Pleistocene chemical weathering history of Asian arid and semi-arid regions recorded in loess deposits of China and Tajikistan

Abstract

Quaternary loess–soil sequences in China and Tajikistan are valuable archives of the chemical weathering history for arid and semi-arid regions of Asia. The development of new proxies independent of grain size are vital to reconstruct the long-term chemical weathering history of eolian deposits. In this study, we analyze major elemental and Rb, Sr, and Ba concentrations of decarbonated residue from representative loess and soil units along a north–south transect on the Chinese Loess Plateau and compare these concentrations among different grain-size fractions. Results show that most of the elemental abundances and ratios vary considerably among different grain-size fractions for both loess and paleosols, indicating transport-driven compositional differentiation of minerals during subaerial transport. However, (CaO + Na 2O + MgO)/TiO 2 ratios show little variation among different size fractions for the loess–soil transect. Loess–soil sequences at Lingtai (northern China) and Chashmanigar (southern Tajikistan) for the past 1.50–1.77 Ma show lower (CaO + Na 2O + MgO)/TiO 2 ratios in soil units than in adjacent loess horizons, in good agreement with greater weathering intensity in soils than in loess units. The (CaO + Na 2O + MgO)/TiO 2 ratio of the decarbonated residue is therefore a reliable proxy for chemical weathering of loess deposits that is independent of grain size. The (CaO + Na 2O+MgO)/TiO 2 ratios of the Lingtai and Chashmanigar sections display a relatively regular oscillation within a narrow range in the early Pleistocene, and a rapid increase in average values and in variance from 0.85–0.6 Ma to the present, indicating lowered chemical weathering intensity in both the dust source regions and the depositional areas. This event may be causally related to the expansion of northern hemisphere ice and/or the regional tectonic uplift of high mountains in Asia since the mid-Pleistocene.