Geochemical proxies of sediment provenance in alluvial plains with interfering fluvial systems: A study case from NE Italy

Abstract

This paper demonstrates that geochemistry is useful for the identification of sediment origin and provenance in alluvial plains characterised by a complex hydrographic evolution. The study is focused on the northeastern Padanian plain (Italy), an area primarily characterised by sedimentary contributions from the two largest Italian river systems (Po and Adige), which intimately interacted during the last millennia. X-ray fluorescence analyses of 120 soils and alluvial sediments define three diverse geochemical affinities that have distinctive siderophile/chalcophile elemental ratios. The sample group characterised by high Ni/Zn and Cr/Pb values conforms to modern Po River sediments, whereas a second group showing low Ni/Zn and Cr/Pb values conforms to the geochemical signature of modern Adige River sediments. A third sample group defines a “transitional” affinity that represents a geochemical mixture of Po (70%) and Adige (30%) sedimentary end-members. Based on these geochemical features, it is possible to distinguish alluvial sediments of the Po River basin (Ni/Zn>1.0 and Cr/Pb>4.2) from those of the Adige River basin (Ni/Zn<0.6 and Cr/Pb<1.9) and to provide evidence of the migration of these rivers during the evolution of the Padanian plain. The interpretation of transitional samples is less constrained and could imply an ancient connection between the two fluvial systems, possibly due to the development of wetlands where both the Po and Adige rivers variably delivered their sedimentary contributions. This study approach, therefore, provides important implications for palaeohydrographic and palaeoenvironmental reconstructions in a complex area that is characterised by significant geomorphological modifications during the last millennia.