LOS ANGELES, Oct. 30 (Xinhua) -- U.S. scientists have found evidence that oxygen production began in Earth's oceans at least 100 million years before the Great Oxidation Event (GOE) around 2.4 billion years ago.
Evidence further demonstrates that even very low concentrations of oxygen can have profound effects on ocean chemistry.
It is widely accepted by scientists that around 2.4 billion years ago, the Earth's atmosphere underwent a dramatic change when oxygen levels rose sharply.
Called the "Great Oxidation Event," the oxygen spike marks the transformation from an oxygen-poor atmosphere to an oxygen-rich one, thus paved the way for complex life to develop on the planet.
But scientists were unable to answer questions on when oxygen production via photosynthesis got started and when it began to alter the chemistry of Earth's ocean and atmosphere.
The new findings were made by geoscientists at the University of California, Riverside.
A team of researchers analyzed 2.5 billion-year-old black shales from Western Australia.
The shales revealed that episodes of hydrogen sulfide accumulation in the oxygen-free deep ocean occurred nearly 100 million years before the GOE and up to 700 million years earlier than such conditions were predicted by past models for the early ocean.
"The conventional wisdom has been that appreciable atmospheric oxygen is needed for sulfidic conditions to develop in the ocean," said Chris Reinhard, a Ph.D. graduate student in the Department of Earth Sciences and one of the research team members.
"We found, however, that sulfidic conditions in the ocean are possible even when there is very little oxygen around, below about 1/100,000th of the oxygen in the modern atmosphere," Reinhard added.
According to Reinhard, at even very low oxygen levels in the atmosphere, the mineral pyrite can weather on the continents, resulting in the delivery of sulfate to the ocean by rivers. Sulfate is the key ingredient in hydrogen sulfide formation in the ocean.
Timothy Lyons, a professor of biogeochemistry who is also in the research team, explained: "A pre-GOE emergence for oxygenic photosynthesis is a matter of intense debate, and its resolution lies at the heart of understanding the evolution of diverse forms of life."
He said his team has found an important piece of that puzzle.
Reinhard said the data collected point to oxygen-producing photosynthesis long before concentrations of oxygen in the atmosphere were even a tiny fraction of what they are today, suggesting that oxygen-consuming chemical reactions were offsetting much of the production.
According to Reinhard, the findings add to growing evidence suggesting that biological production of oxygen is a necessary but not sufficient condition for the evolution of complex life.
He said a planetary atmosphere with abundant oxygen would provide a very promising biosignature. But one of the lessons here is that just because spectroscopic measurements don't detect oxygen in the atmosphere of another planet doesn't necessarily mean that no biological oxygen production is taking place.
According to Reinhard, if sulfidic conditions could occur with very small amounts of oxygen around, then they might have been even more common and widespread after the GOE.
The findings appear in the Oct. 30 issue of Science.
