PEDOSEDIMENTARY DEVELOPMENT AND PALEOENVIRONMENTAL SIGNIFICANCE OF THE S1 PALEOSOL ON THE NORTHEASTERN MARGIN OF THE QINGHAI-XIZANG (TIBETAN) PLATEAU

The S1 pedocomplex, correlated to Oxygen Isotope Stage 5 of the ocean cores, has been traced west of the Loess Plateau on to the extreme nor theastern margin of the QinghaiXizang (Tibetan) Plateau in China. Deta iled micromorphological analysis of closely spaced thin-sections from the pedocomplex at this site provides the basis for reconstruction of the sequence of pedosedimentary processes and associated palaeoenviron mental changes occurring during this time interval. The six pedosedime ntary stages identified are interpreted in terms of temporal variation s in depositional rates, size of particles transported and availabilit y of moisture for pedogenic alteration, as determined by changing bala nces in dominance of winter and summer monsoonal forces. Two main 'soi l-forming intervals' are identified: the more extensive of the two in terms of resultant pedological features reflects pedogenic alteration at a relatively 'stable' land surface under a semi-arid climate during the later part of pedosedimentary stage 2. The other main period corr esponds to pedosedimentary stages 3 and 5, when leaching and bioturbat ion processes were active at aggrading surfaces, leading to developmen t of an accretionary unit without clear differentiation of horizons. P edosedimentary stages 1, 3 and 6 were characterised mainly by rapid ra tes of coarse loess accumulation, with synsedimentary modification res tricted to surface slaking and crust formation, and minor localised re distribution of calcite.