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Rob Sheldon: Why zircons might be evidence for life at earliest formation of Earth


Recently, we picked up on the story that life on Earth may be datable to 4.1 billion years ago:

UCLA geochemists have found evidence that life likely existed on Earth at least 4.1 billion years ago — 300 million years earlier than previous research suggested. The discovery indicates that life may have begun shortly after the planet formed 4.54 billion years ago.

The estimate is based on studies of zircons. Zircons? Champagne gemstones on a beer budget?

Gemstones 160x600
zircons, as most of us know them

Our favourite physicist and physics colour commentator Rob Sheldon kindly writes to explain why zircons matter to origin of life studies:

To answer your question, carbon comes in various isotopes or flavors.

Giving the longest lasting of these isotopes:

Isotope Half-life
——– ———-
C-11 20minutes
C-12 forever
C-13 forever
C-14 5700 years

The zircon crystal, formed under high heat and pressure, was Uranium-Lead dated to 4.1 billion years old. We might quibble about accuracy, but zircons are considered gold-standards in geochronology. The zircon had a graphite speck inclusion, something contaminating the melted rock at formation, hence the graphite is older than the zircon. (They looked for evidence of later reworking but found none.)

But 4.1 billion years is far too long for either C-11 or C-14 to survive, (about 8 half-lives are the time limit for C-14), so the researchers analyzed the C-12/C-13 ratio.

Dr Sheldon
Rob Sheldon

These isotopes are both stable, so presumably this ratio is exactly the same as when the zircon swallowed it up. Now both isotopes are thought to be made in stars, but through different processes (nuclear chemistry). Then this 99:1 ratio is pretty uniform for stellar production, and galaxy abundance. (We can measure isotopes remotely from high-resolution spectroscopy).

However, if plants convert CO2 into carbohydrate, then the heavier 13CO2 gets incorporated more slowly than the lighter 12CO2. If animals then eat the plants and turn it back into CO2, the carbon cycle successively enriches C-12. There’s also a similar effect in O-16, O-17 and O-18 that is even more accurate because it involves 3 stable isotopes.

There are many complications to this picture. For example, forams/coccolithophores sequester CO2 as carbonates and take it to the bottom of the ocean where it becomes sedimentary rock. Thus oceanic carbon has higher C13 ratios after algae blooms. But theoretical complications can be overcome by empirical measurement, by dense sampling. So with sufficient data, the specific ratio for each regime and time period is well-constrained; it is a fingerprint.

The finding was that when the C-12/C-13 ratio of the graphite inclusion in a 4.1 billion year old zircon was measured, it had a fingerprint of the carbon-cycle, suggesting that life was well established before 4.1 billion years.

Why is this significant? The earliest known physical fossils of life, stromatolites, are fossilized cyanobacterial formations which date to 3.65 billion years. This isotope fingerprint is dated to 450 million years before that, to a time before the bombardment of Earth by comets, before the formation of the oceans and seas.

In other words, before life even had a chance to develop on Earth, there was a “fingerprint” in the carbon ratios. I draw two possibilities from this: either life can not only develop but flourish on hot rocks without water (the Hadean), or life didn’t originate on Earth. In either case, the warm-pond Darwinian origin-of-life story gets severely distorted.


See Explanation. Clicking on the picture will download the highest resolution version available.Hadean period?:

No place on Earth was safe. Four billion years ago, during the Hadean eon, our Solar System was a dangerous shooting gallery of large and dangerous rocks and ice chunks. Recent examination of lunar and Earth bombardment data indicate that the entire surface of the Earth underwent piecemeal upheavals, hiding our globe’s ancient geologic history, and creating a battered world with no remaining familiar land masses.

See also: Life on Earth at 4.1 billion years ago?

What we know and don’t know about the origin of life

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Thanks vj, but the first two were the same, a Wikipedia article where the existence of C13 in zircons was taken as evidence that water existed--a rather circular argument. As an aside, I often find scientists oblivious to these circular arguments, evolution just being one egregious example of many. The last link argued that del-O18 (defined by the triple stable isotopes available for Oxygen), suggests a cool, water cycle distinct from the biological carbon cycle. That is, lighter H2O-16 boils off faster than heavier H2O-18, so the ratio of 18/16 (normalized with 17), gives the temperature at which these zircons formed. While at least this isn't a circular argument, there are numerous methods that fractionate O18, including the CO2-carbonate cycle, and even perhaps the olivine-serpentine cycle. Each of these fractionation mechanisms cause triple isotope ratios to lie on a line, and for years the unknown was the biggest reservoir of oxygen in the solar system, the sun's del-18 composition. This was measured for the first time with Roger Wien's experiment on GENESIS (my college roommate, and I contributed to the winning proposal). Nor is this the final word. Last year the triple isotopes for meteorite Polonnoruwa were published and they show huge enhancements of del-O18. So perhaps the cometary bombardments caused the anomaly and not local water. All this to say that the argument for a cool Hadean with water still needs a lot of work, and while I will need to ask Roger what the current sentiment on this theory is, I don't think it refutes my claim that the Hadean was dry (in the sense of lacking oceans.) Robert Sheldon
"Hot rocks without water" is not an accurate way of describing the Hadean. Earth's water is believed to go back 4.4 billion years (see also here and here). That leaves a window of 300 million years for life to develop. Make of that what you will. vjtorley
Or, gee, I dunno. Maybe life was there from the beginning. This requires photosynthesis right? tragic mishap

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