Orbital And Suborbital Climate Variablity Revealed By Pliestocene Eolian-lacustrine Deposits In The Weihe Basin

Orbital to sub-orbital climate variability is supposed to driven by the influence of orbital-induced insolation and bipolar ice-volume changes.However,climate indicators show distinct periodicities and amplitude characteristics identified from aeolian and speleothem records since the Pleistocene.The Weihe Basin,a significant area to discuss the monsooonal evolution,has accumulated thick fluvial-lascustrine depositions since the Cenozoic.This thesis comprehensively analyzes various proxies including grain size,logging（electrical resistivity）,and magnetic susceptibility from 512-m Huxian（HX）fluvial-lascustrine depositions in the Weihe Baisn,discusses the climate change process from orbital to sub-orbital timescales over the past 1.8 Ma,and furtherly discusses the potential driving mechanism with simulation,the proxy indicators were decomposed by the ensemble empirical mode decomposition（EEMD）to explore the characteristics of orbital-suborbital scale climate fluctuations recorded by lascustrine and aeolian sediments.The following results and new findings are obtained:（1）The 1.8-Ma environmental evolutions of Weihe basin recorded by Huxian（HX）lacustrine deposits were revealed.The main lithofacies of HX sediments can be distinguished as upper aeolian,littoral to shallow lake,and subaqueous delta environments.The mean grain size（MGS）of HX core is synchronized with the electrical resistivity（ER）and shows complex and rapid fluctuations and indicates the change of lake level controlled by precipitation.Magnetic susceptibility（MS）is positively correlated with coarse grains but fluctuates with weaker amplitude under complex sedimentary environments.The paleomagnetic and ESR dating results suggest that the HX sequence accumulated after 1.8 Ma.Late Pleistocene（～0.18 Ma,at～25-m depth）saw the end of fluviolacustrine deposition and beginning of subaerial loess accumulation in the HX core,suggesting desiccation of paleolake and integration of Yellow River.（2）Grain size records in HX lacustrine and Weinan（WN）loess are positively correlated while the susceptibility records are generally negatively correlated.The MGS and MS in two sedimentary facies display distinct monsoon variability,these divergences may arise from differences in the dominant mode of climate variability at the geographical locations of these records.The MGS of HX lacustrine deposit and the MS of WN loess were selected as sensitive indicators of summer-mosoon indensity for the further discussion.On orbital timescales,the WN loess MS exhibits dominant peaks around～100 and～41 ka,while the HX MGS was dominated by～23,～41,and～100-ka periodicities.On sub-orbital timescales,the records of WN loess show faint millennial variability,while the HX lacustrine features a pronounced peak centred at the～11-ka half-precessional cycles.（3）The dynamics of distinct orbital monsoon variability and half-precessional cycles are discussed.On the orbital scale,the～23 ka signal in fluvial-lacustrine deposit coincides with the boreal summer insolation,while the～100 ka periods in the loess sediment are mainly related to global ice volume.The simulation results show that the effects of orbital precession,obliquity,CO₂,and ice volume to the temperature and precipitation are spatially different in East Asia.The complex periodic expression of the MS of WN loess and MGS of HX lacustrine deposit was provoked by different sensitivities to changes in precipitation and temperature.On the sub-orbital scale,the half-precessional hydroclimatic changes recorded in HX lacustrine sediments yield good correlations with summer insolation maxima in 30°N and 30°S,which are supposed to be controlled by low-latitude insolation forcing and alternative modulation under the interhemisphric land-air-sea interaction.