Effect of depositional mode on the detectability of microbial fossils in Mars-analog clay-rich sediments

The astrobiological exploration of Mars is ongoing, with multiple missions investigating whether ancient environments could have supported life (Grotzinger et al., 2012) and whether traces of that life could still be preserved in the geological record (Farley et al., 2020). Clay-bearing terrains are regarded as prime targets for these missions because of the strong capacity of some phyllosilicates to adsorb, concentrate, and preserve organic carbon (Hedges and Keil, 1995; Kennedy et al., 2002). Early Earth clay-rich mudstones are also well known for exquisitely preserving delicate morphologies, including cells, filaments and microbial mats (Javaux, 2019). However, Martian surface radiation and oxidizing processes may alter such materials (cf. Fornaro et al., 2018). ESA’s ExoMars mission will therefore extend the search to the subsurface to access materials expected to be less altered (Vago et al., 2017). The selected landing site, Oxia Planum, is a Noachian region dominated by Fe/Mg phyllosilicates (Mandon et al., 2021) that has experienced at least two aqueous episodes, as evidenced by a clay-bearing unit overlain by fan-shaped sedimentary deposits (Quantin-Nataf et al., 2021).

If life ever existed on Mars, potential biomass sources at Oxia Planum could include (i) subsurface communities associated with clay-rich regolith, as observed in hyperarid Earth analogues (e.g., Azua-Bustos et al., 2020), later exhumed and physically reworked, and/or (ii) organisms living in surface or near-surface aqueous settings and locally incorporated into basin-floor clay-rich muds. On Earth, clay-rich sediments can physically shield labile organic matter, reducing its accessibility to microbial degradation within micro- to nano-porosity (McMahon et al., 2016), and low-oxygen bottom waters can further enhance organic carbon preservation in fine-grained deposits (Ritzer et al., 2024). Assuming anoxic conditions in Noachian depositional settings, biosignatures could be well preserved at Oxia. Yet, Oxia’s contrasting sedimentary contexts raise the following question: at constant bulk organic content and under identical diagenetic conditions, to what extent can different pre-diagenetic textures and microstructures bias the morphological and chemical signals, and thus the detectability of fossil biosignatures by vibrational spectroscopy and mass spectrometry in clay-rich sediments?

Here, we investigate this question by conducting laboratory fossilization experiments using saponite (a Mg-rich smectite, synthesized following the protocol of Criouet et al., 2023) and cells from the cyanobacterial strain Synechocystis sp. (PCC6803). Samples were prepared to represent two experimental end-members that differ in their initial texture (wet embedding within a clay-rich mud versus dry physical reworking) while maintaining the same organic content (TOC= 5 wt.%). All samples were then remoistened at the same water-to-rock ratio (W:R=3) and subsequently subjected to accelerated early diagenesis (100 °C, autogenous pressure ~2 bar, 30 days) in a closed system under an early Mars-like (CO₂-rich) atmosphere.

Experimental residues were characterized by SEM-EDS to document fossil morphologies and organo-mineral interactions from micro- to nano-scale, and by complementary spectroscopic (i.e., µRaman, FTIR) and mass spectrometric (i.e., GC-Orbitrap, EA-IRMS) analyses to evaluate associated chemical signals. Altogether, this work aims to provide well-constrained analogs for anticipating how biosignatures may be expressed across Oxia’s contrasting sedimentary contexts and to help validate space instrumentation and protocols.