GRAIN-SIZE OF QUARTZ AS AN INDICATOR OF WINTER MONSOON STRENGTH ON THE LOESS PLATEAU OF CENTRAL CHINA DURING THE LAST 130,000-YR

The Chinese loess-paleosol sequence constitutes an important record of variations in Asian monsoon climate over the past 2.4 myr. Magnetic susceptibility of loess and paleosols has been used as a proxy for summer monsoon intensity, while median grain size has been regarded as a measure of the strength of winter monsoon winds that were responsible for most of the dust transport. However, median grain size is only an approximate index of winter monsoon strength because both paleosols and loess have been modified, to various degrees, by weathering processes that have produced pedogenic clay. The quartz component of loess and paleosols is largely unaffected by weathering processes and therefore constitutes a more reliable proxy index of monsoon wind strength. Median grain size (Qmd) and maximum grain size (Qmax) values of monomineralic quartz isolated from the loess-paleosol section at Luochuan in the central Loess Plateau are characterized by two main intervals during the last ca. 130,000 yr when these parameters were significantly greater than 9 and 85 mu m, respectively, and three main intervals when they were lower. The data imply that the winter monsoon weakened during the intervals with low Qmd and Qmax values, which coincide with marine oxygen isotope stages 5, 3, and 1, and was strongest ca. 67,000 and 20,000 yr ago during isotope stages 4 and 2. However, both quartz grain-size records display second-order high-frequency, high-amplitude variations, which are lacking in the magnetic susceptibility record, that imply rapid and significant changes in atmospheric conditions that affect dust transport and deposition. (C) 1995 University of Washington.