A new Late Holocene sediment record from Lake Grundlsee (Austria): Regional hazard signals and land use history in the Salzkammergut lake district

The lakes of the Salzkammergut region in the Northern Calcareous Alps (Austria) preserve valuable archives of natural hazards and human-environment interactions. Event records from Lake Hallstatt and Lake Altaussee have documented floods, mass movements, and phases of intensified land use over the past millennia – the latter linked to increased catchment erosion and, at Lake Altaussee, to medieval river diversions. However, questions remain regarding the regional coherence of these signals and the causes of pronounced sediment hiatuses observed in some basin sequences. Here, we present the first sediment core analysis from Grundlsee (4.22 km²), contributing a new record to this regional framework.

A total of 18 sediment cores (up to 1.5 m length) were retrieved in November 2025 using a hammer-driven gravity corer. Computed tomography (CT) reveals distinct density variations and event layers throughout the cores. Elevated density values in the upper core sections, estimated to span several centuries, suggest increased catchment erosion potentially linked to medieval land use intensification – a signal also recognized in the neighboring lakes during the 12^th to 17^th centuries. Several discrete high-density layers likely represent flood or mass movement deposits. Radiocarbon dating and subsequent BACON age-depth modelling, complemented by XRF core scanning, will establish a chronological framework and enable correlation with historically documented events.

Sub-bottom profiler data reveal a complex basin architecture. On a western plateau (~9 m water depth) near the lake outlet, Holocene sediment cover is thin (locally <0.5 m), and is separated by an erosional unconformity from an underlying thick sequence of Late Glacial deposits – indicating a significant hiatus. A comparable stratigraphic gap is observed on a shallow platform (~20 m water depth) in Altausseer See, where early to mid-Holocene sediments are absent – a thick debris flow unit directly overlies Late Glacial deposits, with only ~1200 years of Late Holocene sediments preserved above. These hiatuses suggest a regional phenomenon, possibly reflecting periods of lower lake levels relative to both present-day and Late Glacial conditions. Additionally, the sub-bottom profiler data image numerous subaqueous landslides and associated turbidite deposits traceable across the basin floor, indicating recurrent mass movement activity.

By integrating the Grundlsee sediment record into the existing network of studied Salzkammergut lake records, this work aims to improve regional hazard reconstructions and identify common drivers of environmental change in the Late Holocene.