Privileged planet: earliest water even earlier (Introduction)

by David Turell , Tuesday, July 02, 2024, 15:45 (76 days ago) @ David Turell

New zircon study:

https://www.sciencenews.org/article/freshwater-earth-ancient-crystal-years

"Researchers analyzed oxygen molecules within 4-billion-year-old zircon crystals from Western Australia’s Jack Hills, one of the oldest rock formations on Earth. The relative proportions of oxygen’s heaviest and lightest forms, or isotopes, in the zircons are possible only if there been a significant amount of freshwater present, geochemist Hamed Gamaleldien of Khalifa University in Abu Dhabi and colleagues report June 3 in Nature Geoscience.

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'Even if there was a freshwater cycle 4 billion years ago, that doesn’t mean there was necessarily life on Earth that far back, Gamaleldien says. “But at least we have the main ingredient to form life.” Currently, the oldest agreed-upon evidence for life on Earth comes from fossilized microbial mats, or stromatolites, in Australia’s Strelley Pool Chert (SN: 10/17/18). Those stromatolites date to 3.5 billion years ago.

"Cycles of evaporation and rain alter the chemical makeup of water molecules. When water evaporates from the ocean’s surface, leaving the salt behind, the lighter form of oxygen, oxygen-16, tends to evaporate faster than the heavier oxygen-18. That lighter water may then rain out over land, and perhaps evaporate again. Over time, the freshwater becomes more concentrated in oxygen-16 compared with the original seawater.

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"The team then ran thousands of computer simulations to determine the likelihood of different explanations for the observed ratios. “We concluded that the main water on Earth was oceanic,” or salty, Gamaleldien says. “But only when we used freshwater [did] it create the results we see.” Furthermore, he says, the findings also suggest that enough land had emerged above sea level by that time to support a water cycle. Researchers have pondered whether Earth was completely covered by oceans between around 3 billion and 4 billion years ago.

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“'The early Earth is really difficult [to study] because there are so few data points,” Reimink says. Ancient crystals like these remain the only clues scientists have to Earth’s earliest time, he adds. “We need to keep pushing the limits of these zircon grains.'”

Comment: fresh water requires dry land, so it is a matter of when that land appeared. The water allows life, salt or fresh. As a result, life started in the salty oceans and then evolved to fresh water on dry land.