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I was reading this article about how Schopf and Valley’s findings in 2017 confirmed that the 3.5 billion–year-old microfossils found in the Apex chert of Western Australia are indeed remnants of living organisms, as opposed to abiotic geological structures. The most detailed part of the article’s explanation is:

Using SIMS, Valley’s team was able to tease apart the carbon-12 from the carbon-13 within each fossil and measure the ratio of the two compared to a known carbon isotope standard and a fossil-less section of the rock in which they were found.

“The differences in carbon isotope ratios correlate with their shapes,” Valley says. “If they’re not biological there is no reason for such a correlation. Their C-13-to-C-12 ratios are characteristic of biology and metabolic function.”

Can someone explain this in more detail? In particular, why should this ratio be different in biological carbon samples, compared to abiotic carbon samples?

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Via: https://en.wikipedia.org/wiki/Carbon-13#Uses_in_science

Due to differential uptake in plants as well as marine carbonates of 13C, it is possible to use these isotopic signatures in earth science. Biological processes preferentially take up the lower mass isotope through kinetic fractionation.

Via: https://en.wikipedia.org/wiki/Kinetic_fractionation

All organisms preferentially use lighter isotopic species, because "energy costs" are lower, resulting in a significant fractionation between the substrate (heavier) and the biologically mediated product (lighter). As an example, photosynthesis preferentially takes up the light isotope of carbon 12C during assimilation of an atmospheric CO2 molecule. This kinetic isotope fractionation explains why plant material (and thus fossil fuels, which are derived from plants) is typically depleted in 13C by 25 per mil (2.5 per cent) relative to most inorganic carbon on Earth.

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  • $\begingroup$ The second Wikipedia article points out that kinetic fractionation also occurs in at least some known non-biological processes. So why is this being used as evidence of life? $\endgroup$ – WillG Jul 16 at 18:42
  • $\begingroup$ Non-biological processes referred to in the Wiki article appear to discuss oxygen and hydrogen, not carbon. However, I have not read the paper you're referencing. My guess would be that selective uptake of lighter carbon isotopes is evidence of carbon-based life, and that there are other indicators to corroborate that conclusion. $\endgroup$ – Alex Reynolds Jul 16 at 19:26

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