Deltaic burial of authigenic calcite modulates the carbon balance of hardwater lakes

Abstract. Inland waters play an important role in the terrestrial carbon cycle by burying carbon in aquatic sediments while simultaneously releasing CO2 to the atmosphere and laterally exporting carbon along the land-ocean aquatic continuum. Especially in hardwater lakes, the close connection between primary production and calcite precipitation results in a poorly understood balance of carbon burial and release, with stronger coupling of organic and inorganic processes than in softwater lakes. To better understand these dynamics, we analyzed organic and inorganic carbon fluxes in a yearlong (June 2022 to June 2023) sediment trap study in Lake Geneva, the largest natural lake in Western Europe. Two sediment traps – one deployed in the subaqueous delta of the upper Rhône River, the other in the lake's deepest basin – were sampled monthly. Analyzing radiocarbon (14C) signatures of particulate organic and inorganic carbon allowed us to resolve allochthonous (external) and autochthonous (internal) contributions to absolute carbon fluxes. We found that the flux of autochthonous particulate inorganic carbon in the river-proximal deltaic site was approximately four times higher than in the distal one. This is likely the result of calcite precipitation driven by increased fluvial supply of nutrients and suspended carbonate-bearing particles. Sediment core analysis in the same location suggests efficient preservation of this calcite over centennial timescales, which we conservatively estimate around 7–10 Gg C yr−1 lake-wide. This indicates at least partial offset of the CO2 released during calcite precipitation and is an important flux to be considered in mechanistic carbon cycle models.