DISTRIBUTION AND GENESIS OF CALCITIC PEDOFEATURES WITHIN A RAPIDLY AGGRADING LOESS-PALEOSOL SEQUENCE IN CHINA

The distribution of calcitic pedofeatures within part of a buried pale osol complex on Gaolanshan Mountain, near Lanzhou in the western semi- arid part of the Loess Plateau of China, is interpreted in terms of a pedosedimentary model controlled by changes in dominance of regional c limatic (monsoonal) forces. During periods of moderate loess accumulat ion rates and moisture availability, accretionary pedogenesis was char acterised by localised dissolution and redistribution of calcite and d evelopment of compound calcitic depletion/concentration pedofeatures. These took one of two forms: (i) equigranular, hypidiotopic calcitic c oatings (or infillings) (100-350 mu m) and calcitic depletion hypocoat ings ( < 250 mu m) around channels; (ii) calcitic hypocoatings (1.0-2. 5 mm) and depletion infillings (0.5-2.5 mm by 2.5-12.0 mm) of channels . The increasing effects of bioturbation, associated with a gradual di minution in rates of loess accumulation, led to complete disruption of these features and their integration into the groundmass. Residual fr agments of calcitic coatings, in the form of sand-size grains or nodul es of subhedral calcite, provide the only remains of these pedofeature s in the paleosol that developed once dust inputs reached their minimu m. Upon establishment of a relatively stable landsurface and a moister climate, the nodules were subjected to minor dissolution. The solute from the weakly decalcified surface horizon was vertically leached and reprecipitated further down the profile in the form of calcitic coati ngs, hypocoatings and segregations. The reconstruction provides the ba sis for explaining the ubiquitous occurrence of high concentrations of sand-size grains and nodules of calcite within weakly leached horizon s of other paleosols in the western part of the Loess Plateau of China . Their presence reflects not only the influence of semiarid condition s prevailing during the main ''stable'' ''soil-forming intervals'', bu t also preceding (drier) periods of accretionary pedogenesis associate d with transitional changes in relative dominance of dry (northwesterl y) winter monsoons, responsible for the transport of the dust, to that of moist (southwesterly and southeasterly) monsoons. The study emphas ises the general need for a pedose-dimentary approach when attempting to interpret loess-paleosol sequences.