Oxygenated Oceans Go Way, Way Back

16 March 2009 (All day)

Hiroshi Ohmoto

Red tide. Outcroppings of ancient ocean sediments show deposits of the iron oxide, hematite.

Sometimes the evidence for a dramatic find lies right in plain sight. In this case, researchers studying mineral outcrops in northwestern Australia have found deposits of hematite, an oxide of iron, that are 3.46 billion years old. The hematite probably formed from reactions with oxygen molecules in the oceans. If confirmed, the discovery could mean that oxygen-producing photosynthetic organisms originated more than a billion years earlier than previously thought.

Common scientific wisdom says that the first microbes that performed oxygenic photosynthesis--turning sunlight and carbon dioxide into sugar and oxygen--arose about 2.4 billion years ago. Researchers know this because the oldest molecular remnants of bacteria capable of oxygenic photosynthesis were discovered in sedimentary rocks of that age and because hematite crystals that formed from reactions between iron and atmospheric oxygen have been common ever since. Before then, theories go, there was no oxygen in the oceans or the air, so minerals such as hematite could not have been created by processes related to life. Most likely, hematite in older rocks formed from the interaction of iron with small quantities of primordial oxygen in groundwater.

But geochemist Hiroshi Ohmoto of Pennsylvania State University, University Park, wasn't convinced. He and his team searched for sedimentary layers (the bottoms of ancient oceans) older than the 2.4-billion-year-old boundary that contained oxygenated minerals. As they report online this week in Nature Geoscience, they hit pay dirt in the Pilbara Craton formation--once the bottom of an ancient sea and now a rock outcrop in Australia. There, they found the signature red hematite embedded in the 3.46-billion-year-old rock. The mineral probably formed, Ohmoto says, when hot water spewing from hydrothermal vents on the sea floor interacted with oxygen in the seawater--oxygen manufactured by photosynthetic bacteria. Now, he says, the challenge will be to find hematite in other sedimentary rocks older than 2.4 billion years old, adding to the evidence for an earlier start to photosynthesis.

It's "very compelling evidence," says isotope chemist Paul Knauth of Arizona State University, Tempe. The result may go "against the widespread view that [oxygenic] photosynthesis didn't appear" until about 2.4 billion years ago, he says, but the paper's conclusion "is the simplest explanation." He says he hopes the findings will provoke discussion among "all those who argue that the case is closed--surely, we are still learning."

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