TESTING LIQUID DECARBONATION TEMPERATURE AND TIME FOR δ13CORG ANALYSES IN SEDIMENTARY SECTIONS

Carbon isotope shifts often occur in marine and terrestrial strata at mass extinction intervals, and the causes of negative or positive isotope excursions and their relationship to changes in the carbon cycle and extinction mechanisms is of great interest. However, methods for organic carbon isotope analyses (δ¹³C_org) vary substantially for sample preparation, which can lead to differences in the isotopic value reported. In particular, variability in methods for decarbonation to remove inorganic carbon (typically in carbonate mineral form) can lead to incomplete decarbonation. The isotopic difference between organic and inorganic carbon in sedimentary rocks is large, so any fraction of δ¹³C_carbonate remaining in the sample would affect the resulting δ¹³C_org analysis. When put into stratigraphic context, variabilities in the lab methods may lead to different interpretations in deep time.

Liquid decarbonation for organic carbon isotope analysis involves a timed soak of a powdered sample in acid followed by a series of rinses, but is not standardized from lab to lab. We hypothesize that differences in the temperature and length of decarbonation may leave carbonate phases in δ¹³C_org analyses, as different carbonate minerals are more or less recalcitrant during acid digestion (e.g., calcite, dolomite, siderite). Complete removal of carbonate is critical to δ¹³C_org analyses because δ¹³C_carb has a heavy isotopic signature and leads to a more positive δ¹³C_org value if unintentionally left in the sample. Here, we test different decarbonation temperatures (20,70, and 80ºC) and lengths of time (10, 60, 120, 240, and 480 minutes) for several geologic samples, some of which contain dolomite. Our preliminary results indicate samples containing dolomite heated at higher temperatures exhibit a lighter δ¹³C_org isotopic value, suggesting more complete removal of carbonate occurs at higher temperatures. We will report these analyses and discuss the problems associated with this method, including possible solubilization of organic carbon.