The generally held theory regarding the oxygenation of planet Earth can be described as a one-way staircase starting 2.4 billion years ago with what is informally dubbed the Great Oxidation Event, followed almost 2 billion years later with a second irreversible rise in oxygen.
However, a team led by geochemists from the University of California, Riverside are challenging this one-way theory, suggesting that there was a fall in oxygen levels 200 million years after the original oxidation event.
“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 posited irreversible rise favored by many, our new data point convincingly to an equally impressive, and still not well understood, fall in oxygen about 200 million years later.”
According to Lyons, this drop in oxygen may have ushered in more than a billion years that were marked by a return to low-oxygen concentrations 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.
He explained that carbon isotopes are fractionated during photosynthesis. When organic matter is buried, oxygen is released and rises in the biosphere. The burial of organic matter is tracked by the positive or heavy isotopic composition of carbon in the ocean.
“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 sulfur data shows that there were high sulfate concentrations in the ocean that, like today, represent high levels of oxygen in the ocean and atmosphere.
“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 added.
Source: University of California, Riverside
Photo Credit: UCR Strategic Communications.