Rapid measurement of strontium in speleothems using core-scanning micro X-ray fluorescence

Abstract

Speleothem trace element ratios such as Mg/Ca and Sr/Ca are increasingly used in speleothem paleoclimatology as a supplement to stable oxygen and carbon isotope ratios as proxies for past variability in the hydrologic system. Using multiple proxies together allows for a better understanding of both the local and distal hydrologic changes recorded in speleothem chemistry, and therefore of changes in past rainfall. Despite the potential benefits, trace element analysis of speleothems has yet to become widespread, which is likely due to the significant time and costs required by traditional trace element analytical techniques. In this study, we present an in-depth investigation into a rapid, relatively non-destructive and competitively priced technique for measuring Sr/Ca in speleothems: Core-Scanning micro X-ray Fluorescence (CS-μXRF). We show that CS-μXRF reliably and precisely records Sr concentration in speleothems. Ratioed to near-stoichiometric Ca, the Sr/Ca ratio accounts for variations in beam strength and machine settings, producing a more reliable reported measurement for both intra- and inter-run comparisons. CS-μXRF compares favorably with more conventional trace element procedures such as Quadrupole ICP-MS and ICP-AES, giving confidence in the ability of CS-μXRF to produce paleoclimatologically significant Sr/Ca results. We also identify secondary issues relating to speleothem crystallinity, the dominance of Ca spectral peaks, and comparatively lower energy X-rays that can interfere with precise CS-μXRF analyses. If these can be overcome then CS-μXRF may provide an even more useful method of trace element analysis in speleothem studies.