Major Drop in Oxygen After The First Rise in the Evolution of Life

Around 2.4 billion years ago the Great Oxidation Event occurred which had obvious implication or the origins and evolution of the first forms of eukaryotic life. Almost two billion years later the irreversible rise occurred, coinciding with the first appearances and earliest diversification of animals.

A new study challenges the simple notion of an up-only trend for early oxygen and provides the first compelling direct evidence for a major drop in oxygen after the first rise.

This study was led by the team of geochemists at the University of California, Riverside.

"Our group is among a subset of scientists who imagine that oxygen, once it began to accumulate in the ocean-atmosphere system, may have ultimately risen to very high levels about 2.3-2.2 billion years ago, perhaps even to concentrations close to what we see today," said Timothy Lyons, a professor of biogeochemistry and the principal investigator of the project.

 But unlike the irreversible rise theory the new data point convincingly to an equally impressive, and still not well understood, fall in oxygen about 200 million years later.

Lynos predicts that, the drop of oxygen was lead in more than a billion years that was marked by a return to low oxygen concentration at Earth's surface, including the likelihood of an oxygen-free deep ocean.

"It is this condition that may have set the environmental stage and ultimately the clock for the advance of eukaryotic organisms and eventually animals," he said.

"The time window between 2.3 and 2.1 billion years ago is famous for the largest and longest-lived positive carbon isotope excursion in Earth history," said Noah Planavsky, a recent Ph.D. graduate from UC Riverside, current postdoctoral fellow at Caltech, and first author of the research paper.

Noah explains that, the carbon isotopes are fractionated during photosynthesis. Oxygen is released and also rises up to the biosphere when organic matter is buried. The positive or heavy isotopic composition of carbon in the ocean tracks the burial of the organic matter.

"Some workers have attributed the carbon isotope excursion to something other than organic burial and associated release of oxygen," Planavsky said. "We studied the sulfur isotope composition of the same rocks used for the carbon isotope analyses -- from Canada, South Africa, the U.S., and Zimbabwe -- and demonstrated convincingly that the organic burial model is the best answer."

The data that is garnered by the researchers point to the high concentration of sulfur in the ocean. This is a strong proof of high oxygen levels in the ocean and atmosphere.

Sulfate is considered as the second most abundant negatively charged ion in the ocean today. It is high when the mineral pyrite oxidizes easily on the continents and is buried in relatively small amounts in the oxygen-rich ocean.

"What is equally impressive is that the rise in oxygen was followed by a dramatic fall in sulfate and therefore oxygen," Lyons said. "Why the rise and fall occurred and how that impacted the billion years or more of ocean chemistry that followed and the life within that ocean are hot topics of research."

"The idea that oxygen levels at Earth's surface went up and down must be vital in any effort to understand the links between environmental and biological evolution on broad, geologic time scales," Planavsky said.

The study results appear online this week in the Proceedings of the National Academy of Sciences.