Spatiotemporal fluctuations of pyrite sulfur isotope with pyrite morphology during the Ediacaran-Cambrian transition

Abstract

The rapid fluctuations in the bulk-sample sulfur isotope of sedimentary pyrite (δ³⁴S_py) serve as a crucial indicator of marine redox changes during the Ediacaran-Cambrian transition (E-C). However, these bulk δ³⁴S_py values are susceptible to diagenetic alterations, which can alter the ratio of euhedral to framboidal crystals, thereby inducing fluctuations in δ³⁴S_py values. To assess the impact of diagenesis on δ³⁴S_py variability across the E-C transition, we conducted a comprehensive analysis of spatiotemporal variations in δ³⁴S_py across five E-C sections in South China, encompassing diverse environmental settings. Additionally, we characterized pyrite morphologies, distinguishing between framboidal and non-framboidal forms. The results demonstrate a consistent decrease in δ³⁴S_py values with increasing water depth, accompanied by an increase in the proportion of framboids. Specifically, in shallow water platform settings, the mean δ³⁴S_py value is + 34.0 ‰, with a mean framboid proportion of 6 %. Conversely, in the deep water basin environment, the mean δ³⁴S_py value drops to + 5.6 ‰, with a mean framboid proportion of 45 %. A positive δ³⁴S_py excursion, ranging from + 5.1 ‰ to + 51.0 ‰, is observed in the platform at the base of the E-C transition, while a negative excursion, spanning from + 13.0 ‰ to −28.2 ‰ (and from + 14.9 ‰ to −19.5 ‰ in another section), is evident in the basin. Crucially, a consistent negative correlation is observed between framboid proportions and bulk δ³⁴S_py values. This relationship underscores the significance of both the timing of pyrite formation and its local redox condition. Our findings reveal that marine oxidation occurred in uneven pulses during the E-C transition. Furthermore, we emphasize the necessity of considering pyrite morphologies when interpreting δ³⁴S_py records, as framboids may offer a more reliable signal of seawater conditions. Overall, this study provides new insights into the complex interplay between diagenetic alterations, pyrite morphology, and marine redox variations during the E-C transition, underscoring the dynamic nature of marine oxidation during this critical period in Earth’s history.