Effects of organic matter on carbonate stable isotope ratios (13C, 18O values) implications for analyses of bulk sediments

RATIONALE Stable isotope ratio (13C, 18O values) analyses of carbonates can be biased by CO2 release from organic impurities. This is most critical for carbonate isotope analyses from bulk sediments containing comparably high amounts of organic matter (OM). Several methods have been developed to remove OM prior to analyses, but none of them can be universally applied. Moreover, pretreatment methods cause isotopic bias in themselves and should probably best be avoided. Thus, it is essential to have indicators for reliable isotope values of untreated carbonate-OM mixtures. METHODS Artificial mixtures of organic compounds with a standard carbonate were analyzed to investigate the bias on carbonate isotope ratios caused by OM in the sample. The total-inorganic-carbon to total-organic-carbon ratio (TIC/TOC) was used as a measure for the organic impurity of the sample. The target was to evaluate TIC/TOC as a measure for sample quality and to define TIC/TOC thresholds for reliable isotope measurements of mixtures between calcium carbonate and organic compounds. RESULTS The effect of organic impurities on carbonate stable isotope values depended on the specific OM compound and the respective TIC/TOC ratio. Different CO2 release rates were determined for the pure OM compounds. A sample TIC/TOC ratio 0.3 was found to be a threshold for reliable measurements of the isotope composition of calcium carbonate. CONCLUSIONS Bulk carbonate analyses from carbonate-OM mixtures are reliable only if the TIC/TOC values do not fall below certain thresholds. This has implications for carbonate isotope studies from bulk sediments for which the TIC/TOC ratios should be considered as an easy-to-determine measure for sample-quality assessment. Copyright (c) 2013 John Wiley & Sons, Ltd.