Combined µ-XRF and Microfacies Techniques for Lake Sediment Analyses

Abstract

This contribution presents an evaluation of still unexplored potentials, limitations and technical details of µ-XRF element scanning particularly for varved sediment analyses using the vacuum device EAGLE III XL. For this case study a 33 cm long interval of exceptionally well-preserved sub-millimetre scale calcite varves of the interglacial lake deposits from Piànico has been selected. In addition to the demonstration of the very good repeatability of XRF element scans, one focus of this paper is on discussing a suitable scanner resolution in terms of a compromise between short analyses times and a resolution that allows capturing the geochemi-cal signature even of seasonal sub-layers. By combining scanner data with micro-scopic sediment inspection geochemical signatures of various micro-facies as, for example, varves, detrital layers and matrix-supported and clay layers are explored. The potential of counting varves using the seasonal signature of specific elements is discussed and potential error sources are disclosed by comparison with micro-scopic varve counts. Detailed grid-µXRF-scanning provides for the first time an insight into the internal structure of varves presented as high-resolution 3D-images. Finally, an approach of calibrating scanner data through comparison with ICP-MS analyses is introduced. Sensitivity factors for each element have been defined as ratios between mean count rates and bulk element concentrations.