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

Journal Article · Wed Aug 17 00:00:00 UTC 2022 · Geobiology

DOI: https://doi.org/10.1111/gbi.12509 · OSTI ID:1904066

Chen, Mingfei  ^[1]; Conroy, Jessica L.  ^[2]; Geyman, Emily C.  ^[3]; Sanford, Robert A. ^[2]; Chee‐Sanford, Joanne C. ^[2]; Connor, Lynn M. ^[2]

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1. Univ. of Illinois at Urbana-Champaign, IL (United States); OSTI
2. Univ. of Illinois at Urbana-Champaign, IL (United States)
3. California Institute of Technology (CalTech), Pasadena, CA (United States)

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.

• Research Organization: USDOE Joint Genome Institute (JGI), Berkeley, CA (United States)
• Sponsoring Organization: USDOE
• Grant/Contract Number: AC02-05CH11231
• OSTI ID: 1904066
• Journal Information: Geobiology, Journal Name: Geobiology Journal Issue: 5 Vol. 20; ISSN 1472-4677
• Publisher: WileyCopyright Statement
• Country of Publication: United States
• Language: English