A Southern Alps and Northern Pyrenees Holocene record of snowmelt-induced flood events and clastic layers associated with negative NAO phases inWestern Europe
Résumé
The origin of both extreme flood events in Lake Ledro (southern Italian Alps) and coarse sandy layers in two
disconnected lakes from the Bassies valley (Lakes Majeur and Sigriou, northern Pyrenees) have been related
to the impact of snowmelt processes enhancing erosion of mountainous drainage basins (1, 2) throughout the
Holocene. Because of increasing human impact on catchment erosion processes since the mid-Holocene in these
western European mountain ranges, this study compares these well-dated lacustrine sequences in order to further
document the influence of westerlies and of the North Atlantic Oscillation on clastic supply in contrasted lake
basins. 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 identify
clastic sediment source areas. At Ledro, we therefore demonstrated that over the Late Holocene snowmelt-induced
flood events essentially remobilized high altitude pasture areas whereas afterwards the flood events affected
former forested areas from lower altitude1. In the Pyrenees, the southern slopes of lakes Majeur and Sigriou
are characterized by two narrow canyons whose drainage basins are disconnected, relatively small and limited
to the high altitude part of the valley of Bassiès. Our results demonstrated that the mid–late Holocene period
was regularly interrupted by coarse sandy layers affecting both lakes Majeur and Sigriou and reflecting the high
sensitivity of the two active canyons to intense rainfall or snowmelt periods2. While extreme flood deposits in
Lake 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 regional
hydrological events synchronous with European climatic deterioration phases and remarkably matching negative
NAO 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.