Clumped isotope fractionation during phosphoric acid digestion of carbonates at 70 °C

The application of clumped isotopes (Δ47) in carbonate minerals as a sensitive temperature proxy in paleo-environments depends on a well-constrained clumped isotope fractionation for the necessary step of phosphoric acid digestion of the carbonate mineral to produce CO2. Published estimates for clumped isotope fractionations vary, and the effect of different carbonate mineralogies is still under debate. Differences in the sample preparation design and sample digestion temperatures are potential sources for varying acid fractionations and could be a source for discrepant Δ47-temperature calibrations observed in different laboratories. To evaluate the clumped isotope acid fractionation at 70 °C and simultaneously account for a potential cation effect we analyzed a set of eight carbonate minerals (calcite, aragonite, dolomite and magnesite) that were driven to a stochastic isotope distribution by heating them to temperatures of 1000 °C. Our study reveals significant cation- and mineral-specific differences for the Δ47 acid fractionation of carbonate minerals digested at 70 °C or 100 °C. The Δ47 acid fractionation at 70 °C for calcite is 0.197±0.002 ‰, for aragonite 0.172±0.003 ‰, whereas dolomite has a significantly larger acid fractionation of 0.226±0.002 ‰. For magnesite digested at 100 °C we observed a Δ47 acid fractionation of 0.218±0.020 ‰. Projected to an acid digestion at 25 °C, our acid fractionation for calcite of 0.260 ‰ is statistically indistinguishable from existing studies. We further show that the Δ47 of the calcite standards ETH-1 and ETH-2 of 0.265 ‰ and 0.267 ‰, respectively, are in the range of the determined acid fractionation projected to 25 °C suggesting that they have an identical and near stochastic isotope distribution. The observed differences in the Δ47 acid fractionation between calcite and aragonite (ΔΔ47 = −0.025 ‰) and between calcite and dolomite (ΔΔ47 = −0.029 ‰) does not correlate with the phosphoric acid fractionation of oxygen isotopes, but rather depends on the radius of the cation as well as on the mineral structure. Our results reveal that the acid fractionation of dolomite at 70 °C is significantly distinct from the one of calcite, but at 90 °C the two are within error of each other due to the different acid fractionation temperature dependence of calcite and dolomite. Thus it is necessary to use a mineral-specific Δ47 acid fractionation factor for dolomite to avoid differences in the final Δ47 signal from dolomites digested at 90 °C and dolomites digested at lower temperatures. Similar effects may apply also to other carbonates such as magnesite and siderite. However, their mineral specific Δ47 acid fractionation at digestion temperatures around 90 °C might be also similar to the one of calcite so that potential differences could be within the range of the analytical error.