Clumped isotope analysis of carbonates: comparison of two different acid digestion techniques

The kinetic nature of the phosphoric acid digestion reaction enables clumped isotope analysis of carbonates using gas source isotope ratio mass spectrometry (IRMS). In most laboratories acid digestions are performed at 25°C in sealed vessels or at 90°C in a common acid bath. Here we show that different Δ47 results are obtained depending on the digestion technique employed.

Several replicates of a biogenic aragonite and NBS 19 were reacted with 104% H3 PO4 in sealed vessels at 25°C and at 90°C using a common acid bath. The sample size varied between 4 mg and 14 mg. Purification methods that are standard for clumped isotope analyses were applied to the evolved CO2 before measuring the abundances of masses 44 to 49 relative to a reference gas by IRMS.

A systematic trend to lower and more consistent Δ47 values is observed for reactions at 25°C if the sample size is increased. We suggest that secondary re-equilibration of evolved CO2 or reaction intermediates with free water molecules preferentially occurs for relatively small samples (4-7 mg), finally yielding elevated Δ47 values compared with >7 mg aliquots. In contrast, no such sample size effect on Δ47 values is observed for carbonates that are digested at 90°C using the common acid bath.

The determination of Δ47 values of carbonate samples smaller than 7 mg becomes more precise and accurate if digestions are performed at 90°C. Based on our results we propose that the difference in phosphoric acid fractionation factor between 25°C and 90°C is 0.07‰ for both calcite and aragonite.