Sources and sinks of water and elements in the high-CO2 volcanic Laacher See, Germany

The Laacher See (Lake Laach), the largest volcanic lake in Germany, resulted from a massive phreatomagmatic eruption in the Eifel Region ~13,000 years ago. The enclosed lake within a small catchment is still affected by the underlying volcanic activity, providing a unique natural laboratory for investigating the sources and sinks of high dissolved carbon concentrations and associated element cycles in natural chemical gradients. The lake is continuously affected by magmatic CO₂ degassing. A large number of moffettes are distributed not only along the lake shore but also at different depths of the lake, whereby gas seeps can still be found at the deepest point of the lake at 51 m. Here, seasonal investigations of the water column and porewaters from several sediment short cores and a 6 m long core at a reference site show for the first time that the sediment package is an active and special biogeochemical reactor outlining a unique type of diagenesis under the boundary conditions of high dissolved inorganic carbon. With the help of an underwater drone equipped with a temperature & depth sensor, the water column was accurately sampled at regular depth intervals. Additionally, the fingerprinting of surrounding groundwaters as potential water and elemental sources, allows for a first assessment of the cycling of dissolved carbon, water, major and trace elements. Stable H and O isotope signatures provide insight into the water sources and the seasonal water balance of the lake. The C isotopic composition of dissolved inorganic carbon (DIC) indicates its sources and fate, and explains signatures reported for authigenic sedimentary carbonates, e.g. siderite. Sulfate is consumed by microbial sulfate reduction in the upper few centimeters of the sediments, and the SO₄ isotopic signature from the lake water is close to that of the moffette solution indicating similar influence of these benthic processes. The examination of porewaters from a ca. 7-meter-wide pockmark provides evidence of enhanced diagenesis under high DIC fluxes potentially affecting metal accumulation and liberation from sediments.