SWELLING CLAY PALEOSOLS AS INDICATORS OF SEASONAL DRYNESSS DURING PLIO-QUATERNARY IN ANKARA, TURKEY

Soils are strongly influenced by the agents as precipitation, temperature, parent material, morphology of landscape and time. Paleosols ,the ancient soils, therefore can provide excellent information on climate of the past and can be useful proxies of weathering and climate conditions at the time of formation. In paleoclimotology studies, paleosols and paleosol carbonates are widely used to do such interpretations. Clay minerals, one of the most important components of soils, can be used as a good indicator of long-term climatic fluctuations because the clay mineral compostion of weathering profiles and soils largely depends on the climatic conditions.

In this study, a succession of Plio-Quaternary age from Gölbaþý-Karahamzalý area is selected. The selected fluvial succession includes reddish-brown mudstones with alternating horizons of massive calcretes and channel deposits. Calcretes formed as massive accumulations of carbonates between clasts. Mudstones represent blocky-subangular blocky- prismatic and granular micromorphological structures. Additionally common pedofeatures of clay coatings, ped structures, floating grains and crack systems are well developed within the mudstones. The mineral and rock fragments, voids and dissolution channels are surrounded by carbonate spars which is a typical feature of calcretes. Manganese oxide also rims some grains and also seen as dense compound infillings and loose discontinous clusters within the voids. Voids in paleosols of the Kulu succession can be classified as planar voids observed as crack patterns and chamber connected with channels. Main carbonate mineral of the paleosols is dolomite in the succession. Clay minerals are dominantly smectite and little amounts of detrital illite, chlorite and kaolinite. Smectite contents show an increase up to succession suggesting an increase in aridity.

Dolomitic calcretes occurring within the paleosol have δ18O values ranging from -3.97‰ to -5.02‰ VPDB, reflecting their formation under the infuence of meteoric water and δ13C values between -7.11‰ and -7.8‰ VPDB. The stable isotope compositions of the massive calcretes in the succession is consistent with the clay mineral assemblages. The increase in the frequency of calcretes toward the top of the succession, increase in the smectite content of the paleosols up to the section and the stable isotope data represent the alternating wet and dry seasons.