Organic matter removal for continuous flow isotope ratio mass spectrometry analysis of carbon and oxygen isotope compositions of calcite or dolomite in organic‐rich samples

Abstract

Abstractδ13C and δ18O values of carbonate in organic‐rich samples such as soils, biofilms, and lake sediments have been scarcely used so far in paleo‐environmental, biomineralization, or diagenetic studies because organic matter in high proportions is suspected to alter carbonate isotope analysis. Yet, with the improvement of analytical capabilities and in particular the use of CF‐IRMS, this may not be an issue anymore. To evaluate this, δ13C and δ18O values of calcite or dolomite mixed in various proportions with yeast (used here as a model for immature organic matter) were measured. The results indicate that measurements of calcite or dolomite δ13C and δ18O by CF‐IRMS require organic matter removal only when its weight proportion exceeds that of carbonates. Yeast generated an unidentified molecule during phosphoric acid digestion at 25°C (none at 80°C), which shifted the carbonate δ18O values for yeast proportions higher than 50%. It also generated CO2, but with a noticeable shift in δ13C and δ18O values only at 80°C (none at 25°C) and for yeast proportions higher than 75%. Three methods of organic matter removal were tested: the well‐established NaOCl and H2O2 wet treatments as well as low temperature plasma ashing (LTA) so far seldom examined. LTA removed the isotope shifts, although imperfectly for yeast proportions of 95%, without carbonate dissolution. NaOCl efficiently removed the isotope shifts in all cases but dissolved part of the calcite and dolomite. H2O2 treatment caused severe dissolution of calcite and dolomite while worsening the δ13C and δ18O shifts and should thus be avoided.