Refining the temperature dependence of the oxygen and clumped isotopic compositions of structurally bound carbonate in apatite

Clumped isotope data from carbonated apatite from in vivo and in vitro samples are presented to refine the relationship between mineral growth temperature and carbonate clumped isotopic composition (Δ₄₇). Δ₄₇, δ¹⁸O and δ¹³C data were obtained from phosphoric acid digestion (T = 110 °C) of chemically untreated teeth from an African elephant, Greenland sharks, sand tiger sharks and synthetic apatites. These data cover a temperature range between 1 °C and 80 °C and enlarge the calibration dataset presented in Wacker et al. (2016) by a factor of five. Taxon-specific analyses of tooth enamel(oid) and dentine reveal that both tissues show identical Δ₄₇ values even though the content of organic matter differs by an order of magnitude. The following Δ₄₇ temperature calibration for (bio)apatite is derived (R² = 0.9924, p-value < 0.0001, n = 122; 8 samples):

Δ_{47 CDES 110} = 0.0325 (± 0.0012) ×10⁶ /T² + 0.2137 (± 0.0124) (with T in K andΔ₄₇ in ‰)

This calibration becomes indistinguishable from a reprocessed empirical calibration of calcite made in the same laboratory if a difference of the acid fractionation factors (AFF) of 0.110‰ between 25 °C and 110 °C is considered. The measured AFF for bioapatite matches the one that is extrapolated from experimental data on calcite and aragonite. The oxygen isotope fractionation between structural carbonate in the synthesized carbonated hydroxylapatites (CHAP) and water between 7 °C and 80 °C closely follows the temperature dependence for the calcite-water system. It is described by the following (CHAP-water) equation (R² = 0.997, p-value < 0.04, n = 17; 3 samples):

1000 ln (α_{CHAP - water}) = 17.23(± 0.59) ×10³ ×T^-1 - 27.28 (± 1.73) (with T in K)

Both calibrations are applied to shark teeth from a modern Greenland shark and a fossil megatooth shark (Carcharodon megalodon) specimen to reconstruct the apparent Δ₄₇-based habitat temperature of C. megalodon (19 ± 4 °C) and the oxygen isotopic compositions of seawater.