Unraveling the Enigma of Earth’s Early Atmosphere Through the Great Oxidation Event

Microscopic mineral samples offer unprecedented insights into oxygen accumulation in the atmosphere and its ensuing effect on Earth’s mantle. Source: Hugo Moreira / Nature Geoscience

By employing synchrotron technology, researchers have revealed vital data about the Great Oxidation Event. Their work involved the examination of apatite inclusions within zircon crystals taken from ancient magmas via the ESRF – Exceptionally Bright Source.

Approximately 2.4 billion years in the past, Earth experienced a watershed moment known as the Great Oxidation Event. During this time, there was a marked increase in atmospheric oxygen levels. This surge fundamentally transformed the planet’s atmospheric composition and led to significant chemical changes, paving the way for the evolution of complex multicellular life and a profound reorganization of Earth’s ecosystems.

The Influence of Plate Tectonics on Earth’s Chemical Composition

Plate tectonics serve as an efficient conduit for the transfer and cycling of elements between Earth’s surface, atmosphere, and mantle. As mountains erode through their interaction with water and air, they decompose into sedimentary particles. These particles are then partially reabsorbed into the Earth’s mantle via subduction mechanisms (where one tectonic plate descends under another). The generation of magmas in the mantle regions above these subduction zones offers a unique opportunity to understand how the Earth’s atmosphere could influence its mantle by integrating materials from these subducted sediments.

Innovative Techniques for Investigating Geological Interplay

Studying the complex relationship between Earth’s mantle and its atmosphere has been an ongoing endeavor for scientists. This task is especially challenging when considering Earth’s early history, marked by rapidly changing atmospheric and tectonic conditions. Researchers from the University of Montpellier and the University of Portsmouth partnered with the ESRF – The European Synchrotron – to circumvent these obstacles, focusing their efforts on the study of apatite inclusions in zircon extracted from subduction zones.

In a 2017 publication, Hugo Moreira, a CNRS postdoctoral researcher at the University of Montpellier and lead author of the study, pointed out that apatite, when formed under reduced conditions (lack of free oxygen), displayed a unique sulfur signature. Conversely, under oxidized conditions, the sulfur content within the apatite exhibited a markedly different profile, making apatite an effective indicator for redox conditions. Moreira and his team investigated phosphate-mineral apatite inclusions in zircon that crystallized in magmas of an ancient subduction zone. They measured the sulfur valence states using X-ray absorption near edge structure (XANES) at the ESRF, renowned as the brightest synchrotron light source.

Crucial Discoveries and Their Implications

The state of sulfur within apatite is intrinsically tied to the oxygen fugacity of the surrounding magma, making it an ideal candidate for assessing oxidation states during magmatic development. “The advantage of studying apatite inclusions in zircon, as opposed to apatite from the rock matrix, is their preservation within extremely durable zircon crystals, safeguarding their original composition,” states Moreira.

The findings revealed that the sulfur in apatite inclusions from magmas that formed prior to the Great Oxidation Event displayed a relatively reduced state. Post-event samples exhibited a more oxidized state. The zircon analysis indicated that these magmas originated from similar sources, but the younger samples had integrated a sediment component. This underscores the transformative impact that an increasingly oxidizing atmosphere had on modifying the Earth’s mantle and in shifting magmatic fugacity towards more oxidized conditions.

Directions for Future Research

“Our research confirms that synchrotron X-ray examination of apatite inclusions in zircon is an invaluable methodology for constraining critical magmatic parameters,” concludes Moreira.

The team’s future efforts will focus on investigating magmas that formed during other significant epochs of Earth’s history, such as the Neoproterozoic Oxidation Event (commencing 850 million years ago) and during the emergence of the first traces of oxygen in the Archean era.

Reference: “Sub-arc mantle fugacity shifted by sediment1 recycling across the Great Oxidation Event,” 31 August 2023, Nature Geoscience.
DOI: 10.1038/s41561-023-01258-4

Frequently Asked Questions (FAQs) about Great Oxidation Event

More about Great Oxidation Event

• Nature Geoscience Publication
• University of Montpellier Research
• University of Portsmouth Research
• ESRF – The European Synchrotron
• Overview of the Great Oxidation Event
• Introduction to Plate Tectonics
• Explanation of Synchrotron Technology
• Apatite as a Mineral Indicator
• Neoproterozoic Oxidation Event
• The Archean Eon