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

Impact of an 0.2 km3 Rock Avalanche on Lake Eibsee (Bavarian Alps, Germany) – Part I: Reconstruction of the paleolake and Effects of the Impact

Rock avalanches destroy and reshape landscapes in only a few minutes and are among the most hazardous processes on Earth. The surface morphology of rock avalanche deposits and the interaction with the underlying material are crucial for runout properties and reach. Water within the travel path is displaced, producing large impact waves and reducing friction, leading to long runouts. We hypothesize that the 0.2 km3 Holocene Eibsee rock avalanche from Mount Zugspitze in the Bavarian Alps overran and destroyed Paleolake Eibsee and left a unique sedimentological legacy of processes active during the landslide. We captured 9.5 km of electrical resistivity tomography (ERT) profiles across the rock avalanche deposits, with up to 120 m penetration depth and more than 34 000 datum points. The ERT profiles reveal up to ~50 m thick landslide debris, locally covering up to ~30 m of rock debris with entrained fine‐grained sediments on top of isolated remnants of decametre‐wide paleolake sediments. The ERT profiles allow us to infer processes involved in the interaction of the rock avalanche with bedrock, lake sediments, and morainal sediments, including shearing, bulging, and bulldozing. Complementary data from drilling, a gravel pit exposure, laboratory tests, and geomorphic features were used for ERT calibration. Sediments overrun by the rock avalanche show water‐escape structures. Based on all of these datasets, we reconstructed both position and size of the paleolake prior to the catastrophic event. Our reconstruction of the event contributes to process an understanding of the rock avalanche and future modelling and hazard assessment. Here we show how integrated geomorphic, geophysical, and sedimentological approaches can provide detailed insights into the impact of a rock avalanche on a lake. © 2020 The Authors. Earth Surface Processes and Landforms published by John Wiley & Sons LtdThe Eibsee rock avalanche detached from Mount Zugspitze and impacted and destroyed Paleolake Eibsee. Paleolake Eibsee was larger than modern Lake Eibsee; the rock avalanche deposit covers the northern half of the paleolake. The complementary application of geomorphology, electrical resistivity tomography (ERT) and sedimentology allows for ERT calibration at seven different sites, where materials (rock avalanche, bedrock, lake clay, mixed sediments) and effects of the impact (bulldozing, bulging, overriding of secondary lobes, splashing of boulders) can be distinguished.Studienstiftung des Deutschen Volkes e.V. (German National Academic Foundation): http://dx.doi.org/10.13039/50110000435

Impact of an 0.2 km3 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 analyses 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 fluidized 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 subaquatic 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 geomorphology. © 2020 The Authors. Earth Surface Processes and Landforms published by John Wiley & Sons LtdSedimentological, geophysical and geomorphological investigation in and around Lake Eibsee allows to decipher three rock‐slope failures from Mount Zugspitze: (i) the Eibsee rock avalanche ~4000 cal yr BP; (ii) a debris avalanche in the aftermath; and (iii) a large debris flow ~3740 cal yr BP. The Eibsee rock avalanche was re‐dated to a refined age of 4089–3876 cal yr BP. The coincidence with major events in the Fernpass rockslide cluster increases the likelihood of a prehistoric earthquake trigger.Studienstiftung des Deutschen Volkes e.V. (German National Academic Foundation): http://dx.doi.org/10.13039/501100004350British Society for Geomorphology http://dx.doi.org/10.13039/10001016

Impact of climatic extremes on Alpine ecosystems during MIS 3

The effects of climatic extremes on Alpine ecosystems, such as in the last glacial period, are poorly understood. The recently discovered Nesseltalgraben site is currently the best dated and most complete high-resolution sedimentary sequence in the northern Alps covering the Marine Isotope Stage (MIS) 3 around 59-28 calibrated kiloyears before present (ka cal BP). The MIS 3 is a period with frequent climatic fluctuations known as Dansgaard-Oeschger cycles or Greenland interstadials-stadials. Here, we present pollen assemblages, bryophyte macrofossils, and stable isotopes (δ2H, δ13C, δ18O) from the Nesseltalgraben to elucidate palaeoenvironments and palaeoclimatic variability during that period. In addition to bulk sedimentary cellulose, also cellulose extracted from fossil wood, monocots, and bryophytes was analysed isotopically. Among the terrestrial pollen, Poaceae and arboreal pollen show an antithetic behaviour reflecting interstadial-stadial variations. Arboreal pollen are dominated by Pinus sylvestris-type, with admixtures of Picea, Betula, Alnus, and Salix. The arboreal pollen record exhibits several maxima indicating milder climatic conditions tentatively assigned to Greenland interstadials 17–14, 12/11, 8, and 6. During Heinrich events 5 and 4, the arboreal pollen record shows distinct minima underlining a severe impact of these events on regional climate and vegetation. Bryophyte assemblages show predominant wetland conditions at the site during the entire MIS 3. The sudden occurrence of the bryophyte Drepanocladus turgescens after 31.6 ka cal BP indicates a change from a fen to a frequently drying wetland environment. This habitat change is presumably linked to enhanced glaciofluvial action caused by glaciers approaching the site. Carbon, hydrogen, and oxygen stable isotope records of bulk sedimentary cellulose exhibit comparably stable conditions from 59 until 52 ka cal BP and increased values around 51 ka cal BP followed by a period of almost absent cellulose until 39 ka cal BP. Thereafter, and lasting until 30 ka cal BP, bulk sedimentary cellulose isotope records (δ2H, δ13C, and δ18O) reveal strongly fluctuating values. These isotope variations are interpreted as variable mixtures between terrestrial lignified plants and monocots on the one, and wetland bryophyte sources on the other hand. A strong negative isotope excursion in the bulk sedimentary and the bryophyte cellulose records around 36.4 ka cal BP is contemporaneous with maximum Cyperaceae pollen and best explained by progressively waterlogged soils due to permafrost. The rise in the bryophyte δ18O record thereafter, accompanied by likewise increasing δ13C values, most likely indicates an enhanced evaporation of source waters

Geochemistry, grain size and radiocarbon ages of MIS 3 sediment record from Nesseltalgraben, Germany

Continuous sediment profiles were taken from ravine slopes at the Nesseltalgraben site in the Northern Calcareous Alps (SE Germany, 47.6567°N 13.0467°E, 560-582 m a.s.l.) in October 2016. The profile consists of fine-grained lacustrine-palustrine sediments overlain by several metres of glacifluvial gravels and lodgement tills of the Last Glacial Maximum and underlain by a diamicton. High-resolution (2 mm steps) element counts (Ca, S, Si, K, Ti, Mn, Fe, Zn, Rb, Sr, Zr) were obtained with an XRF core scanner (Itrax, Cox Analytical Systems, Sweden). Organic geochemistry (total organic and inorganic carbon, total nitrogen, total sulphur) was analysed with an elemental analyser (Euro EA, Eurovector, Germany), grain size with a laser diffractometer (Beckman-Coulter LS 200). The sediment profiles were compiled to a composite record of 21 m length. The age model bases on 29 radiocarbon analyses of macroscopic terrestrial plant remains (byrophytes, plant debris, monocots, wood, and twigs) and a previously discovered paleomagnetic anomaly assigned to the Laschamp event. The age model covers the period 59 to 29.6 ka cal BP and assigns the record to Marine Isotope Stage (MIS) 3. The sediment record shows rapid changes in lithology, sedimentology, and geochemistry related to Dansgaard-Oeschger climatic events