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Unique Neoproterozoic carbon isotope excursions sustained by coupled evaporite dissolution and pyrite burial

Abstract

The Neoproterozoic era witnessed a succession of biological innovations that culminated in diverse animal body plans and behaviours during the Ediacaran–Cambrian radiations. Intriguingly, this interval is also marked by perturbations to the global carbon cycle, as evidenced by extreme fluctuations in climate and carbon isotopes. The Neoproterozoic isotope record has defied parsimonious explanation because sustained 12C-enrichment (low δ13C) in seawater seems to imply that substantially more oxygen was consumed by organic carbon oxidation than could possibly have been available. We propose a solution to this problem, in which carbon and oxygen cycles can maintain dynamic equilibrium during negative δ13C excursions when surplus oxidant is generated through bacterial reduction of sulfate that originates from evaporite weathering. Coupling of evaporite dissolution with pyrite burial drives a positive feedback loop whereby net oxidation of marine organic carbon can sustain greenhouse forcing of chemical weathering, nutrient input and ocean margin euxinia. Our proposed framework is particularly applicable to the late Ediacaran ‘Shuram’ isotope excursion that directly preceded the emergence of energetic metazoan metabolisms during the Ediacaran–Cambrian transition. Here we show that non-steady-state sulfate dynamics contributed to climate change, episodic ocean oxygenation and opportunistic radiations of aerobic life during the Neoproterozoic era.

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Fig. 1: Carbonate carbon isotope record.
Fig. 2: Negative δ13C excursion driven by net oxidation of a DOC reservoir via coupled evaporite weathering and pyrite burial.
Fig 3: Feedback diagram illustrating the effects of evaporite weathering on ocean oxygenation and δ13C.
Fig. 4: COPSE model forced with sulfate input and including differently sized DOC reservoirs.

Data availability

The authors declare that data supporting the findings of this study are available within the article and Supplementary Information.

Code availability

MATLAB code for COPSE is freely available at https://github.com/sjdaines/COPSE/releases.

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Acknowledgements

This work was supported by the NERC-NSFC programme ‘Biosphere Evolution, Transitions and Resilience’ through grant NE/P013643/1 to G.A.S. and M.Z. and NE/P013651/1 to T.M.L., grant NE/R010129/1 to G.A.S. and B.J.W.M., a University of Leeds Academic Fellowship to B.J.W.M., and the National Natural Science Foundation of China (41661134048) and Strategic Priority Research Program (B) of the Chinese Academy of Sciences (XDB18000000) to M.Z.

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G.A.S., B.J.W.M. and M.Z. conceived the project. B.J.W.M. created the model, which was revised from previous versions created by T.M.L., B.J.W.M. and S.J.D. All authors contributed to data interpretation and the writing of the manuscript.

Corresponding authors

Correspondence to Graham A. Shields or Benjamin J. W. Mills or Maoyan Zhu.

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Shields, G.A., Mills, B.J.W., Zhu, M. et al. Unique Neoproterozoic carbon isotope excursions sustained by coupled evaporite dissolution and pyrite burial. Nat. Geosci. 12, 823–827 (2019). https://doi.org/10.1038/s41561-019-0434-3

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