A sedimentary record of Holocene surface runoff events and earthquake activity from Lake Iseo (Southern Alps, Italy)

Abstract

International audienceThis study presents a record of Holocene surface runoff events and several large earthquakes, preserved in the sediments of pre-Alpine Lake Iseo, northern Italy. A combination of high-resolution seismic surveying, detailed sediment microfacies analysis, non-destructive core-scanning techniques and AMS 14C dating of terrestrial macrofossils was used to detect and date these events. Based on this approach, our data shed light on past seismic activity in the vicinity of Lake Iseo and the influence of climate variability and human impact on allochthonous detrital matter flux into the lake. The 19 m long investigated sediment sequence of faintly layered lake marl contains frequent centimetre- to decimetre-scale sandy-silty detrital layers. During the early to mid Holocene, these small-scale detrital layers, reflecting sediment supply by extreme surface runoff events, reveal a distinct centennial-scale recurrence pattern. This is in accordance with regional lake-level highstands and minima in solar activity and thus apparently mainly climate-controlled. After c. 4200 cal. yr BP, intervals of high detrital flux occasionally also correlate with periods of enhanced human settlement activity. In consequence, deposition of small-scale detrital layers during the late Holocene apparently reflects a rather complex interplay between climatic and anthropogenic influences on catchment erosion processes. Besides the small-scale detrital layers, five up to 2.40 m thick large-scale detrital event layers, composed of basal mass-wasting deposits overlain by large-scale turbidites, were identified, which are supposed to be triggered by strong earthquakes. The uppermost large-scale event layer can be correlated to a documented Mw=6.0 earthquake in ad 1222 in Brescia. The four other large-scale event layers are supposed to correspond to previously undocumented local earthquakes. These occurred around 350 bc, 570 bc, 2540 bc and 6210 bc and most probably also reached magnitudes in the order of Mw = 5.0-6.5

Tracking Holocene glacial and high-altitude alpine environments fluctuations from minerogenic and organic markers in proglacial lake sediments (Lake Blanc Huez, Western French Alps)

“…The acoustic signature of this deposit together with the successive sedimentary facies identified on core are typical of a subaqueous mass wasting deposit (MWD, figure 2) in clastic alpine lakes having steep slopes (Chapron et al, 2004;Schnellmann et al, 2006;Fanetti et al, 2008;Lauterbach et al, 2012). This MWD is identified along the southern slope and in the axis of the main sub-basin where it thins toward the North (figure 2), suggesting that its main sediment source was originating from the southern slope of the basin.…”

“…Bourget dated between 9400 and 9700 cal BP and attributed to a large paleo-earthquake (van Rensbergen et al, 1999;Chapron et al, 1996Chapron et al, , 2004Debret et al, 2010;Arnaud et al, 2012). Following the classification of Mulder and Cochonat (1996), SE 1 and SE 11 are interpreted as slide deposits, while SE 9 and SE 10 are graded beds typical from thin distal turbidites.…”

“…It is covering most of the Holocene and is established based (i) on a corrected composite depth with the thickness of the SEs being subtracted (figure 6B), since these event layers correspond to instantaneous events (cf. Lauterbach et al, 2012) and (ii) on eight chronological markers: two AMS 14 C dates on macroremains, three AMS 14 C dates on bulk sediment corrected from the occurrence of fossil organic matter, two radiocarbon dates from the nearby archaeological site correlated with Ag/Sr peaks tracking former local mining activities, and the stratigraphic correlation of SE 11 with the slump deposit from core LBH03 dated by radionuclide and related to the impact of the AD1962 Corrençon earthquake located at 47 km from LBH . This age model ( figure 6D) allows estimating the chronology of SEs (table 2) and is comforted by the correlation of SE 10 with the AD1881 Allemond earthquake (Nomade et al, 2005;Guyard et al, 2007;Wilhelm et al, 2011) located at 6 km from LBH and SE 9 either with the largest regional earthquake located next to Lake Le Bourget at 80 km from LBH in AD1822 (the Chautagne event, Chapron et al, 1999;Guyard et al, 2007), or with a smaller but local event in AD1782 (the Uriage earthquake; Nomade et al, 2005;Wilhelm et al, 2011) located at 20 km from LBH ( figure 1B) .…”

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

“…Among them, lake sediments have shown to be valuable archives as they record past events in a continuous and high-resolution mode (e.g. Lauterbach et al, 2012;Wilhelm et al, 2012a;Strasser et al, 2013;Wirth et al, 2013;Amman et al, 2015;Van Daele et al, 2015). The greater hydraulic energy of flooded rivers increases their capacity to erode and transport sediments.…”

“…return period of ∼ 35 years) suggests a high sensitivity of Lago Inferiore de Laures to earthquake shaking, as such a high frequency of earthquake-induced deposits has rarely been observed in the region (e.g. Guyard et al, 2007;Lauterbach et al, 2012;Simonneau et al, 2013;Strasser et al, 2013;Kremer et al, 2015;Chapron et al, 2016;Wilhelm et al, 2016b). All historical earthquakes are plotted in a "distance vs. epicentral MSK intensity" diagram (e.g.…”