| Under the background of global climate change,past climate evolutions in different regions over the world have been studied for a long period.Scientists obtained past climate informations from various geologic materials,such as marine sediments,glacial ice cores,lake deposits,loess-paleosol sections,tree-rings,fossil corals and cave speleothems.These materials provided researchers with different climate changing processes and environmental responses to the driving factors on different timescales,i.e.decadal,centennial,millennial,and orbital scales.Over the past two decades,speleothem records have been developed as one of the climatic benchmarks in climate change research due to their advantages of precise dating(U-series),multiple and high-resolution proxies,and large distributions of caves and available stalagmites.In China,the longest speleothem-inferred climatic curves,spanning the past 640,000 years,have been recently established utilizing oxygen isotopes of four famous caves in central and southern China.On the other hand,a large amount of speleothem-based climate records were built using multiple proxies,for instance,oxygen isotope(?18O),carbon isotope(?13C),trace element concentration and ratio,layer thickness,brightness,magnetism indicators and biomarkers.However,studies on speleothem records in northwestern China and arid central Asia are still needed because of the poor understanding of past climate evolutions and lack of well-dated materials in this huge and arid/semi-arid region.In this thesis,a cave named Baluk in southern Tianshan Mountains of central Xinjiang region was surveyed and four calcite-deposited stalagmites were collected from the cave in September,2012.Besides stable isotope ratios(?18O,?13C),the trace element ratios(Mg/Ca,Sr/Ca,Ba/Ca,U/Ca)of Baluk Cave stalagmites were analyzed for reconstructing the regional climatic evolutions during the last two thousand years,the Holocene,and the MIS5 c to 5a stages.The main results were summarized as follows:(i)The modern precipitation oxygen isotope(?18Op)variations of observatory stations in central Xinjiang region were firstly investigated on monthly and annual time scales.The analyzing results indicated a prominent ?Temperature effect‘ on monthly variations of precipitation ?18Op values,namely the ?18Op values of summer rainfall were quite higher(~-5‰)while it was quite lower in winter snowfall(~-20‰).On annual timescale,there occurred a negative correlation between mean annual temperature and annual weighted average precipitation ?18Op values.This anti-correlation relationship was proved by the inverse varying trends in regional annual temperature and ice core oxygen isotopes from two mountain glaciers(during the past ~50 years)in middle and eastern Tianshan Mountains.Moreover,the calculation of relative differences of two distinct years(1997/1988)indicated that moisture sources and transporting pathways to the study area were variable between these two years.It was possible that more water vapor originating from high-latitude areas with relative lower ?18Op values,i.e.north Atlantic and Artic oceans,were transported to Xinjiang by westerlies during the warmer year(1997).For the cooler year(1988),the proportion of moistures from mid-latitude region with relative higher ?18Op values,i.e.Mediterranean,Caspian and Aral seas,were increasely transported to central Xinjiang.(ii)During the past two thousand years,an 87-mm long stalagmite(BLK12C)deposited in Baluk Cave according to its 230 Th ICP-MS dates.The oxygen and carbon isotope curves of BLK12 C illustrated a positive relationship with each other and varied in very similar trends on decadal to centennial scales.Together,the Mg/Ca ratio showed similar varying pattern with stable isotope profiles,especially the abrupt increasing and decreasing events around the beginning and termination of Medieval Climate Anomaly(MCA,950-1250AD),accompanied by a relative lower state of those three ratios occurred in the duration of MCA.However,other three trace element ratios(Sr/Ca,Ba/Ca,U/Ca)did not show remarkable shifts during the MCA instead of relative stable values during the period.It was noted that all these indexes displayed valley values during the 14 th century,possibly suggesting a multi-decadal wetter period in the study region,which could be related to the development and collapse of Mongolia and Yuan Empire in Asia.During the Little Ice Age(LIA,1300-1900AD),the ratios of ?18O and ?13C varied inapparently,while Sr/Ca,Ba/Ca,U/Ca ratios showed dramatic fluctuations which in turn could respond to the cooler and unstable climate conditions during the LIA.Meanwhile,there was a persistent increasing trend in Mg/Ca ratio since the middle 18 th century,which was potential to record the influence of human activity-affected atmospheric aerosol growth since the industrial revolution.(iii)During the Holocene,the Baluk Cave provided two stalagmites for researching,BLK12 A and combined BLK12B/C,which grew during the past 9500 years based on 38 precisely dated U-Th ages.The ?18O records of Baluk Cave stalagmites showed relative lower ?18O values during early to middle Holocene(9.5-3.0ka),while relative higher ?18O values were observed after the ~3ka shift.This varying pattern followed northern hemisphere summer insolation(NHSI)on orbital timescale.By the way,an obvious hiatus occurred on BLK12 A stalagmite during 2.5~1.3ka,which confirmed the weak depositing condition of the cave.In addition,the trace element ratios,Sr/Ca,Ba/Ca,U/Ca,illustrated remarkable higher ratios during the early Holocene and lower ratios during the middle to late Holocene.The variations of above-mentioned ratios possibly suggested a dry early Holocene,a humid middle Holocene and a wettest late Holocene in the study area,which is consistent with regional precipitation/effective moisture variations constructed by lake sediments and loess-paleosol sections.Additionally,the climatic shifting recorded in Baluk Cave could be related to the glacier advancing event and westerlies circulation adjustment around 3ka in arid central Asia.(iv)During the MIS5c-5a stages,two stalagmites,BLK12 E and BLK12 F,were occupied to establish proxies on the basis of 24 precise U-Th dates,showing a growth time interval of 78.0-101.5ka,an exact time window of a precessional cycle(~23ka)covering the time interval of late-MIS5 c to MIS5 a stages.The stable isotope ratios of BLK12 E and BLK12 F showed similarities on orbital variations,which showed relative lower ?18O and ?13C values during 5c/5a and higher values during 5b(~3‰ shifting).Although quite lower resolution measurements for trace element wereperformed,the ratios(Mg/Ca,Sr/Ca,Ba/Ca,U/Ca)indicated orbital synchronizations with stable isotopes.Compared with regional cave records in Xinjiang and Uzbekistan,Baluk Cave records confirmed the driving role of precession on orbital timescale climate evolutions in northwestern China and arid central Asia.The MIS5b/5a transition or MIS5.2/5.1 event recorded by Baluk records provided us continuous record in studying such an abrupt climatic shift and environmental transformation in the arid/semi-arid region.The MIS5.2/5.1 event in central Xinjiang could possibly respond to climatic variations in both hemispheres according to the similarities between Baluk Cave and Greenland/Antarctic ice core records.Notably,the time point of increasing temperature recorded in Greenland was earlier(~85.5ka)than Antarctic record(~84.5ka),which also preceded Asian summer monsoon and westerlies-dominated climatic shiftings.Our findings suggested the coupled paleoclimatic variations of different climatic cells in Asia continent climatic system,and the possible contribution of monsoonal moisture to Xinjiang region on orbital timescale.Furthermore,the moisture transportations from tropical and intertropical regions will significantly impact the environment and ecosystems in such an arid/semiarid region. |
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