Event-Scale Stratigraphy and Anthropogenic Overprint in Danube Sediments Downstream of Vienna (Stopfenreuth-Schwalbeninsel -Section, Lower Austria)

Floodplain deposits of the Danube downstream of Vienna can be interpreted as high-resolution archives of recent environmental change, recording fluvial dynamics as well as anthropogenic influence at different scales. In our study. we present sedimentological data from a recently exposed outcrop at Stopfenreuth (STR1025, close to “Schwalbeninsel”, Lower Austria), extending the existing stratigraphic frameworks for the Danube river basin. We document a laterally variable succession of fine-grained material showing alternating sandy- and clayey-silty deposits, starting with the youngest (2024 and 2002) flood layers on top. Detailed logging documenting a strong lateral facies variability, reflecting shifting depositional settings within a floodplain system. This variability complements the previous observations of channel infilling, point-bar development, and overbank aggradation in the Vienna Basin. Lateral variations in sedimentology within individual depositional (in particular flood-) units, indicates that single events may generate spatially variable sedimentary architectures (like the interplay of deposition/erosion and channel formation). This has direct implications for stratigraphic correlation and event reconstruction in fluvial settings. The STR1025 outcrop refines the stratigraphic framework of Danube floodplain deposition downstream of Vienna by resolving event-scale sedimentation within an Anthropocene setting. We logged magnetic susceptibility through STR1025. The signal shows distinct short- and long-wavelength variability. Being (amongst other factors like weathering and detrital input) grain size dependent, this signal was interpreted to reflect stacks of individual floods and the longer-term trends in sediment supply, where the latter illustrates subtle changes in grain size through the section. This long-term trend could reflect ongoing changes in sediment supply caused by river regulation and hydroelectric- dam construction, which have modified grain-size distributions and changed the hydrodynamic behaviour of the river. The record of the transition from fine-grained deposits to increasingly massive, sand-dominated flood layers, is consistent with the behaviour of regulated fluvial systems. The assessment of granulometry shows a variability from mud-rich (up to ~60% <63µm) to fine sand-dominated deposits. The mineralogical compositions in contrast are rather uniform (quartz-dominated with subordinate carbonates, feldspar, and mica), suggesting stable provenance and consistently similar mineral assemblages. Changes in grain size and mineralogy do not correlate and are thus likely to reflect sediment transport dynamics linked to event-related changes in river geomorphology and connectivity, and aligns with broader Anthropocene shifts toward sandier, more massive floodplain deposits in regulated river systems. The STR1025 section provides a high-resolution archive of flood dynamics and anthropogenic overprint in the Danube system. Ongoing radionuclide analyses will further constrain the chronology.