Abstract
Geological records suggest that marine phytoplankton might have arisen in the Proterozoic while zooplankton remained absent, and marine productivity was not excessively low. However, quantitative estimates of phytoplankton biomass and net primary productivity remain elusive. Here, we use the Community Earth System Model version 1.2.2, modifying biological module and boundary conditions, to simulate marine biogeochemical cycles in the Proterozoic. The simulations demonstrate that, within the expected range of nutrient levels, phytoplankton at sea surface was more than 2 times denser than present, sustaining a greener ocean due to the absence of predators. Heavier surface chlorophyll in the Proterozoic would block sunlight from reaching subsurface layers. This so-called self-shielding effect would decrease subsurface net primary productivity significantly. Simulations show that, through the combined influence of low nitrate level under a low-oxygen environment, the absence of diatoms, and self-shielding, the Proterozoic net primary productivity was only approximately 60% and 30% of the present level in warm (almost ice-free) and cold (sea-ice reaches around 30°N/S) periods, respectively. These findings are subject to uncertainties in model framework and Proterozoic nutrients levels; a slightly less green ocean or more productive ocean was possible if the phosphorus level was much lower or higher than the present level.
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Data availability
The simulation results used for the present study are archived on Zenodo with the identifier https://doi.org/10.5281/zenodo.18489467. Source data are provided as a Source data file. Source data are provided with this paper.
Code availability
The source code of CESM1.2.2 can be accessed at https://github.com/ESCOMP/CESM. The NCAR Command Language (NCL) version 6.2.2 (https://doi.org/10.5065/D6WD3XH5) is used for graphing.
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Acknowledgements
Y.L. is supported by the National Natural Science Foundation of China under grant 42225606. P.L is supported by the National Natural Science Foundation of China under grant 42475052, the National Key Research and Development Program of China under grant 2024YFF0808000, Fundamental Research Funds for the Central Universities under grant 202541010, and Young Talent of Lifting Engineering for Science and Technology in Shandong, China under grant SDAST2024QTA022. S.L. is supported by the National Natural Science Foundation of China under grant 42121005. The simulations were performed on the High-performance Computing Platform of Peking University and the Marine Big Data Center of the Institute for Advanced Ocean Study of Ocean University of China. The authors thank the technical support of the National Large Scientific and Technological Infrastructure “Earth System Numerical Simulation Facility” (https://cstr.cn/31134.02.EL).
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Y.L. proposed the project. P.L. and Y.L. performed modeling analyses. P.L. and Y.L. wrote the manuscript. L.D., J.Z., S.L., and Y.C. contributed to the discussion and manuscript revision.
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Liu, P., Liu, Y., Dong, L. et al. Earth system simulations suggest that the Proterozoic ocean was greener but less productive. Nat Commun (2026). https://doi.org/10.1038/s41467-026-69654-z
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DOI: https://doi.org/10.1038/s41467-026-69654-z
