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Evidences of PaLeo-Environmental Changes in Representative Soils of Iran

Authors:
[ Givi ] - Assistant professor, Soil Dept., Agric College, Shahrekord Univ
[ Stoops ] - Full Professor, Geological Inst., Gent Univ

Abstract:

Fsr a lon-e time. it was believed that high, so called "pluvial" lake levels occurred during pcrr~ods of glaciation, as a result of increased precipitation. However. this interpretation is now considered a mistake and instead, it is believed that during the last -g laciation. vast areas of the earth's surface were affected by pronounced aridity.For elucidation of paleo-environmental change in Iran, representative alluvial soils of Caspian coastal plain and the Iranian central desert were studied. The obtained results were commented on the base of literature review, field survey, physico-chemical and micromorphological analysis. The Caspian coastal plain was under influence of dust deposition at the end of Pleistocene epock when the climate was cold and dry. Two sources for the loess material can be assumed : the flood plains present at the coast and the continental shelf of the Caspian Sea. As the sea level was lower than today during the late glacial maximum, the sediments on the shelf were dry and exposed and consequently subjected to wind erosion. Lower level of the Caspian Sea during the last glacial period can be explained by its proximity to the Eurasian ice sheet. During the transitional stage between the glacial and the interglacial period, expansion of lake in the Isfahan area was caused by an increase in runoff water, as a result of ice melting and low evaporation. At that time, the present-day old Zayanderud river terraces were waterlogged. Due to evaporation of a saline groundwater, transportation of S04- and Ca++ (originated from gypsiferous formations of the area) by i runoff water, gypsum precipitated in the soils of the old terraces. The presence of gypsum reveals that the climate was dry during the glacial period. In the soils of the playa lowlands, below a depth of 65 cm, stratification is pronounced and considerable amounts of charcoal can be seen under the microscope. At this depth, a buried horizon is also darker compared to the overlying and underlying horizons and contains highest amount of shells, partly fragmented. Evidences of biological activity, such as passage features, granular excrements and channel microstructure are more frequent below a depth of 65 cm. These characteristics as well as the presence of cross bedding of fine and coarse materials, suggest that the 65 cm limit is the upper boundary of lake deposits. Carbonate pendants and weathered marl fragments suggest considerable dissolution of marl in the soils of the alluvial fan. The clay present in the marl was mobilised and redeposited as coatings. A somewhat more humid paleo-climate can be proposed based on the observation of gypsum pendants, superimposed on carbonate pendants.

Keywords:

Paleo - Climate , Iranian Soils