Ancient Earth was a barren waterworldDry land may be something of a novelty. Until around 2.5 billion years ago our planet was almost completely covered by water, a model of the early Earth suggests. Today, some 28 per cent of Earth's surface is above sea level. Exactly how the ratio of land to sea has varied through Earth's history is unclear, but it is generally agreed that the amount of continental crust has increased over time.
Now, calculations by Nicolas Flament of the University of Sydney, Australia, and colleagues suggest that Earth was a water-world until about 2.5 billion years ago, with land making up only 2 to 3 per cent of its surface.
The team assumed that back then Earth's mantle was up to 200 °C hotter than it is now, mainly because there was then a larger quantity of radioactive elements decaying and producing heat.
A hotter mantle would have made the crust beneath the oceans hotter and thicker than it is today, buoying it up relative to the continents. The resulting shallower ocean basins would have held less water, leading to the flooding of what is now land. In addition, the hotter mantle would cause the continental crust of the time to spread laterally, making it lower-lying and flatter than today, and so more likely to flood.
Then, as the mantle cooled, land would have gradually appeared as the oceans became deeper and regions of high relief on the continental crust formed. The team believe that this transition may help to explain why levels of oxygen in the atmosphere rose around this time. During the water-world period, any oxygen produced by photosynthesising bacteria would have been quickly used up through reactions with decaying organic matter in the oceans. When the newly emerged land eroded, it produced sediment that, once washed into the oceans, would have buried the organic matter, preventing any further reactions with oxygen, and so allowing it to build up in the atmosphere.
This would have allowed oxygen-breathing organisms to flourish, say the team. The eroded sediment would also have caused an explosion in life by fertilising the oceans with phosphorus - an important nutrient. And newly formed coastal regions would have provided plenty of shallow habitats for photosynthesis.
Stephen Mojzsis of the University of Colorado, Boulder, agrees that the early continental regions could have been mostly flooded at this time. However, he suspects the land fraction was not quite as low as 2 to 3 per cent because many rocks of this age appear to have formed from sediment washed off dry land.
Copyright: New Scientist
http://www.newscientist.com/article/mg20126882.600-ancient-earth-was-a-barren-waterworld.html
