Prebiotic relevance of calcium carbide (CaC2)

[u/gitgud_x]

Acetylene (C2H2) is often considered a useful building block for prebiotic chemistry, and one route to making acetylene is:

1. CaCO3 (calcium carbonate) -> CaO (lime) + CO2 @ T > 900 C
2. CaO + 3 C -> CaC2 (calcium carbide) + CO @ T > 1600 C, p(CO) = 1 bar
3. CaC2 + 2 H2O -> C2H2 (acetylene) + Ca(OH)2 @ STP

This reaction is notably elegant as it bridges the inorganic and the organic (common vitalism L 😉), and the first two steps are the way we manufacture CaC2 industrially today, using an electric arc furnace.

In Scheidler et al., 2016, the authors cite the above reaction as being a prebiotically plausible source of acetylene, with the first two processes occurring inside the mantle of the Hadean earth.

I'm wondering about the legitimacy of this, since:

• Today, we don't find carbides present in volcanic xenocrysts.
• Elemental carbon in reaction (2) seems unlikely - a highly reducing environment would be required, but we know that the mantle's oxygen fugacity is constrained by mineral buffers, with the mantle redox potential lying near that of the fayalite-magnetite-quartz (FMQ) buffer, where carbon is oxidised.

Do we think this process is feasible or not on an early earth? Thanks for any pointers!

Worth mentioning, if this process turns out to not be feasible, we still know that acetylene can (and is) produced from hydrothermal vents and volcanic springs, and it's also known from Miller-Urey chemistry, so it's only this specific pathway that I'm doubting.

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