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

δ 13 C and δ 18 O 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, δ 13 C and δ 18 O 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 δ 13 C and δ 18 O 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 δ 18 O values for yeast proportions higher than 50%. It also generated CO 2 , but with a noticeable shift in δ 13 C and δ 18 O 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 H 2 O 2 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. H 2 O 2 treatment caused severe dissolution of calcite and dolomite while worsening the δ 13 C and δ 18 O shifts and should thus be avoided.