During the Neoproterozoic–Cambrian transition, seawater carbon isotopic composition experienced multiple dramatic fluctuations. Among them, the negative excursion of carbonate carbon isotopes (with δ13C dropping from ~+5‰ to a minimum of ≤-12‰) during the mid-Ediacaran represents the largest seawater carbon isotope anomaly in Earth’s history. Widely recorded across global continents, it is known as the Shuram (or DOUNCE) event. The Shuram event is widely regarded as a global marine oxygenation event and is thought to be closely linked to the rapid radiation and evolution of early complex life (Fig. 1).
However, the driving mechanism behind the extremely negative seawater carbon isotope signal of the Shuram event remains a topic of intense debate. The most widely accepted hypothesis suggests that the event was driven by the oxidation of a massive “dissolved organic carbon (DOC) pool” in the ancient ocean. The oxidation of DOC delivered a vast amount of light carbon isotopes, resulting in the extreme negative shift of seawater dissolved inorganic carbon isotope composition. Yet, this raises a crucial scientific question: oxidizing such a large DOC pool would require consuming immense quantities of oxidants. Given that atmospheric oxygen levels at the time were likely only 1%–10% of modern levels, oxygen alone was far from enough to sustain such large-scale DOC oxidation. Therefore, the source and budget of these oxidants have become one of the core bottlenecks in validating the DOC oxidation hypothesis.
To investigate the total quantity and sources of oxidants required for DOC oxidation during the Shuram event, Dr. ZHANG Yinggang, a postdoctoral fellow at the Nanjing Institute of Geology and Paleontology, Chinese Academy of Sciences (NIGPAS), under the supervision of Prof. ZHU Maoyan, conducted quantitative Earth system modeling in close collaboration with Prof. Benjamin Mills and Dr. Fred Bowyer from the University of Leeds. The findings were recently published in Communications Earth & Environment.
In 2019, a joint Sino-UK research team co-led by Prof. Maoyan ZHU proposed that a massive flux of terrestrial evaporites (sulfate) could have provided sufficient oxidants to fuel DOC oxidation and constrained the potential oxidant flux through forward approach using the COPSE model. However, that model relied on a carbon isotope curve derived solely from Oman, which could not fully represent the global trend. To address this limitation, the team first constructed a global seawater carbon isotope evolution curve based on the global carbonate carbon isotope database coupled with previously published age constraints (Fig.1). Subsequently, using the SCION Earth system evolution model coupled with Monte Carlo inversion techniques, the team quantitatively modeled the DOC oxidation flux and total oxidant demand required to drive this carbon isotope evolution trajectory (Fig.1).
The results show that DOC oxidation can successfully explain the severe negative shift in seawater carbon isotopes; however, during the recovery phase of the event, merely “ceasing DOC oxidation” is insufficient to meet the requirement for a rapid rebound in carbon isotopes (Fig. 2). Quantitative modeling demonstrates that a rapid recovery of seawater carbon isotopes can be achieved through the additional effects of enhanced primary productivity and increased organic carbon burial flux (Fig. 2). The Monte Carlo inversion quantitatively calculated that the upper limit of total oxidants required for the Shuram event is approximately 9.3 × 1019 mol. Although this value far exceeds the combined reservoirs of atmospheric oxygen and seawater sulfate at the time, it is well matched by the flux of sulfate that could be supplied by terrestrial evaporites during that period (Fig. 3).
This study provides quantitative Earth system evolution modeling evidence confirming that terrestrial sulfate supply was sufficient to meet the oxidant budget required for the Shuram event, offering quantitative constraints for validating the DOC oxidation hypothesis and understanding carbon cycle evolution during this critical transition.
This work was jointly funded by the National Key R&D Program of China, the National Natural Science Foundation of China (NSFC), the Jiangsu Excellent Postdoctoral Program, and UK Research and Innovation (UKRI).
Reference: Zhang, Y.*, Zhu, M.*, Bowyer, F. T., & Mills, B. J. W. (2026). Revisiting the oxidant budget of the DOUNCE event. Communications Earth & Environment. https://doi.org/10.1038/s43247-026-03855-8.
Shields, G. A., Mills, B. J. W., Zhu, M.*, Raub, T. D., Daines, S. J., & Lenton, T. M. (2019). Unique Neoproterozoic carbon isotope excursions sustained by coupled evaporite dissolution and pyrite burial. Nature Geoscience, 12. https://doi.org/10.1038/s41561-019-0434-3.
Zhang, Y., Zhu, M.* (2025). Meta-analysis of the DOUNCE event (Shuram/Wonoka excursion): pattern, variation, causal mechanism, and global correlation. Earth-Science Reviews, 261. https://doi.org/10.1016/j.earscirev.2024.105000.
Fig.1 Major macroevolutionary events, a reconstructed global seawater δ13C curve, and an inverse modeling strategy within the SCION earth system evolution model.
Fig.2 Modeled seawater δ13C variations, their offsets from the reconstructed seawater δ13C curve, and required DOC oxidation rates under baseline and elevated phosphorus-input scenarios.
Fig.3 Comparison of required DOC and potential oxidant supply under the DOC-oxidation hypothesis.
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