Fast and high-resolution LA-ICP-MS imaging for improved characterization of speleothems as paleoclimate archives

Speleothems are important environmental archives due to their potential of recording variations in temperature, rainfall regimes, soil/vegetation dynamics and many other processes through changes in growth speed, isotopic signature and trace elements concentration. From the geochemical point of view, the vast majority of approaches rely on the analysis of replicate transects along speleothems' main growth axis, where lateral variability is averaged out as a disturbing feature. However, lateral variability is an important component of individual growth conditions which are fundamental for any environmental interpretation. Extracting such information requires an extrainvestment in developing integrated analytical approaches (multi-technique and multi-disciplinary) specifically optimized to elucidate the relationship between chemistry and microstructure in two dimensions at least. We propose a novel LA-ICP-MS method for fast high-resolution elemental imaging specifically adapted and assessed for the analysis of speleothems, derived from a method we recently developed for ice cores [1]. Instrumental settings were optimized to achieve artifact-free mapping at 20 ?m lateral resolution of 5-7 masses per run at the speed of about one square mm per minute. Quantitative analysis is achieved by combining internal normalization, external calibration with multilevel pellets of aragonite obtained from KNI-51 stalagmites (Kimberley, Australia), and validated by standard liquid ICP-MS analysis. The method was applied to the pilot elemental mapping of slow growing (1 mm/ka) subaqueous calcite deposits from Devils Hole (Nevada, USA), previously used to date groundwater oscillations at orbital and multi-millennial timescales [2]. Preliminary results show dramatic differences in the 2D spatial distribution amongst Mg, Sr, Ba and U; particularly associated to the microstructural features of dense mammillary calcite (formed under water) and porous folia (formed at the water surface), including structurally complex mineralogical boundaries, and transition zones undetected by microscopy alone. Integrating longitudinal and lateral variability in the microstructural investigation of speleothems opens the way to elucidate how reconstructing minerogenic processes can bring to light the effects of past climate changes on delicate cave environments. Additionally, more accurate linear (temporal) records can be extracted as reductions of high-resolution maps being able to visualize, characterize and eventually avoid irregularities post-acquisition.