Primitive Oceans and Atmosphere Pulsed in Sync for 200 Million Years
Researchers have mapped out when Earth's oceans transitioned from being oxygen-free to oxygen-rich. This transition occurred over many millions of years, during which the oxygen levels in the oceans rose or fell - always in close sync with the oxygen levels in the atmosphere.
If you know where to look—and have the right techniques—Earth's oldest rocks and cliffs hold many insights into Earth's past and development.
Kasper Primdahl Olesen, a PhD student in geochemistry at the Department of Biology, knows how to look and he has been involved in analyzing sediments from Transvaal in South Africa, which in Earth's youth was a seabed. This work has contributed to new insights into the crucial period of Earth's evolution when the atmosphere and oceans came to contain enough oxygen for complex life to develop.
The analyses are part of a study led by assistant professor Chadlin Ostrander from the Department of Geology and Geophysics at the University of Utah and published in the scientific journal Nature. Other researchers behind the study are from Woods Hole Oceanographic Institution, University of California, University of Johannesburg, University of Leeds, and Université de Lorraine.
First Life on Earth
The first life on Earth arose about 3.8 billion years ago. However, it was far from the life we know today and consisted of simple, single-celled organisms that lived in an oxygen-free world.
Science has long known that one change was crucial for the evolution of complex life: the rise of oxygen in Earth's atmosphere. Researchers have traditionally called this change the Great Oxidation Event, but it was not a single event in the atmosphere, the researchers conclude in their new study.
Instead, the oxidation occurred over a long period of about 200 million years, it happened in pulses, and it also happened in the oceans.
South Africa's Ancient Rocks
The analyzed sediments come from the Transvaal Supergroup, an area in northern South Africa and southern Botswana. It contains ancient seabed sediments that are approximately 2.5 billion years old.
Pulses of Oxygen in Ocean and Atmosphere
- Our study shows that there was a close connection between the oxidation of the atmosphere and the oxidation of the oceans. Both the atmosphere and the oceans went through several pulses of oxygenation and deoxygenation over a period of 200 million years, and this happened synchronously. The key word here is synchronous. When oxygen disappeared from the atmosphere, it also disappeared from the ocean—and vice versa, said Kasper Primdahl Olesen and continues:
- This is fundamentally new knowledge because we previously did not have the ability to determine the oxygen content and oxidation patterns in the oceans during this period of Earth's history. This new knowledge significantly contributes to our understanding of a crucial event for the evolution of multicellular life.
After several pulses of oxygenation and deoxygenation, the oxygen content finally reached the threshold that allowed complex life to evolve.
Oxygen in the Sea and Atmosphere Followed Each Other
The researchers reached their results by analyzing thallium isotopes from the Transvaal sediments. Thallium is a heavy metal found in different variants called isotopes. The research team found enrichments of a thallium isotope in the Transvaal sediments, and since thallium isotopes are affected when manganese oxidizes on the seabed, the isotopes can indicate the oxygen content in the sea in which the Transvaal sediments were once deposited. These analyses thus tell researchers that oxygen was globally accumulated in seawater during some periods and not during others.
Two of the study's co-authors, Simon Poulton from the University of Leeds and Andrey Bekker from the University of California, have previously conducted similar analyses of sulfur isotopes with Professor Don Canfield from the Department of Biology. These analyses similarly indicated oxygen levels in the atmosphere. They also showed that the concentration of oxygen in the atmosphere fluctuated for about 200 million years before reaching a threshold that allowed for the evolution of complex life.
When the research team compared the results of their thallium and sulfur isotopes, they saw that the pulses of oxygenation and deoxygenation of the oceans and atmosphere followed each other.
Oxygen Balance Tipping Point
The first life forms on Earth probably arose in the sea long before life evolved on land. At that time, cyanobacteria lived in the sea and produced oxygen through photosynthesis, but much of this oxygen disappeared when reacting with minerals on land and volcanic gases, so only very small amounts of oxygen found its way to the atmosphere.
In periods, "oxygen oases" appeared in limited parts of the surface waters of the oceans. But these were depleted several times without leading to oxygen accumulation in the atmosphere.
Over time, cyanobacteria produced more oxygen than minerals and volcanic gases could absorb, leading to the tipping point in Earth's oxygen balance that gave us the oxygen-rich oceans and atmosphere we have today.
The study was supported by NASA Exobiology, WHOI Postdoctoral Scholarship program, and Petroleum Foundation of the American Chemical Society.
Meet the researcher
Kasper Primdahl Olesen is a PhD student at the Department of Biology. His research focus is geochemistry.