Distribution and genesis of calcitic pedofeatures within a rapidly aggrading loess-paleosol sequence in China

Abstract

The distribution of calcitic pedofeatures within part of a buried paleosol 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 climatic (monsoonal) forces. During periods of moderate loess accumulation rates and moisture availability, accretionary pedogenesis was characterised by localised dissolution and redistribution of calcite and development of compound calcitic depletion/concentration pedofeatures. These took one of two forms: (i) equigranular, hypidiotopic calcitic coatings (or infillings) (100–350 μm) and calcitic depletion hypocoatings (< 250 μm) 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 diminution in rates of loess accumulation, led to complete disruption of these features and their integration into the groundmass. Residual fragments of calcitic coatings, in the form of sand-size grains or nodules of subhedral calcite, provide the only remains of these pedofeatures in the paleosol that developed once dust inputs reached their minimum. 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 coatings, hypocoatings and segregations. The reconstruction provides the basis for explaining the ubiquitous occurrence of high concentrations of sand-size grains and nodules of calcite within weakly leached horizons of other paleosols in the western part of the Loess Plateau of China. Their presence reflects not only the influence of semi-arid conditions prevailing during the main “stable” “soil-forming intervals”, but also preceding (drier) periods of accretionary pedogenesis associated with transitional changes in relative dominance of dry (northwesterly) winter monsoons, responsible for the transport of the dust, to that of moist (southwesterly and southeasterly) monsoons. The study emphasises the general need for a pedosedimentary approach when attempting to interpret loess-paleosol sequences.