Crystals from 4 billion years ago contain the oldest evidence of the existence of fresh water
A new study confirms that fresh water on the planet appeared about 500 million years earlier than previously believed. The work was done on crystals dating from the beginning of the Precambrian in western Australia.
The result of a recent investigation provides a fact that threatens to change certain established paradigms. And it is that our planet Earth could have had fresh water about 500 million years earlier than previously thought, as revealed by Eos. The study in question, published by Nature Geoscience, points out that this revealing information was obtained from tiny zircon grains from Australia dated about 4 billion years ago.
The research managed to determine that about 4 billion years ago the planet had oceans but also exposed crust that allowed the hydrological cycle and the appearance of fresh water necessary for life as we know it.
According to the scientists who worked on this investigation, the minerals contain signs of continental rain, which provides new evidence that the early Earth had a continental crust in addition to oceans. The generalized interaction between the meteoric (fresh) water and the continental crust that emerged on the early Earth may have been key to the emergence of life, although it is not known precisely when the hydrological cycle began.
For the research they used the isotopic composition of zirconium crystals oxygen in Western Australia, to determine when the hydrological cycle began. It is a dive of the Hadean eon, one of the first stages of the Precambrian period, which ended about 3.8 billion years ago. At that time, a very young planet housed little more than rock, magma and water. Data on this ancient period is scarce, so this new analysis is a good contribution to the understanding of scientists about how our planet was formed, says Eos.
Looking at the beginnings of the Earth
This discovery could help scientists more accurately determine when life on Earth emerged. Fresh water and the exposed continental crust are two of the conditions that were necessary for life to arise.
Today, the first evidence of life could point to Australia's 3.5 billion-year-old stromatolites. But now this new work shows that "we have the same conditions about 4 billion years ago," as indicated by the co-author of the study Hamed Gamaleldien, a geochemist at Khalifa University in the United Arab Emirates.
In conclusion, the finding goes back about 500 million years the timeline corresponding to the beginning of life on the planet. Said that way, it's just a number, but if we compare that number with the existence of the human being, it's an enormity. The existence of our species does not go beyond 2 thousandths of that time. And of that primitive land there is very little or it is difficult to access.
The Earth of that Hadean era has almost faded in time. Most of the rocks have been worn or have been subducted in the depths of the crust, says Eos. Some of the only minerals left of the Hadean are zircon crystals embedded in younger rocks. The zircon is hard and resistant to chemical alterations, and contains small fragments of uranium that allow scientists to date them. It also contains oxygen, which was the key to the new discovery.
The key is in oxygen
Technically, the researchers explain that oxygen has three stable isotopes, which are O-16, O-17, and O-18. The relationship between light oxygen O-16 and heavy oxygen O-18 is influenced by terrestrial processes such as evaporation and condensation, and deviations from the standard ratio have long been used by paleo climatologists as an indirect indicator of temperature.
Delta-O-18 is a measure of the deviation in the ratio of the stable isotopes O-18 and O-16. Now, the delta-O-18 of some Australian zircons has been used in this research. It was possible to show that it is highly likely that the planet had a solid crust at that time, as well as oceans of liquid water, unlike a rotating balloon full of magma oceans as previously imagined. For the study, samples were taken from Jack Hills, Australia, a region known for containing the oldest zircon grains on Earth.
They dated some of the grains collected to obtain ages of 3.4 billion years and others about 4 billion years ago, before the end of the Hadean. They managed to demonstrate that these zircons were formed by interacting with fresh water resulting from the evaporation of ocean water. They are rocks that interacted with fresh water, not with ocean water. And if the rocks interacted with fresh water, it must mean that there was land protruding above the ocean, which suggests that the Earth had a continental crust 4 billion years ago.
News reference:
Gamaleldien, H., Wu, LG., Olierook, H.K.H. et al. Onset of the Earth's hydrological cycle four billion years ago or earlier. Nat. Geosci. 17, 560-565 (2024). https://doi.org/10.1038/s41561-024-01450-0