Micro X-ray fluorescence (μXRF) line scanning on Cretaceous rudist bivalves: A new method for reproducible trace element profiles in bivalve calcite

The reconstruction of palaeoclimate on a sub-annual scale requires the measurement of chemical proxies in fossil material at a high spatial resolution. While various methods for trace element analysis on the micrometre scale are available, they are usually destructive to the sample or not widely accessible. This study evaluates the performance of a table-top micro X-ray fluorescence scanner in obtaining reproducible trace element profiles on bivalve calcite. Standard calibration and repeatability tests demonstrate the robustness of results obtained with this new generation micro X-ray fluorescence device that uses a comparatively high excitation energy of 30 W. Results show that elemental abundances measured using this fast, readily accessible and non-destructive analytical set-up are reproducible on the μg g−1 level, and therefore suitable for the analysis of the abundance of commonly studied elements in bivalve calcite (for example, Sr, Mn and Fe). Spectra obtained in this study show considerable improvement in terms of signal to noise ratio compared to earlier table-top micro X-ray fluorescence studies. Reliability of the measurements is tested using a conservative detection and quantification limit. Count rates of Ca are used to check for point measurements disturbed by irregularities on the sample surface. Furthermore, the method allows semi-quantitative two-dimensional element mapping, which is a convenient tool for the detection of diagenetic alteration in fossil samples. The method reveals records of Sr/Ca, Fe/Ca and Mn/Ca ratios in a Late Campanian Vaccinites vesiculosus shell, with Sr/Ca ratios showing a trend opposite to Mn/Ca and Fe/Ca. Resulting trace element records are discussed in terms of seasonal cyclicity in the Late Cretaceous and diagenetic alteration of the fossil shell