Isotopic resetting of CO2 in the presence of carbonate samples: Implications for oxygen- and clumped-isotope analyses

Exchange between CO 2 and water-containing phases is a consistent problem during oxygen and clumped isotope analysis of carbonate-derived CO 2 . Here, we have identified numerous natural and synthetic carbonates and carbonate-bearing materials that consistently reset the δ 18 O, ∆ 47 and ∆ 48 values of analyte CO 2 during acidification in a common acid bath and, thereby, contribute to preparation noise. From a mass-balance perspective, this resembles the isotopic exchange between CO 2 and a small water contaminant: indicating that certain samples cannot be effectively dried, even after several months in a vacuum drier. This effect can be replicated in off-line experiments, allowing for problematic samples to be unambiguously identified without endangering day-to-day lab operations. We further describe laboratory best-practices wherein the risk of this effect is effectively minimized, allowing for rapid identification of problematic samples: for example, the use of carbonate standards with extremely low ∆ 47 values (e.g. ETH-1 and ETH-2) before an analytical sequence allows such issues to be rapidly identified. This highlights the importance of analyzing both carbonate and equilibrated CO 2 standard materials, as these effects could not otherwise be easily identified and eliminated. We present this report in expectation that it will assist other laboratories to improve data quality, as well as to start a conversation about the potential necessity for additional carbonate standards with “exotic” bulk and clumped isotope compositions.