An ideal terrestrial thermometer using carbonate clumped isotopes from
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).