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Geo-Joint: Ancient Zircons

Posted on December 23 2015


I’ve been keen on geology and biology for a long time, but despite what some say, I didn’t actually witness the initial development of single celled life. Nor was anybody there to start a stopwatch when the Earth was formed, but with clever methods, geo-chemists have pegged the age of the  Earth at about 4.54 billion years. That’s a lot of candles on the birthday cake. But even that many candles wouldn’t match the heat that our early planet was giving off. Planetary construction is hot work, and even at age 40 million years, Earth was presumed to be cooking. To further boil the pot, a collision between the Earth and some other celestial body is theorized to have knocked loose the material for our moon, opening up the molten interior, making for a world-wide lava ocean. The assumption has been that the conditions for life couldn’t have existed much before 3.8 billion years ago.


That view may be changing, with some recent discoveries in rocks from the Jack Hills of Western Australia. The sandstone in this locale contains some crystals of extraordinary age. Researchers crush samples of this rock, turning the sandstone back into sand and then putting it in a very dense solution. Only the heaviest parts of the mixture can fall to the bottom of the container, and that’s what the geologists want to see. Tiny crystals of zircon have the needed weight, and these grain-sized crystals reveal a host of secrets about the early state of Earth. By measuring the amount of uranium in each crystal that has decayed into lead, they can be dated. A small percentage of the zircons are in the neighborhood of 4.1 billion years old, and some contain fragments of graphite, a carbon mineral. By studying the kind of carbon in the graphite, a ratio of carbon-12 to carbon-13 can be determined. What use is that? Well, life processes on Earth increase the quantity of carbon-12, and as it turns out, some of these samples read out with carbon-12 values that are similar to carbon contents in modern-day materials. That is, materials that exist in a biologically active world. It’s a convoluted investigatory process and the evidence is no slam-dunk for pushing the age of life back by an additional 300 million years, to 4.1 million years, but the prospect is intriguing and bears further study.


If it turns out to be accurate, our concept of the state of the Earth so soon after its formation will be greatly changed. Life in whatever form, even if it was only the kind that lives around steam vents, would require a world much, much cooler than scientists have long assumed. Solid land and oceans would have had to form, even if there were still tons of volcanoes belching lava and ash. The long process of the evolution of life may have had 300 million more years to work its magic than we had imagined.

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