Volcanic degassing and risk implications at the dormant Laacher See volcano in the East Eifel, Germany

Dormant volcanic areas are defined by long periods of quiescence often exceeding 10 ka since the last eruption. Although these systems may appear inactive, a prolonged dormancy does not imply volcanic extinction, and this misconception can obscure the true eruptive hazard. The eruption of the Laacher See volcano represents the most recent volcanic activity in the East Eifel volcanic field (13 ka BP) and resulted in the formation of a caldera lake. The most prominent indicators of ongoing activity are the continuous release of gases in the form of mofettes and soil degassing, as well as strong gas seepages at various depths (8 – 52 m) within the lake, some of which reach the surface. Previous studies have shown that the gas consists of ≈ 99% CO₂ and that the gases originate from the upper mantle, as indicated by elevated helium isotope ratios (>5 Ra).

Although Laacher See is a holomictic lake, events such as earthquakes, landslides or a drop in the water level can cause a sudden release of gas stored at depth. This poses a particular risk from spring until late autumn when the lake is stratified and the region is visited by many tourists. For this reason, three CO₂ flux campaigns covering the entire lake surface have been conducted in summer (June/August 2024) and in winter (February 2025) to obtain an initial estimate about the total amount of CO₂ released by the lake. Results indicate that winter CO₂ emissions are an order of magnitude higher than those measured in summer, suggesting substantial accumulation of dissolved CO₂ in the hypolimnion during the stratification period. In parallel to that, summer water column profiles of the dissolved inorganic carbon (DIC) and its respective δ13C signature show gradients from up to 10mM (deep waters) to 6mM (surface waters), with a correspondent water signature from 4 to 8 mUr VPDB, independently supporting our interpretations. In temperate climate zones such as the Eifel, lake stratification persists for approximately eight months. Based on our CO₂ flux output calculations, we estimate that 1.6 x 10⁹ moles of CO₂ accumulate in the hypolimnion during the stratification period. We therefore discuss possible scenarios under which the gas pressure can exceed the hydrostatic pressure resulting in the sudden release of gas stored at depth.

Alongside the CO₂ flux measurements, we sampled the free gas phase directly from gas emission points at different depths to determine the origin of gases using δ¹³C-CO₂ and noble gas isotopic ratios. For this purpose, we tested our newly developed gas sampler mounted on a remotely operated vehicle. Helium ratios range from 4.9 up to 5.3 R_A, while δ¹³C-CO₂ range between -1.7 and -0.17‰. The methods presented here are part of an ongoing monitoring study for Laacher See that aims to understand changes in the magmatic plumbing system related to increased volcano-tectonic activity in the region.