PhD: Earth’s Great Oxidation: Redox and nutrient controls on the transition to a permanently oxygenated atmosphere

The rise of oxygen on the early Earth has been recognised as one of the most significant episodes in the history of our planet, and this initial rise set in motion a prolonged chain of events that ultimately resulted in the habitable conditions we now enjoy. In recent years, the topic of early Earth oxygenation has received a huge amount of attention, and almost without exception, traditional views on the timing, causes and consequences of oxygenation have been shown to be incorrect (e.g., Krause et al., 2018; Alcott et al., 2019). The first stages of significant planetary oxygenation are believed to have started sometime after the evolution of oxygenic photosynthesis, and the environmental consequences of this are seen early in the geological record, perhaps (but controversially) as far back as 3.0 billion years ago (Crowe et al., 2013). However, a significant rise in atmospheric oxygen, from essentially nothing to a few percent of modern levels, occurred much later. This rise has been dubbed the Great Oxidation Event (GOE; Holland, 2002), and for many years was considered to be just that – a single event. However, it is now apparent that early Earth oxygenation was much more complex than this, with the GOE occurring over a protracted period of time, with major rises and falls in oxygen levels (e.g., Gumsley et al., 2017; Poulton et al., 2021).

These fluctuations in atmospheric oxygen were linked to some of the most extreme perturbations in climate ever to affect the Earth, resulting in the first major glaciations of possible global extent – the so-called ‘Snowball’ Earth. Following these glaciations, it now appears that the GOE extended until the Lomagundi C isotope excursion ~2.1 billion years ago, which represents the largest positive C isotope excursion in Earth history. However, controls on the Lomagundi excursion, including its role in planetary oxygenation, are essentially unknown.

One key parameter that impacts greatly on the course of oxygenation is the availability of key nutrients, through their impact on primary productivity, carbon burial and oxygen release. The bioavailability of nutrients depends, to a large extent, on the supply of nutrients to the ocean through weathering and the subsequent behaviour of nutrients during deposition and diagenesis under different redox states. In this context, the precise redox state of the ocean has a huge impact on whether nutrients such as P and bioessential trace metals are fixed in the sediment or recycled back to the water column, where they can stimulate further productivity (e.g., März et al., 2008). Thus, a detailed understanding of nutrient availability during key intervals of planetary oxygenation also requires that geochemical studies are placed firmly in the context of ocean redox reconstructions. However, our understanding of ocean redox conditions, as well as P and trace metal micronutrient cycling and bioavailability after the final glaciation of the GOE and through the Lomagundi excursion, are almost non-existent. This hugely limits our ability to decipher links between the Lomagundi excursion and the most significant change in atmospheric chemistry in Earth’s history.

This project will take a new approach to unravelling nutrient availability by examining the speciation of phosphorus and trace metals in marine shales deposited across the final ~150 million years of the GOE (i.e., across the Lomagundi excursion). All of the nutrient analyses will be set in the context of the precise redox conditions encountered in the overlying water column and during diagenesis. These data will be combined with state-of-the-art biogeochemical modelling to test oxygenation hypotheses and to upscale local geochemical data to the global scale. There will be several key aims, including:

What was the nature of ocean redox chemistry at the height of the Lomagundi excursion and how did this evolve during its termination?
Is there evidence for enhanced nutrient bioavailability driven by changes in weathering and/or redox conditions?

Did changes in nutrient bioavailability cause the Lomagundi excursion, and what controlled its termination? How high did oxygen levels rise at this time and were these levels maintained after the Lomagundi excursion?

In answering these questions, this project will address several major outstanding issues related to the early evolution of Earth’s surficial environment and the ultimate development of a planet habitable for oxygen-breathing organisms.