Unaltered evidence: comparing oxygen isotopes in conodonts from deep-sea chert analysed in situ and extracted using NaOH

The oxygen isotope composition (δ18O) of conodont biogenic apatite is a critical proxy for reconstructing past marine temperatures and environments. This study compares δ18O values of Triassic conodonts embedded in chert with those extracted from the same samples using sodium hydroxide (NaOH), a novel alternative to traditional hydrofluoric acid (HF) extraction. Using high-resolution secondary ion mass spectrometry (SHRIMP) combined with statistical analyses, we show that no measurable effect of NaOH extraction on the original isotopic signal was detected within analytical uncertainty. Conodonts extracted using NaOH yielded δ18O values indistinguishable from those of conodonts still embedded in chert, supporting the geochemical integrity of this less invasive technique. Further, comparisons of Misikella sp. extracted using two NaOH protocols (1 M at 100°C and 3 M at 50°C) showed no significant differences in δ18O values, indicating that no measurable isotopic bias was detected under the extraction conditions investigated. Comparison of elemental compositions of specimens treated with NaOH vs HF shows that HF treatment significantly alters the original apatite composition, creating fluorite conodont pseudomorphs devoid of oxygen. These findings support NaOH extraction as a reliable and non-destructive method for conodont geochemical analysis, opening up new possibilities for studies using rare or morphologically delicate specimens preserved in deep-sea sediments.