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

海洋生物沉淀磷酸盐（生物磷灰石）的氧同位素（δ18O）研究为重建（古）海水温度提供了有价值的工具。传统的生物磷灰石氧同位素分析通过IRMS（同位素比质谱）需要大量的样品体积，而在原位SIMS（二次离子质谱）分析中，这一体积基本上减少了。这种样本量的减少也使初级氧同位素信号受到污染的风险降到最低。尽管有这种明显的优势，但一些关键点可能会影响数据的最终解释：(1)SIMS和IRMS产生的氧同位素值之间似乎存在偏移，因为SIMS可能不仅释放与PO43−基团相关的氧同位素。非po43−相关的氧同位素是否（如果是，在多大程度上）对最终的δ18O值有偏差，或者这种报告的偏移是否只是样品蚀变的人工产物，都需要澄清。(2)重建（古）海水温度所需的方程基于IRMS分析数据。但是，这些温度计方程也适用于SIMS数据吗？(3)为了校准SIMS生成的生物磷灰石的氧同位素值，通常使用Durango磷灰石作为主要同位素标准。即使它在δ18O中的均匀性被普遍接受，但对这一假设仍然存在质疑。该项目旨在澄清这些关键方面，以校准和建立用于生物磷灰石SIMS分析的磷酸盐δ18O温度计，从而精确计算海水温度。为了这个项目的成功实现，我们想分析在标准化海水条件下长大的鲨鱼的牙齿。商业水族馆给出了必要的要求。为什么我们要用鲨鱼？鲨鱼是海洋生物磷灰石的理想提供者，因为：(1)鲨鱼的牙齿主要由面粉磷灰石组成，最能抵抗次生蚀变；(2)鲨鱼的牙齿是不断变化的，是海洋和陆地环境的重要化石。为了确定牙齿生物磷灰石与海水之间的氧同位素分异，将对同一水族馆的不同鲨鱼进行调查。另一方面，海水温度、ph值和氧同位素组成的变化将通过四个商业水族馆的八个盆地来检测不同鲨鱼种类的影响。我们的研究结果将为利用SIMS分析生成的化石生物磷灰石的氧同位素计算地质过去的海水温度提供基础依据。因此，我们的项目对更好地理解地球历史上大气、水圈和生物圈之间的相互作用做出了重要贡献。

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.