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 detai led loess-paleosol sequence yet known for the Brunhes chron, and the centra l and lower parts of the Chashmanigar section provide similar detail for mo st of the Matuyama chron. To enable paleoclimates to be deduced, the primar y and secondary minerals in the silt and clay fractions must be determined separately to evaluate the type and intensity of mineral weathering and cla y mineral formation. To distinguish between inherited and pedogenetically f ormed clay minerals, the original petrographic homogeneity of the parent ma terial 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 pal eosols (S) and pedocomplexes (PK) in Karamaydan and Chashmanigar, except S XVII in which large amounts of smectites were formed. There is little diffe rence in the type and amount of pedogenic clay mineral formation between th e Holocene soils and the paleosols in the Brunhes epoch at Karamaydan as we ll as during most of the Matuyama epoch at Chashmanigar. These results indi cate that the climates of the interglacials represented by the B or Bt hori zons of the buried paleosols of young, mid and old Pleistocene age were sim ilar to that of the Holocene. (C) 1998 Elsevier Science B.V. All rights res erved.