GSA Connects 2024

Abstract

The Ediacaran marks a critical transition in Earth history, beginning at the termination of the Marinoan glaciation and ending with the emergence and subsequent downfall of the earliest complex macroscopic organisms, the Ediacaran biota. This period also saw significant shifts in the global carbon cycle, as evidenced by the highest magnitude negative δ13C excursion in Earth history, known as the Shuram Excursion (579 to 564 Ma). The Portfjeld Group of North Greenland is an Ediacaran to Lower Cambrian carbonate succession which records a large negative δ13C anomaly of ~ −12‰ that has been correlated with the Shuram Event. The Portfjeld Group also contains exceptionally well-preserved microfossils, including phosphatized animal-like eggs and embryos comparable to the Weng’an biota in South China. The relationship between negative δ13C anomalies and biological innovation remains poorly understood, although recent data suggest that a transient interval of widespread ocean oxygenation during the Shuram Excursion could be an environmental link between these phenomena. Here, we present a new carbonate uranium isotope (δ238U) record of the Portfjeld Group as a proxy for global ocean redox. The interval prior to and during the early decreasing limb of the δ13C excursion is characterized by very low δ238U values (median = −0.72‰), indicating widespread global ocean anoxia. As δ13C values rapidly drop from ~−2‰ to ~−8‰, however, there is a pronounced shift towards higher δ238U values (median = −0.16‰, indistinguishable from modern carbonates). These δ238U data suggest an episode of global ocean oxygenation that coincides with the nadir of δ13C values. Above this interval, δ238U values slightly decrease again as δ13C values rise back to ~−4‰, before a major subaerial unconformity truncates the Ediacaran section. When carbonate deposition was reestablished in the basal Cambrian, δ238U values are lower and more variable (median = −0.51‰). Overall, our data indicate a transient episode of near-modern levels of ocean oxygenation during the Shuram Excursion. Importantly, however, the animal-like egg and embryo fossils in the Portfjeld Group are stratigraphically well below the positive δ238U shift, suggesting that there was not a simple cause-and-effect relationship between ocean oxygenation and metazoan evolution.