PEDOGENIC CARBONATE CUTANS ON CLASTS IN SOILS AS A RECORD OF HISTORY OF GRASSLAND ECOSYSTEMS

Abstract

Pedogenic carbonate cutans (coatings, pendants, rinds) on coarse clasts are common in stony soils of arid and semi-arid regions dominated by open vegetation. This paper gives an overview of the potential of the carbonate cutans for paleograssland research using data from literature and personal observations. Carbonate coatings are stratified into distinct microlamina, which can provide a record of local environmental changes. Lamina made of a rather pure calcium carbonate with better formed and parallel oriented crystals seem to be indicative of relatively dry phases that were less favorable for biological activity. A high content of admixtures, poorer formed and randomly oriented calcite crystals presumably mark wetter periods characterized by climatic amelioration. At a most general level, δ13C of the cutan carbonate can provide information on the type of photosynthesis of local vegetation, whereas δ18O in carbonate material of the coatings correlates with oxygen isotopic composition of precipitation water at the site and is sensitive to fluctuations in temperature. To obtain chronological evidence on cutan formation the 14C and Th/U methods can be applied with certain limitations. Additionally, cutan growth rates can be used to evaluate the age of these features.