Palaeoflood activity and climate change over the last 1400 years recorded by lake sediments in the north‐west European Alps

Abstract

International audienceA high-resolution sedimentological and geochemical study of a high-altitude proglacial lake (Lake Blanc, Aiguilles Rouges, 2352m a.s.l.) revealed 195 turbidites, 190 of which are related to flood events over the last 1400 years. We used the coarsest sediment fraction of each turbidite as a proxy for the intensity of each flood event. Because most flood events at this locality are triggered by localized summer convective precipitation events, the reconstructed sedimentary record reveals changes in the frequency and intensity of such events over the last millennium. Comparisons with other temperature, palaeohydrological and glacier reconstructions in the region suggest that the most intense events occurred during the warmest periods, i.e. during the Medieval Climate Anomaly (AD 800-1300) and the current period of global warming. On a multi-decadal time scale, almost all the flood frequency peaks seem to correspond to warmer periods, whereas multi-centennial variations in flood frequency appear to follow the regional precipitation pattern. Consequently, this new Alpine flood record provides further evidence of a link between climate warming and an increase in the frequency and intensity of flooding on a multidecadal time scale, whereas the centennial variability in flood frequencies is related to regional precipitation patterns

A new CT scan methodology to characterize a small aggregation gravel clast contained in a soft sediment matrix

“…Analysis of median grain size (Q50) and the coarser 10th percentile (Q90) parameters from within graded beds leads us to consider these to be turbidites caused by heavy rainfall in the watershed (Støren et al, 2010;Giguet-Covex et al, 2012;Wilhelm et al, 2012Wilhelm et al, , 2013Wilhelm et al, , 2015Gilli et al, 2013). Gravels were found in the upper part of the flood deposit that are associated with receding torrential activity .…”

“…Fluvial events such as floods are able to transport very large quantities of sediment in a short period of time (Sturm and Matter, 1978;Jenny et al, 2014). In recent years, they have also been largely identified as a major sedimentary source in high-elevation lakes (Giguet-Covex et al, 2012;Wilhelm et al, 2013Wilhelm et al, , 2015Wirth et al, 2013). As floods are formed by heavy precipitation events, the torrential stream transporting sediment in suspension will enter into the lake and create a density current resulting in a characteristic deposit called turbidite .…”

“…Wet avalanches in lacustrine deposits have been identified by Vasskog et al (2011) using grain size analysis to identify layers of poorly sorted grain accumulation associated with gravels resulting in a multi-modal grain size distribution. Such deposits on lake ice would result in drop stones at thaw season (Luckman, 1975), and may contain such deposits on lake ice would result in drop stones at thaw season, and may contain terrestrial organic matter (OM; Irmler et al, 2006;Wilhelm et al, 2013;Korup and Rixen, 2014).…”

A multi-centennial record of past floods and earthquakes in Valle d'Aosta, Mediterranean Italian Alps

“…Grain-size analyses of the detrital material were performed to characterize the transport-deposition dynamics of the deposits (e.g. Passega, 1964;Wilhelm et al, 2013Wilhelm et al, , 2015. …”

“…These mass-movement-induced GBs are also known to be generally thicker than those induced by flood events because mass movements may mobilize much larger quantities of sediments than floods (e.g. Shiki et al, 2000;Schnellmann et al, 2005;Fanetti et al, 2008;Wilhelm et al, 2013). Accordingly, the rare GB-III characterized by large thicknesses may be associated with mass movements.…”