Episodic river flooding revealed by geochemical proxies from modern age deposits off the Po Delta, Italy

Deciphering sediment dispersal pathways and their evolution through time in the fluvial-to-marine transition zone depends on our ability to discern the relative contribution of fluvial versus marine processes, which in homogeneous muddy successions can hardly be carried out through conventional facies analysis. Using high-resolution XRF-scanner profiles and point analyses across a hierarchy of Upper Holocene deltaic and shallow-marine deposits, located 4-6 km off the Po River mouth, we demonstrate that key geochemical indicators and terrestrial biomarkers may allow the separation of episodic river-flood deposits preserved below fairweather wave base from mud accumulated on the shelf through normal marine depositional processes. High Ni/Ca ratios, associated with low δ13C values delineate sediment provenance from ultramafic rocks exposed in the Po River catchment and an abundance of terrestrial organic matter, which can be used as elemental fingerprints of punctuated fresh water fluvial input (flood layers). Low Ni/Ca values coupled with relatively high δ13C values instead suggest a reduced Po River influence and an increasing marine contribution, driven by marine productivity and the delivery of carbonate-rich sediment from North Adriatic fluvial sources. Peaks in Cr, an element scarcely detected in the study units by the XRF-scanning technique, provide additional evidence for Po-derived flood layers. Comparison of hydrographs of the 2000 Po River flood with Ni/Ca peaks from the associated sedimentary layer is used to delineate short-lived flood pulsations to an unprecedented level of temporal (monthly to daily) and spatial (laminaset) scales.