Orbital And Millennial-scale Climate Changes During The Middle Pleistocene From Western Tarim Basin,Northwest China

The climate in Asia can be roughly divided into three distinct regions: the humid eastern-southern region that is mainly influenced by monsoon circulation,the inland arid/semi-arid western region that is dominated by the westerlies,and the Tibetan Plateau that is primarily influenced by the Westerlies and the Indian summer monsoon.Previous paleoclimatological research has mainly focused on sedimentary records that document the Asian monsoon and the changing climate on the Tibetan Plateau.There are comparatively few studies on the inland arid regions of Asia that were affected by the changing patterns of westerly circulation through the Quaternary,most of them focused on the climate change during the Holocene,on shorter time scales.Westerly winds that link the North Atlantic and East Asian monsoon area have a major impact on the climate of East Asia.The interaction between the westerly winds and the Tibetan Plateau also plays an important role in the climate of western China,especially in the Tarim Basin.The Tarim Basin is part of the largest arid zone in the northwestern region of China.It is bounded by the Tian Shan range to the north,the Pamir Plateau to the west,and the Kunlun Mountains to the south.Its environment is characterized by scarce water,limited vegetation and fragile ecosystems,thereby making it very sensitive to climate change.The Quaternary sediments in the Tarim Basin are very sensitive to climate changes,but there are almost no studies on the Quaternary in the western Tarim Basin.As a result,the sedimentary pattern and paleoenvironmental characteristics of the Quaternary in this area are still not well known,and chronostratigraphic and paleoclimate research work is needed.Previous studies on the progressive stepwise aridification history of the Asian interior have utilized the widespread thick loess deposits in the Tarim Basin.However,there is a lack of outcrop and core research about the aridification during the late Quaternary in the western Tarim.This work focused on builting the chronology of the Quaternary boreholes in the western Tarim Basin and reconstructing the palaeoclimate on orbital and suborbital scales based on magnetic susceptibility,geochemical elements,gamma-ray,total organic carbon,grain size,and pollen-spore.Then using the LOVECLIM model to explore the mechanism of the climate change at the glacial-interglacial scale.The main conclusions are as follows:1)The cores from the 800-m-long KT11 borehole into the Kashgar region at the westernmost part of Tarim Basin of NW China are mainly composed of fluvial fan sediments.To explore whether Milankovitch cyclicity might also be present in fluvial successions,we carried out high-resolution gamma ray(GR)from downhole logging,magnetic susceptibility(MS),Rb/Sr and total organic carbon(TOC)analysis of a core from an 800-m borehole(KT11)from the Kashgar region in the western Tarim Basin,China.Spectral analyses of the GR,MS and Rb/Sr data reveal cycles with ?70 m,?30 m and ?14 m wavelengths.These pertain to relative variations in grain size.Electron spin resonance(ESR)dating and magnetostratigraphy imply that the cored interval spans the past 1.15 Myr.This age-span constraint,coupled with the comparison of the ratios of these cycle wavelengths to expected ratios of Milankovitch frequencies,suggests that these cycles represent ?100-kyr short eccentricity,?40-kyr obliquity and ?20-kyr precession frequencies,respectively.We constructed an astronomical time scale(ATS)spanning the past 1.15 Myr using the ?100-kyr short-eccentricity period.The average accumulation rate is about 70 cm/kyr for this 800-m core succession,and the evolutive spectrum emphasizes that long-term sedimentation rates were stable.We also applied correlation coefficient(COCO)analysis to identify the astronomical forcing.2)We conclude that orbitally forced climate change was the main driver of stratigraphic alternations between sand-and clay-rich intervals in the Kashgar region.During maxima of the 100-kyr short-eccentricity cycles,which correspond to interglacial intervals,more chemical weathering and increased seasonal runoff delivered a larger amount of relatively coarser-grained sediments to the fluvial fan in the Kashgar region,and vice versa.The Mid-Pleistocene Transition(MPT)at ca.0.9 to 0.65 Ma is also well recorded in our GR and Rb/Sr data,and is characterized by a shift in dominant cycle period from 41 kyr obliquity to 100 kyr eccentricity.After 0.65 Ma,the average TOC content decreases whilst Rb/Sr values increase,which suggests that there was an increase in regional aridity of the Kashgar region in the western Tarim Basin following the MPT.This regional change is synchronous with the trend toward aridification observed throughout central Asia.3)Dating by OSL established a chromoly of a 48.5 m interval in the KT11 borehole from the Kashgar Basin spanning 70 to 14 ka.The GR log of relative clay to sand abundances is a record of warm-humid climate with increased clay influx alternating with cold-dry climates.The GR and mean grain size data reveal that the influence of temperature and the hydrological cycles play significant role on the climate in Xinjiang region,support the climatic variation presents an in-phase pattern between central Asia and mid-latitude monsoonal Asia during last glacial.The GR record has a goocorrelation with summer insolation,and the power spectrum also shows semi-precession and 4th harmonics of the precession cycles.The six of the Heinrich cold events and D-O cycles of the last glacial were also recorded by the fluvial system on the Kashgar Basin.This demonstrates that rapid and high frequency climate changes in the North Atlantic also had profound effects on the western Tarim Basin region.And we also suggest that the ?2 kyr and 1 kyr climate variabilities from the western Tarim Basin were primarily the results of solar activity.4)The B/N(broadleaf/needleleaf,warm index)ratios with higher values in MIS 9,7,5 could be the warmer interglacials.And AP/NAP ratios(Arboreal Pollen /Non-Arboreal Pollen NAP,humid index)show higher values during MIS 5 and MIS 15.The results from the LOVECLIM suggest that mean temperature in the western Tarim Basin is mainly controlled by GHG and the mean precipitation is controlled by the insolation,which is in reasonable agreement with the pollen-spore records.The B/N record and GR data show changes in the magnitude of interglacial after the Mid-Brunhes Event.