Clumped isotope analysis of (bio)apatite using a Kiel IV device in long-integration dual-inlet mode

Abstract

Rationale: Clumped isotope (Δ47) analysis of bioapatite-derived CO2 is a powerful tool to determine body temperatures of extinct vertebrates. The common acid bath technique in combination with dual-inlet-based mass spectrometric measurements has been the preferred method of choice for this purpose, but the large amount of material necessary and the presence of secondary calcite represent obstacles. Methods: We analyzed the Δ47 composition of carbonate-bearing (bio)apatites using a Kiel IV device, which – in general – allows a reduction of sample replicate size by a factor of ~40 over dual-inlet-based techniques. The Kiel IV device was tested in two different modes: without and with additional water sinks for improved water removal. Furthermore, we tested a pretreatment technique based on 1 M acetic acid (pH = 5) to selectively remove secondary calcite from the carbonate-bearing (bio)apatite phase. Results: Significantly lower Δ47 values were obtained for a given bioapatite after the installation of the two water sinks. With this setup, Δ47 of (bio)apatites followed a temperature relationship that is indistinguishable from the unified one for pure carbonates, provided a dentine sample, rich in organic matter, was excluded. The original bioapatite Δ47 value was restored from a bioapatite/calcite mixture if the mixed material was treated for 1 h with 1 M acetic acid (pH = 5). Conclusions: (Bio)apatites having low organic matter content such as enamel(oid) can be analyzed accurately for Δ47 using a Kiel IV equipped with water sinks that ensure effective removal of water. Secondary calcite can be effectively removed from carbonate-bearing apatite by pretreatment with 1 M acetic acid (pH = 5) for 1 h.