ORGANIC MATTER PRESERVATION IN CLAY-RICH SURFACE PALEOENVIRONMENTS OF EARTH AND MARS

Thousands of locations on Mars have sedimentary rocks containing Fe/Mg phyllosilicate clay minerals, which indicate widespread aqueous alteration of Martian crustal rocks. Many of these rocks exhibit spectral signatures of dioctahedral phyllosilicate clays, which probably formed from pedogenesis. Phyllosilicates preserve organic matter adsorbed onto clay mineral surfaces and interlayer spaces, often over geologic time scales. Importantly, dioctahedral phyllosilicate clay-rich paleosols on Mars were recently named a high priority site for NASA Mars Sample Return. Not well understood, however, is how clay mineralogy influences organic matter preservation in paleosols of Earth and Mars over geologic time. Archean (2.6 Ga) clay-rich paleosols preserve organic carbon at detectable levels (0.36 wt %), but the influence of clay mineralogy on organic matter preservation has not yet been examined, and terrestrial analogs are needed.

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.