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

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 (“Totes Gebirge”). 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.

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  and are further interpreted based on observations on  subbottom profiling data and high-resolution multibeam bathymetry.

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.

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  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.

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.