ORGANIC MATTER PRESERVATION IN CLAY-RICH SURFACE PALEOENVIRONMENTS OF EARTH AND MARS
Paleosol sequences at the John Day Fossil Beds National Monument in eastern Oregon have vertical profiles of dioctahedral Al-smectite clays overlain by Fe / Mg clays that exhibit striking spectral similarity to Noachean (4.1-3.7 Ga) sequences at Mawrth Vallis, Nili Fossae, and elsewhere on Mars. Here we measured the total organic carbon (TOC) content of 21 different paleosol types. We identified clay minerals with visible/near infrared (VNIR) spectroscopy and powder X-ray diffraction. TOC was correlated with dominant clay mineralogy, clay content, and depth in profile. We find evidence for clay mineralogy and depth in profile as principal controls on the preservation of organic matter in clay-rich paleosols. The highest amounts of TOC were detected in the surface horizons of paleosols rich in Fe/ Mg smectites (> 80 wt %) that formed under reducing conditions. The lowest amounts of TOC were associated with oxidized kaolinitic paleosols. These results suggest that smectite-rich paleosols with evidence of reducing conditions should be prioritized in the search for biosignatures on Mars.