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

Chen, Mingfei and Conroy, Jessica L. and Geyman, Emily C. and Sanford, Robert A. and Chee-Sanford, Joanne C. and Connor, Lynn M. (2022) Stable carbon isotope values of syndepositional carbonate spherules and micrite record spatial and temporal changes in photosynthesis intensity. Geobiology, 20 (5). pp. 667-689. ISSN 1472-4677. doi:10.1111/gbi.12509.

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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.

Item Type:Article
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URLURL TypeDescription
Chen, Mingfei0000-0002-6281-2480
Conroy, Jessica L.0000-0003-3652-3199
Geyman, Emily C.0000-0003-4349-9350
Additional Information:© 2022 The Authors. Geobiology published by John Wiley & Sons Ltd. This is an open access article under the terms of the Creative Commons Attribution-NonCommercial License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited and is not used for commercial purposes. Version of Record online: 18 July 2022. Manuscript accepted: 26 June 2022. Manuscript revised: 27 May 2022. Manuscript received: 30 November 2021. This research was funded by ACS-PRF 57417-DNI2 and NSF-EAR 1602590 to JLC. We thank the Environmental Ministry of the Republic of Kiribati for the research permit to complete this work. We thank A. Wyman, M. Higley, C. Karamperidou, N. Murray, and N. Meghani for field assistance, C Field, M. Band, and G. Rendon for the technical support, and A. Maloof, B. Fouke, and T. Johnson for valuable comments and advice. We thank the UIUC Roy J. Carver Biotechnology Center, specifically the High-throughput Sequencing and Genotyping Unit, for DNA sample analysis. Sequence data were produced by the US Department of Energy Joint Genome Institute ( in collaboration with the user community. We thank the editor and two anonymous reviewers for helping to improve this manuscript. The authors have no conflict of interest to report. DATA AVAILABILITY STATEMENT. The data that support the findings of this study are available from the corresponding author upon reasonable request.
Funding AgencyGrant Number
American Chemical Society Petroleum Research Fund57417-DNI2
Subject Keywords:carbonate; microbial mat; stable isotopes; carbon cycle; metagenomics; geochemical modeling
Issue or Number:5
Record Number:CaltechAUTHORS:20220722-768932000
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Official 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, 667–689.
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:115777
Deposited By: George Porter
Deposited On:26 Jul 2022 19:54
Last Modified:11 Nov 2022 23:09

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