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Boost for evidence of early ocean
The theory that oceans covered the Earth four billion years ago has received a boost from a study of crystals found in Greenland.

The research, published in the current issue of the journal Lithosphere, also sheds light on the processes that formed the continents and crust in the Archaen Era, between 2.5 and 4 billion years ago.

Because of weathering and recycling of the Earth's crust by subduction into the mantle, most of the rocks of this era have been lost.

However, clues are preserved in the ancient isotopic signatures of chemicals in crystals called zircons.

Research on zircons from Jack Hills in Western Australia, published in Science in 2005, pointed to the existence of an ocean as far back as four billion years ago.

The Australian and Swedish researchers, led by geochronologist Dr Chris Kirkland, from the Western Australian Department of Mines and Petroleum, have found evidence from sandstones in the Morænesø Formation in North Greenland, which confirms the presence of oceans on the early Earth.
Oxygen isotopes

The researchers analysed the ratio of heavy to light isotopes of oxygen in zircons ranging from 900 to 3900 million years old. They compared this isotopic ratio to the current average isotopic ratio of oceans called the 'standard mean ocean water'.

"The nice thing is there is one grain that confirms the Jack Hills results and that is really critical in science," says study co-author Dr Martin Van Kranendonk, also from the Department of Mines and Petroleum.

"Before we only had that data from one locality, now we have the same result literally from the other side of the world."

The isotopic composition of this grain shows that it must have been altered by low temperature, near surface conditions, which points to weathering by liquid water.

"Rain is probably not enough to give this sort of a signature because we are dealing with large areas of exposed rocks and they have been significantly altered [by weathering]," says Van Kranendonk.

"The volume of water must have been significant."
Plate tectonics

Since subduction is needed to drag water into the crust, the finding also confirms that plate tectonics, the cycling of the Earth's crust, was happening at this time, albeit in a different way, say the researchers.

Van Kranendonk says the evidence points to a weaker, hotter crust sinking at a shallower angle into the underlying mantle.

The research also confirms a suspected shift in the composition of the Earth's crust 2.5 billion years ago.

"We think this oxygen isotope value shows changes in the style of continental crust, and reflects the continents getting stiffer," Van Kranendonk says.

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