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12 December 2013 1:00 pm ,
Vol. 342 ,
Evolutionary biologists have long studied how the Mexican tetra, a drab fish that lives in rivers and creeks but has...
Victorian astronomers spent countless hours laboriously charting the positions of stars in the sky. Such sky mapping,...
In an ambitious project to study 1000 years of sickness and health, researchers are excavating the graveyard of the now...
Stefan Behnisch has won awards for designing science labs and other buildings that are smart, sustainable, and...
The iconic 125-year-old Lick Observatory on Mount Hamilton near San Jose, California, is facing the threat of closure...
Recent results from the Curiosity Mars rover have helped scientists formulate a plan for the next phase of its mission...
A new, remarkably powerful drug that cripples the hepatitis C virus (HCV) came to market last week, but it sells for $...
In pretoothbrush populations, gumlines would often be marred by a thick, visible crust of calcium phosphate, food...
- 12 December 2013 1:00 pm , Vol. 342 , #6164
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Oxygenated Oceans Go Way, Way Back
16 March 2009 (All day)
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."