A Southern Alps and Northern Pyrenees Holocene record of snowmelt-induced flood events and clastic layers associated with negative NAO phases inWestern Europe

International audienceThe origin of both extreme flood events in Lake Ledro (southern Italian Alps) and coarse sandy layers in twodisconnected lakes from the Bassies valley (Lakes Majeur and Sigriou, northern Pyrenees) have been relatedto the impact of snowmelt processes enhancing erosion of mountainous drainage basins (1, 2) throughout theHolocene. Because of increasing human impact on catchment erosion processes since the mid-Holocene in thesewestern European mountain ranges, this study compares these well-dated lacustrine sequences in order to furtherdocument the influence of westerlies and of the North Atlantic Oscillation on clastic supply in contrasted lakebasins. The integrative approaches performed on each site allow us to show that organic and minerogenic markers,such as non-pollen microsfossils, Rock-Eval pyrolysis or X-ray microfluorescence, are powerful tools to identifyclastic sediment source areas. At Ledro, we therefore demonstrated that over the Late Holocene snowmelt-inducedflood events essentially remobilized high altitude pasture areas whereas afterwards the flood events affectedformer forested areas from lower altitude1. In the Pyrenees, the southern slopes of lakes Majeur and Sigriouare characterized by two narrow canyons whose drainage basins are disconnected, relatively small and limitedto the high altitude part of the valley of Bassiès. Our results demonstrated that the mid–late Holocene periodwas regularly interrupted by coarse sandy layers affecting both lakes Majeur and Sigriou and reflecting the highsensitivity of the two active canyons to intense rainfall or snowmelt periods2. While extreme flood deposits inLake Ledro during the Bronze Age period may result from the combination of both climate and human activities,contemporaneous extreme flood events in Ledro and coarse sandy layers in the Bassiès lakes, dated to AD 1710,AD1530, AD1360, AD940, AD570 and 1850, 1050, 1410, 1850, 2690, 4190, 4800 cal BP, testify of regionalhydrological events synchronous with European climatic deterioration phases and remarkably matching negativeNAO phases and solar minima over the Mid-Late Holocene3,4.(1) Simonneau et al., 2013, Climate of the Past, 9: 825-840.(2) Simonneau et al., 2013, The Holocene, 23: 1764-1777.(3) Olsen et al., 2012, Nature Geoscience, 5 : 808-812.(4) Delaygue and Bard, 2011, Climate Dynamic, 36: 2201-2218