What is the baseline for carbon isotope ratios?

In the Martian atmosphere, the ratio between concentrations of carbon-13 to carbon-12 has been measured, for example reported in

Mahaffy, Paul R., et al. "Abundance and isotopic composition of gases in the Martian atmosphere from the Curiosity rover." Science 341.6143 (2013): 263-266.

The value of this ratio, 45 parts per thousand, is in need of explanation, according to this paper

The isotopic signature $$\delta^{13}$$C from CO2 of ~45 per mil is independently measured with two instruments. This heavy isotope enrichment in carbon supports the hypothesis of substantial atmospheric loss. [where the lighter carbon-12 is preferentially lost to space]

If we are surprised by the value 45 parts per thousand and call it "heavy isotope enrichment", there must be some baseline to compare with, right? What is this baseline?

Ratio on earth is 1.109% (11 per mil)

Note " per mil" means Part per Thousand, not part per million.

Earth has no significant processes that would alter this concentration, so it can be assumed to be very close to natural universal abundance.

On Mars, the CO2 that contains C13 moves a bit slower at the same temperature than the C12-containing CO2 moves, thus is less likely to be lost by Mars' atmospheric leakage mechanism.(which operates thousands of times faster than for Earth, this the same process is negligible on Earth)

• Thanks! Really? We're taking Earth, where $\delta^{13}$C is influenced by life, apparently by enough that we can read the evolution of plants and various mass extinctions in the $\delta^{13}$C record, as the baseline? Nov 2 '20 at 8:30
• C13 is absorbed by life (slightly) differently than C12. And even then, the historical range is within a few percent of the baseline. On Mars, the ratio is 410% as high as on Earth.
– user38044
Nov 2 '20 at 9:19
• @Wouter: The variation is tiny. Note that we have 11‰ $^{13}C$ on Earth, and the variation is some 60‰ of this value. That is, the actual amount in a sample varies from 10.4‰ to 11.6‰. Nov 2 '20 at 10:01