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Chemical disequilibrium in planetary atmospheres has been proposed as ageneralized method for detecting life on exoplanets through remotespectroscopy. Among solar system planets with substantial atmospheres, themodern Earth has the largest thermodynamic chemical disequilibrium due to thepresence of life. However, how this disequilibrium changed over time and, inparticular, the biogenic disequilibria maintained in the anoxic Archean or lessoxic Proterozoic eons are unknown. We calculate the atmosphere-oceandisequilibrium in the Precambrian using conservative proxy- and model-basedestimates of early atmospheric and oceanic compositions. We omit crustal solidsbecause subsurface composition is not detectable on exoplanets, unlikeabove-surface volatiles. We find that (i) disequilibrium increased through timein step with the rise of oxygen; (ii) both the Proterozoic and Phanerozoic mayhave had remotely detectable biogenic disequilibria due to the coexistence ofO$_{2}$, N$_{2}$, and liquid water; and (iii) the Archean had a biogenicdisequilibrium caused by the coexistence of N$_2$, CH$_4$, CO$_2$, and liquidwater, which, for an exoplanet twin, may be remotely detectable. On the basisof this disequilibrium, we argue that the simultaneous detection of abundantCH$_{4}$ and CO$_{2}$ in a habitable exoplanet's atmosphere is a potentialbiosignature. Specifically, we show that methane mixing ratios greater than0.001 are potentially biogenic, whereas those exceeding 0.01 are likelybiogenic due to the difficulty in maintaining large abiotic methane fluxes tosupport high methane levels in anoxic atmospheres. Biogenicity would bestrengthened by the absence of abundant CO, which should not coexist in abiological scenario.

Disequilibrium biosignatures over Earth history and implications for detecting exoplanet life