Stable carbon isotope values of syndepositional carbonate spherules and micrite record spatial and temporal changes in photosynthesis intensity

Marine and lacustrine carbonate minerals preserve carbon cycle information, and their stable carbon isotope values (δ¹³C) are frequently used to infer and reconstruct paleoenvironmental changes. However, multiple processes can influence the δ¹³C values of bulk carbonates, confounding the interpretation of these values in terms of conditions at the time of mineral precipitation. Co-existing carbonate forms may represent different environmental conditions, yet few studies have analyzed δ¹³C values of syndepositional carbonate grains of varying morphologies to investigate their origins. Here, we combine stable isotope analyses, metagenomics, and geochemical modeling to interpret δ¹³C values of syndepositional carbonate spherules (>500 μm) and fine-grained micrite (<63 μm) from a ~1600-year-long sediment record of a hypersaline lake located on the coral atoll of Kiritimati, Republic of Kiribati (1.9°N, 157.4°W). Petrographic, mineralogic, and stable isotope results suggest that both carbonate fractions precipitate in situ with minor diagenetic alterations. The δ¹³C values of spherules are high compared to the syndepositional micrite and cannot be explained by mineral differences or external perturbations, suggesting a role for local biological processes. We use geochemical modeling to test the hypothesis that the spherules form in the surface microbial mat during peak diurnal photosynthesis when the δ¹³C value of dissolved inorganic carbon is elevated. In contrast, we hypothesize that the micrite may precipitate more continuously in the water as well as in sub-surface, heterotrophic layers of the microbial mat. Both metagenome and geochemical model results support a critical role for photosynthesis in influencing carbonate δ¹³C values. The down-core spherule–micrite offset in δ¹³C values also aligns with total organic carbon values, suggesting that the difference in the δ¹³C values of spherules and micrite may be a more robust, inorganic indicator of variability in productivity and local biological processes through time than the δ¹³C values of individual carbonate forms.