title: Effects of organic matter on carbonate stable isotope ratios (δ13C, δ18O values)--implications for analyses of bulk sediments.
abstract: BACKGROUND
  Stable isotope ratio (δ(13)C, δ(18)O values) analyses of carbonates can be biased by CO(2) 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 CO(2) 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.
authors: Department of Earth and Environmental Sciences, Ludwig-Maximilians Universität München, 80333, Munich, Germany. m.oehlerich@lrz.uni-muenchen.de
authors: Markus
authors: M
authors: Oehlerich
authors: Marlene
authors: M
authors: Baumer
authors: Andreas
authors: A
authors: Lücke
authors: Christoph
authors: C
authors: Mayr
mesh_terms: D002247:Carbon Isotopes / Q000032:analysis*; D002254:Carbonates / Q000032:analysis / Q000737:chemistry*; D019015:Geologic Sediments / Q000737:chemistry*; D013058:Mass Spectrometry; D009930:Organic Chemicals / Q000737:chemistry; D010103:Oxygen Isotopes / Q000032:analysis*; D010756:Phosphoric Acids / Q000737:chemistry
chemical_list: D002247:Carbon Isotopes; D002254:Carbonates; D009930:Organic Chemicals; D010103:Oxygen Isotopes; D010756:Phosphoric Acids; C030242:phosphoric acid