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332-11 Alkalinity or Oxygenation? A Re-evaluation of the Uranium Isotope Proxy Provides a Unified Mechanism for the Ediacaran Shuram Excursion

Session: Integrated Stratigraphic, Geochemical, and Paleobiologic Perspectives on Earth System Evolution: A Session in Honor of the Contributions of Bradley B. Sageman (Posters)


Poster Booth No.: 224

Presenting Author:

Matthew Pedersen

Authors:

Pedersen, Matthew G.1, Gan, Tian2, Gilleaudeau, Geoffrey J.3, Segessenman, Daniel C.4, Thompson, Samuel Madison5, Chen, Xinming6, Piccoli, Philip M.7, Evans, Michael N.8, Bykova, Natalia9, Maximenko, Vadim10, Grazhdankin, Dmitriy V.11, Kaufman, Alan Jay12r> (1) Department of Geological, Environmental, and Planetary Sciences, University of Maryland, College Park, Mount Airy, MD, USA, (2) Department of Atmospheric, Oceanic & Earth Sciences, George Mason University, Fairfax, Virginia, USA; Department of Geological, Environmental, and Planetary Sciences, University of Maryland, College Park, College Park, Maryland, USA, (3) Department of Atmospheric, Oceanic & Earth Sciences, George Mason University, Fairfax, VA, USA, (4) Department of Atmospheric, Oceanic & Earth Sciences, George Mason University, Fairfax, Virginia, USA, (5) Department of Atmospheric, Oceanic & Earth Sciences, George Mason University, Fairfax, Virginia, USA, (6) School of Oceanography, Shanghai Jiao Tong University, Shanghai, China, (7) Department of Geological, Environmental, and Planetary Sciences, University of Maryland, College Park, College Park, MD, USA, (8) Department of Geological, Environmental, and Planetary Sciences, University of Maryland, College Park, College Park, Maryland, USA; Earth System Science Interdisciplinary Center, University of Maryland, College Park, College Park, Maryland, USA, (9) Department of Geosciences, University of Missouri, Columbia, MO, USA; Trofimuk Institute of Petroleum Geology and Geophysics, Novosibirsk, Russia, (10) Trofimuk Institute of Petroleum Geology and Geophysics, Novosibirsk, Novosibirsk Obl, Russia, (11) Trofimuk Institute of Petroleum Geology and Geophysics, Novosibirsk, Russia; Department of Geology, Northwest University, China, (12) Department of Geological, Environmental, and Planetary Sciences, University of Maryland, College Park, Maryland, USA; Earth System Science Interdisciplinary Center, University of Maryland, College Park, College Park, Maryland, USA,

Abstract:

The appearance of the enigmatic Ediacara biota in the rock record represents an early experiment in multicellularity and has long been related to the oxygenation of the oceans and atmosphere associated with the Ediacaran Shuram Excursion (SE). The anomalously depleted δ13C values characteristic of the SE reflects a profound change in the redox landscape of Ediacaran seawater and has been interpreted as the crescendo of the protracted Neoproterozoic Oxidation Event (NOE), spanning the Tonian through the Cambrian. A main line of evidence for the SE as a ventilation event comes from carbonate uranium (U) isotopes, which record a shift to near-modern carbonate δ238U values coincident with the negative carbon isotope excursion in multiple SE sections worldwide. This observation directly contrasts with shale-based thallium (Tl) isotope data suggesting deep-water anoxia during the SE. Sedimentological observations of SE bearing carbonates globally and a Macrostrat-based carbonate quantity increase in North America during the SE suggest it occurred during a period of extremely high carbonate saturation, likely driven by intense continental weathering, supported by emergent carbonate lithium isotope data. In this study, we present new high resolution U isotope data from Siberia and model the effects of Ca2+ and CO32- ion concentrations on aqueous U speciation and thus its isotopic fractionation. Our results indicate that the neutral calcium-uranyl-carbonato species (Ca-UO2-CO3) dominates under conditions of high carbonate saturation, thereby inhibiting the reduction of U(VI) to U(IV) and producing crustal δ238U values in seawater despite widespread ocean anoxia which is normally expected to drive seawater δ238U to lower values. We thus re-interpret the carbonate δ238U record of the SE as a reflection of high carbonate saturation in anoxic seawater associated with an increase in neutral Ca-UO2-CO3 species (and not deep ocean oxygenation). This novel interpretation also provides a unified explanation for the time-series variations in other stable isotope systems (C, O, Li, Sr, Ca) during the SE that cannot be solely accounted for by diagenesis and contributes to our understanding of Earth system dynamics at the dawn of animal life.






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Alkalinity or Oxygenation? A Re-evaluation of the Uranium Isotope Proxy Provides a Unified Mechanism for the Ediacaran Shuram Excursion

Category

Topical Sessions

Description


Session Format: Poster

Presentation Date: 10/14/2026

Presentation Room: CCC, Hall F

Poster Booth No.: 224

Author Availability: 9:00 to 11:00 a.m.

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