Rossberg landslide history and flood chronology as recorded in Lake Lauerz sediments (Central Switzerland)

The southern slopes of Rossberg mountain, Central Switzerland, on which one of the largest historic landslides of the Alpine region was released in 1806 ad (Goldauer Bergsturz), are prone to large-scale mass wasting processes. This has led to numerous sliding events, which are well-recognizable in the modern topography but lack accurate dating. In order to provide new insights into the timing and the processes associated with past landslides as well as into the frequency of exceptional flood events, long sediment cores were retrieved from the subsurface of Lake Lauerz that lies in the pathway of these landslides and that records strong runoff events with typical flood layers. Analyses of the recovered cores display a sedimentologic succession with variable fingerprints of past landslides and flood events, depending on the coring location within the lake. The landslide signature can be calibrated using the 1806 ad event: An organic-rich peaty unit, which is found in two cores located close to the rockmass impact, points towards a sudden, gravity spreading-induced lateral displacement of the swampy plain where parts of the rock mass were accumulating. This rapid lateral mobilization of soft sediments, and not the rock masses, acted as ultimate trigger for the reported ~15m-high impulse waves on the lake. In the more distal areas, the 1806 ad event led to the deposition of a thick, organic-rich redeposited layer. The 10m-long core from the distal basin covers a radiocarbon-dated ~2,000years sedimentation history and contains a highly similar event layer that was deposited in 810±60 ad. This layer is most likely the product of a major historic landslide, known as Röthener Bergsturz, which, based on scarce historical reports, was commonly dated to 1222 ad. In the 2,000years record, we identify three periods with enhanced occurrence of flood turbidites dated to 580-850 ad, 990-1420 ad, and 1630-1940 ad. Among the 54 detected flood layers, 6 probably mark exceptionally heavy rainfall events that are dated to ~610, ~1160, ~1290, ~1660, ~1850, and ~1876 ad, the latter being associated to one of the most intense rainfall events ever recorded instrumentally in the regio

Mapping basin-wide subaquatic slope failure susceptibility as a tool to assess regional seismic and tsunami hazards

This study of subaquatic slope failures in Lake Lucerne, central Switzerland, presents a new concept for evaluating basin-wide slope stability through time as a potential tool for regional seismic and tsunami hazard assessments. Previously acquired high-resolution bathymetry and reflection seismic data, as well as sedimentological and in situ geotechnical data, provide a comprehensive data base to use this lake as a "model basin” to investigate subaquatic landslides and related geohazards. Available data are implemented into a basin-wide slope model. In a Geographic Information System (GIS)-framework, a pseudo-static limit equilibrium infinite slope stability equation is solved for each model point representing reconstructed slope conditions at different times in the past, during which earthquake-triggered landslides occurred. Comparison of reconstructed critical stability conditions with the known distribution of landslide deposits reveals minimum and maximum threshold conditions for slopes that failed or remained stable, respectively. The resulting correlations reveal good agreements and suggest that the slope stability model generally succeeds in reproducing past events. The basin-wide mapping of subaquatic slope failure susceptibility through time thus can also be considered as a promising paleoseismologic tool. Furthermore, it can be used to assess the present-day slope failure susceptibility, allowing for identification of location and estimation of size of future, potentially tsunamigenic subaquatic landslide

Sedimentological and stratigraphic framework of the several hundred thousand years old lacustrine record from Lake Van, Turkey

Within the frame of the International Continental scientific Drilling Program (ICDP) project PALEOVAN, a long and continuous sediment record from Lake Van, a closed lake situated in a climatically sensitive semiarid and tectonically active region in Eastern Anatolia, has been drilled in summer 2010. At two sites, Ahlat Ridge and Northern Basin, sedimentary records of 220 and 140 m were recovered, respectively. With basal ages possibly around 500'000 years, these records span several glacial-interglacial cycles and reach back until the lake’s initial transgression in the Middle Pleistocene. First results from ongoing analysis of core-catcher samples and newly opened cores document the sedimentological and geochemical succession. Two composite profiles of the drill sites were defined. Core catcher-based geochemical data such as proxies of lake’s productivity and catchment alterations show large variations and reflect a rich paleoenvironmental history. Most of the 220 m thick succession consists of carbonate mud, mostly sub-mm-thick laminated and interbedded by either homogenous mud or pyroclastic cm-thick layers. The lowermost sediments from the Ahlat Ridge site represent the initial lake transition as the drilling could not penetrate further and the seismic data indicates coincidence with the ‘acoustic’ basement. Such an early transgressive state of the lake’s history is also supported by the lithology consisting of a gravel unit as an indicator of a beach-like environment, which is overlain by sand deposits containing fresh-water gastropods (Bithynia). Above 200 mblf, the laminated mud clearly indicates that the lake was already deep enough to form anoxic bottom water as the laminations were preserved. This unique paleoclimate archive indicates that great changes of the depositional conditions occurred that hint to a fascinating evolution of the environment and has ideal prerequisites for the investigation of the Quaternary climate evolution in the Near East

15,000 Years of mass-movement history in Lake Lucerne: Implications for seismic and tsunami hazards

