Dear Micha and Michael,
I am far from an expert of your subject, but might it be that during heating at
500 °C of natural samples, isotopic changes are due to a loss of naturally
occuring organic impurities or even could you loose some CO2 enriched in light
12C???
Kind regards
Dr. Bruno Glaser
Institute of Soil Science and Soil Geography
University of Bayreuth
D-95440 Bayreuth
Germany