18 Apr (GEOL) Thesis Defense: Thompson, Ediacaran oxygenation

Madison Thompson, Earth System Science MS Thesis Defense

Ediacaran ocean oxygenation: Insights from the Portfjeld Group, North Greenland

Fri, 18 Apr, 2:00pm, Exploratory Hall 1309

Advisor: Geoff Gilleaudeau

Rocks of the Ediacaran Period record the emergence, rise, and subsequent downfall of the Ediacaran biota, Earth’s oldest known communities of complex macroscopic organisms. This pivotal period of Earth history is also characterized by significant shifts in the global carbon cycle, with middle Ediacaran carbonates worldwide recording one of the largest negative δ¹³C excursions in the geologic record, known as the Shuram-Wonoka Excursion (SWE), from 579 to 564 Ma. While the relationship between negative δ¹³C anomalies and biological innovation remains poorly understood, recent data suggests that a transient interval of widespread ocean oxygenation during the SWE could be an environmental link between these phenomena. However, Ediacaran-age rocks that are fossiliferous and conducive to carbonate-based paleoredox analysis are exceedingly rare. 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 SWE. The Portfjeld Group also contains exceptionally well-preserved microfossils, including putative phosphatized animal-like eggs and embryos comparable to the Weng’an biota in South China. Here, we present a new carbonate uranium isotope (δ²³⁸U) record of the Portfjeld Group as a proxy for global ocean redox during the SWE. In the interval prior to and during the early decreasing limb of the δ¹³C excursion, δ²³⁸U values are very low (median = −0.72‰), indicating widespread global ocean anoxia. However, as δ¹³C values rapidly drop from ~−2‰ to ~−8‰, there is a pronounced shift towards higher δ²³⁸U values (median = −0.16‰, indistinguishable from modern carbonates). Overall, our data indicate a transient episode of near-modern levels of global ocean oxygenation during the SWE. Notably, the animal-like egg and embryo fossils in the Portfjeld Group are stratigraphically well below the positive δ²³⁸U shift (as is the case in South China). Our findings suggest that there was not a simple cause-and-effect relationship between ocean oxygenation and the evolution of complex organisms during this time, and that other local or global factors influenced the appearance and preservation of these organisms.