The search for the most ancient traces of life on Earth has always been fraught with controversies because of the inevitable degradation undergone by fossilized biomolecules. Laboratory experiments may provide unique clues to achieve a better mechanistic understanding of the key processes involving (biogenic or abiotic) organic carbon during a geological history. The Earth atmosphere has changed over geological times, from a CO 2-rich atmosphere during the Hadean and Archean to the O 2-rich atmosphere of the present day, with a direct impact on the nature of the gas phase trapped within the sediment porosity. Yet, the influence of the nature of this gas phase on fossilization processes has almost never been investigated. Here, we conducted a series of fossilization experiments using an emblematic biomolecule (i.e. RNA) and clay minerals at 200°C for 7 days in closed systems in equilibrium with two different gas phases (e.g. CO 2 versus N 2 /O 2). The multiscale characterization of experimental residues using a suite of advanced microscopy and spectroscopy techniques showed that the final organomineral assemblages strongly depend on the nature of the gas phase. In addition to the nature of the mineral phases, results showed that the nature of the gas phase impacts the chemistry of the residual N-rich organic compounds trapped within the interlayer spaces of Mg-smectites (e.g. mainly aliphatic-rich under CO 2 vs dominated by heterocycles under N 2 /O 2). Altogether, the present study demonstrates the necessity to take into account the nature of the gas phase composition when experimentally simulating fossilization processes aiming at better constraining which biosignatures may be preserved in ancient rocks. Finally, the experimental results reported here may serve to identify the potential biosignatures that should be searched for on other planetary bodies.