The Study On The Climatic And Environmental Changes Over The Last 2300 Years: Speleothem Records From Qixing Cave, Guizhou Province

Qixing Cave is located in the slop zone between the southeast edge of Yunnan-Guizhou Plateau and the North Guangxi hills. Qixing Cave belongs to the plateau Karst area of Subtropical monsoon climate zone. The study area is of a typical subtropical monsoon climate and is affected by both Indian and East Asian summer monsoons. This area is an ideal location to study the relationship between global change and the regional response by using the stalagmite. In this thesis, we use stalagmite QX-3 which is collected from Qixing Cave as the research material. Using the scanned image, the grayscale of the polished section of the stalagmite QX-3 was attained. We then discussed the driving mechanism of grayscale change by observing the microscopic structure characteristics of the mineral crystal structure. Based on 16 230 Th age and 1403 oxygen isotope records, along with the grayscale features, we reconstructed climatic and environmental changes over the last 2300 years in the study region. By using Ensemble Empirical Mode Decomposition(EEMD) method, we analyzed and compared the variations of the speleothem δ18O time series of QX-3 and DA(a stalagmite collected from Dongge Cave) on different times-scales.. We explored the characteristic of climate fluctuations which is recorded by stalagmite oxygen isotope, and the dominant driving factor of the climatic cycles’ feature. Weobtained the following conclusions:1. The changes in grayscale of QX-3 stalagmite mainly reflect the porosity and impurity content within the intercrystal of calcite which is related to the environmental changes inside the cave during the growth of stalagmite. It means that the stalagmites’ grayscale can be used as a proxy index for paleoclimatic and paleoenvironmental reconstruction in this cave. The grayscale profile of the stalagmite is highly correlated with the δ18O series and δ13C series, suggesting that both grayscale and isotope composition were controlled by the common environmental factors, and documented the climatic and environmental changes.2. The δ18O time series of stalagmite QX-3 indicate that the δ18O values were relatively high during the 100~200a, 600~800a and 1500~2000a(AD), implying the relatively dry climate; and the δ18O values were relatively low during the 400BC~100a, 200~600a and 800~1500a(AD), suggesting the relatively wet climate. The profiles of δ13C and grayscale show the similar variations to the the δ18O profile, further confirming the climatic and environmental conditions inferred from the δ18O record.3. The spectrum analysis of QX-3 δ18O sequence showed that the periods of 24-18 years and 14 years are significant(above the 99% confidence level). These periodscorrespond well with that found in the tree-ring reconstructed △14C series, furtherconfirming the forcing of solar activity on the Asian monsoon climate.4. The comparisons of last 2000 years QX-3 δ18O record and other speleothem δ18O records, i.e., the records from Wanxiang Cave, Huangye Cave, Heshang Cave, and Dongge Cave, indicate that besides the similarities, the differences on centennial todecadal time-scales are also significant, implying the diverse responses of regional precipitation to the Asian monsoon changes on these time-scales.5. The resulting components of EEMD analysis on QX-3 and DA δ18O time series showed the consistent variations of these two records on decadal time-scales, which are likely associated with the changes of Pacific decadal oscillation(PDO), indicating that both records were influenced by the same forcing factors on decadal time-scales. The similarity of these two records suggest that the solar activity plays a important role in modulating the Asian summer monsoon changes on these time-scales, through direct or indirect forces...