Seismic and sedimentological characterization of the post-eruption infill of the Laacher See caldera lake in Germany

The East Eifel Volcanic Field (EEVF) in the west of Germany has received increased scientific attention in recent years due to new findings on ongoing deep magma-related seismicity and regional uplift. The most recent volcanic event in the EEVF was the eruption of the Laacher See Volcano around 13 ka BP, which is well-studied in central Europe due to the Laacher See Tephra frequently being used as a regional chronostratigraphic marker. This eruption event featured several phreatomagmatic and Plinian phases, as well as vent migration and magma chamber collapse, resulting in the formation of a caldera which subsequently filled with groundwater to form a lake. Present-day activity of the Laacher See system is evidenced by degassing vents in the lake and along its shores, emitting CO₂ of magmatic origin. Although the Laacher See Eruption has been the topic of many studies in the past, the structure and infill of the caldera has not yet been fully documented. During two surveys at Laacher See in 2019 and 2021, several types of geophysical data were collected to investigate the lake floor and subsurface. High-resolution seismic reflection profiles were acquired with different acoustic sources, using different frequencies. These profiles were used to construct a seismic stratigraphy of the lake's sedimentary infill. Additionally, a multibeam echosounder was used to produce a high-resolution bathymetric map of the lake floor. Our results show two vent-shaped subbasins within the caldera depression, which are mostly filled with acoustically laminated sediment, reaching a thickness of at least 50 m in the northernmost subbasin. Several stratigraphic units can be distinguished, which are not always evenly distributed across the different subbasins, pointing at different phases of basin infill. In the central part of the lake, we identify a large gas accumulation zone through enhanced reflections and acoustic blanking, preventing visualization of the basin infill and structure underneath. Along the slopes of the lake, the lake sediments often contain mass-transport deposits, occurring along at least 9 different stratigraphic horizons. In order to reconstruct the sedimentation history of the lake since the 13 ka BP eruption, a total of 4 sediment cores were taken during coring surveys in 2019 and 2023, with recoveries between ~3.5 and ~8.5 meter below the lake floor. These sediment cores will be integrated with the reflection seismic data to further characterize different phases of sedimentation in the lake. The acquired data shows promising results that will help to reconstruct the sedimentary evolution of Laacher See since its eruption and aid in a better understanding of the caldera formation and structure, and its sedimentary infill history.