TELLURIUM ISOTOPES AS A PALEOREDOX PROXY: AN INITIAL ASSESSMENT

The Precambrian transition from an anoxic to an oxic atmosphere was once described as a unidirectional step-wise function, with the Great Oxidation Event (2.5-2.3 Ga) and Neoproterozoic Oxidation Event (0.8 Ga) marking two increases in atmospheric and marine O₂ levels. The period of low atmospheric O₂ in between was dubbed the “boring billion”, but recent research supports a more dynamic evolution influenced by exchanges between the sources and sinks of O₂. The lack of Cr isotopic fractionation in ironstone formations until 0.8 Ga has been suggested to indicate insufficient O₂ (<0.1% of present atmospheric level, PAL) to oxidize and weather continental Cr.¹ This work has important implications for the delayed rise of animals, as the earliest metazoans and last common ancestors of animals had O₂ requirements of 0.5% PAL.² Surface ocean oxygenation, though, allowed significant evolution to occur including the rise of modern eukaryotes.³ Additionally, recent estimates of mid-Proterozoic O₂ levels, based on redox-sensitive element concentrations in deepwater shales, are as high as 4% PAL.⁴ Different isotopic paleoredox proxies could resolve these arguments. However, the range of sensitivity to oxygenation for other proxies is either too high or low for the mid-Proterozoic.

Tellurium (Te) isotope ratios (¹³⁰Te/¹²⁵Te) may provide greater detail about atmospheric O₂ levels over the mid-Proterozoic. Te isotope ratios would be invariant as long as Te remained in reduced forms, but would “light up” once conditions promoted its oxidation to the +4 and/or +6 valence. The reduction of Te(VI) to Te(0) produces a kinetic isotope effect mediated abiotically (ε^130/125: -4‰) or biologically (ε^130/125: -5‰).^5,6 Based on known thermodynamic data, the redox (Eh) threshold at which isotopic fractionation would begin is greater than that of U and less than that of Cr. We have established an improved method for measuring ¹³⁰Te/¹²⁵Te and are analyzing samples from Precambrian paleosols and marine ironstones to begin developing this new paleoredox indicator.

[1] Planavsky et al. (2014) Science 346,235-238

[2] Mills et al. (2014) PNAS 111,4168–4172

[3] Knoll (2014) Perspect. on Biol. 6,a016121

[4] Zhang et al. (2016) PNAS,in press

[6] Smithers and Krause (1968) Can. Journ. Chemistry 46,583-591

[7] Baesman et al. (2007) AEM 73, 2135-2143