Pronounced changes in atmospheric circulation and dust source area during the mid-Pleistocene as indicated by the Caotan loess-soil sequence in North China

Abstract

The Quaternary loess-soil sequence in North China is of significant value for tracing variations in atmospheric circulation. In this study, we measured the anisotropy of magnetic susceptibility (AMS) of the Caotan (CT) loess-soil sequence from the northeastern margin of the Tibetan Plateau (TP) and analyzed the surface textural features of quartz grains of representative samples. The AMS results indicate a primary sedimentary magnetic fabric without significant disturbance by pedogenesis. The Dec-Kmax values indicate a significant shift of the dominant surface wind direction from nearly N–S to W–E since ∼300 ka, while the surface textural features of the quartz grains indicate an increase in the contribution from fluvioglacial sediments derived from the TP at the same time. As there is no evidence of significant climatic changes around 300 ka, we attribute the coeval changes of atmospheric circulation and dust source area to the uplift of the TP during the mid-Pleistocene and the resulting climatic cooling. Cooling of the TP may have led not only to the enhancement of the plateau monsoon but also to the significant accumulation of plateau glaciers, which would have resulted in accelerated physical erosion of the underlying rocks, thus increasing the quantity of fine-grained fluvioglacial sediments to be transported to the study site.