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An ideal terrestrial thermometer using carbonate clumped isotopes from gar scales
  • Katelyn Gray,
  • Mark Brandon
Katelyn Gray
University of Delaware

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Mark Brandon
Yale University
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Carbonate clumped isotope thermometry has been calibrated for a wide variety of carbonates, including calcite, aragonite, dolomite, siderite, and many of their biogenic forms. The clumped isotope composition of the carbonate group substituting for phosphate or hydroxyl in bioapatite (Ca(PO4,CO3)(OH,F)) has also been temperature calibrated using vertebrate tooth enamel from a range of endothermic body temperatures. We apply this method to other bioapatite-bearing taxa and the calibrated temperature range is extended to lower paleoclimatologically relevant temperatures. Furthermore, because relatively large bioapatite samples are required for carbonate clumped isotope measurements (Δ47), replicate sampling of thin tooth enamel may not be feasible in many situations. Here, we use gar fish (Lepisosteus sp.) scales to extend the calibration. These fish are unique in that they are entirely covered in ganoine scales, which are >95% hydroxyapatite. Their enamel structure also makes them resistant to diagenesis. Additionally, gar fossils are common in lacustrine, fluvial, and near-shore facies, and have a wide distribution in time (Cretaceous to modern) and location (North America, South America, Europe, India, and Africa). We have developed a reliable lab protocol for measuring Δ47 in gar bioapatite. We estimate the standard error (SE) for a single measurement as 0.027‰, which is based on replicate analyses and Student T-distribution to account for sample size. We report results for modern gar scales from seven North American localities with mean annual water temperatures (MAWT) ranging from 9 to 26 °C. These data give a temperature calibration curve for gar scales of Δ47 = (0.1095 ± 0.0159) x 106/T2 – (0.5941 ± 0.0548) (R2 = 0.74) and a curve for pooled bioapatite of Δ47 = (0.1003 ± 0.0144) x 106/T2 – (0.4873 ± 0.0495) (R2 = 0.76).