Freshwater first appeared on Earth 4 billion years ago, ancient crystals hint

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.

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.

The finding suggests that freshwater may have been actively cycling on Earth hundreds of millions of years earlier than previously thought. Past studies have found evidence that a robust water cycle, one that involved rain and evaporation from the land back to the atmosphere and then rain again, existed by at least 3.2 billion years ago.

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.

When that rainwater percolates through the ground, it can chemically react with the rocks themselves, or with magma within the rocks, imparting those lighter isotopic oxygen values — indelible clues that freshwater was present.

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.

In the 3.4-billion-year-old zircons, the team measured ratios of oxygen-18 to oxygen-16 that were as low as 0.1 per mil — a measurement of the ratio of those isotopes when compared to a standard oxygen isotopic ratio from ocean water. That 0.1 value is very low compared with the average oxygen isotope of rocks at that time, about 5 parts per mil. The 4-billion-year-old zircons, meanwhile, had oxygen isotopic values that were about 2 parts per mil.

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.

Gamaleldien and colleagues present a convincing case that there was freshwater cycling on Earth 3.4 billion years ago, corresponding to previous evidence for freshwater on Earth, says geochemist Jesse Reimink of Penn State. But “the jury’s still out” on whether that was the case 4 billion years ago.

It’s not clear that there would need to be large volumes of freshwater, such as would indicate an active water cycle, to get the observed isotopic values, Reimink says. Still, “that doesn’t rule it out,” he says.

“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.”