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Palaeocliniatic Changes In Central Asian Arid Area During Past50000Years Based On Loess Records

Posted on:2013-03-04Degree:DoctorType:Dissertation
Country:ChinaCandidate:M RanFull Text:PDF
GTID:1220330398469440Subject:Physical geography
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The loess-covered area of Central Asia is surrounded by other three loess areas including European loess area, Siberia loess area and Chinese Loess Plateau. Therefore, the loess deposits in Central Asia is of great significance in clearifying the similarities and differences of loess deposits between Central Asia and other loess areas and in understanding the regional climate importance in terms of loess deposition and paleosol development in loess. Climatologically, Central Asian Arid zone is affected by the Asian monsoon and the westerlies, understanding the regional paleoclimate change allows us to clarify the interacting history of the Asian monsoon and the westerlies. According to field strata investigation, in combination with laboratory analyses with chronological supports of AMS14C dates, the climate change in Central Asia during past50,000years is reconstruted. The conclusions are as follows:1. From~48,00014C yr BP to~30,00014C yr BP,the loess deposited under poorly-drained conditions and the loess deposition was interrupted by three weakly developed paleosols that were formed at~45,00014C yr BP, at~40,00014C yr BP and at~35,00014C yr BP, respectively. From~30,00014C yr BP to~18,00014C yr BP, loess deposition absolutely dominated under poorly-drained conditions with some gleyed characteristics. From~18,00014C yr BP to~10,00014C yr BP, a weakly developed and gleyed paleosol was developed in VA section that is located in the western portion of the studied area. Holocene sequence in Central Asian region includes an early loess layer (~10,00014C yr BP—~5,00014C yr BP) and a late soil layer (since5,00014C yr BP).2. Comparison of the loess deposits in the study area with those in the surrounding area (i.e., Europe, Siberia, China Loess Plateau) demonstrates that MIS3was unamously characterized by2-3paleosol development in all of mentioend loess areas. The typical loess of MIS2was deposited under poorly-drained conditions in Central Asia, being similar to those in Europe and Siberia and being differene from those in Chinese Loess Plateau. The Holocene sequence Central Asia is out-of-phase with that in East Asia (i.e., Chinese Loess Plateau). That is, Holocene sequence in East Asia loess region includes an early soil layer (~9,00014C yr BP—~4,00014C yr BP) and a late loess layer (past~4,00014C yr BP), while Holocene sequence in Central Asia loess region includes an early loess layer (~9,00014C yr BP—~4,00014C yr BP) and a late soil layer (past~4,00014C yr BP), Generally speaking, the loess stratigraphies in central Asia are similar to those in Europe.3. Organic carbon isotope analysis revealed that δ13Corg values vary between-22%o and-26%o at VA and TR sectios, the indicative vegetation was composed of C3and C4plants with C3plants being dominated. The δ13Corg values were more negative during MI S3than MIS2and paleosol layers have more negative813Corg values than loess layers. Further analysis demonstrates that the major control factor on the δ13Corg values was probably precipitation or/and soil moisture. The δ13Corg values in ZKT section show an opposite trend in comparison with those in VA and TR section. That is, the δ13Corg values were more negative in loess layers than in soil layers.4. Our analysis also found that the proxy "<10μm (%)" in grain size is extremely sensitive to climate change and that the "<10μm(%)" in VA and TR sections was significantly low at-12800cal. yr BP, at-16000cal. yr BP, at~24000cal. yr BP and at~39000cal. yr BP, seemingly being correspondent with YD, H1, H2and H4events retrieved in North Atlantic regiom during the last glaciation.5. The Holocene variations in "<2μm(100%)" in grain size in VA section is similar to the Holoene winter precipitation trend in Norwegian Sea and to the Holocene moisture evolution trend in Xinjiang. That is, early Holocene was relatively dry and late Holocene was relatively humid.
Keywords/Search Tags:Central Asia Arid raea, Loess, Climatic Change, Last Glaciation, Holocene
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