Oxygen production began in Earth's oceans 100 mln yrs earlier than believed
30 Oct
2009, 1206 Hrs
Washington, October 30 In a new study, scientists have come across evidence that suggests oxygen production began in Earth's oceans at least 100 million years before the "Great Oxidation Event" (GOE).
When the GOE happened around 2.4 billion years ago, the Earth's atmosphere underwent a dramatic change, with oxygen levels rising sharply.
The oxygen spike marks an important milestone in Earth's history, the transformation from an oxygen-poor atmosphere to an oxygen-rich one paving the way for complex life to develop on the planet.
Two questions that remain unresolved in studies of the early Earth are when oxygen production via photosynthesis got started and when it began to alter the chemistry of Earth's ocean and atmosphere.
Now, a research team led by geoscientists at the University of California, Riverside, corroborates recent evidence that oxygen production began in Earth's oceans at least 100 million years before the GOE, and goes a step further in demonstrating that even very low concentrations of oxygen can have profound effects on ocean chemistry.
To arrive at their results, the researchers analyzed 2.5 billion-year-old black shales from Western Australia.
Essentially representing fossilized pieces of the ancient seafloor, the fine layers within the rocks allowed the researchers to page through ocean chemistry's evolving history.
Specifically, 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.
Scientists have long believed that the early ocean, for more than half of Earth's 4.6 billion-year history, was characterized instead by high amounts of dissolved iron under conditions of essentially no oxygen.
"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," he added.
According to Timothy Lyons, a professor of biogeochemistry, whose laboratory led the research; the hydrogen sulfide in the ocean is a fingerprint of photosynthetic production of oxygen 2.5 billion years ago.
"Our data 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," said Reinhard. (ANI)
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