Pedogenic, palustrine and groundwater dolomite formation in non-marine bentonites (Bavaria, Germany)

Dolomite and calcite in Bavarian bentonites, southern Germany, were investigated using petrography, field-emission scanning electron microscopy and stable isotope geochemistry to explore the role of authigenic carbonate formation during bentonitization. Pedogenic, palustrine and groundwater carbonates were distinguished on the basis of X-ray diffraction, micromorphological and stable isotope analysis. The δ13CV-PDB and δ18OV-PDB values of dolomite range from -8.0 ppm to -6.1 ppm and -5.4 ppm to -3.4 ppm, respectively. Calcites show a range from -11.9 ppm to -8.1 ppm for carbon and from -9.1 ppm to -6.2 ppm for oxygen. Carbon isotope compositions imply a C3-plant-dominated carbon source and repeated wetting and drying cycles. The oxygen isotope data points to an evaporation and temperature controlled δ18OV-SMOW value of meteoric water of -7.0 ppm to -4.8 ppm. A syngenetic to early diagenetic timing of dolomitization is indicated, suggesting both dolomite and bentonite formation in non-saline, non-arid and repeatedly partially-oxygenated and reducing soil and groundwater environments during pedogenesis.