A CARBONATE CLUMPED ISOTOPE CALIBRATION OF GAR SCALE APATITE

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 in bioapatite (Ca(PO₄,CO₃)(OH,F)) has also been temperature calibrated using shark, reptile, and mammal tooth enamel precipitated at body temperatures of 25 to 37 °C (Eagle et al., 2010). However, because relatively large bioapatite samples are required for carbonate clumped isotope ratio (Δ₄₇) measurements, replicate sampling of thin tooth enamel may not be feasible in many situations. Furthermore, there is a need to both test this method on other apatite-bearing taxa and extend the calibrated temperature range to lower, paleoclimatologically-relevant temperatures. Gar fish (Lepisosteus sp.) are unique in they are entirely covered in ganoine scales, which are >95% hydroxyapatite, thus replicate Δ₄₇ measurements of single fish are readily attainable. Their enameloid structure also makes them resistant to diagenesis, a process that can significantly affect the geologic preservation of clumped isotopes in bone. Furthermore, gar fossils extend back to the Cretaceous in North America, South America, Europe, India, and Africa. Here we present clumped isotope data from modern gar scales from five North American localities with mean annual water temperatures (MAT) that range from 9 to 28°C. Our results indicate that gar scale bioapatite data follow the Zaarur et al. (2013) calibration trend (i.e., aligned with previous Yale calibration results) when compared with MAT, but not with summer (JJA) temperatures. This is consistent with the observation that gar retain their scales throughout their lives, which can encompass decades, and grow scales year-round. We also present a framework for enamel preparation for clumped isotope analysis, including sample pretreatment and extended water trapping during the acid reaction.