Oct 24, 2023

Why is Earth so oxygenated? New 'mantle' link discovered


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An international team of scientists recently uncovered a significant connection between Earth's early atmosphere and the chemistry of its deep mantle.

They achieved this by investigating ancient magmas formed in subduction zones during the Great Oxidation Event (GOE), which occurred 2.1 to 2.4 billion years ago.

The findings, published in Nature on August 31, offer essential insights into Earth's geological evolution and reveal how the deep Earth and its mantle are closely related to changes in the atmosphere.

The Earth's atmosphere, upon which we rely for breathing, consists of 21 percent oxygen. However, have you ever paused to consider where and when this crucial element originated? Well, that's where this recent study comes in.

“With these findings, our understanding of Earth's ancient ‘breath’ has taken a significant leap forward. Not only does it provide crucial insights into Earth's geological evolution, but it also sheds light on how the deep Earth and its mantle are intimately connected to atmospheric changes," said lead author Dr. Hugo Moreira from the University of Montpellier in a press release.

"It provides us a better understanding of the relationship between Earth's external and internal reservoirs."

Additionally, she emphasized that the findings prompt intriguing questions about the role of oxygen in shaping Earth's history and creating conditions conducive to life as we know it.

One of its key findings revolves around the role of plate tectonics – the process through which the Earth's outer shell shifts and reshapes its surface.

While much has been learned about the effects of atmospheric changes, understanding how these changes impacted the Earth's mantle has remained relatively unexplored.

The research aimed to bridge this gap by investigating the intricate relationship between the Earth's deep interior and the evolving atmosphere.

The investigation involved analyzing ancient magmas that crystallized before and after the GOE. The team's experiments revealed a shift from magmas with reduced properties to those with higher oxidation levels.

Hugo Moreira / Nature Geoscience

This transformation, they say, was driven by the deep subduction of oxidized sediments – remnants of mountains that underwent weathering and erosion.

These sediments were recycled into the Earth's mantle through subduction processes, effectively creating a pathway for atmospheric elements to interact with the mantle.

The implications of this discovery extend to our understanding of Earth's geological evolution. Even small fluctuations in oxygen levels during the GOE could have triggered an increase in the oxidation of specific magma types.

This shift likely contributed to changes in the composition of the Earth's continental crust and played a role in forming valuable ore deposits.

The research team employed advanced techniques, including analysis using the ID21 beamline at the European Synchrotron Radiation Facility in France.

They examined the sulfur states within minerals trapped in two-billion-year-old zircon crystals from Brazil's Mineiro Belt. These ancient crystals acted as time capsules, preserving clues about the Earth's distant past.

The team noted a clear transition: minerals formed before the GOE showed a reduced sulfur state, while those created afterward exhibited a more oxidized state.

The study's implications are not confined to scientific understanding alone. It opens up new avenues of research, shedding light on the complex relationship between geological processes and atmospheric changes.

Co-author Professor Craig Storey from the University of Portsmouth highlighted the study's significance, stating that it offers "a deeper understanding of the Earth's ancient past and its profound connection to the development of our atmosphere."

As we continue to explore the depths of Earth's geological history, it's evident that there is much more to uncover beneath the surface. Dr. Moreira aptly summed up this sentiment, highlighting that the study's findings underscore the ongoing quest to unravel the mysteries of our planet's past.

The complete study was published in Nature on August 31 and can be found here.

Study abstract:

The chemical exchange between the atmosphere, crust and mantle depends on sediment recycling via subduction. However, it remains unclear how atmospherically modified sediment may affect mantle oxygen fugacity through time. The Great Oxidation Event, among the most important atmospheric changes on Earth, offers an opportunity to investigate changes in magmatism related to surface–mantle interactions. Here we use sulfur K-edge micro X-ray absorption near-edge structure spectroscopy to measure the relative abundances of S6+, S4+ and S2− state in apatite inclusions hosted in 2.4–2.1-billion-year-old igneous zircons from the Mineiro Belt, Brazil. The host magmas record intracrustal melting of juvenile crust and the involvement of recycled sediments in the sub-arc mantle wedge. Unaltered apatite inclusions reveal a change from reduced to more oxidized magmas from pre- to post-Great Oxidation Event during the early Proterozoic. We argue that this change is a direct result of deep subduction of oxidized sediments and thus evidence of mantle–atmosphere interaction across the Great Oxidation Event. This suggests that the onset of sediment recycling in the Archaean provided atmospheric access to the mantle, and early ‘whiffs’ of oxygen may have already contributed to a localized increase of calc-alkaline magmatism and related ore deposits on Earth.

Earth's ancient oxygen levels Two-billion-year-old zircon crystalsStudy abstract: