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

Recent studies have shown that the red clay sequence underlying the Quatern ary 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 addr ess 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-int erglacial loess records, and the other to observe particle changes of the T ertiary red clay underlying the Quaternary loess. The loess transect consis ts of nine sections, and the red clay transect of four sections. Analyses o f closely spaced samples show that there is a strong southward decrease in grain size of both loess and palaeosol horizons of the Late Pleistocene, wh ich is consistent with the idea that the aeolian materials of the Quaternar y in the Loess Plateau are transported by the northerly winter monsoonal wi nds. Grain size distribution of the red clay sequences, however: does not s how such a change. From north to south along the red clay transect, the par ticle size distribution is almost identical in the four sections, suggestin g that the winter monsoonal winds might have played a less important role i n 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 no rthwestern China onto the Loess Plateau. A remarkable re-arrangement of atm ospheric 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 He misphere. (C) 2000 Elsevier Science Ltd. All rights reserved.