Study reveals that early Earth may have been an ocean planet

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New research, conducted by American scientists and published in the journal Nature Geoscience, suggests that Earth was covered by a global ocean that turned our planet into a “water world” around 3.2 billion years ago.

Dr. Boswell Wing, researcher in the Department of Geological Sciences at the University of Colorado Boulder said: “Our findings could help scientists to better understand how and where single-cell organisms first emerged on Earth.

“The history of life on Earth tracks available niches. If you’ve got a waterworld, a world covered by ocean, then dry niches are just not going to be available.”

Imagine an Earth in which there are no continents and without imposing mountain ranges and deep valleys.

A world full of water

The researchers performed an isotopic analysis of ancient oceanic crust found at a geological site called Panorama in Australia. Locked inside the ancient crust are chemical clues about the seawater that covered Earth 3.2 billion years ago.

They described the research as a 'once-in-a-lifetime opportunity'. 

Specifically, they analysed the relative amounts of two isotopes, oxygen-16 and the slightly heavier oxygen-18, in more than 100 samples of the stone.

Scientists tried to establish the temperature of the Earth at this time, something that had proven difficult to determine.

The ratio of two isotopes of oxygen, which are natural variations of the element, can be linked to the temperature of ancient oceans due to their different molecular weights. Low temperature water contains more oxygen-16 compared to oxygen-18, and vice versa. “Though these mass differences seem small, they are super sensitive,” Dr. Wing noted.

After reconstructing a temperature profile of the region 3.2 billion years ago, they discovered that there were 3.3% more oxygen-18 atoms than is the case currently on Earth.

“The process is like analysing coffee grounds to gather information about the water that poured through it.

“Today’s land masses are covered by clay-rich soils that disproportionately take up heavier oxygen isotopes from the water — like mineral vacuums for oxygen-18,” explained Dr. Benjamin Johnson, scientist at Iowa State University.

The findings suggest that the most likely explanation for the excess of oxygen-18 in the ancient oceans is because there were no continents to absorb the isotopes. However, that does not mean there were no areas of dry land around.

Wing pointed out: “we just don’t think that there was global-scale formation of continental soils like we have today.”

Dr Johnson went on to explain: “There are no samples of really ancient ocean water lying around, but we do have rocks that interacted with that seawater and remembered that interaction.” This could be the basis for further research.

The researchers will continue investigations and plan to explore more recent rock formations at sites spanning from Arizona to South Africa, to try to discover the exact moment that land masses first emerged on Earth.

Was ancient Earth a much hotter world than it is today? By understanding how our planet’s complex geology has been shaped over time can shed light on how life could have formed.

Reference: Benjamin W. Johnson & Boswell A. Wing. Limited Archaean continental emergence reflected in an early Archaean 18O-enriched ocean, Nature Geoscience, 2020 DOI: 10.1038/s41561-020-0538-9

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