Tracking Marine Deoxygenation during the Cambrian SPICE Event Using Thallium Isotopes

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The stable isotopic composition of thallium (ε205Tl) in organic-rich, sulfidic sediments has recently emerged as a new proxy for tracking changes in global marine redox conditions in the geologic record [1, 2]. Here we employ this proxy to interrogate the record of the Cambrian SPICE (Steptoean Positive Carbon Isotope Excursion) — a putative Paleozoic oceanic anoxic episode that coincides with a marine extinction event. We examined two stratigraphic successions which span the SPICE: the Alum and Outwoods Shales (from Sweden and the U.K, respectively). Both records reveal a shift to less negative ε205Tl values that correspond with the onset of the SPICE and extinction event. This suggests a reduction of Mn-oxide burial and the expansion of anoxic bottom waters as previously hypothesized from other global geochemical records. Interestingly, the shift to less negative ε205Tl values during the SPICE continues after the carbon isotope peak, suggesting expanded anoxia may have continued after the organic carbon burial event — a pattern also observed during OAE 2 of the Cretaceous [1]. This suggests a comparable timeline for organic carbon burial relative to marine deoxygenation during each event. Our results demonstrate that the Tl isotope proxy captures a detailed view of marine redox changes not recognized using previously employed proxies and improves our understanding of the biological response to marine deoxygenation during the SPICE and other similar events.