Re-arrangement of atmospheric circulation at about 2.6 Ma over northern China: evidence from grain size records of loess-palaeosol and red clay sequences

Abstract

Recent studies have shown that the red clay sequence underlying the Quaternary loess of the Chinese Loess Plateau is wind-blown in origin. Continuous atmospheric dust deposition in the past 7.0 Ma has been documented. To address the wind system that transported the Tertiary red clay, two north–south transects were studied in the Chinese Loess Plateau. One of the transects was designed to study spatial changes in grain size of the last glacial–interglacial loess records, and the other to observe particle changes of the Tertiary red clay underlying the Quaternary loess. The loess transect consists of nine sections, and the red clay transect of four sections. Analyses of closely spaced samples show that there is a strong southward decrease in grain size of both loess and palaeosol horizons of the Late Pleistocene, which is consistent with the idea that the aeolian materials of the Quaternary in the Loess Plateau are transported by the northerly winter monsoonal winds. Grain size distribution of the red clay sequences, however, does not show such a change. From north to south along the red clay transect, the particle size distribution is almost identical in the four sections, suggesting that the winter monsoonal winds might have played a less important role in transporting the red clay material. It is suggested that the red clay may have been transported by the westerlies from the dust-source regions of northwestern China onto the Loess Plateau. A remarkable re-arrangement of atmospheric patterns at about 2.6 Ma, therefore, has been recorded by the red clay-loess shift. It is speculated that this re-arrangement of atmospheric patterns may have been caused by the onset of glaciation in the Northern Hemisphere.