The Holocene sediment record of Lake Altaussee (Salzkammergut, Austria): A perspective on mass movements, lake level change and varying karst spring activity in an inner-Alpine setting

<p>Inner-Alpine lakes typically present a complex sedimentary record, with changing sediment dynamics and event deposits related to different natural hazards. Lake Altaussee (712 m asl; 2.4 x 1.0 km; max. 72 m deep) is a moderately sized lake located within the Northern Calcareous Alps. It is mainly fed by several subaqueous springs associated to a regional karst plateau (&#8220;Totes Gebirge&#8221;). This spring activity produced several craters (up to 60 m in diameter and 22 m deep) on the lake bottom. Within the framework of the Walter Munk Foundation for the Oceans (WMFO), a multidisciplinary research effort is undertaken to understand the lake system.</p><p>Here we present first results based on sedimentological and geochemical analysis (XRF core scanning) of four long sediment cores (3-9 m long) collected in summer 2021. The cores were taken in different depositional environments, such as the deep central basin, a shallow plateau in the western part and on the outer slopes of the largest karst crater. The sediment cores were dated by ¹⁴C &#160;and are further interpreted based on observations on &#160;subbottom profiling data and high-resolution multibeam bathymetry.</p><p>The deep basin core (9 m long; 52 m water depth) reveals a ~2.5 m thick megaturbidite (MT) characterized by an overall normal grading and a thin fine-grained cap layer. The MT is overlying a ~1.3 m thick mass-transport deposit consisting of a mixture of remobilized organic-rich lake-internal sediment and coarser cm-scale pebbles. Morphologic and seismostratigraphic mapping indicate that this megaturbidite was formed by massive sediment remobilization due to multiple (synchronous?) rockfalls and/or sudden remobilization of accumulated slope deposits impacting the eastern part of the lake basin around 724-931 CE.</p><p>This event deposit overlies several brighter-colored, rather homogenous units, which are separated by a 20 cm thick dark organic-rich laminated interval. Preliminary dating of these lower units to about 8-10 kyrs BP suggest the existence of a hiatus of several millennia between the MT and the lower units. To verify whether this is caused by erosion related to the mass-transport event or a period of non-deposition, we analyzed a long core (6 m long; 22 m water depth) on the western plateau. This core exhibits a Late Glacial clastic varve sequence (~14-15 kyr BP), overlain by &#160;a unit of poorly-sorted debrite(s) and an organic-rich lacustrine sediment sequence deposited during the past ~1.2 kyr which can be traced in all short cores and seismic profiles throughout the lake. Altogether these observations suggest highstand conditions during Late Glacial times, a significantly lower lake level during large parts of the Holocene, and again highstand conditions during the past 1.2 kyrs.</p><p>The identification and stratigraphic position of numerous clastic outflow deposits on the outer slope of the largest karst crater hint at a sudden onset or intensification of spring activity in the Late Holocene at this location. Whether this can have contributed to the hypothesized lake level rise in this inner-alpine basin remains unclear and forms the focus of ongoing multidisciplinary investigations.</p>