3 Billion Years Ago, the World Might Have Been a Waterworld, With No Continents At All
Evidence from an ancient section of the Earth's crust suggest that Earth was once a water-world, some three billion years ago. If true, it'll mean scientists need to reconsider some thinking around exoplanets and habitability. They'll also need to reconsider their understanding of how life began on our planet.
[...] The work is focused on an area in the Australian Outback called the Panorama district. In that region in northwestern Australia there's a slab of ocean floor 3.2 billion years old, that's been turned on its side. The chunk of crust holds chemical clues about ancient Earth's seawater.
[...] Marine sediments have been well-studied over time, but the authors of this study looked at the ancient crust instead. The ancient oceans held different types of oxygen that were then deposited into the crust. The scientists gathered over 100 samples of the ancient rock and analyzed it for two oxygen isotopes: oxygen-16 and oxygen 18. They wanted to find the relative amount of each isotope in the ancient crust, to compare it to the amounts in the sediment.
Their results showed more oxygen-18 in the crust when it was formed 3.2 billion years ago, meaning the ocean at that time had more oxygen-18. The pair of researchers say that means that when that crust formed, there were no continents. This is because when continents form, they contain clays, and those clays would have absorbed the heavier oxygen-18. So if there had been continents 3.2 billion years ago, their crust samples would have held less oxygen-18. The over-arching conclusion of their work is that the Earth's oceans went through two distinct states: one prior to continents forming, and one after continents formed.
Also at University of Colorado Boulder and Popular Mechanics.
Limited Archaean continental emergence reflected in an early Archaean 18O-enriched ocean (DOI: 10.1038/s41561-020-0538-9) (DX)
(Score: 2) by NotSanguine on Wednesday March 04 2020, @05:19PM (1 child)
Once the Earth was cool enough for liquid water to pool on its surface, the processes driving land formation likely hadn't had a chance to synthesize lighter continental crust [wikipedia.org], rather than the denser oceanic crust [wikipedia.org] which subducts [wikipedia.org] under the lighter continental crust, causing uplift and creating land at higher altitudes than sea level.
Since such continental crust didn't exist yet, it makes sense that the Earth's crust would be fully below sea level.
No, no, you're not thinking; you're just being logical. --Niels Bohr
(Score: 2, Interesting) by nitehawk214 on Wednesday March 04 2020, @08:32PM
Makes sense. I think I read somewhere that if the earth was a perfect sphere with no continental shelfs (shelves?), the ocean would be over 2 kilometers deep. On geologic timescales continental crust is like the buoyancy of ice cubes floating in water.
"Don't you ever miss the days when you used to be nostalgic?" -Loiosh