Calcite precipitation: The forgotten piece of lakes' carbon cycle.

Lakes emit substantial amounts of carbon dioxide (CO) into the atmosphere, but why they do remains debated. The long-standing vision of lakes as solely respirators of the organic matter leaking from the soils has been challenged by evidence that inorganic carbon produced by weathering of the catchment bedrock could also support lake CO emissions. How inorganic carbon inputs ultimately generate lake CO outgassing remains a blind spot. We develop and introduce a calcite module in a coupled one-dimensional physical-biogeochemical model that we use to simulate the carbon cycle of the large Lake Geneva over the past 40 years. We mechanistically demonstrate how the so-far neglected process of calcite precipitation boosts net CO emissions at the annual scale. Far from being anecdotal, we show that calcite precipitation could explain CO outgassing across various lakes globally, including some of the largest lakes in the world.