North-Central Section - 50th Annual Meeting - 2016

Paper No. 2-2
Presentation Time: 8:20 AM


WASSERMAN, Naomi L. and JOHNSON, Thomas M., Geology, University of Illinois, 156 Computing Applications Building, 605 E. Springfield Ave, Champaign, IL 61820,

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 (ε130/125: -4‰) or biologically (ε130/125: -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