Luminescence Chronology Of Drilling Cores In Ulan Buh Desert And Its Inferred Lake-desert Evolution During Late Ouaternary

Desert is an important landscape in arid and semi-arid area of the world, and is recognized as the symbol of aridity and desertification. Desertification mainly affected by climate changes during Pleistocene and desertification could be affected by both climate changes and human activities During the Holocene. Therefore, it is important to study the evolution of typical deserts during late Quaternary to understand how the desert environmental changes response to the global climatic changes and what relationship between human activities and desertification during the Holocene. This study chose Ulan Buh Desert and Western Hetao Plain, located in the fringe of Asian summer monsoon, where environmental changes are sensitivity to monsoon strength. Two areas experienced dramatic environmental changes during Late Quaternary and frequent human disturbances in the late Holocene. It is of scientific significance to study environmental changes during the Late Quaternary in this region.We investigated Ulan Buh Desert and Hetao Plain six times between September2007and October,2010.6profiles in northern Ulan Buh Desert were taken DGPS measurement and OSL samples collection. In July2009, OSL dating samples and proxy samples were collected in the western Hetao Plain. In Oct2010, two cores in the northern Ulan Buh Desert were drilled and continuous cores WL10ZK-1and WL10ZK-2were collected. In the OSL laboratory, different K-feldspar methods were conducted on Holocene samples and the reliability of K-feldspar methodology for Holocene samples were discussed, especially its application in north China samples. The Infrared stimulated luminescence (IRSL) dating and OSL dating were applied to establish the chronology of core samples. Combing OSL chronology, grain size, carbonate content, Organic content, and pollen, environment changes during last150ka in Ulan Buh Desert reconstructed, and the possible driving forces of environmental changes are also discussed.We explored using the multi-elevate-temperate Post-IR IRSL (Met-pIRIR) of coarse Potassium feldspar grains to obtain reliable ages for samples beyond Quartz signal saturation zone. Combing with reliable Optical stimulated luminescence (OSL)dating of coarse quartz grain, the chronology of drilling core during last150ka were established. IRSL dating of the K-feldspar fraction is an alternative way for Holocene samples when quartz-fraction luminescence signals are comparatively dim. In this study, OSL dating was applied using quartz and K-feldspar fraction respectively to four Holocene samples. K-feldspar present age using single aliquot regeneration dose protocol (SAR), corrected feldspar age using g value correction, and isochron IRSL age using isochron dating method were obtained and compared with reliable quartz age using SAR dating method. It is found that K-feldspar present age has relative deviation less than10%than quartz age and can be used to establish Holocene dating framework. The measured g values were used to correct the conventional IRSL ages and K-feldspar correction age with g value corrected cannot be used to obtain reliable ages due to the introduction of systematic errors when measuring g values in laboratory. Isochron dating method is effective to measure samples with dose rate problems, which was caused by related to changes in past dose rate due to post depositional migration of radionuclides or/and changes in water content when water-lain sediments dry out. However, it may introduce a relatively greater error for different grain sizes K-feldspar equivalent dose curve fitting and therefore it could be used to build a low-resolution framework for Holocene samples.The high lake level period in northwest china appeared at MIS3or MIS5has been in controversy. The main reason for this is lake of reliable absolute age control.14C dating only could be used to date samples less than50ka. The error bar of traditional OSL dating with quartz is large when measure samples old than70ka because of low De saturation of quartz signal. In this study, the chronology of drilling core is established by Met-pIRIR dating using coarse K-Feldspar. The stratum chronology and index analysis shows lake-desert changes occurred at Ulan Buh Desert during late Quaternary. The-10m thickness lacustrine sediments chronology shows a stable and continuous paleolake appeared at MIS5c-5e with a high lake period appeared during130-87ka. The warm and moisture-conditioned climate during the Last Interglacial is likely responsible for the paleolake appearance at Ulan Buh Desert.The core chronology study also indicates the paleolake shrank and broke apart at the Last Glacial. Thereafter, Aeolian activity prevailed during Last glacial to early Holocene. The short duration lakes were appeared at LGM. The lake appearance at this period likely due to low evaporates near surface induced by cold climate of this period.The evolution of Ulan Buh Desert has long been debated, due to the lack of reliable absolute age data and/or depositional evidence. During the field work in May2008, we found widely distributed blue-grey sandy clay between dunes in northern Ulan Buh Desert. We anticipated that these lacustrine outcrops to have same altitudes and ages if they were deposited by a unified paleolake. We dated these lacustrine sediments using optically stimulated luminescence (OSL) dating and measured their accurate altitudes using different global position system (DG-PS) technology. In combination with proxies such as grain size and carbonate content, we reconstructed the lake-desert changes of Ulan Buh Desert and Western Hetao Plain.The OSL ages of aeolian sand at the top of profiles in Ulan Buh Desert and western Hetao Plain indicate an aeolian sand dune landscape were formed at12-8ka BP. In this study, OSL dating, along with DGPS measurements were successfully applied to derive the lake-desert evolution in Ulan Buh Desert. The results show that another paleolake covering the whole northern Ulan Buh desert formed during8-7ka. The possible mechanism is that there was increased precipitation in this area as a result of the increasing intensity of Asia summer monsoon from early Holocene to middle Holocene. Then, the Ulan Buh paleolake shrank and broke into parts during7-6.5ka BP. The formation of modern Ulan Buh Desert and Jilantai Salt Lake was synchronous with the disappearance of this paleolake. The modern Ulan Buh Desert was formed after7-6.5ka, when human activity was quite limited. The natural impacts are considered to be the main force for this desertification process. Specifically, the drying event at the fringe of monsoon zone induced by the decreasing summer monsoon strength is responsible for the desert evolution. The enviroment become optimum during5-2ka charactered with the dune stabilization and paleosoil formation. Human activities such as farming and following abandonment and overgrazing during Han dynasty at2ka BP induced the dune activity of the region again. High dunes formation at-2ka in the northwestern Ulan Buh Desert might correlate to human activities at around area.The evolution of ancient Tushenze Lake was synchronous with the development of Ulan Buh paleolake. At the beginning of the Holocene, the entire desert and wesetern Hetao Plain was in a desert environment. OSL dating results and grain size analysis results show that high lake levels of ancient Tushenze Lake occurred at～7.6ka. The lake environment disappeared before6.5ka BP and dune activity appeared at the western Hetao Plain at the same period. Dune activities were limited with in certain areas, which was different from the aeolian sand dune geomorphology in northern Ulan Buh Desert.