MAGNETIC PARAMETER VARIATION OF THE CHAONA LOESS/PALEOSOL SEQUENCES IN CENTRAL CHINESE LOESS PLATEAU AND ITS SIGNIFICANCE FOR MIDDLE PLEISTOCENE CLIMATE TRANSITION

A high-resolution rock magnetic investigation has been carried out on the Chaona Quaternary loess/paleosol sequence in Central Chinese Loess Plateau (CLP). Statistical analyses of magnetic parameters show that the loess layers are characterized by relatively higher coercivity (Bc), remanent coercivity (Bcr), and lower magnetic susceptibility (X), whereas the paleosol layers are characterized by relatively higher Xlf, saturation magnetization (Ms), and remanent saturation magnetization (Mr). We suggest that Mr, Ms in addition to Xlf can be regarded as summer monsoon intensity proxies, whereas Bc and Bcr can be regarded as winter monsoon intensity proxies, although there are some differences between Bc (Bcr) and traditional coarse grain size winter monsoon proxies. On the basis of a newly developed independent time scale (not orbitally-tuned), we reconstruct the history of the East Asia summer and winter Monsoons inferred from magnetic records during the last 3Ma. Spectrum analysis of our records, unlike most previous studies, indicates that there are various periods in addition to orbital periodicities, but no significant periodic signals observed during the interval 880-530ka. The results suggest that the forcing mechanism for the Pleistocene glacial cycles may be more complicated than the Milankovitch theory predicted, or global ice volume change. Wavelet power spectrum analyses indicate that the appearance of the 100ka period occurred at the middle of L15 (about 1260ka), not at the beginning of L15. The time interval 880~530ka indicates a nonlinear response of the global climate system to solar insolation. After 530ka, the ~100 ka period recorded in most magnetic parameters becomes strong and stable. If the 100 ka period appearance is the onset of the Middle Pleistocene Transition (MPT), then this climate transition in the loess paleosol sequence occurred at L15 (~1260ka) and ended at the beginning of S5-1 (~530ka), which differs with the duration obtained by previous investigations based on orbitally-tuned timescales.