251-4 Thallium Isotopes Reveal Protracted Global Anoxia Through the Late Stages of the Early Silurian Ireviken Extinction Event

Session: Phanerozoic Earth System Shifts in the Marine Sedimentary Record

Presenting Author:

John Goodin

Authors:

Goodin, John¹, Wang, Yuxuan², Loydell, David K.³, Wignall, Paul B.⁴, Poulton, Simon W.⁵, Owens, Jeremy D.⁶, Young, Seth A.⁷

(1) Department of Earth, Ocean & Atmospheric Science, National High Magnetic Field Laboratory, Florida State University, Tallahassee, FL, USA, (2) School of Earth and Environment, University of Leeds, Leeds, United Kingdom; GFZ German Research Centre for Geosciences, Potsdam, Germany, (3) School of the Environment, Geography and Geosciences, University of Portsmouth, Portsmouth, United Kingdom, (4) School of Earth and Environment, University of Leeds, Leeds, United Kingdom, (5) University of Leeds, School of Earth and Environment, Leeds, United Kingdom, (6) Department of Earth, Ocean & Atmospheric Science, National High Magnetic Field Laboratory, Florida State University, Tallahassee, FL, USA, (7) Department of Earth, Ocean & Atmospheric Science, National High Magnetic Field Laboratory, Florida State University, Tallahassee, FL, USA,

Abstract:

The Silurian was an environmentally unstable period of the Paleozoic, featuring multiple perturbations to the carbon cycle and biological crises. The early Wenlock interval was characterized by a positive carbon isotope excursion (up to +5‰), the early Sheinwoodian carbon isotope excursion (ESCIE), currently hypothesized to result from increased marine organic carbon burial. Preceding and coincident with the carbon isotope excursion are extinctions of marine groups including conodonts, graptolites and trilobites known as the Ireviken Extinction Event (IEE). The cause(s) of the IEE are not well understood, but recent evidence has suggested that an increase in the extent of marine anoxia played a role. Sulfur isotope evidence has suggested an expansion of global euxinia, while I/Ca ratios from various sites provide evidence of local anoxia. However, further testing of this hypothesis is necessary, particularly with global redox proxies that are responsive to changes in the marine redox ladder above sulfate reduction.

Here, we present a thallium isotope investigation of the early Wenlock succession of the Nant-ysgollon Shales Formation from the Banwy River outcrop, Wales, United Kingdom. Thallium isotopes are uniquely suited to track the extent of global marine anoxia, as variations of marine ε²⁰⁵Tl values on <1 Myr timescales are determined by the burial of manganese oxide minerals which dissolve in anoxic waters or sediments. Crucially, because Mn reduction occurs shortly after oxic reduction, ε²⁰⁵Tl responds relatively rapidly to changes in global marine oxygenation. During the early Wenlock, the ε²⁰⁵Tl composition of Banwy River mudstones remains high (~ -2.0) within the firmus Biozone, suggesting little burial of manganese oxide minerals and widespread marine anoxia. Up the section, at levels close to where C. rigidus has been recorded, ε²⁰⁵Tl reaches heavy values of ~ -1.0, before declining to nearly -3.0. Values then increase to -1.0 prior to the first occurrence of M. belophorus. These results suggest that the nadir and falling limb of the ESCIE here were characterized by low oxygen marine environments. Expansive global anoxia may have driven high extinction rates in the early Wenlock.

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

doi: 10.1130/abs/2025AM-8854

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