Eolian sedimentary records for the evolution of monsoon and westerly circulations of northern China in the last 2.6 Ma

Abstract

Systematic grain-size analysis has found that Chinese loess generally shows bimodal distribution composed of overlapped coarse and fine components. Sedimentary dynamic illustra-tion indicates that the coarse component was mostly the product of low-level monsoonal circulation, and the fine component was mainly transported by high-level circulation dominated by westerly in northern China. Grain-size series of coarse component consistently indicates that the monsoonal circulation strengthened in glacial stages and weakened in interglacial stages over Loess Plateau. The latitude differentiation of fine component series suggests that the westerly circulation intensi-fied and shifted to about 35oN in glacial stages, conversely, weakened and shifted to 37oN in inter-glacial stages. Grain-size variation trends of coarse and fine components revealed by Luochuan loess-paleosol suggest that the contribution of westerly circulation to eolian loess decreased and its intensity weakened in the last 2.6 Ma, which is accompanied by strengthening intensity and increasing contribution of the monsoonal circulation. This variation trend was marked by an abrupt change in the transition period of 1.20.9 Ma, which probably reflected the controlling role of Tibet Plateau uplift on the atmospheric circulation of northern China.