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Cycles and forcing mechanism of wet-dry variations in north China during the past 11 million years revealed by wind-blown silt deposits

LU Hua-yu, HU Ting, Wang XIAN-yan   

  1. School of Geographysical and Oceanographical Sciences, Nanjing University, Nanjing 210093, China
  • Received:2009-06-20 Revised:2009-06-20 Online:2009-06-20 Published:2009-06-20

Abstract: There occur thick wind-blown silt deposits which are regarded as a good geological archive of paleoclimatic changes in Late Cenozoic in North China and the central Asia. In these aeolian sequences, the Quaternary loess deposits were widely investigated and the Milankovitch cycles were found in the paleoclimatic changes. However, the frequencies and forcing mechanism of wet-dry changes in Pliocene and Miocene are not clear to date. In this study, the representative aeolian silt sequences in Chinese Loess Plateau and northeastern Qinghai-Tibetan Plateau are systemically sampled and measured for grain size distribution. On the basis of the previous optically stimulated luminescence dating and magnetostratigraphic investigations,the time series of the grain size variations of the past 11.0 Ma is established, which is a good proxy indicator of the wet-dry changes in North China and the interior Asia. We slice the new time series into 11~7.3 Ma, 8.0~5.0 Ma, 5.3~2.5 Ma, 2.5~1.2 Ma and 1.2~0 Ma, and utilize Blackman-Turkey spectrum analysis and wavelet transfer to detect cycles of the long-time climatic changes. Our results show that the cycles of ~20 ka of the precession, ~40 ka of the obliquity and~100 ka of the eccentricity have been dominated in the time series, demonstrating that insolation has forced the paleoclimatic changes in the past 11.0 Ma. Atthe same time, the non-orbital cycles found in this new time series might be caused by harmonics of the orbital cycles and/or the spurious time scale, induced by a vulnerable depositional process of the aeolian sediments. We suggest perturbation of the Earth orbital trajectory, which caused the insolation changes in the north Hemisphere, might drive the replacement of the Westerlies and the Asian monsoon changes, and controlled the wet-dry variations in North China and the central Asia. Further investigation on seeking for complete aeolian records and strengthening modeling work would improve our understanding of environment changes in North China and the central Asia.