AC magnetic susceptibility studies of Chinese red clay sediments between 4.8 and 4.1 Ma: Paleoceanographic and paleoclimatic implications

Abstract

Recent studies show that measuring AC magnetic susceptibility (with in-phase X′ and quadrature X″ components) versus temperature provides an effective way to map the grain-size distribution of pedogenic maghemite. Applying this method, we mapped the pedogenic maghemite grain-size distribution of samples of the red clay sediments deposited on the Chinese Loess Plateau (CLP) during the interval 4.8-4.1 Ma, during which the validity of magnetic susceptibility as an East Asian summer monsoon precipitation intensity proxy has been questioned. The results show that the pedogenic maghemite grain-size distribution is the same as in the overlying loess-paleosol sequence and in the other intervals of the red clay sequence, suggesting similar pedogenesis mechanisms and marked precipitation seasonality on the CLP between 4.8 and 4.1 Ma compared with the other intervals. We argue that the seasonal position shift of the polar westerlies produces the required precipitation seasonality and thus produces a uniform pedogenic grain-size distribution during this time interval. Reconstructing paleo-rainfall on the central CLP on the basis of surface calibration results suggests that the intensification of the East Asian summer monsoon might be a key trigger for the subsequent significant ocean and cryosphere adjustments during the Pliocene. Copyright 2008 by the American Geophysical Union.