Comparative Study On Aeolian Loess-paleosol Sequences And Environmental Changes Since Late Pleistocene In Maqu Valley And Luoyang Basin

Aeolian loess-paleosol sequence is an important carrier for recording paleoclimate cycles and monsoon evolution during Quaternary glacial-interglacial period.Compared with the detailed loess deposits studies in the central of the Loess Plateau,the study of typical aeolian loess-paleosol profiles on the northeast Tibetan Plateau and Luoyang Basin is still not in-depth.In this paper,two typical aeolian loess-paleosoil sequences（ZHK and WCG profile）were found through detailed filed investigations in the Yellow River source area and Luoyang Basin,respectively.The chronology(AMS ¹⁴C and optically stimulated luminescence（OSL）dating)and sedimentology（magnetic susceptibility,hygroscopic water,geochemical elements,micromorphology,grain-size and heavy mineral）were analyzed in the laboratory.The Bacon age-depth modelling,end-member modelling analysis and cluster analysis were combined for comprehensive analysis.The main conclusions of comparative analysis are as follows:（1）The stratigraphic sequences of the ZHK profile from the bottom to top are listed as:Glacial outwash deposits（>14000 yr BP）→Late Pleistocene aeolian loess,Glacial outwash/flashflood deposits（14000～11500 yr BP）→Early Holocene transitional loess（11500～9000 yr BP）→Mid-Holocene paleosol（9000～3100 yr BP）→Modern soil（3100～0 yr BP）.The stratigraphic sequences of the WCG profile from the bottom to top are listed as:Late Pleistocene Malan loess（>11500 yr BP）→Early Holocene transitional loess（11500～8500 yr BP）→Mid-Holocene paleosol（8500～3100 yr BP）→Late Holocene recent loess and topsoil（3100～0 yr BP）.（2）Most of samples of the ZHK profile are weakly weathered,while the paleosol is moderately weathered.Weathering is at an initial stage of Na⁺and Ca²⁺leaching,and is dominated by plagioclase weathering.In terms of pedogenic intensity the stratigraphic units can be ordered as follows:Mid-Holocene paleosol>Modern soil>Early Holocene transitional loess>Late Pleistocene aeolian loess.The weathering intensity of each sedimentary layer in the WCG profile is different.The paleosol was moderately weathered,while the loess（Malan loess,transitional loess and recent loess）was weakly weathered.The profile is dominated by plagioclase weathering,the weathering intensity of the stratigraphic units is as follows:Mid-Holocene paleosol>Late Holocene recent loess>Early Holocene transitional loess>Late Pleistocene Malan loess.（3）The ZHK profile of Maqu valley records four stages of paleoclimate evolution on the northwest Tibetan Plateau since the Late Pleistocene.1)The late Pleistocene（14000～11500yr BP）was characterized by a cold and dry climate dominated by the Westerlies.2)The climate gradually warmed with the strengthened East Asian summer monsoon and weakened Westerlies in the early Holocene（11500～9000 yr BP）.3)Warm-wet climate was affected by the strong East Asian summer monsoon in the middle Holocene（9000～3100 yr BP）,however,the Westerlies increased abnormally and the East Asian summer monsoon weakened,resulting in a short-term cold event during 5000～3500 yr BP period.4)The climate gradually shifted to dry-cold with the weakened East Asian summer monsoon and strengthened Westerlies in the late Holocene（3100～0 yr BP）.The WCG profile in the Luoyang Basin also records four stages of paleoclimate evolution since the late Pleistocene.1)Dry-cold climate was under the intensive East Asian winter monsoon during the late Pleistocene.2)The climate gradually warmed with the strengthening of the East Asian summer monsoon and the weakening of East Asian winter monsoon in the early Holocene（11500～8500 yr BP）.3)Warm-wet climate was affected by the strong East Asian summer monsoon in the middle Holocene（8500～3100 yr BP）.4)The climate changed to dry and cool with the weakening of the East Asian summer monsoon in the late Holocene（3100～0 yr BP）.（4）The grain-size end-member model and heavy mineral composition analysis showed that there are five end-member（EM）components in the ZHK profile of Maqu valley.EM1 is secondary clay mineral produced by the weathering and pedogenesis under the influence of East Asian summer monsoon.EM2 is dust of distant source transported by high altitude Westerlies.EM3 and EM4 represent moraines and periglacial sediments carried by the Westerlies and Plateau monsoon from middle and far areas.And EM5 is river deposits and slope deposits transported by valley-wind from nearby areas.The grain-size end-member model analysis showed that the WCG profile have four end-member（EM）components.EM1represents the secondary clay minerals formed by the weathering under the effect of East Asian summer monsoon;EM2 is dust component from distant source carried by the East Asian winter monsoon;EM3 is coarse silt from the Yellow River floodplain carried by the northeast wind;EM4 is sand grain from the nearby area carried by local airflow.（5）The typical aeolian loess-paleosoil sequences on the northeast Tibetan Plateau and Luoyang Basin have great regional differences.There are also significant differences in soil and sedimentology between different layers in the both regions.The sediments are dominated by plagioclase weathering and are still in the primary weathering stage of Na⁺and Ca²⁺leaching,but the pedogenesis in Luoyang Basin is stronger than that in the Maqu valley in the Yellow River source area.In addition,the Maqu valley in the Yellow River source area may be located on the Tibetan Plateau and is highly sensitive to climate change,so it entered the middle Holocene 500 years earlier than the Luoyang Basin.Moreover,the paleoclimate evolution was dominated by the East Asian summer monsoon and Westerlies,while the Luoyang Basin was dominated by the East Asian summer monsoon and the East Asian winter monsoon.The main provenance components of the Maqu valley are moraines and periglacial deposits in the middle and distant areas carried by the Westerlies and Plateau monsoon,while the Luoyang Basin is mainly dust from distant source carried by the East Asian winter monsoon.