Cryogenian Oceans Oxygenated by 700 Million Years Ago

Mo isotopic composition of the mid-Neoproterozoic ocean: An iron formation perspective

Authors:

1. Geoffrey J. Baldwin (a)
2. Thomas F. Nägler (b)
3. Nicolas D. Greber (b)
4. Elizabeth C. Turner (a)
5. Balz S. Kamber (c)

Affiliations:

a. Department of Earth Sciences, Laurentian University, Sudbury, ON, Canada P3E 2C6

b. Institut für Geologie, Universität Bern, Baltzerstr. 1, 3012 Bern, Switzerland

c. Department of Geology, Trinity College Dublin, Dublin 2, Ireland

Abstract:The Neoproterozoic was a major turning point in Earth's surficial history, recording several widespread glaciations, the first appearance of complex metazoan life, and a major increase in atmospheric oxygen. Marine redox proxies have resulted in many different estimates of both the timing and magnitude of the increase in free oxygen, although the consensus has been that it occurred following the Marinoan glaciation, the second globally recorded “snowball Earth” event. A critically understudied rock type of the Neoproterozoic is iron formation associated with the Sturtian (first) glaciation. Samples from the 716 Ma Rapitan iron formation were analysed for their Re concentrations and Mo isotopic composition to refine the redox history of its depositional basin. Rhenium concentrations and Re/Mo ratios are consistently low throughout the bottom and middle of the iron formation, reflecting ferruginous to oxic basinal conditions, but samples from the uppermost jasper layers of the iron formation show significantly higher Re concentrations and Re/Mo ratios, indicating that iron formation deposition was terminated by a shift towards a sulfidic water column. Similarly, the d98Mo values are close to 0.0‰ throughout most of the iron formation, but rise to ~+0.7‰ near the top of the section. The d98Mo from samples of ferruginous to oxic basinal conditions are the product of adsorption to hematite, indicating that the Neoproterozoic open ocean may have had a d98Mo of ~1.8‰. Together with the now well-established lack of a positive Eu anomaly in Neoproterozoic iron formations, these results suggest that the ocean was predominantly oxygenated at 700 Ma.