Understanding the complexities of Proterozoic redox using carbon, nitrogen and trace metal composition of organic rich shales

Abstract

The Great Oxidation Event (GOE, Paleoproterozoic ca. 2.34 Ga) marked a time of substantial increase in oxidation. However, since this discovery the evidence for a Neoproterozoic Oxygenation Event (NOE, ca. 800 Ma–580 Ma) has flourished. More controversial and most recently, the suggestion of a potential Mesoproterozoic Oxygenation Event has been put forward. The aim of this thesis is to obtain a coupled data set using carbon and nitrogen isotopes in conjunction with molybdenum and vanadium trace metals as proxies for oceanic redox conditions throughout the Proterozoic. Collectively, samples involved span the late Paleoproterozoic through to late Neoproterozoic (ca. 1.74 Ga – 0.635 Ga), with a keen focus on the Mesoproterozoic. Carbon isotopes range from -32.99‰ (Barney Creek Formation) to -26.33‰ (Tent Hill Formation), suggesting a change in fraction of organic carbon burial from 0.12 and 0.40. This is almost a three-fold increase in organic carbon burial, which would predict an overall increase in production of O2. This is supported by an increase in V/Mo ratios coupled with decreasing Mo abundances through the Paleoproterozoic to Neoproterozoic, again a likely result of increasing of ocean oxygenation. With this in mind, a trend to lighter nitrogen isotopes followed by an increase to heavier δ15N can be analysed effectively. This trend in nitrogen isotopes is thought to be associated with an increase in Mo-based nitrogen fixation efficiency, followed by increased availability of aerobic nitrogen cycling (i.e. an abundance of NO3- and NO2-) to support the dominance of denitrification and anammox processes. Previous models that call for no significant growth in atmospheric oxygen post GOE or a singular Mesoproterozoic Oxygenation Event are not supported by this coupled data set. Carbon, nitrogen and trace metals present convincing evidence of an overall increase in ocean redox, and by proxy, atmospheric oxygen throughout the Mesoproterozoic and Neoproterozoic.