Sedimentary Evidence of High Sublacustrine Groundwater Discharge and Subaqueous Landslide Events in Hintersteiner See, Austria

Sublacustrine spring activity in karstic lakes, including variations in discharge rates and patterns, has been shown in previous studies to correlate with both climate change and local neotectonic activity. This study aims at better understanding the influence of the interplay between karst groundwater dynamics and environmental drivers on sediment deposits in the small groundwater-fed lake Hintersteiner See (lake surface = 56.6 ha, max. depth = 39.78 m), located at the foothills of the mountain massif Wilder Kaiser in the Northern Calcareous Alps.

Recent multibeam bathymetry mapping has revealed multiple pockmarks and the lake´s sedimentary record is hypothesized to comprise a high-resolution record for paleo-environmental, paleo-hydrogeological and, possibly, paleoseismic activity.

Here, we present initial results from high-resolution reflection seismic, water column sampling and sediment coring surveys, along with subsequent multi-method stratigraphic, sedimentological and geochemical analyses, to investigate sedimentary evidence of high sublacustrine groundwater discharge and subaqueous landslides events in Hintersteiner See.

Mapped pockmarks are up to 8.5 m in diameter and 3 m deep and occur predominantly on the northwestern, more gently dipping and terraced slope, characterized by buried scarps and irregular bulges. A seismic profile across this pockmark evidences reflector truncation and interbedded high-amplitude reflection adjacent to the pockmark, indicating variable ground-water discharge activity eroding overlying lake sediments and possible sediment extrusion events. In the deep and flat depocenter of the lake seismic profiles indicate several stratigraphic horizons comprising Mass Transport Deposits (MTDs), some of which indicating that multiple simultaneous slope failures have occurred, possibly caused by past earthquakes.

Sediment cores recovered adjacent to the pockmarks, on both sides, show a 5 cm thick layer with increased density and magnetic susceptibility values, comprising a sharp coarser base and upward fining, and composed mainly of detrital carbonate silt. This layer stratigraphically correlates with the high-amplitude reflection in the seismic data and is interpreted to be genetically linked to an elevated groundwater discharge and sediment expulsion event in the pockmark. Ongoing analyses, including radiocarbon dating, aim to establish a precise temporal framework for the observed sedimentary events, to improve our understanding of high groundwater discharge and subsurface sediment mobilization processes and test links to environmental and tectonic drivers.