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264-8 Exploring Ediacaran Biogeochemistry Through a Multi-Proxy Lens: New Insights from U-Li-Sr Isotopes in the Portfjeld Group, North Greenland

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

Presenting Author:

Samuel Thompson

Authors:

Thompson, Samuel Madison¹, Pederson, Matthew G.², Gan, Tian³, Remirez, Mariano N.⁴, Farkas, Juraj⁵, Rugen, Elias⁶, Segessenman, Daniel C.⁷, Dahl, Tais W.⁸, Frei, Robert⁹, Kaufman, Alan Jay¹⁰, Gilleaudeau, Geoffrey J.¹¹r> (1) Atmospheric, Oceanic, and Earth Science Department, George Mason University, Fairfax, VA, USA, (2) Department of Geology, University of Maryland, College Park, MD, USA, (3) School of Physics, Chemistry, and Earth Sciences, Adelaide University, Adelaide, Australia; Atmospheric, Oceanic, and Earth Science Department, George Mason University, Fairfax, VA, USA, (4) Institut for Geovidenskab og Naturforvaltning, University of Copenhagen, Copenhagen, Denmark, (5) School of Physics, Chemistry, and Earth Sciences, Adelaide University, Adelaide, Australia, (6) UCL Earth Sciences, University College London, London, United Kingdom, (7) Atmospheric, Oceanic, and Earth Science Department, George Mason University, Fairfax, VA, , (8) Institut for Geovidenskab og Naturforvaltning, University of Copenhagen, Copenhagen, Denmark, (9) Institut for Geovidenskab og Naturforvaltning, University of Copenhagen, Copenhagen, Denmark, (10) Department of Geology, University of Maryland, College Park, MD, USA, (11) Atmospheric, Oceanic, and Earth Science Department, George Mason University, Fairfax, VA, USA,

Abstract:

The Ediacaran Period records profound changes in ocean chemistry, climate, and early ecology, including the appearance of the first complex macroscopic life and major perturbations to the global carbon cycle. The period is characterized by multiple large negative δ¹³C excursions, including the Shuram-Wonoka Excursion (SWE; ~579–564 Ma) and the Basal Cambrian Excursion (BACE, ~539 Ma), both of which are preserved in marine carbonate successions from a variety of paleoenvironments across multiple paleocontinents. The dolostone-dominated Portfjeld Group of North Greenland preserves a pronounced negative δ¹³C excursion comparable to either the SWE or BACE, along with phosphatized microfossils below the nadir resembling purported early embryo-like metazoans from the late Ediacaran Doushantuo Formation of South China. Bulk-carbonate δ²³⁸U analysis of the Portfjeld Group reveals a pronounced positive shift to values comparable to modern carbonates near the nadir of the δ¹³C excursion, then return to more negative values. Similar inverse relationships between δ¹³C and δ²³⁸U in other SWE-bearing sections have been interpreted as evidence of transient global ocean oxygenation. However, this relationship could also reflect changes in uranium reduction kinetics related to oceanic carbonate saturation state, the expansion of ferruginous ocean conditions, or even diagenetic alteration. Here, we expand the Portfjeld Group record using δ⁷Li, ⁸⁷Sr/⁸⁶Sr, δ⁸⁸/⁸⁶Sr, and new δ²³⁸U measurements from chromatographically separated U(VI) and U(IV) fractions. This approach allows us to further evaluate potential causes of the positive bulk carbonate δ²³⁸U shift recorded in the Portfjeld Group. Parallel U(VI) and U(IV) analyses of the Doushantuo Formation enables comparison between two similar, yet paleogeographically distant successions, with initial analysis showing both sections are largely dominated by the U(VI) fraction. A pronounced negative excursion in δ⁷Li to near crustal values at the nadir of the δ¹³C excursion suggests intense congruent weathering of the continents, an expression similar to the SWE of South China and Siberia. δ⁸⁸/⁸⁶Sr values remain elevated throughout the succession (some of the highest recorded in Earth history) and are potentially even more enriched coincident with the nadir of the δ¹³C excursion, suggesting the existence of a large ⁸⁸Sr-depleted sink somewhere in the oceans potentially related to rapid authigenic carbonate precipitation. Together, these proxies provide a framework for distinguishing global ocean redox change from local depositional effects and post-depositional alteration.

Geological Society of America Abstracts with Programs. Vol. 58, No. 5, 2026

doi: 10.1130/abs/2026AM-21526

Exploring Ediacaran Biogeochemistry Through a Multi-Proxy Lens: New Insights from U-Li-Sr Isotopes in the Portfjeld Group, North Greenland

Description

Session Format: Oral

Presentation Date: 10/13/2026

Presentation Start Time: 03:35 PM

Presentation Room: CCC, Bluebird Ballroom 3A

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