Precious little gems

Microdiamonds may hold signs of ancient life on Earth

Ancient microdiamonds embedded inside ancient zircons found in western Australia suggest that life may have existed on Earth up to 4.25 billion years ago.

Chemical analyses indicate that the mini-gems contain higher-than-average concentrations of the carbon-12 isotope, researchers report in the July 3 Nature.

Experts strongly debate whether that anomaly is evidence that life existed on Earth so soon after the planet formed, 4.6 billion years ago. If true, however, the findings would put life’s earliest appearance at least 400 million years sooner than previously thought.

“If the light carbon signature is from life, then this is very big indeed,” says Craig O’Neill, a geoscientist at MacquarieUniversity in Sydney. “The trouble is, there are quite a few other mechanisms that can form light carbon signatures.”

Zircons, tiny crystals of zirconium silicate, are hard, durable, and chemically inert. The rocks that contain these crystals eventually erode, but often the zircons simply end up incorporated into younger rocks, says Martin Whitehouse, an isotope geochemist at the SwedishMuseum of Natural History in Stockholm.

Trace elements in zircons, especially radioactive isotopes of uranium and thorium, enable scientists to determine the age of the crystals. The oldest known crystals on Earth are zircons found in the JackHills of Western Australia, the remnants of rocks that originally solidified about 4.4 billion years ago.

Small bits that are trapped in zircons when they cool, also known as inclusions, typically are protected even as the zircons’ host rocks are degraded over time. As such, the crystals are tiny time capsules that hold clues about the ancient environment. Whitehouse and his colleagues recently measured the ratios of carbon isotopes that make up micrometer-sized diamonds embedded within some of the JackHills zircons.

In all, the researchers analyzed 22 microdiamond inclusions found inside 18 zircons that ranged in age from 3.05 billion to 4.25 billion years. There were no signs that the microdiamonds formed inside the crystals, so the tiny gems must have formed first and then been incorporated into the zircons themselves.

The concentration of the heavier isotope, carbon-13, in some of the microdiamonds was around six parts per thousand lower than normal, says Whitehouse. That’s about the same isotope ratio typically found deep in Earth’s mantle, where most natural diamonds form (SN: 6/30/07, p. 412). However, concentrations of carbon-13 in some of the microdiamonds were as low as 58 parts per thousand below normal, the researchers report in their Nature paper. “That’s a very unusual carbon signature,” says Whitehouse.

In fact, that carbon-isotope ratio is far below that found in other diamonds and in other reservoirs of isotopically light carbon, including carbon-rich meteorites and interplanetary dust.

Metabolic processes that take place in an organism’s cells, and especially in microorganisms, produce isotopically light carbon. A much-higher-than-average concentration of carbon-12, the lightest of carbon’s stable isotopes, often is a sign that the carbon was generated by biologic activity, says Whitehouse.

Whitehouse agrees that other mechanisms can lead to higher levels of the lighter carbon. For example, some inorganic chemical reactions — including those between carbon oxides, methane, hydrogen and water, all constituents in Earth’s early atmosphere — can yield isotopically light carbon.

The carbon-isotope ratios found in the JackHills zircons “are among the lightest ever measured,” says Steven Shirey, a geochemist at the Carnegie Institution of Washington (D.C.). “These materials are so unique and unusual, they have to lead us to insights about a time on Earth when there were no large continents,” he notes.

Although the isotopically light carbon isn’t a sure sign that life existed on Earth 4.25 billion years ago, it’s impossible to discount the notion, Shirey says. “Geology is a very frustrating field, because often two or three processes can give you the same result.”

The new findings are “fascinating,” says Andrew Steele, an astrobiologist at Carnegie. “Even if there wasn’t life on Earth at the time, this research gives us insight into Earth’s prebiotic chemistry,” he says.

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