How 17O excess in clumped isotope reference-frame materials and ETH standards affects reconstructed temperature

Carbonate clumped isotope measurements (Delta(47)) have been widely applied as a paleothermometer, but various sources of error typically limit their application to anomalies greater than about +/- 3 degrees C. Calculation of Delta(47) requires a correction for mass interference from O-17, which traditionally assumes all analytes follow a linear relationship between O-17/O-16 and O-18/O-16 with a slope of 0.528 and a O-17 excess (Delta O-17) of zero. Here, we evaluate these assumptions by measuring Delta O-17 in Delta(47) reference-frame gases and waters, our mass spectrometer working gas, and ETH standards 1, 2, 3 and 4. We systematically evaluate how the Delta O-17 of these materials influence Delta(47) values, and the magnitude of Delta(47) error introduced by assuming a Delta O-17 of zero. We find most Delta(47) reference-frame materials and ETH standards have negative Delta O-17 values, ranging from -300 to +38 permeg, which can largely be explained by equilibration with surface water at Earth surface temperature. Exceptions include CO2 equilibrated with evaporatively enriched water, which has a more negative Delta O-17 than other reference-frame materials, and CO2 derived from fossil fuel combustion, which has a less negative Delta O-17 and greater variance. CO2 heated to 1000 degrees C in quartz glass tubes shows a small, 5-10 permeg, increase in Delta O-17 that may reflect contamination in the quartz. CO2 evolved from ETH standards exhibits mean Delta O-17 values of -151 to -123 permeg, similar to most natural carbonates, but ETH 2 and 4 have greater variance. Our results show that assuming Delta O-17 = 0 can overestimate or underestimate measured Delta(47) values, with the direction of change dependent on whether sample Delta O-17 falls above or below the Delta O-17 of the working gas. Similarly, assuming Delta O-17 = 0 can also overestimate or underestimate Delta(47) in the carbon dioxide equilibration scale (CDES), with the direction of change dependent on whether sample Delta O-17 falls above or below reference-frame materials. The magnitude of this effect is significant, and is equivalent to an error on reconstructed temperature of 0.6-1.7 degrees C for Earth surface conditions and 1.9-5.8 degrees C in the shallow crust. While analytical and calibration errors are also important, the effect of Delta O-17 needs to be considered for Delta(47) thermometry to achieve its highest possible accuracy. The same is true for the recently introduced dual clumped isotope thermometer. We provide suggested laboratory protocols that best account for Delta O-17 when measuring Delta(47).

Automated summary

The study evaluates the assumptions made in Delta(47) measurements by analyzing Delta O-17 in reference frame materials and standards. The results show that assuming Delta O-17 = 0 can lead to overestimation or underestimation of Delta(47) values, depending on the sample's Delta O-17 relative to the working gas. This effect can introduce significant errors in reconstructed temperatures, emphasizing the importance of considering Delta O-17 for accurate Delta(47) thermometry.