The oxygenation of the Earth's atmosphere and oceans, as well as the emergence and evolution of life, are essential episodes in our planet's history. An abundance of geochemical and sedimentological evidence supports the increase in the oxygen level in the atmosphere.
However, the time when marine habitats became oxygenated and the relationship between such oxygenation and evolutionary changes are still debated. Hypothesized ocean oxygenation timelines range from an oxygen increase between the Neoproterozoic and Cambrian periods to minor episodic oxygenation before the late Paleozoic-early Mesozoic eras.
Reconstructing Geological Timeline
To test these hypotheses, the timeline and extent of oxidation of marine habitats from redox-sensitive geochemical proxies need to be reconstructed. At the Hebrew University of Jerusalem, a team of researchers has introduced a new approach to reconstructing the rise in oxygen levels in ancient marine environments.
Dr. Uri Ryb and Dr. Michal Ben-Israel from the Institute of Earth Sciences led the pioneering application of dolomite U-Pb geochronology. The details of their work are discussed in the paper "Oxygenation of Marine Environments Supported by Dolomite U-Pb Dating."
In this study, the scientists used uranium and lead measurements in dolomite rocks spanning the last 1.2 billion years to create a reliable proxy for revealing the levels of oxygen within ancient marine environments.
Experts usually study the composition of 'redox-sensitive' elements preserved in ancient sedimentary rocks when estimating the oxygen levels in ancient oceans. However, these compositions can be altered over time.
To overcome this challenge, the research team used dolomite U-Pb dating to detect oxygenation signals resistant to these alterations. This gives them an unbiased perspective on marine oxygenation dynamics.
The record reveals a dramatic increase in ocean oxygenation during the Late Paelozoic era, hundreds of millions of years after the first animals emerged. This aligns with other evidence that indicates the oxygenation of the ocean at the same time. It also supports the hypothesis that animals evolved in oceans that were mostly oxygen-limited, suggesting that changes in ocean oxygen were brought by evolution.
According to Dr. Ryb, their discoveries enhanced our understanding of ancient Earth ecosystems and can have implications for the search for extraterrestrial life. Revealing the dynamics between evolution and oxygen levels in early Earth environments can provide insights into the atmospheric composition of exoplanets. It specifically suggests that low oxygen levels can be sufficient to allow complex life forms to thrive.
How Does U-Pb Geochronology
Uranium-lead dating is one of the oldest and most refined radiometric dating schemes used to date rocks and minerals.
U-Pb geochronology relies on two separate decay chains: the uranium series from U-238 to Pb-206, with a half-life of 4.47 billion years, and the actinium series from U-235 to Pb-207, with a half-life of 710 million years.
This method can date rocks that formed and crystallized from about 1 million years to more than 4.5 billion years ago with routine precisions in the 0.1-1% range. It is usually applied to zircon, which incorporates uranium and thorium atoms into its crystal structure but strongly rejects lead when it forms. The newly-formed zircon crystals do not contain lead, so any lead detected in the mineral is radiogenic. Since the exact rate at which uranium decays into lead is known, the current ratio of lead to uranium in a mineral sample can be used to determine its age.
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