Oxygen ratios in ancient zircon crystals suggest that the planet’s water cycle got started hundreds of millions of years earlier than thought.
Earth may have had fresh, not just salty, water as soon as 600 million years after the planet formed — a mere blink of an eye in geologic time.
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,SN: 10/17/18Cycles 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.
The researchers analyzed oxygen isotopic ratios of more than 1,300 zircons. Most of the zircons had relatively heavy oxygen isotope values, as would be expected from seawater. But at two time periods, around 3.4 billion years ago and 4 billion years ago, the ratios indicated a greater proportion of lighter oxygen.
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 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.
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