Chinese scientists trace phased rise of oxygen that shaped Earth's habitability

BEIJING -- Chinese scientists have found that Earth's atmosphere experienced three major oxygen surges over the past 2 billion years as it shifted from an anoxic to an oxygen-rich state, offering fresh insights into the origins of life, its evolution and the planet's long-term habitability.

The study, led by researchers from Chengdu University of Technology and Nanjing University, was recently published in Nature. It presents a record of the triple oxygen isotope composition of sulfate trapped in sedimentary rocks.

"For the first time, our research provides the most direct atmospheric evidence to date that Earth's oxygen levels reached modern levels around 410 million years ago, confirming the three-stage oxygenation process of the planet's surface," said Li Chao, a professor at the Institute of Sedimentary Geology, Chengdu University of Technology.

Li explained that triple oxygen isotopes refer to the three stable isotopes of oxygen, and their relative abundance in various natural substances is not fixed but undergoes slight variations due to biogeochemical processes in the atmosphere and oceans. By measuring these subtle differences -- much like reading the "fingerprints" of the Earth -- scientists can deduce how the ancient Earth's surface environment evolved.

However, scientists have long lacked effective tracer techniques and data to directly constrain the changes in Earth's surface oxygen reservoirs, their sources and sinks, as well as the key controlling factors. This study bridges the gap by combining systematic sampling analysis and integration of literature data.

The new record reveals that the first oxygen surge occurred during the Paleoproterozoic era (2.4 to 2.1 billion years ago), the second surge took place in the Neoproterozoic era (about 1 billion years ago), and the third in the Paleozoic era (about 440 million years ago).

These changes indicate that Earth's oxygen rose from negligible levels in a phased manner, approaching the modern stable oxygen-rich state around 410 million years ago, a process that spanned nearly 2 billion years.

Meanwhile, during the Neoproterozoic, sharp, interconnected shifts in carbon, sulfur and oxygen isotopes show that after atmospheric oxygen increased, it began periodically oxidizing the largely oxygen-poor oceans.

"This research not only lays a solid foundation for understanding the phased evolution of complex eukaryotic life but also offers new perspectives for exploring the formation of habitable planets and ancient hydrocarbon source rocks," Li said.

"It also reveals the interactive relationship between the atmosphere and oceans throughout this prolonged process, taking a crucial step toward answering the fundamental question of when and how the Earth became suitable for life," he added.