Stuck in the clay: Organic matter preservation in clay-rich surface paleoenvironments of Earth and Mars

On Mars, Noachean (4.1-3.7 Ga) sedimentary rocks containing Fe/Mg phyllosilicate clay minerals have been detected in thousands of locations across the planet. Many of these rocks exhibit spectral signatures of dioctahedral phyllosilicate clays, which may have formed from pedogenic weathering. The presence of these clay minerals suggests subaerial aqueous alteration of the Martian crust with circumneutral-pH fluids under low temperatures. Importantly, dioctahedral phyllosilicates can facilitate organic matter preservation by adsorption to clay mineral surfaces and interlayer spaces, which may protect organic matter from both oxidation and radiation over geological time scales. However, it is unclear how clay mineralogy influences organic matter preservation in subaerial paleoenvironments, which appear to be widespread on Mars. Archean (2.6 Ga) clayey paleosols can preserve macromolecules of organic matter well above detectable levels, but the influence of clay mineralogy on organic matter preservation in paleosols has not been investigated, and terrestrial analogs are needed.

Eocene / Oligocene (42-28 Ma) volcanic paleosol sequences at the John Day Fossil Beds National Monument in Oregon have vertical profiles of dioctahedral Al-smectite clays overlain by Fe / Mg clays that exhibit striking spectral similarity to Noachean clay sequences at Mawrth Vallis, Oxia Planum, and Nili Fossae. In these paleosols we measured the total organic carbon (TOC) content of 21 different pedotypes. We identified clay minerals with visible/near infrared (VNIR) spectroscopy and X-ray diffraction. We correlate TOC with clay content, dominant clay mineralogy, and depth in profile. The highest amounts of TOC were detected in the surface horizons of Fe/Mg smectite-rich (> 70 wt %) paleosols that formed under reducing conditions. The lowest amounts of TOC were associated with oxidized kaolinitic paleosols. In addition, we find that many clay-rich paleosols (> 80 wt %) have resisted diagenetic alteration. These results suggest that smectite-rich paleosols with evidence of reducing conditions should be prioritized in the search for biosignatures on Mars.