A THALLIUM ISOTOPIC RECORD OF THE CAMBRIAN SPICE EVENT FROM THE ALUM SHALE, ANDRARUM, SWEDEN

Changes in the availability of oxygen in marine waters is often invoked as a contributing factor in biological crises and radiations throughout Earth history. However, tracking global changes of oxygenation in the ancient record remains a challenge due to the inherent limitations of available geochemical proxies. Recently, the geochemical behavior of the stable isotopes of thallium (²⁰³Tl and ²⁰⁵Tl) has emerged as a new proxy for tracking changes in global marine redox. Its suitability is rooted in observations from the modern ocean: (1) the ocean reservoir of Tl is isotopically homogeneous due to a long residence time; (2) the inputs of Tl are nearly isotopically identical, while only two major outputs fractionate Tl: sorption to Mn oxides, and the alteration of oceanic crust- the rate of which is unlikely to vary significantly on short timescales; (3) authigenic Mn oxides, only preserved in oxygenated sediment-water interfaces , are enriched in ²⁰⁵Tl relative to seawater; (4) pyrite formed in anoxic bottom waters inherits the Tl isotopic composition of coeval seawater (ε²⁰⁵Tl_SW) and can therefore be used to track the magnitude of the global Mn oxide removal flux, a reflection of the extent of oxic bottom waters.

To evaluate the Tl isotope proxy, we investigated the Cambrian Alum Shale, a black shale deposited along the margin of the paleocontinent of Baltica. The studied interval contains the SPICE (Steptoean Positive Carbon Isotope Excursion), a putative marine anoxic event associated with significant extinctions. Iron speciation analysis indicates anoxic conditions throughout the sampled interval, making it an ideal target for the Tl isotope proxy. Our data record a positive shift of ~3 ε units over the SPICE, suggesting reduced Mn oxide burial. This trend is in agreement with other global geochemical records which support the hypothesis that the SPICE represents an interval of expanded oceanic anoxia and euxinia. However, a change to less negative ε²⁰⁵Tl_SW precedes the initiation of the SPICE and continues after its peak. This suggests that the expansion of reducing conditions occurred prior to and continued after the carbon burial event. Our results demonstrate that the Tl isotope proxy can provide new insights about the timing and extent of redox changes not reflected in other more commonly applied redox proxies.