Lakes can emit carbon dioxide but also store carbon in their sediments. In hardwater lakes like Lake Geneva, calcite precipitates in the water column, releasing CO₂ to the atmosphere, but upon sinking these particles also transport carbon to the sediment. Using sediment traps and radiocarbon isotopes, we show that much of the precipitated calcite is buried, highlighting an overlooked carbon sink that partly offsets the CO₂ outgassing and should be included in lake carbon budgets.