Authigenic CaCO₃ precipitation contributes to the stabilization of algal-derived organic carbon in karst lake

The carbonate weathering carbon sink constitutes a significant component of the global carbon budget. A growing body of research has identified the role of biological carbon pumps in the sequestration of inorganic carbon produced through carbonate weathering. Nevertheless, there has been considerable controversy surrounding whether carbon fixed by phytoplankton photosynthesis is stable over the long term. The present study speculated that authigenic calcium (Ca) precipitation induced by primary production facilitates the practical preservation of organic carbon (OC) in the karst lake sediment, which is important for the long-term stabilization of carbonate weathering carbon sinks. The primary sources of OC in surface and core sediments were analyzed in Lake Fuxian, a deep alpine lake in the karst region. Their molecular composition and potential interaction mechanism with Ca were investigated. The δ¹³C isotopic signature and OC:N molar ratio indicate that the sedimentary OC is predominantly derived from autochthonous photosynthesis. The significant positive correlation between the contents of Ca and total organic carbon suggestes that Ca sedimentation is related to primary production. High-resolution mass spectrometry analyses reveal that molecules with abundant nitrogen atoms are bound to Ca-containing minerals in the sediment. The coprecipitation of amino acids during CaCO₃ formation in water facilitates the deposition and preservation of autochthonous OC in the sediment. Our finding demonstrated that the natural photosynthesis-induced calcium carbonate precipitation can substantially enhance the stability of carbonate weathering carbon sinks within karst regions, thus leading to a significant expansion in the existing understanding of carbon sink processes in these environments. © 2025 Elsevier Ltd.