Geochemical normalization of magnetic susceptibility for investigation of floodplain sediments

Abstract

Magnetic susceptibility (MS) is commonly used as a proxy for industrial pollution in natural sediments or as a proxy for the percentage of detrital components in peat or carbonates. The MS may also reflect the geology of the sediment source and post-depositional processes in sediments, such as soil development. The aim of our research was to test the usefulness of Fe-normalized mass-specific MS (χ) and Ti-normalized Fe in a study of floodplain sediments. We sampled 27 floodplain sediment cores from several geologically different catchments throughout the Czech Republic, analysed their Fe and Ti concentrations using X-ray fluorescence spectroscopy, and determined their χ. To decipher sediment grain-size dependence and possible magnetic enrichment, background functions for χ were constructed using similar approach as that used for geochemical background functions of the risk element concentrations with Fe concentrations as an independent variable. It provides a mechanism to calculate χ of sediments as it would be “pristine”, i.e. without post-depositional changes and pollution. Sediments derived from “mafic” source rocks had χ/Fe larger by two orders of magnitude than sediments derived from “felsic” rocks. Sediments derived from “mafic” source rocks also exhibit lower mean Fe/Ti ratio in pristine sediment strata than the average upper continental crust. The magnetic carriers inherited from mafic rocks are stepwise destroyed by pedogenesis in the floodplains and thus slowly approach χ of sediments derived from felsic rocks. Gleying processes may change χ/Fe, Fe/Ti ratio allows identifying a past action of those post-depositional processes.