X-ray fluorescence core scanning of wet marine sediments: methods to improve quality and reproducibility of high-resolution paleoenvironmental records

Abstract

X-ray fluorescence (XRF) core-scanning is a convenient non-destructive tool to rapidly assess elemental variations in unprocessed sediments. However, substantial analytical deviations may occur in such data due to physical sedimentary properties. The consequences of these artifacts on element intensities are important for paleoclimatic studies, but as yet have not been adequately studied for high resolution (≤ 1-cm interval) records. We have done a high resolution comparison between results from XRF scanning and from two more conventional methods: XRF with beads and inductively coupled plasma optical emission spectrometry (ICP-OES), both on discrete samples. Our XRF scan data concord reasonably well with those from the conventional methods for most elements. In several discrete intervals large deviations occur that could have been attributed to major paleoceanographic events. However, these deviations appear to be mainly related to water content and distribution. We show that the variability in the water film thickness underneath the plastic foil is highly variable, causing significant apparent variability in all elements from Al to Fe. The unverified use of these elements in ratios can thus lead to deviations in the paleoenvironmental interpretation. Erroneous element (ratio) variability can, however, be recognized and corrected using conventional analytical data of a few discrete samples. The antipathetic behavior of Ca to K or Ti, and their similar behavior to water absorption, leaves their ratios relatively unaffected at variable water film thicknesses. Hence, Ca/K and Ca/Ti ratios are recommended for a preliminary down-core overview of sedimentary variability and for stratigraphic correlations between unprocessed marine cores.