In the geological community, a long and intense debate revolves around the cause of the largest negative carbon isotope excursion in Earth's history, known as the Shuram excursion (SE). The SE occurred approximately 570 million years ago, during the Ediacaran period (635 to 539 million years ago). It is extensively recorded on different global plates and roughly coincides with the rapid evolution of early animals on Earth, making it the focal center of interest. Some scientists believe that the event is related to a significant rise in the global ocean's oxygen (O₂) levels, leading to a massive oxidation of organic carbon. This represents a major disturbance in the carbon cycle of global supergene processes. On the other hand, other scientists argue that the event could be a result of diagenesis, rather than being an original phenomenon. This issue is of great importance because the SE is correlated with the rapid radiation of early Earth's animals, which influences our understanding of the mechanisms behind early animal evolution.

In order to unravel the mystery of the SE, Professor Bao Huiming's team from Nanjing University and Professor Li Chao's team from the Institute of Sedimentary Geology of CDUT collaborated to conduct in-depth research on the Ediacaran strata from three different ancient continents. They focused on studying the sulfur-oxygen isotope composition of carbonate-bound sulfate ions. Excitingly, the sulfate ions from all three regions during the SE period exhibited significant negative anomalies in ¹⁷O (a unique signal of atmospheric O₂; Figure 1). Furthermore, there was a strong positive correlation between the ¹⁷O signal and the δ³⁴S, δ¹⁸O, and δ¹³C_carb. This means that as δ¹³C_carb declined, δ³⁴S and δ¹⁸O also decreased synchronously, while ¹⁷O showed a notable negative anomaly. As the later diagenetic processes make it difficult for atmospheric O₂ to enter sedimentary carbonate rocks, this finding undoubtedly provides support for the SE's primary origin. Through further quantitative analysis and model deductions, the research team proposed that atmospheric O₂ oxidized the deep-water bodies of the ocean, which were oxygen-deficient and rich in hydrogen sulfide and organic carbon during the SE period. This resulted in the production of a large amount of sulfate with negative ¹⁷O anomaly, eventually leading to the occurrence of the SE (Figure 1). Since the oxidation of sulfides by atmospheric O₂ is the only process that can cause the negative ¹⁷O anomaly in marine sulfates, this study provides the most direct evidence for the primary cause of the largest-known carbon isotope excursion in Earth's history (i.e., the ancient ocean oxidation event) and confirms the occurrence of a global ocean oxidation event involving atmospheric O₂ 570 million years ago. Consequently, it establishes a connection between the oxidation of the Earth's supergene environment and the radiation of early animals.

Although further in-depth exploration is still needed for this ocean oxidation event, the research has provided the scientific community with new directions and a solid foundation for further investigation. We look forward to more scientists delving into this field and contributing to resolving the enigmas of Earth's history.

This study has been published online in the prestigious, international, comprehensive journal Nature Communications (https://www.nature.com/articles/s41467-023-39962-9). In the publication, CDUT’s Researcher Wang Haiyang is listed as the first author, Researcher Cheng Meng as a co-author, and Professor Li Chao as a corresponding co-author.

Figure 1: Formation mechanism of sulfate ions with negative ¹⁷O anomaly during the SE. Atmospheric O₂ exhibits a characteristic non-mass-dependent negative ¹⁷O anomaly, which is generated through stratospheric photochemical reactions involving O₂-O₃-CO₂ processes. It is the sole source of the negative ¹⁷O anomaly in sulfate ions (via processes like terrestrial iron pyrite weathering or hydrogen sulfide oxidation in seawater).