Calcretes aus jungpaläozoischen Bodenbildungen - ein möglicher Proxy für die CO<sub>2</sub>-Konzentration der Paläoatmosphäre

Abstract

The use of pedogenic carbonates (calcretes) in estimating CO2 concentrations in the Late Paleozoic atmosphere has been examined. For this purpose Cerling (1991,1999) submitted a theory based on the 13C/12C isotopic composition in calcretes. According his model the relationship of 13C/ 12C in calcretes is mostly coupled with the soil type and CO2 concentration in the atmosphere. By comparing time equivalent sections from different localities, it is attempted in the present study to separate calcretes altered due to diagenesis from unaltered carbonates.20 sections were examined, containing Late Paleozoic sediments. Thin sections, cathodoluminescence and x-ray diffractometry prove that a great number of different carbonates exist in these exposures. Comparing the Paleozoic samples with recent and subrecent calcretes (Knox 1977, Khadkikar et al. 2000), it can be demonstrated that anhedral, cryptocrystalline (<10 µm) and subhedral, microcrystalline (<40 µm) carbonates are clearly of pedogenic origin. On the other hand, crystals of greater size, showing a poikilotopic texture, were derived from groundwater or altered by diagenesis. Macro- and micromorphological features, typical of recent calcretes, occur in several of the investigated soil profiles. In contrast, thin section microscopy reveals a strong diagenetic alteration of the pedogenic carbonates. Time equivalent sections including comparable soil types (protosols, vertisols) offer a great range in their carbon isotopic composition. On the other hand, data sets in several sections do not differ very much. Therefore a mesogenetic change of isotopic composition is assumed, leading to an equalisation of carbon isotopic data. As a consequence, most of the sections studied are not suitable for the estamination of CO2 in paleoatmosphere. Only two sections (Campsie, Scottland, Lower Devonian, Vatterode, Permocarboniferous, Germany) show unaltered calcretes in wich carbonates not differ from modern pedogenic carbonate. However isotopic data sets derived from this sections do not fit exactly in the Cerling model.