99 Evolution of Life in the Cambrian Seas: Biotic, Biogeochemical, and Sedimentological Contexts, Part I

This session is dedicated to studies that illuminate the biotic, geobiological, sedimentological, and geochemical contexts of the evolution of early animal life in the oceans, with a particular focus on the Cambrian Period.

Monday, 20 October 2025
8:00 AM - 12:00 PM
HBGCC, 304B

Redox Revelations from Utah’s Wheeler Formation: A Multi-Proxy Reconstruction during the Rise of Animals

Paper number: 99-13
Program: Technical Sessions
Time: 11:00 AM – 11:15 AM

Authors

Abstract

Reconstructions of Earth’s ocean oxygenation history during the Cambrian Period (538-486 Ma) provide insight into the role of free oxygen (O2) in early animal evolution. Many Cambrian oxygenation reconstructions are derived from sedimentary sequences with no material evidence of animals. While there are benefits to this approach, it requires indirect correlations to the fossil-bearing strata, complicating temporal animal-O2 connections. 

 

This presentation will summarize our efforts to reconstruct local- and global-scale ocean redox conditions from mid-Cambrian siliciclastic-dominated sedimentary rocks with rare and exceptional soft-body faunal preservation. Our sample targets are fossil-bearing calcareous shales of the ~503 million-year-old Wheeler Formation from western Utah recovered from drill core DM-15-1. We have applied a long (and still growing) list of paleoredox proxies to these shales. This includes iron speciation and redox-sensitive trace metals (e.g., Mo, U, V) to constrain local redox, and thallium (Tl) isotopes to potentially track global-scale changes in deep ocean oxygenation.

 

Geochemical data generated thus far reveal novel paleoenvironmental information. Iron speciation data suggest a highly anoxic and at-times even sulfidic (euxinic) local environment, whereas trace metal data support suboxic to oxic conditions. All data considered, local scale redox conditions seemed to straddle the oxic-anoxic boundary, with perhaps short-term redox oscillations but tending toward reducing conditions. This broadly reducing characterization is consistent with the local fossil preservation, in that soft-bodied fauna can only be preserved in reducing sediments.

 

Locally reducing conditions are ideal for applying Tl isotopes as a global-scale paleoredox proxy. Sedimentary sulfides formed under such conditions should, by analogy with today, reliably capture the globally homogenous seawater Tl isotope composition (e205Tl). Seawater e205Tl values are very sensitive to Mn oxide burial on the seafloor because preferential 205Tl removal during this process drives a complimentary seawater 203Tl enrichment. Wheeler shale e205Tl values rest primarily between averaged ocean inputs and modern seawater, implying that the Cambrian global deep ocean was more reducing than today.

 

Our analyses provide important information about ancient local conditions for a soft body fossil-rich Cambrian shale unit, as well as insight into global-scale deep ocean oxygenation during one of the most explosive periods of animal diversification in Earth history.

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

doi: 10.1130/abs/2025AM-414285

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