Terrestrial mollusc records from Xifeng and Luochuan L9 loess strata and their implications for paleoclimatic evolution in the Chinese Loess Plateau during marine oxygen isotope stages 24–22

Abstract. Marine Isotope Stages 24–22 is a key period of the Mid-Pleistocene Transition, however, its climate variability is still unclear. The coarse-grained loess unit L9, one of the most prominent units in the Chinese loess stratigraphy, yields a high potential terrestrial record of paleoclimatic and paleoenvironmental changes during this period. In this study, two high-resolution terrestrial mollusc records of L9 loess strata from the Xifeng and Luochuan sequences in the Chinese Loess Plateau were analyzed. Our mollusc results show that the MIS 24, the early and late parts of MIS 22 were dominated by cold and dry climate. Relatively mild-humid climate occurred in MIS 23 and the middle part of MIS 22. The climatic conditions at Xifeng region were cooler and more unstable compared to Luochuan region. A comparison of mollusc species composition and other proxies of L9 strata (MIS 24–22) with those of L1 loess units (MIS 4–2) reveals that the L9 loess was not deposited under the most severe glacial conditions in Quaternary climate history as suggested by previous studies. Our study shows that climatic conditions in the Loess Plateau during the L9 loess forming period were like that of gentle glacials (MIS 24 and MIS 22) and interglacial (MIS 23), as suggested by marine δ18O record. Three cooling fluctuations occurred at ~940–923 ka, 905–895 ka and 885–875 ka, which might be hints of the global "900 ka cooling event". The "900-ka event" in the Loess Plateau seems not a simple long glaciation but a complex of several climatic fluctuations superposed on a general cooling trend. The uplift of Tibetan Plateau and its adjacent regions during this period resulted in a rapid increase in source materials of desert and loess and an enhancement of winter monsoon, which would have amplified the effects of cooling climate on dust production and transport during MIS 24–22, as indicated by increased grain size and mass accumulation rate of L9 loess.