The provenance of Danubian loess

With one of the largest watersheds in Europe, draining complex geological terrains within the Alps, Bohemian Massif, Carpathians, Dinarides, and the Balkan Mountains, the Danube River valley has long been linked to the formation of thick loess deposits, particularly within the Middle and Lower Danube basins. However, uncertainty over the provenance of loess-palaeosol sequences along the Danube impacts our understanding of sediment generation mechanisms and hinders interpretation of paleoenvironmental proxies preserved in loess. To date, most of the studies investigating loess provenance in Europe have not attempted a standardised characterisation and synthesis of loess deposits with potential source rocks. Further, despite clear links identified between loess and rivers in Asia and Europe, detrital zircons have not yet been used systematically to investigate the relationship between loess and the fluvial sediments of the Danube and its tributaries. Finally, in European loess research, provenance fingerprinting has often been conducted using indirect approaches or bulk sample geochemical analyses, which have been shown to have a limited application in well mixed sedimentary bodies such as loess. This provenance study of loess along the Danube River integrates existing zircon U-Pb ages and Hf datasets for loess, rivers, and bedrock, with new loess zircon U-Pb and Hf results from loess sequences in Croatia, Serbia and Bulgaria. The results show that all surrounding mountain belts (i.e. the Alps, Bohemian Massif, Carpathians, Dinarides Alps, and Balkan Mountains) contribute primary sediment to loess deposits in the Danube valley via its modern tributary network. Critically sedimentary sources remain relatively homogenous along the river, with no apparent major change in source with tributary confluence, further highlighting the role of fluvial transport in homogenising sediment prior to final aeolian deposition. Whilst some small site variations can be observed, they are likely explained by contributions restricted to very local rock outcrops. Moreover, geomorphological results support floodplain sediments as the proximal sediment source and suggest that short-distance aeolian transport dominates sediment delivery to loess sequences, challenging distant sources hypotheses such as major Saharan sources. The identification of sediment sourced from lower elevation regions such as the Bohemian Massif, Dinarides, and Balkans, which did not support ice-caps, suggests that the role of glacial action in silt-size sediment production has been previously exaggerated. Therefore, the glacial and desert loess division inadequately separates and describes sediment generation processes. This research supports and furthers previous work, which suggests “mountain sourced and transported by-rivers” as a more appropriate term for the particles forming loess, at least in the Danube basin