Spatial pattern of grain size in the Late Pliocene ‘Red Clay’ deposits (North China) indicates transport by low-level northerly winds

Abstract

The confirmation of an eolian origin of the Tertiary ‘Red Clay’ from North China would make it a valuable terrestrial paleoclimatic record. However, it is still controversial as to which wind primarily transported the dust forming the ‘Red Clay’ onto the Loess Plateau. Careful consideration of this question is a prerequisite for better understanding the history of atmospheric circulation in central and East Asia during the Miocene and Pliocene epochs. In this paper, the grain-size distributions of bulk samples and chemically isolated quartz grains of the Late Tertiary ‘Red Clay’ were investigated to explore the potential transportation mechanisms of the dust as well as the atmospheric circulation at the time of deposition. The samples were taken from six localities spanning across 550 km on the Loess Plateau. The results show that the mean grain size of both bulk and quartz samples decreases southward, as does the coarse fraction content (>42 μm). These data not only give new evidence for the eolian origin of the ‘Red Clay’, but also indicate that the ‘Red Clay’ was probably transported mainly by northerly low-level winds, or the winds at least had a strong northerly component. Therefore, the spatial distribution of grain size of the ‘Red Clay’ is comparable to that of the overlying Quaternary eolian loess, which demonstrates a remarkable northwest–southeast decrease in grain size, sedimentation rate, and thickness during the Last Glacial, coinciding with the dominant wind direction of the northwesterly East Asian winter monsoon. However, the grain size of the ‘Red Clay’ is generally finer than the corresponding loess. This indicates that the ‘Red Clay’ may have been transported by a weaker winter monsoon associated with stronger pedogenic modification, and/or the dust source area was not extensive compared to that during the Quaternary. Moreover, the high content of grains coarser than 19 μm, transported mainly by energetic low-level winds over short distances, suggests that the dusts were not transported by upper-level westerly.