Evolution of Earth’s oxygenation and temperature depends on surface carbonate accumulation

There are no good models for the chemical evolution of the Earth’s surface over the planet’s lifetime, because models typically overlook the progressive build-up of carbonate rocks in the crust. A new model that includes this accumulation enables the reconstruction of major oxygen and temperature trends throughout Earth’s history.

The problem

Oxygenation of the Earth’s atmosphere and oceans played a pivotal part in the evolution of the surface environment and life¹. Numerous processes have been proposed to drive oxygenation, but obtaining a reasonable approximation of Earth’s oxygenation history by using simulations that account for the evolution of all these processes has not been possible. The same is true for Earth’s temperature: although we have a good understanding of the processes that control global temperature over Earth’s history, we have few models that can reconstruct changes in temperature over all of Earth’s history, and none that can reproduce both temperature and oxygenation trends using a system that connects all the cycles of the main elements (for example, carbon and oxygen).

A key part of this problem is the rate of carbon dioxide (CO₂) input to the surface system from the deep Earth (mantle). This input was probably larger — perhaps much larger billions of years ago — than it is now, because the mantle was hotter. But this factor then implies more carbon might have been cycling through the atmosphere, oceans and biosphere on the early Earth than at present, which should have led to high CO₂ levels and temperatures, as well as high rates of photosynthesis and abundant oxygen. However, we know from geological evidence that the early Earth was devoid of oxygen and experienced severe glaciations.

The solution

An often-overlooked process in these models is the accumulation of carbon in the form of carbonate in the crust through both inorganic and biologically controlled mechanisms². This build-up potentially led to increased amounts of CO₂ outgassing from volcanoes throughout Earth’s history, which might have been able to counter the mantle cooling trend and led to a speeding up of the global carbon cycle over time, rather than a slowing down.

To understand how these different processes affect the Earth’s habitability, we further developed a theoretical framework for the long-term oxygen, phosphorus and carbon cycles³ by incorporating potential trajectories for the emergence of continents above the sea surface, which has implications for the amount of nutrients that can be released by weathering (the break-down of surface rocks), CO₂ degassing from volcanoes, and the increase in the size of the crustal carbonate reservoir. This is the first model to incorporate all of these potential drivers of oxygenation and at the same time consider their effects on global oxygen, carbon and phosphorus cycles (Fig. 1).

Fig. 1: Simulated oxygen and carbon dioxide concentrations.

a, Estimated atmospheric oxygen concentrations over the past 4,000 million years (Ma) according to our model; dashed boxes show oxygen concentrations estimated from geochemical proxies⁴. PAL, Present Atmospheric Level. b, Estimated CO₂ (ppm) concentration; dashed lines denote 95% confidence interval from a carbonate–silicate cycle model⁵. Simulated 95% confidence intervals in both panels are shown as a shaded area with the median value plotted. © 2024, Alcott, L. J. et al., CC BY 4.0.

We find that without accounting for the growing crustal carbonate reservoir, we are unable to simulate the known geological record of Earth, caused by high degassing rates of CO₂ on the early Earth due to the hotter and more active mantle. These elevated temperatures lead to increased rates of weathering and nutrient delivery for photosynthesis, meaning that an anoxic Earth is not achievable under these conditions.

However, the inclusion of a progressively increasing continental carbonate reservoir acts to diminish CO₂ degassing rates on the early Earth (when carbonates were rare), leading to cooler global temperatures and much lower levels of atmospheric oxygen — consistent with the geological record. Our model was also able to reproduce isotopic signatures (which show the balance between inorganic and organic carbon) and nutrient burial records (the amount of phosphorus found in rocks) over long timescales, strengthening our conclusions.

The implications

Our model shows that simulating Earth’s surface chemical evolution is only possible when we include the accumulation of carbonates in the crust. This carbonate build-up is a key factor missing from previous models linking Earth’s climate and nutrient and oxygen evolution, and it might be of importance for reconstructing various events over Earth’s history and for potential exoplanet biogeochemistry.

These results suggest that the evolution of an oxygenated Earth-like planet elsewhere in the galaxy might not require a specific biological revolution or tectonic event beyond the inception of photosynthetic bacteria, and we might expect high-oxygen worlds to be more commonplace than anticipated. However, the long timeframe required to sufficiently build up carbonates in the crust might mean that oxygenation — and therefore the permissive environmental conditions required for complex life as we know it — might only be possible on older worlds.

Lewis Alcott¹ & Benjamin Mills²

¹University of Bristol, Bristol, UK.

²University of Leeds, Leeds, UK.

Expert opinion

“To my knowledge, the fundamental concept presented in this paper is novel and has the potential to advance the understanding of the Earth’s surface evolution. I’m not aware that the concept of carbonate accumulation over time has previously been considered in models of nutrient fluxes and redox changes. I would therefore expect this paper to have high impact and result in numerous follow-up discussions.” Eva Stüeken, University of St Andrews, St Andrews, UK.

Behind the paper

This work came about because we wanted to know whether it was possible to include all the key factors involved in oxygenation in a simple planetary model. The progression to this model is the culmination of decades of proposed drivers of planetary environmental changes. Probably the most difficult part of the research was convincing the reviewers of the paper that despite the substantial error bars on our model’s simulations, our work still represented a big step forwards in understanding. On shorter timescales, we are often able to obtain simulations that are very close to geological data by making assumptions about the long-term background state of the Earth System, but here we are testing that background state itself, which results in major paradoxes over deep time and is very difficult to model. L.A. & B.M.

From the editor

“This work by Alcott et al. stood out to me because their model can simulate the oxygenation trajectory of the early Earth in both the atmosphere and oceans, which is difficult to achieve. Their findings pose new questions on the importance of crustal carbonates in helping to create habitable conditions on this planet and others.” Alison Hunt, Associate Editor, Nature Geoscience.