Oceans now cover three quarters of our planet. But in the early days of the earth, its surface could have been almost completely covered by water. Researchers have now found new evidence of this in a 3.2 billion year old rock formation in Australia. The ratio of oxygen isotopes measured in these samples suggests that the primeval seas hardly experienced any chemical influences from solid land masses at that time. Accordingly, our planet probably experienced two phases of its development: first it was an almost pure water world, only then did the first continents appear.
Researchers have speculated for a long time about what our earth looked like in the early archaic era – 3.6 to 3.2 billion years ago. Previous studies show that our planet has had a solid rock crust for about four billion years – before that, the surface of the earth was largely glowing and was repeatedly heated and broken up by violent asteroid strikes. After the crust solidified and cooled, extensive oceans formed. Because plate tectonics had probably not started at that time, there were hardly any mountains and increased land masses that displaced the water. Therefore, scientists have long suspected that at this time the earth could have been almost completely covered by water – a planet with an ocean spanning the earth and without large continents.
Oxygen isotopes as windows in the Urozean
Benjamin Johnson and Boswell Wing from the University of Colorado in Boulder now provide new evidence for this water world hypothesis. For their study, they examined the isotope values ​​of rock samples from the Pilbara Kraton in Australia. This rock formation is one of the oldest crustal stones on earth and therefore opens a window into the early archaic. “There are no samples of really old sea water, but we have rocks that interacted with the sea water at that time and preserved these interactions,” explains Johnson. These include a 3.24 billion year old crust section in the Pilbara area, which was part of the primeval seabed at the time. The researchers took more than 100 samples from this rock and analyzed them for the content of oxygen isotopes 16 and 18. The ratio of these two oxygen variants can provide information about past temperature conditions, but also about geochemical processes such as weathering or sediment deposition ,
The analyzes showed that prehistoric seawater must have contained around 3.3 per thousand more of the heavier oxygen isotope O-18 than today’s seawater. But why? In order to investigate the possible cause of this excess, the researchers developed a model that simulates the geochemical and physical processes on early Earth and went through various scenarios. The result: “The O-18-infested paleo-ocean can be reproduced if the water cycle of the early Archaic was characterized by oceanic oxygen recycling,” report Johnson and Wing. To put it more simply: the isotope values ​​measured in the prehistoric rock could have come about because there were no extensive land areas at that time. As a result, the chemical processes that increasingly absorb and bind the heavier oxygen were missing.
First water world, then continents
According to the researchers, their results suggest that our planet could have been an almost pure water world around 3.2 billion years ago. There were probably no larger land masses at that time. “There is nothing to contradict the fact that the first mini continents already protruded above the surface of the water,” emphasizes Wing. But a noteworthy occurrence of continental soils at the time was rather unlikely due to their results. “It was only around 2.5 billion years ago that continental weathering and oxygen recycling rose from zero to modern rates, did the oxygen-isotope ratio gradually drop to modern levels. “The Earth’s water cycle could have experienced two different states of equilibrium – one before and one after the emergence of the continents,” stated Johnson and Wing.
Your scenario might match that of a previous study relatively well. After analyzing 2.7 billion year old rocks, scientists came to the conclusion that the first large land masses had already appeared at this time. Taking the two studies together, the formation of the first continents could have taken place between 3.2 and 2.7 billion years ago.
Source: Benjamin Johnson and Boswell Wing (University of Colorado, Boulder), Nature Geoscience, doi: 10.1038 / s41561-020-0538-9