A guide to synchrotron hard X-ray fluorescence mapping of annually laminated stalagmites: Sample preparation, analysis and evaluation

Abstract

Annually laminated stalagmites are an exceptional archive of high-resolution past hydroclimate changes and provide robust annually-resolved age models. Stalagmite chemical annual laminae are invisible through optical and fluorescence microscopy, but are revealed by X-ray fluorescence or mass-spectrometry techniques. Synchrotron radiation based micro-X-ray fluorescence (S-μXRF) has the unique advantage of creating 2-dimensional maps of element distribution, which allows visualizing lateral heterogeneity and porosity that pose problems when one dimensional line-scanning techniques, such as LA-ICP-MS, are used. Here, we show how S-μXRF fast element mapping allows obtaining robust age models and palaeoclimate data from stalagmites otherwise difficult to date. Analytical challenges and solution to problems are discussed for both thick polished slabs and thin sections. The elements detectable through the fast acquisition mode (dwell time 1 to 4 ms) are typically Ca, Sr, Br, while Fe, Zn, Ni, Cu, Mn, Y, and Rb are occasionally detected in few distinctive layers or, in association with grain inclusions and porosity. Only Sr, however, exhibits a consistent and clear annual cyclicity. At incident angles of 90° the attenuation depth for Sr is ca. 240 μm and, therefore thick samples are preferable as they produce stronger fluorescence signal and sharper images unaffected by non-linear drift. On the other hand, thick samples are more affected by geometric and edge effects that can be particularly severe in the presence of irregular lamination and when the laminae are dipping with variable angle within the sample. The Sr concentration in the tropical stalagmites analysed in this study varied from 15 to 600 ppm, which strongly influences the dwell time needed to obtain high-quality maps. For Sr concentration below 20 ppm the ideal dwell time exceeds 20 ms and this renders the acquisition of large maps (> 20 Megapixels) hardly feasible. For concentrations above 20 ppm the fast mapping (dwell time < 5 ms) through S-μXRF is by far the best methodology to visualise chemical annual cycles in stalagmites.