Weathering and clay mineral formation in two Holocene soils and in buried paleosols in Tadjikistan: towards a Quaternary paleoclimatic record in Central Asia

The upper part of the Karamaydan section, Tadjikistan, shows the most detailed loess-paleosol sequence yet known for the Brunhes chron, and the central and lower parts of the Chashmanigar section provide similar detail for most of the Matuyama chron. To enable paleoclimates to be deduced, the primary and secondary minerals in the silt and clay fractions must be determined separately to evaluate the type and intensity of mineral weathering and clay mineral formation. To distinguish between inherited and pedogenetically formed clay minerals, the original petrographic homogeneity of the parent material from which a soil developed must be established. The main sources of pedogenic clay minerals are phyllosilicates in the silt fractions. Illites and vermiculites are the dominant pedogenetically formed clay minerals in the B or Bt horizons of the Holocene climaphyromorphic soils and in all paleosols (S) and pedocomplexes (PK) in Karamaydan and Chashmanigar, except S XVII in which large amounts of smectites were formed. There is little difference in the type and amount of pedogenic clay mineral formation between the Holocene soils and the paleosols in the Brunhes epoch at Karamaydan as well as during most of the Matuyama epoch at Chashmanigar. These results indicate that the climates of the interglacials represented by the B or Bt horizons of the buried paleosols of young, mid and old Pleistocene age were similar to that of the Holocene. (C) 1998 Elsevier Science B.V. All rights reserved.