VARIATIONS OF EASTERN ASIAN MONSOON OVER THE LAST 140,000 YEARS

The Weinan loess-soil sequence in central China provides litho,genic, geochemical and palaeopedological evidence of past changes in the eastern Asian monsoon climate. The chemical weathering index as defined by Nesbitt and Young [1982] and the SiO2/TiO2 ratio are found to be sensitive indicators of the strengths of the summer and winter monsoons, respectively. Over the last climatic cycle, strong summer monsoon occurred during six time intervals with an apparent similar to 20,000 years cycle, the major component of the Earth's orbital precession. Strengthening of the summer monsoon and weakening of the winter monsoon is approximately in phase. Micromorphological study of the palaeosols reveals a landscape variability from north temperate steppe (Haplic Chernozems) to subtropical forest (Chromic Luvisol) environments for the interstadial and interglacial periods. Changes in both orbitally produced north summer insolation and glacial age boundary condition are necessary to explain the major shifts of the monsoon climate. Within the constraints of the used lime scale, the variations of the monsoon climate are basically coeval with the variations of the global ice volume as indicated by the marine delta(18)O record. Moreover, the amplitudes of monsoon variability and the landscape evolution display however striking discrepancies with the global ice volume, which may be better explained by the variations of the summer insolation in the northern hemisphere with an apparent time lag of several thousands years for the loess sequence.