Abstract.: A chronological catalogue of Late Glacial and Holocene mass-movement deposits in Chrüztrichter and Vitznau Basins of Lake Lucerne (Vierwaldstättersee) reveals a complex history of natural hazards affecting the lake and its shores. Ninety-one mass-flow and six megaturbidite deposits have been identified and mapped out with a grid of more than 300 km high-resolution seismic profiles. An age model based on the analyses of 10 long piston cores, 2 tephra layers and 32 AMS-14C ages allowed building up a chronological event catalogue covering the last 15,000 years. Most of the identified mass-flow deposits relate to subaqueous sliding and subaerial rockfalling, as indicated by slide scars and rockfall cones on the lake floor. Deposits related to subaqueous sliding occur at the toe of slopes with inclinations >10° and reach their largest extents below slopes with inclinations between 10 and 20°. On subaqueous slopes >25° there is hardly any sediment accumulation. Rockfall cones and related mass-flow deposits in the northeastern and southern part of Vitznau basin evidence repeated rockfall activity, particularly along two zones on the northern face of Bürgenstock Mountain. Historic examples indicate that such rockfalls, as well as large subaqueous slides, can induce considerable tsunami waves in the lake. All of the 91 identified mass-flow deposits are associated with 19 seismic- stratigraphic event horizons. One of them comprises 13 mass-flow deposits and 2 megaturbidites and relates to the 1601 A.D. Mw ~ 6.2 earthquake. By analogy, five older multiple mass-movement horizons, each including six or more coeval deposits, are interpreted to result from strong prehistoric earthquake

Environmental History of Lago di Tovel, Trento, Italy, Revealed by Sediment Cores and 3.5kHz Seismic Mapping

Proxy data from a total of 30 sediment cores and information from a seismic survey show that the sedimentological and limnological history of Lago di Tovel (1178m a.s.l.) has been significantly influenced by slope dynamics of its mountainous catchment. The lake represents a dead-ice lake with pro-glacial deposits at the base of its sedimentary record. A prominent lake level rise in 1597/1598 that increased maximum water depth from ∼20 to 39m caused slope instabilities, leading to the deposition of mass-flow sediments with a maximum thickness of 2.5m in the northern part of the lake and less than 50cm in the southern part, resulting in a total volume of more than 113,000m3. Consequently, a rough lake bottom morphology was produced, which led to distinct differences in sedimentation rates of 0.07cmyr−1 on sills and 0.18cmyr−1 within depressions. The age of the top of the mass-flow deposits was used to validate the ages of the younger, laminated sediments, which were dated by 210Pb and 137Cs. Lithological investigations showed that the sediments below the mass-flow deposits are also laminated and that they were not bioturbated. The long-term meromixis of Lago di Tovel is therefore mainly due to a combination of its topographic setting and the 5-month period of ice cover. Both prevent effective mixing of the lake by strong winds during spring and autumn. Distinct spatial differences in sediment distribution within the lake show that it is risky to interpret proxy data from only one coring site, even if the lake is very small. This is especially true in mountainous areas, where rock falls, mass movements, and slope instabilities of a significant size may have considerable effects on lake

The last 1300years of environmental history recorded in the sediments of Lake Sils (Engadine, Switzerland)

Abstract.: Proglacial Lake Sils is located in the Upper Engadine (south-eastern Swiss Alps) at 1800 m a.s.l. and is the uppermost of four lakes in the valley. A highresolution seismic survey combined with geophysical and chemical data of 22 short sediment cores, and 137Cs, 210Pb, and 14C AMS dating provide insight into the sedimentological development of the lake during the last 1300years. The deposits consist of diffusely laminated clayey silts. In contrast to nearby Lake Silvaplana, no varves could be detected. Sedimentation rates are on average 1.1mm per year (40mg cm−2a−1) which is much lower than in the adjacent Engadine lakes. The most prominent sediment feature is a turbidite that was deposited around cal AD 700, has a thickness of up to more than 6 metres and a total estimated volume of 6.5*106 m3, which is more than the total cumulated sediment mass deposited since that time (4.5 * 106 m3). The sediments deposited after around AD 1880 show higher contents of organic carbon (Corg) and biogenic silica (bSi), which suggests enhanced primary production due to increasing tourism in the area and subsequent higher nutrient supply to the lake. Sediments with distinctly lower Corg and bSi concentrations, but with larger grain-size medians and higher mica concentrations were accumulated between AD 1500 and 1880. These features are related to a late period of the ‘Little Ice Age' when major regional glacier advances occurre

Impact of an 0.2 km 3 Rock Avalanche on Lake Eibsee (Bavarian Alps, Germany) – Part II: Catchment Response to Consecutive Debris Avalanche and Debris Flow

The ~0.2 km3 Eibsee rock avalanche impacted Paleolake Eibsee and completely displaced its waters. This study anal- yses the lake impact and the consequences, and the catchment response to the landslide. A quasi‐3D seismic reflection survey, four sediment cores from modern Lake Eibsee, reaching far down into the rock avalanche mass, nine radiocarbon ages, and geomorphic analysis allow us to distinguish the main rock avalanche event from a secondary debris avalanche and debris flow. The highly flu- idized debris avalanche formed a megaturbidite and multiple swashes that are recorded in the lake sediments. The new calibrated age for the Eibsee rock avalanche of ~4080–3970 cal yr BP indicates a coincidence with rockslides in the Fernpass cluster and sub- aquatic landslides in Lake Piburg and Lake Plansee, and raises the possibility that a large regional earthquake triggered these events. We document a complex history of erosion and sedimentation in Lake Eibsee, and demonstrate how the catchment response and rebirth of the lake are revealed through the complementary application of geophysics, sedimentology, radiocarbon dating, and geo- morphology