Groundwater induced sediment diagenesis in LakeCadagno, Switzerland

Lake Cadagno, a meromictic, redox stratified lake located in Val Piora in Switzerland has extensively been studied serving as a modern analogue of the Archean/Proterozoic ocean. Euxinic conditions were established after a short transition interval immediately following lake formation ~10’000 years ago. Overall elevated but very variable Mn concentration during this transition period have been thought to reflect fluctuating bottom water redox condition with authigenic accumulation of sedimentary Mn from the water column in line with observation from deep basins of the Baltic Sea.

To test this hypothesis we measured the Mo isotopic composition of Lake Cadagno sediments with a focus on the transition interval following lake formation between 7.4 and 7.8 m depth. With a δ^98/95Mo_NIST+0.25 composition ranging from -1.72 to +3.77‰ the Mo isotopic variability of the transition interval sediments is significantly larger compared to Cadagno sapropels deposited under euxinic conditions and significant larger compared to the entire Mo isotopic variability observed in marine reservoirs. Aside this large variability in δ^98/95Mo_NIST+0.25 a gradual change from isotopically very light to very heavy Mo isotopic composition upcore is observed. This transient sedimentary Mo isotope pattern cannot be explained by fluctuating bottom water redox conditions at the beginning of the lake and cannot be a primary signature caused by changes in the depositional environment. Likely, the inflow of oxic groundwater from subsurface karst aquifers present in Cadagno into organic-matter-rich lake sediments, a likely active and currently ongoing process, causes the formation of a chemocline at depth in the sediment column. Here the reduction of sulphate and the precipitation of isotopically light Mo from the dissolved oxic groundwater reservoir causes progressively higher δ^98/95Mo_NIST+0.25 upwards in the sediment column.