The historical timeline of ancient ocean oxygenation: a look into Earth's past

The oxygenation of the Earth’s atmosphere and oceans and the emergence and evolution of life are pivotal episodes in the planet’s history. Now, a pioneering application of dolomite uranium–lead dating (U–Pb dating) – one of the oldest and most sophisticated radiometric techniques for determining the age of rocks and other objects – has been developed by scientists at the Hebrew University of Jerusalem (HU).

It can be used to date rocks that formed and crystallized from about a million years to over 4.5 billion years ago with routine precisions in the 0.1–1 percent range – shedding new light on the evolution of ancient marine environments. 

By uncovering discrepancies in dolomite samples’ U-Pb ratios, the team developed a reliable proxy for reconstructing the levels of oxygen within ancient marine habitats in which the first animals emerged and evolved. 

Dr. Uri Ryb and Dr. Michal Ben-Israel from HU’s Institute of Earth Sciences, along with their collaborators. revealed a significant rise in marine oxygenation during the Late Paleozoic era (400 million years ago), hundreds of millions of years after the emergence of animal-life. 

These findings suggest that early animals have evolved in oceans that were mostly oxygen-poor and deepened our grasp of interactions among ecosystems and the evolution of complex life forms. 

Comprehending these relationships provides critical context for future observations of exoplanet’s atmospheres using the new generation of space-telescopes in search for extra-terrestrial life.

This important discovery in the field of earth sciences has just been published in the prestigious journal Nature Communications under the title “Late Paleozoic oxygenation of marine environments supported by dolomite U-Pb dating.” 

The new approach spans 1.2 billion years

The new approach reconstructs the rise of oxygen in ancient marine environments using such measurements in dolomite rocks spanning the last 1.2 billion years. 

Most earth scientists had estimated the oxygen levels in ancient oceans from the composition of “redox-sensitive” elements preserved in ancient sedimentary rocks – but these compositions can be easily changed in the course of geological history. 

The team overcame this challenge by developing a new approach that uses dolomite U-Pb dating to detect signals of oxygenation that are resistant to such alteration, giving us an unbiased perspective on marine oxygenation dynamics.

Their record indicates a dramatic increase in the oxygenation of the oceans during the Late Paleozoic era, hundreds of millions of years after the emergence of the first animals. 

This aligns with other evidence indicating the oxygenation of the ocean at the same time, support the hypothesis that animals have evolved in oceans that were mostly oxygen-limited, and suggests that changes in ocean oxygen were driven by evolution. 

According to Ryb, these discoveries not only enhance our understanding of ancient Earth ecosystems but also have implications for the search of extra-terrestrial life. 

“Revealing the dynamics between evolution and oxygen levels in early Earth environments can put observations on the atmospheric composition of exoplanets that now become available through the new generation of space telescopes in context. Specifically, suggesting that low levels of oxygen are sufficient for complex life-forms to thrive.”