Effects of pre-treatments and organic matter on oxygen and carbon isotope analyses of skeletal and inorganic calcium carbonate

The oxygen and carbon isotope composition of skeletal and inorganic calcites and aragonites was measured with or without pre-treatments (vacuum roasting, NaOCl and H2O2 treatments) used for removing organic matter. In addition, mixtures of pure calcium carbonate and organic matter (simple organic compounds, coal and kerogen) were analyzed with or without the thermal and oxidizing treatments. The studied organic substances have no effect on isotope analyses of untreated samples carried out by using the phosphoric acid method. The experimental results indicate that the roasting treatment may produce carbon isotope exchange between calcium carbonate and organic matter. Changes in original δ18O and δ13C values of calcites and δ13C values of aragonites were observed after the NaOCl treatment of the skeletal and inorganic samples. The H2O2 treatment causes minor deviations in original δ values, although may cause partial dissolution of calcium carbonate. Unnecessary usage of pre-treatment methods may be a source of errors in stable isotope analyses and palaeotemperature determinations based on the oxygen isotope composition of skeletal carbonates.