The Great Oxidation Event: A 200-Million-Year Pathway to Oxygenation

Recent findings indicate that Earth’s “Great Oxidation Event” unfolded over a span of 200 million years.

Fresh research underscores the intricate nature of the Great Oxidation Event, demonstrating that the increase in atmospheric and oceanic oxygen was a dynamic process lasting over 200 million years. This evolution was shaped by geological and biological factors crucial for the development of life.

Around 2.5 billion years ago, oxygen, or O2, began accumulating in Earth’s atmosphere at significant levels, marking a crucial milestone in the planet’s evolution towards supporting complex life forms.

Unveiling the Complexity of the Great Oxidation Event (GOE)

Known as the Great Oxidation Event (GOE), this process was far more complex than previously thought, as revealed by recent research led by a geochemist from the University of Utah.

According to Chadlin Ostrander, an assistant professor in the Department of Geology & Geophysics and the lead author of the study published in Nature on June 12, the GOE spanned at least 200 million years. Understanding the accumulation of oxygen in Earth’s oceans has posed significant challenges until now.

“Emerging data indicate that the initial rise of oxygen in Earth’s atmosphere was dynamic and occurred intermittently until approximately 2.2 billion years ago,” Ostrander explained. “Our findings support this hypothesis and further extend these dynamics to include the oceanic environment.”

His international research team, backed by the NASA Exobiology program, focused on marine shales found within South Africa’s Transvaal Supergroup. Their study provided insights into the dynamics of ocean oxygenation during a pivotal era in Earth’s history.

Through analysis of stable thallium (Tl) isotope ratios and elements sensitive to redox conditions, they identified fluctuations in marine oxygen levels that coincided with changes in atmospheric oxygen.

These discoveries contribute significantly to our understanding of the intricate processes that influenced Earth’s oxygen levels during a critical period, setting the stage for the evolution of life as we recognize it today.

“We still lack a clear understanding of the oceanic conditions, which likely nurtured Earth’s earliest life forms and their evolution,” said Ostrander, who recently joined the University of Utah from the Woods Hole Oceanographic Institution in Massachusetts. “Knowing the oxygen levels in the oceans and how they changed over time is probably more critical for early life than understanding the atmosphere.”

Building on Insights from Previous Research on Earth’s Oxygenation

This research builds on previous work by Ostrander’s colleagues, Simon Poulton of the University of Leeds in the UK, and Andrey Bekker of the University of California, Riverside. Their 2021 study revealed that oxygen didn’t permanently accumulate in the atmosphere until about 200 million years after the global oxygenation process began, a much later timeline than previously thought.

The presence of rare, mass-independent sulfur isotope signatures in sedimentary records before the GOE serves as compelling evidence of an anoxic atmosphere. These sulfur isotope patterns are difficult to generate through natural processes on Earth, and their persistence in the rock record strongly implies a lack of atmospheric O2.

For the first half of Earth’s history, both its atmosphere and oceans were largely devoid of O2. Cyanobacteria in the oceans began producing this gas before the GOE, but the early O2 was quickly consumed in reactions with exposed minerals and volcanic gases.

Poulton, Bekker, and their colleagues observed that these rare sulfur isotope signatures disappear and reappear, indicating multiple fluctuations in atmospheric O2 levels during the GOE. This era was marked by successive increases and decreases in oxygen levels, rather than a singular, isolated event.

Challenges in Earth’s Oxygenation

According to Ostrander, Earth underwent significant biological, geological, and chemical evolution before becoming suitable for oxygenation. He likens this process to a teeter-totter, where oxygen production and destruction were in equilibrium, preventing significant atmospheric change. Researchers are still investigating when Earth reached a point of irreversible oxygenation, transitioning away from anoxic conditions.

Despite oxygen now constituting 21% of Earth’s atmosphere by weight, second only to nitrogen, it remained a minor component for hundreds of millions of years following the GOE.

Advanced Isotopic Analysis Techniques

To study oceanic oxygenation during the GOE, Ostrander’s team employed stable thallium isotopes, benefiting from recent advancements in mass spectrometry. Isotopes, variants of the same element with different neutron counts, play a crucial role in fields like archaeology and geochemistry.

Thallium Isotopes and Oxygen Indicators

Thallium isotope ratios reflect manganese oxide burial on the ocean floor, a process requiring O2 in seawater. Analysis of thallium isotopes in marine shales, which also tracked rare sulfur isotope fluctuations in atmospheric O2 during the GOE, revealed enrichments in the lighter-mass thallium isotope (203Tl).

These enrichments coincide with periods when the atmosphere was oxygenated, as indicated by the absence of rare sulfur isotope signatures. Conversely, the enrichments disappear when the atmosphere returned to anoxic conditions, aligning with findings from redox-sensitive elements, another tool used to study ancient O2 variations.

Ostrander concludes, “When sulfur isotopes signal atmospheric oxygenation, thallium isotopes indicate oceanic oxygenation. The synchronized oxygenation and deoxygenation of atmosphere and ocean provide new insights into ancient Earth, revealing previously unknown dynamics.”

Read the original article on: Scitech daily

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