SSS 2009-04-13（在线收听）

This is Scientific American's 60-Second Science. I'm Cynthia Graber. This will just take a minute.

The Earth’s original atmosphere would have been unpleasant—deadly in fact—to any organisms that breathe oxygen. There wasn’t any. Not until about 2.4 billion years ago anyway. That’s the time of what scientists call the Great Oxidation Event. Now researchers believe they’ve found clues as to what may have caused the change. They published their report in the April 9 edition of the journal Nature.

Researchers analyzed trace elements in sedimentary rock from dozens of sites. Turns out nickel was 400 times more abundant in primordial oceans than in today’s waters. Microorganisms called methanogens love nickel-rich water, and they release methane into the atmosphere. Methane prevents a buildup of oxygen.

Scientists testing the rocks saw that around 2.7 to 2.4 billion years ago, ocean-dissolved nickel dropped off. This corresponds to the Great Oxidation Event. Lack of nickel could have killed off methanogens and left room for algae and other life forms that release oxygen during photosynthesis.

Researchers don’t know exactly why nickel decreased—possibly because of the cooling and solidifying of the Earth’s mantle. But the nickel disappearance is one more clue about how the planet went from suffocating to a place where a terrestrial tetrapod could take a deep breath.

Thanks for the minute for Scientific American's 60-Second Science. I'm Cynthia Graber.