175-8 Do Uranium Isotopes in Carbonates Reveal Global Ocean Oxygenation During the Ediacaran Shuram Excursion? Insights from the Portfjeld Group, North Greenland

Session: The Neoproterozoic Earth and Life Co-evolution, Part I

Presenting Author:

Samuel Thompson

Authors:

Thompson, Samuel Madison¹, Gan, Tian², Pederson, Matthew³, Doerrler, Andrew⁴, Segessenman, Daniel C.⁵, Remirez, Mariano N.⁶, Rugen, Elias⁷, Frei, Robert⁸, Kaufman, Alan Jay⁹, Gilleaudeau, Geoffrey J.¹⁰

(1) Atmospheric, Oceanic, and Earth Sciences Department, George Mason University, Fairfax, VA, USA, (2) University of Maryland-College Park, Greenbelt, MD, USA; George Mason University, Fairfax, VA, USA, (3) University of Maryland-College Park, College Park, MA, USA, (4) University of Maryland -College Park, Damascus, MD, USA, (5) George Mason University, Fairfax, VA, USA, (6) George Mason University, Fairfax, VA, USA; University of Copenhagen, Copenhagen, Denmark, (7) University College London, London, United Kingdom, (8) University of Copenhagen, Copenhagen, Denmark, (9) University of Maryland -College Park, College Park, MD, USA, (10) George Mason University, Fairfax, VA, USA,

Abstract:

The Ediacaran Period records a pivotal transition in Earth history, including the rise and fall of the Ediacara biota and large-scale perturbations to the global carbon cycle, such as the Shuram-Wonoka negative δ¹³C excursion (SWE; ~579–564 Ma). A growing body of geochemical evidence suggests that transient episodes of ocean oxygenation occurred throughout the Ediacaran Period and may link negative δ¹³C excursions to evolutionary innovations. However, stratigraphic sections that preserve Ediacaran fossil assemblages in facies such as carbonates that are amenable to global paleoredox analysis remain scarce. The Portfjeld Group of North Greenland is a unique Ediacaran to Lower Cambrian carbonate succession that records a large negative δ¹³C anomaly (~−12‰) that corresponds to the SWE, which archives exceptionally preserved phosphatized putative animal embryos. Here, we present a carbonate uranium isotope (δ²³⁸U) record from the Portfjeld Group to track ocean redox across this critical transition. Our δ²³⁸U data reveal initially low values (~−0.72‰) prior to and during the early stages of the δ¹³C excursion, consistent with widespread global ocean anoxia. A pronounced positive shift in δ²³⁸U values (peaking at −0.01‰, with average values indistinguishable from modern carbonates) coincides with the nadir of δ¹³C values (~−8‰), which is seen in SWE successions worldwide and has been used as evidence for a transient interval of near-modern levels of global ocean oxygenation. Following this interval, δ²³⁸U values decline once more, averaging ~−0.51‰ in the overlying strata, suggesting a return to widespread anoxic conditions. Although this pronounced positive δ²³⁸U shift would traditionally be interpreted as a global ocean oxygenation event, we are also exploring the role of increased oceanic carbonate saturation states during the SWE that could have suppressed U(VI) to U(IV) reduction, and thus muted δ²³⁸U fractionations. Notably, the purported animal egg and embryo fossils in the Portfjeld Group occur stratigraphically well below the positive δ²³⁸U excursion, also observed in the Doushantuo Formation of South China. This stratigraphic offset challenges models linking metazoan evolution directly to global ocean oxygenation events, implying that other local environmental or biological factors played significant roles in the appearance and preservation of early complex life. Overall, these findings reinforce the SWE as a global oceanographic phenomenon and demonstrate that carbonate δ²³⁸U can capture transient oxygenation or alkalinity events in the Ediacaran oceans.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-9950

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