Molybdenum (Mo) and Iron (Fe) Isotope Evidence of Tepla-Barrandian Black Shales Against Widespread Deep Ocean Oxygenation in the Late Neoproterozoic

Abstract:

The late Neoproterozoic was a period of major environmental perturbations including tectonic reorganizations, biologic evolution and environmental oxygenation (Neoproterozoic oxygenation event). Authigenic enrichments in redox-sensitive elements such as Mo, V and U in late Neoproterozoic black shales prior to the appearance of the first metazoan fossils indicate that increasing oxygen levels in the atmosphere-hydrosphere system have facilitated the evolution and diversification of multi-cellular life. The isotopic composition of these elements is another tool to trace (possibly global) changes in the oceanic redox state. For example, significantly higher δ⁹⁸Mo of seawater and black shales are expected, when the sink of isotopically light Mo in oxic deep marine settings increased. Accordingly, modern anoxic sediments in the Black Sea as well as the well oxygenated seawater show high δ⁹⁸Mo of 2.3 ‰. However, Mesoproterozoic black shales show relatively low δ⁹⁸Mo values up to 1.4 ‰. To test if the enrichment of redox-sensitive elements and metazoan evolution temporally correlate with an increase in seawater δ⁹⁸Mo, we present Mo and Fe isotope data of slightly younger late Neoproterozoic black shales of the Tepla-Barrandian, Czech Republic.

We observe a perfect correlation of Fe/Al ratios with δ⁵⁶Fe that is best explained by mixing of detrital derived Fe with δ⁵⁶Fe of ~0.1 ‰ and hydrothermal sourced Fe with δ⁵⁶Fe of ~-0.7 ‰. Hydrothermally dominated samples with low δ⁵⁶Fe are also enriched in heavy metals such as Ni, Cu and Zn as well as hydrothermally derived Mo (with δ⁹⁸Mo of ~0.6 ‰). Samples with minor hydrothermal influence show authigenic Mo enrichment from seawater with a maximum δ⁹⁸Mo of 1.2 ‰. This estimate indicates no significant increase in the seawater δ⁹⁸Mo during the Neoproterozoic and after the evolution of metazoan life. Thus, our Mo isotope dataset provides no evidence for deep ocean oxygenation during the Neoproterozoic oxygenation event, which might have acted as a trigger for the evolution of metazoan life.