Characteristics Of Surface Sediments In The Taklamakan Desert And Their Implications For Surface Processes And Paleoclimate

Deserts are an important part of the global ecosystem.The geochemical composition of eolian sediments not only can be used to fingerprint the source area,but also has a sensitive response to climate change and regional environmental evolution.In recent years,a large number of studies have focused on identifying the sediment provenance based on the geochemical characteristics of sediments,and great progress has been made.However,at present,most of the studies investigating eolian sediment provenances are qualitative,and quantitative research is relatively scarce.Additionally,many studies reveal modern surface processes by analyzing the geochemical characteristics of eolian sediments in arid areas,but the effects of modern environmental changes on sediment properties needs further research.The Taklamakan Desert,the largest desert in China,is a significant dust source in the Northern Hemisphere which affects the global biogeochemical cycle.However,the transport of Taklamakan Desert dust to the ocean and its response to global climate change are still unclear.The scientific goal of this thesis is to understand the provenance of eolian sediments in the Taklamakan Desert,and its response to both modern environmental change and paleoclimate variability.The first aim is to identify the provenance of different sediments and quantitatively calculate the relative contributions of the source areas to sediments in the Taklamakan Desert.This thesis examined the sedimentological and geochemical characteristics of different sediments collected from different geomorphological regions in the Taklamakan Desert.The second key goal was to understand the interaction between sediments and surface processes and environmental change by monitoring changes in sediment composition as the intensity of desertification varied since2000.Finally,this thesis examined how the transport of desert dust responded to paleoclimate change by comparing desert dust from the Taklamakan Desert to the geochemical characteristics of eolian dust in deep-sea sediment cores collected from the Pacific Ocean.This thesis examined the grain size,major,trace and rare earth elements of surface sediments collected from loess,sandy loess,fluvial sands,vegetated dunes and mobile dunes in the Keriya River Basin on the southern edge of the Taklamakan Desert.The sediment provenance of different grain size fractions and different geomorphological types were identified and the relative contribution of different sediment sources was quantitatively calculated.Major and trace element analysis found that although the properties of the bulk samples were similar to those from the Kunlun Mountains,the sediments within different grain size fractions have different source areas.The discriminant function analysis method was used to quantitatively calculate the relative contribution of the sediment sources,and the results show that the contribution of the Kunlun Mountain and Tianshan Mountain to the sand fractions are 68% and 32% respectively,while the contribution to the dust fractions are 40% and 60% respectively.The provenance of different types of sediments within the same grain size fractions remains unchanged,but the relative contributions is slightly different.The contribution from the Kunlun Mountains to the sand fractions of mobile dunes and fluvial sands is about 66%～77%,which is higher than that of the Kunlun Mountains contribution to other types of sediments.Modern environmental changes affect the surface processes and the properties of sediments.This thesis analyzed the vegetation change,the Palmer Drought Severity Index and the Surface Wetness Index in the Keriya River Basin since 2000,with results showing that the intensity of desertification processes weakened.The grain size and major element characteristics of sediments from the same sampling location collected in different years were compared,and the regional wind field strength was analyzed.The results show that the grain size composition and major element characteristics of sediments responded to vegetation change,wind field strength and desertification intensity in the past 20 years.This study suggests that with the strengthening of westerlies and precipitation,the vegetation coverage and degree of surface moisture in the Keriya River Basin increased.This means that more clay components may be added to the loess located on the northern slope of the Kunlun Mountains.Since 2000,the weakening of desertification and the decreased in near-surface wind speed resulted in an increased grain size and a relative increase in the mafic mineral content of aeolian sediment on the surface of mobile dune in the Keriya River Basin.Dust plays an important role in global climate in both the past and present.This thesis compared the Sr-Nd-Hf characteristics of Pacific Ocean terrigenous clastic sediments and desert dust from different sources during the Paleocene.The aim was to identify the source of Pacific eolian dust through time on a tectonic scale and its paleoclimate significance.The result show that between65～34 Ma,a semi-humid climate occurred in some parts of the mid-latitudes of Asia,and the Taklamakan Desert had not yet formed.Eolian loess deposition had started to form by 34 Ma in southwestern Mongolia,and the marine eolian dust in the North Pacific Ocean came from Mongolian dust and American volcanic ash.Since 25 Ma,drought occurred in the Taklamakan Desert and the region provided dust to the northwest Pacific Ocean.With the periodic uplift of the Tibet Plateau,aridity was gradually enhanced in central Asia.This caused the contribution from the central and western deserts,dominated by the Taklamakan Desert,to the marine dust in the Northwest Pacific to gradually increase.This finding provides new insight into the response of desert dust release and transport due to tectonic-scale paleoclimate change.