Mass-movement and flood-induced deposits in Lake Ledro, southern Alps, Italy: implications for Holocene palaeohydrology and natural hazards

Abstract

High-resolution seismic profiles and sediment cores from Lake Ledro combined with soil and riverbed samples from the lake's catchment area are used to assess the recurrence of natural hazards (earthquakes and flood events) in the southern Italian Alps during the Holocene. Two well-developed deltas and a flat central basin are identified on seismic profiles in Lake Ledro. Lake sediments have been finely laminated in the basin since 9000 cal. yr BP and frequently interrupted by two types of sedimentary events (SEs): light-coloured massive layers and dark-coloured graded beds. Optical analysis (quantitative organic petrography) of the organic matter present in soil, riverbed and lacustrine samples together with lake sediment bulk density and grain-size analysis illustrate that light-coloured layers consist of a mixture of lacustrine sediments and mainly contain algal particles similar to the ones observed in background sediments. Light-coloured layers thicker than 1.5 cm in the main basin of Lake Ledro are synchronous to numerous coeval mass-wasting deposits remoulding the slopes of the basin. They are interpreted as subaquatic mass-movements triggered by historical and pre-historical regional earthquakes dated to AD 2005, AD 1891, AD 1045 and 1260, 2545, 2595, 3350, 3815, 4740, 7190, 9185 and 11 495 cal. yr BP. Dark-coloured SEs develop high-amplitude reflections in front of the deltas and in the deep central basin. These beds are mainly made of terrestrial organic matter (soils and lignocellulosic debris) and are interpreted as resulting from intense hyperpycnal flood event. Mapping and quantifying the amount of soil material accumulated in the Holocene hyperpycnal flood deposits of the sequence allow estimating that the equivalent soil thickness eroded over the catchment area reached up to 5 mm during the largest Holocene flood events. Such significant soil erosion is interpreted as resulting from the combination of heavy rainfall and snowmelt. The recurrence of flash flood events during the Holocene was, however, not high enough to affect pedogenesis processes and highlight several wet regional periods during the Holocene. The Holocene period is divided into four phases of environmental evolution. Over the first half of the Holocene, a progressive stabilization of the soils present through the catchment of Lake Ledro was associated with a progressive reforestation of the area and only interrupted during the wet 8.2 event when the soil destabilization was particularly important. Lower soil erosion was recorded during the mid-Holocene climatic optimum (8000–4200 cal. yr BP) and associated with higher algal production. Between 4200 and 3100 cal. yr BP, both wetter climate and human activities within the drainage basin drastically increased soil erosion rates. Finally, from 3100 cal. yr BP to the present-day, data suggest increasing and changing human land use

Orbital changes, variation in solar activity and increased anthropogenic activities: controls on the Holocene flood frequency in the Lake Ledro area, Northern Italy

“…The grading pattern in combination with the elemental and organic composition observed in these deposits is interpreted as the typical signature of hyperpycnal flood deposits in a sub-aquatic basin (Mulder and Alexander, 2001; Gilli et al, 2013;Simonneau et al, 2013). Several hundred of these flood deposits are recorded within core LL08-1 and LL08-2 (Table 2).…”

“…These are the same particles that also characterise the laminated facies, indicating their origin in the lake's biological productivity. Thus, these light grey-brown homogeneous deposits are of lacustrine origin and may correspond to subaquatic mass-movements or masswasting deposits (MWD) reworking and mixing lacustrine sediments from the slopes and the shores (Simonneau et al, 2013). This second type of sedimentary event is indicated by a layer thickness ranging from 0.1 mm to 13 cm.…”

“…2a). The mean granulometry of these event layers is finer, better sorted and no graded (mean grain-size < 25 µm; Simonneau et al, 2013); therefore, petrographic observations show that this facies is mainly composed of organic matter of lacustrine origin (grey amorphous particles; Millet et al, 2007). These are the same particles that also characterise the laminated facies, indicating their origin in the lake's biological productivity.…”

“…2c) and varies the organic matter content. Laser grain-size analyses revealed that all these sedimentary events are graded (mean grain-size < 60 µm; detailed data are presented in Simonneau et al, 2013;Fig. 2c) and some of them contain an inverse (coarsening upward) grading at the base followed by normal (fining upward) grading (see event E; Fig.…”

“…This second type of event deposit is much less common than the dark-brown one; 11 and 13 deposits (of more than 1.5 cm thickness) have been identified, respectively, in LL08-1 and LL08-2 records, while several hundred flood event deposits have been observed. These MWDs were previously identified (Simonneau et al, 2013) and manually removed from the record before the image analysis.…”