Studying the Significance of Subaqueous Slides in Lake Hallstatt. (UNESCO World Heritage Cultural Landscape, Austria). S4LIDE - Hallstatt

This ÖAW S 4 LIDE-Hallstatt project was conducted within the broader context of the IUGS-UNESCO International Geoscience Programme (IGCP) Project 640 -S 4 LIDE: "Significance of Modern and Ancient Submarine Slope LandSLIDEs" and piggy-backed the Hipercorig-Hallstatt-History (H 3 ) Drilling Project that logged and sampled Lake Hallstatt´s complete Late Pleistocene-to-Holocene stratigraphic succession with a new innovative hydraulic coring system.With a dedicated focus on subaquatic landslide deposits we aimed at systematic investigation of mass transport deposits (MTD) embedded in the sedimentary sequence of Lake Hallstatt by state-of-the-art reflection seismic surveys, borehole logging and core scanning and dating techniques.We achieved establishment of an unprecedented long-term (spanning >11.500 years), high-resolution core-log-seismic-integrated event stratigraphy record of subaqueous mass movements in this intra-mountainous lake characterized by clastic input and very high sedimentation rates.Considering Lake Hallstatt as "natural laboratory" representative for other high-sedimentation rate environments, the new well-dated long-term record of subaquatic landslides will be added to the global datasets contributing to and continuing the international research efforts to address some of the most pressing research questions on geohazard, environmental implication and economic significance of submarine landslides, as tackled by the broad international research community within the IGCP-640 S4LIDE project.Main results and achievements of this ÖAW S 4 LIDE-Hallstatt project are (1) a detailed characterization and dating of 10 (up to 5.1 m thick) MTDs emplaced within different Holocene lithostratigraphic units, that evidence recurrent natural hazards at the UNESCO World Heritage site;(2) advanced understanding of emplacement mechanism and provenance of two large-scale mass movements recorded in the sediments of Lake Hallstatt, one of which occurred in the mid-4 th century BCE likely coinciding (and is thus hypothesized to be related) with the abandonment of the famous Iron Age cemetery of the prehistoric salt-mining culture in Hallstatt high valley; (3) establishing the initial high-resolution physical-and element chemical proxy dataset for the entire H 3 core and providing proof-of-concept results as basis for follow up research projects beyond subaquatic massmovement objectives (4) submission and publication of above results 1, 2 and 3 as original peerreviewed papers lead-authored by project members (Ortler et al., in review; Lauterbach et al., 2023); (5) preparation and submission of project plans as grant proposals for follow-up research projects (Ortler, subm.; Kowarik et al., in prep); and (6) several public outreach activities reaching a wide range of nation and international audiences to communicate about the science and ÖAW research project.Graphical Abstracts: Detailed representative core sections showing photographic assemblages (computed tomography (CT), smart cube and ITRAX optical and radiodensity scanner for each major event deposits resulting from subaquatic massmovement throughout the Holocene in Lake Hallstatt (Figure after Ortler et al., in review) 1) SUMMARY OF SCIENTIFIC CONTEXT AND ADDRESSED RESEARCH OBJECTIVESIt is widely recognized that subaqueous landslides are of common occurrence in lacustrine and marine environments (Mountjoy et al., 2020 1 ).However, knowledge is limited about the recurrence periods for landslides, how they cluster in space and time, and how different driving factors can affect this timing.It is also still problematic to link potential causal mechanisms to specific events because it is known that subaqueous landslides can be initiated by a wide range of triggers.Thus, key fundamental and societal-relevant scientific questions on causes and consequences of subaquatic landslides cannot yet conclusively be answered.The lack of well-calibrated diagnostic features in the stratigraphic record and the limited knowledge from only few, well-dated long-term landslidestratigraphic records has negatively impacted our predictive capability, which remains an outstanding challenge for the geoscience community.The IGCP Project 640 -S 4 LIDE was established as international and multidisciplinary platform allowing geoscientists from academia and industry to sustain a dialogue conducive to the integration of findings resulting from research and development efforts into a more cohesive understanding of subaquatic landslides (Moscardelli et al., 2014).Limnogeological research on mass movements in lakes revealed that general characteristics of Mass Transport Deposits (MTDs), as well as their underlying transport and initiation processes are often comparable to those described in the classical submarine landslide literature (e.g.Sammartini et al., 2020).Given that lakes have well-constrained boundary conditions, smaller sizes and offer the possibility to be investigated on a complete basin-wide scale, studying mass movements in lacustrine environments offers a series of advantages.Coring from mobile platforms at comparably low logistical costs can reach down to critical depths, crossing gliding surfaces and sampling multiple-stacked MTD successions.This allows a complete sampling, characterization and dating of MTDs within evolving stratigraphic sequences, and can provide a solid data base for constraining frequency and magnitude relation on relevant timescales, as key requirement for testing mass transport genesis models.The ÖAW ESS-IGCP S 4 LIDE-Hallstatt project investigated Lake Hallstatt´s complete stratigraphic succession imaged by reflection seismic surveys and sampled and logged by a new innovative hydraulic coring and borehole logging system (Harms et al., 2020; Ortler et al., in review) to investigate subaquatic landslides and the event stratigraphy in this steeply-incised intra-mountainous lake with very high sedimentation rates.The aim was to integrate data from previous short coring campaigns (Lauterbach et al., 2017;Strasser et al., 2020) with new data to be elaborated from the deep drilling to establishing an unprecedented long-term high-resolution record of subaqueous mass movements.Furthermore, provided by the setting of Lake Hallstatt within the UNESCO World Heritage Cultural Landscape Hallstatt-Dachstein/Salzkammergut, which is famous for its well-documented medieval and prehistoric human settlement and mining history, the project also aimed at producing ultra-high resolution data and proof-of-concept results that eventually will allow for deciphering the sedimentary record for natural vs. anthropogenic causes of landslides and to quantify system perturbation effects from and resiliencies to the occurrence of extreme event.The main objectives were: Objective O-1: Apply state-of-the-art high-resolution core logging, scanning and analyses methodology to identify and systematically characterize different types of mass-movement event beds in the new Late Glacial-to-Holocene long record of Lake Hallstatt.Objective O-2: Establish detailed facies characterization of event deposits for advanced interpretation of transport, emplacement and deposition processes to infer source and driving mechanisms of subaquatic mass movements in Lake Hallstatt.Objective O-3: Date and establish quantitative constraints on magnitude-frequency relation of the different MTDs in the Lake Hallstatt long record.Objective O-4: Characterize and constrain physical and chemical proxies of "background" sediments immediately overlying MTDs and constrain source provenance, emplacement processes and explore the on-and-offshore system responses after the occurrence of extreme events that lead to MTDs.1 References by S 4 LIDE-Hallstatt collaborators and directly resulting from this project are marked in italics and bold.

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