Regional Climate And Environmental Changes Documented By Sediments From Toson Lake In The Qaidam Basin During The Late Holocene

Being influenced of the westerlies and the Asian summer monsoon,Northeast Tibetan Plateau(NE TP)is an ideal region for studying the history of the Asian summer monsoon and the westerly during the late Holocene.In this thesis,Lake Toson in the eastern margin of the Qaidam Basin in the NE TP was selected as the object of study.Core TSLA(37°08.185?N,96°57.387? E,381 cm in length)and TSLC(37°07.060?N,96°57.017?E,35.5cm in length)were recovered at the water depth of 23.6m in Lake Toson and were dated using AMS14 C and 210 Pb,respectively.Proxies included carbonate content,carbonate isotope(?18Ocarb and ?13Ccarb),carbonate mineral composition,grain size,total organic carbon content(TOC)and trace-element.The climatic and environmental significance of each proxy was clarified on the basis of modern water isotopes,surface and recent sediments and the meterological data since 1956.Results show that carbonate content indicates the desalination and salinization of the lake water.?18O and?13C value were attributed to changes in precipitation/evaporation(P/E)ratio(effective humidity).The TOC content indicates temperature change of the lake area.The Zr/Rb ratio and the change of Ti content indicate the change of chemical weathering and erosion intensity,and the stage of the hydromagnesite corresponded to the cold and dry climate environment.The climate change during the late Holocene can be divided into three stages.During the period of4633~2990cal a B.P.,the relative humidity of the lake area fluctuated obviously,there were two dry(4633~4310 and 3750~3520cal a B.P.)and two wet(4310~3750 and 3520~2990cal a B.P.)periods.During the period of 2990~1400cal a B.P.,the effective humidity in the lake area slightly decreased,the effective humidity and temperature were relatively low.From 1400 cal a B.P.to recent,the climate is more humid,relatively wet environment during the periods of 1380~1250cal a B.P.and 1080~800cal a B.P.correspond to the Dark Ages Cold Period and Medieval Warm Period,respectively.The climate had obvious dry and wet stage during the Little Ice Age(670~100cal a B.P.).The sediment of Lake Toson reveals a significant phase of recent human activities during the period of 1975~2004A.D..The inconsistency between the reduced effective humidity recorded by proxies and the increased humidity by the observational data were caused by regional agriculture development.Increasedwater consumption for irrigation during this period reduced the water supply for Lake Toson through the Bayin River.Our result suggests that recent human impacts should be evaluated before the relationship between proxies and climate factors are used for paleoclimate reconstruction in regions with intensive human activities.The effective humidity change in the Lake Toson area is in good agreement with the westerly-influenced region.The effective humidity of Lake Toson increased when the NAO anomaly was negative.Negative NAO anomaly was advantageous for atmospheric water vapour transport through westerlies to the Qaidam Basin.Meanwhile,high total solar irradiance(positive ?TSI)would intensify the monsoon and then strengthens the subsidence of air mass on the Qaidam Basin.Therefore,high total solar irradiance strengthens drier climate on the Lake Toson area.