Calibration of the phosphate δ18O thermometer for SIMS analyses to reconstruct ancient seawater temperatures

• Applicant: Professor Dr. Thomas Wotte
• Subject Area: Geology
• Term: since 2021
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 470134482

Oxygen isotope (δ18O) studies on marine biological precipitated phosphate (bioapatite) provide a valuable tool for reconstructing temperatures of (paleo)seawater. Whereas the conventional oxygen isotope analysis of bioapatite via IRMS (isotope ratio mass spectrometry) requires large sample volumes of > 1 mg, this volume is essentially reduced during in situ SIMS (secondary ion mass spectrometry) analysis. Such a reduction of sample size also minimizes the risk of contamination of the primary oxygen isotope signal. Despite this obvious advantage, some critical points probably influencing the final interpretation of data: (1) There seems to be an offset between oxygen isotope values generated via SIMS and IRMS, as SIMS probably not only liberates oxygen isotopes related to the PO43− group. Whether (and if so, to what extent) non-PO43− related oxygen isotopes bias the final δ18O values or whether this reported offset is only an artefact of sample alteration, needs clarification. (2) Equations necessary for reconstructing the (paleo)seawater temperatures are based on data from IRMS analyses. But, are these thermometer equations also applicable for SIMS data? (3) In order to calibrate the oxygen isotope values from bioapatite generated by SIMS, Durango apatite is commonly used as the primary isotope standard. Even if its homogeneity in δ18O is generally accepted, there are still doubts questioning this presumption. This project aims to clarify these critical aspects to calibrate and establish a phosphate δ18O thermometer for SIMS analyses from bioapatite, allowing a precise calculation of seawater temperatures. For the successful realization of the project, we want to analyze teeth of sharks grown up during standardized seawater conditions. Requirements necessary, are given by commercial aquariums. Why we want to use sharks? Sharks are the ideal provider for marine bioapatite, as (1) their teeth are principally composed of flour-apatite, which is most resistant against secondary alteration, and (2) shark teeth are changed continuously and are thus important fossils from marine and terrestrial environments. To determine oxygen isotope fractionation between bioapatite of teeth and seawater, different shark species of the same aquarium will be investigated. On the other hand, the influence of variations in seawater temperature, pH-value, and oxygen isotopic composition on distinct shark species will be detected by using eight basins of four commercial aquariums.Results of our research will be a fundamental base for calculating seawater temperatures of the geological past, using oxygen isotopes of fossil bioapatite generated via SIMS analysis. Therefore, our project is an important contribution towards a better understanding of the interaction between atmosphere, hydrosphere, and biosphere throughout the Earth’s history.

• DFG Programme: Research Grants