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

Authors: CHEN, MINGFEI - University Of Illinois; CONROY, JESSICA - University Of Illinois; GEYMAN, EMILY - Princeton University; SANFORD, ROBERT - University Of Illinois; Chee Sanford, Joanne; Connor, Lynn

Submitted to: Geobiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/22/2022
Publication Date: 7/18/2022
Citation: Chen, M., Conroy, J.L., Geyman, E.C., Sanford, R.A., Chee Sanford, J.C., Connor, L.M. 2022. Stable carbon isotope values of syndepositional carbonate spherules and micrite record spatial and temporal changes in photosynthesis intensity. Geobiology. 20(5):667-689. https://doi.org/10.1111/gbi.12509.
DOI: https://doi.org/10.1111/gbi.12509

Interpretive Summary: Global climate change conditions have impacted environmental conditions that largely govern important biogeochemical cycles like carbon (C) cycling. While microbial communities are now being characterized more frequently using sequencing technology, only recently has such data been applied to provide deeper explanations for how resident microbes cause seasonal and diurnal shifts in the physicochemical environment of their habitats. In this study, a “simple” natural sediment system was analyzed using d13C isotopic patterns in minerals together with microbial mat community sequences to ascertain the origins of carbonate deposition that distinguished those indicating temporal photosynthetic- and heterotrophic microbial activities apart from abiotic processes. The results showed microbial populations and their corresponding activities predicted from seasonal and diurnal conditions of light and organic C led to patterns of change in d13C that better indicated variability in productivity and local biological processes through time. The impact of this study is a better understanding of how to integrate contemporary biological data to study the modern impacts of factors such as rising temperatures and changing rainfall patterns on nutrient (carbon and nitrogen) cycling. Such approaches are critical to develop studies that can be applied to complex systems like soils to better interpret global nutrient cycles that impact plant growth and climate change across multiple scales of time and space.

Technical Abstract: Marine and lacustrine carbonate minerals preserve ancient and modern carbon (C) cycle information, and their stable carbon isotope values (d13C) are frequently used to infer paleoenvironmental changes. However, multiple processes can influence the d13C values of bulk carbonates, confounding 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 d13C values of syndepositional carbonate grains of varying morphologies to investigate their origins. Here, we combine stable isotope, metagenomics, and geochemical modeling to interpret d13C values of syndepositional carbonate spherules (> 500 µm) and fine-grained carbonate mud (< 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 d13C values of spherules are high compared to the syndepositional matrix 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 d13C value of dissolved inorganic carbon is elevated. In contrast, we hypothesize that the fine-grained carbonate more continuously precipitates in both the water and deeper, heterotrophic layers of the microbial mat. Both metagenome and geochemical model results support a critical role for photosynthesis in influencing carbonate d13C values. The down-core offset in d13C values also aligns with total organic carbon values, indicating that the difference in the d13C values of spherules and syndepositional carbonate matrix is a more robust inorganic indicator of variability in productivity and local biological processes through time than the d13C values of individual carbonate forms.