Most researchers imagine the initial oxygenation of the ocean and atmosphere to have been something like a staircase, but with steps only going up. The first step, so the story goes, occurred around 2.4 billion years ago, and this, the so-called Great Oxidation Event, has obvious implications for the origins and evolution of the first forms of eukaryotic life. The second big step in this assumed irreversible rise occurred almost two billion years later, coinciding with the first appearances and earliest diversification of animals.From the point of view of the early inhabitants of earth, the first rise of the oxygen atmosphere was more dire than anything global
Now a team led by geochemists at the University of California, Riverside 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.
“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.
Or maybe a comet or ice age did it. Speculation is fun.