol sequence on the Chinese Loess Plateau is a good recorder of East Asianmonsoon oscillations. The Chinese loess started its deposition since2.5Ma ago, and theprocess continues since then. The geological and biological evidences from Quaternaryloess-paleosol sequences show that when the winter monsoon prevails with a cold and dryclimate, relatively unweathered loess is deposited whereas the interbedded paleosol is formedrepresenting strong summer monsoon with a warm and humid climate. Since the completeloess-paleosol sequence on the Chinese Loess Plateau uninterruptedly preserves the EastAsian monsoon evolution, which is the alternative history between winter and summermonsoon dominant periods, it can be seen as the reflection of global climatic cycles in theEast Asia region. In this thesis, two20m sampling pits were excavated at Jingyuan (36.35°N,104.6°E,2210m above sea level) and Gulang (37.49°N,102.88°E,2400m above sea level),respectively, to obtain climatic data such as the particle size distributions, magneticsusceptibility, chemical elements contents, and factional dimensions of particle sizedistributions. Key methods such as the optically stimulated luminescence, cumulativedeparture, Mann-Kendall trend test, wavelet analyses are employed in this thesis to genaeratean independent chronology of Jingyuan and Gulang loess-paleosol sequences. These analysesaim to test the stages, trend, periodical, and abrupt changes of the East Asia Monsoon, and todetermine the driving factors of abrupt climatic changes in the East Asian monsoon system onthe northwestern Loess Plateau since. The key findings presented in this research are outlinedas follows:1. The suitable winter and summer monsoon intensity proxies are determined. The meanparticle size, relatively coarse particle (32-63μm) content, Si/Al, Zr/Al, and fractionaldimensions of particle size distributions are identified as the reliable proxies of the wintermonsoon intensity at the Jingyuan and Gulang sections. The magnetic susceptibility, fineparticle (<2μm) content, Sr/Al, and Ca/Al are sensitive proxies of the summer monsoon intensity during the Holocene at the research sections which are not sensitive in the lastglacial period because of the brief weak summer monsoon intensity.2. The stage and trend characteristics of the winter monsoon intensity at the Jingyuansection are analysed. Optically stimulated luminescence dating shows that the20m pitaccumulated over48kyr, the uppermost1.7m (from14.8kyr BP) unit represents the HoloceneS0soil,1.7-20m (47.3-14.8kyr BP) layer for the late last glacial period L1loess, and noweakly weathered paleosol layers are identified in the L1loess layer. All of the winter andsummer monsoon intensity proxies show clear glacial-interglacial scale differences.Compared to the mean of monsoon intensity proxies in the interval of the recent47.3kyrperiod, the monsoon intensity since the late last glacial period can be divided into4stages:(1)the period47.3-30.1kyr BP at which the winter monsoon was weak but showed a significantincreasing trend;(2)30.1-24.2BP, the winter monsoon intensity steadily was equal to theaverage of the monsoon intensity with a significant increasing trend, which slowed downrelatively compared to the increasing trend in the first stage;(3)during the period24.2-14.8kyr BP, the winter monsoon was quite strong but appeared a significant decreasingtrend;(4) during the period14.8kyr BP till now, the summer monsoon is strong with asignificant increasing trend, but winter monsoon is weak with a significant decreasing trend.3. The stage and trend characteristics of the winter monsoon intensity at the Gulangsection are analysed. Optically stimulated luminescence dating shows that the20m pit inGulang accumulated over60kyr, the uppermost1.6m (from13.7kyr BP) unit represents theHolocene S0soil,1.6-20m (60-13.7kyr BP) layer for the late last glacial period L1loess, andno weakly weathered paleosol layers are identified in the L1loess layer. The winter andsummer monsoon intensity proxies in Gulang also show distinct differences between S0andL1layers. Compared to the mean of monsoon intensity proxies in the interval of the recent60kyr period, the monsoon intensity since the late last glacial period can be separated into5stages:(1) the period60-50.1kyr BP was a time when winter monsoon was weak with noincreasing or decreasing trend;(2) at the period50.1-37.1kyr BP the winter monsoonremained at the average level with an insignificantly decreasing trend;(3) at the stage37.1-26.8kyr BP the winter monsoon was weak but significantly increased;(4) during thestage26.8-13.7kyr BP, the winter monsoon was quite strong, and still exhibited a significantincreasing trend; and (5) at he period13.7kyr BP till now the summer monsoon is strong witha significant increasing trend, but winter monsoon is weak with a significant decreasing trend.4. The periodical characteristics of the winter monsoon intensity at the Jingyuan andGulang sections are analysed. At the Jingyuan section, there is no periodicity over the wholeresearch period since the late last glacial period. During the periods of48-30,48-25, and 48-20kyr BP, the winter monsoon intensity was mainly characterised by the periodicities of,8-12,15-20, and30-35kyr, respectively. The winter monsoon intensity at the Gulang sectionhas obvious periodicities which are about10kyr,21kyr, and43kyr, and the first mainperiodicity is about43kyr. At the time scales of43kyr and21kyr, the winter monsoonintensity shows clear periodic variations since the late last glacial period, but the10kyrtime-scale only characterised the winter monsoon intensity during the late last glacial period.The wavelet transform coefficient of the winter monsoon intensity reaches zero, which means,in the future, that the winter monsoon intensity will get to the strong stage, which will resultin a cold-dry climate in the northwestern part of theChinese Loess Plateau.5. The differences of deposition and pedogenesis environments between Jingyuan andGulang are analysed. There is no distinct spatial climatic difference between these two regions,which are characterised by a similar deposition and pedogenesis environment. Jingyuanlocates farther from the dust source area, however, it has a higher mean particle size, highercoarse particle size content, and even higher accumulation rates (10-130cm kyr-1) than thosein Gulang (9-63cm kyr-1). The reason for these differences is the proximity of Jingyuan to theYellow River which is a likely local dust source. Since the Holocene, both annual meantemperature and precipitation are slightly higher in Jingyuan than Gulang, resulting in ahigher magnetic susceptibility, and stronger leaching of elements Sr and Ca in Jingyuan.6. The driving forces of abrupt monsoonal changes at the northwestern Chinese LoessPlateau are determined. The two20m pits clearly record the abrupt climatic events includingthe Younger Dryas event, the Heinrich events, and the Dansgaard-Oeschger cycles. The rapidchanges in the fractal dimensions of particle size distributions at the Gulang section are wellaligned with those abrupt events recorded in the North Greenland ice core and Chinese HuluCave speleothems, especially over the interval of60-27kyr BP. The evidence is frompublished simulated results from the coupled climate model CCSM3, which shows that anadded fresh water flux into the North Atlantic slowed down the Atlantic meridionaloverturning circulation, consequently an enhancement of the winter monsoon circulation anda decrease of summer monsoon precipitation are found across the dust supply regions of theLoess Plateau. As a result, the Atlantic meridional overturning circulation is deemed as thedriving force of the abrupt climatic events in the northwestern Loess Plateau during the lastglacial period.