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The ratio of lithium isotopes in the crystals bore the signature of heavy weathering. "Because we see it so commonly, it suggests that weathering was really extensive on the early Earth," Valley said. "People have thought for a long time that the early atmosphere might be carbon-dioxide-rich," Valley added. High CO2 levels -- perhaps 10,000 times more -- would have led to extreme acid rain, which would have dissolved the less-resistant minerals of the Earth's early crust, and could explain why rock from more than 4 billion years ago has so far been impossible to find. If liquid water existed on Earth that long ago, the conditions could already have supported life. "What our samples suggest is that the Earth was habitable. It was a relatively friendly environment," Valley said. "The basic observations are almost certainly correct that these very old zircons are rich in lithium and that is a distinctive signature of silicate minerals that have been exposed to weathering," said John Eiler of the California Institute of Technology in Pasadena, who was not involved in the study. But because the minerals are so old, there are a couple of other possible explanations for how lithium could have been incorporated later that need to be ruled out, he added. "It's another brushstroke in our repainting of the picture of early Earth and maybe it will hold up and maybe it won't," he said. But Valley's team's explanation is "very plausible," he said. "These more-than-4-billion-year-old zircons are, as far as we know, the only samples of the first half-billion years of Earth's history. They're so intrinsically interesting that anything new and novel that you do is important." Related Links: Jessica Marshall's Blog: EnvironMental Case Larry O'Hanlon's blog: Earth Impacts |
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