Detection of organic carbon in Mars-analog paleosols with thermal and 1 evolved gas analysis 2 3 4

37 Decades of space exploration have shown that surface environments on Mars were habitable 38 billions of years ago. Ancient, buried surface environments, or paleosols, may have been preserved 39 in the geological record on Mars, and are considered high-priority targets for biosignature 40 investigation. Studies of paleosols on Earth that are compositionally similar to putative martian 41 paleosols can provide a reference frame for constraining their organic preservation potential on 42 Mars. However, terrestrial paleosols typically preserve only trace amounts of organic carbon, and 43 it remains unclear whether the organic component of paleosols can be detected with Mars rover44 like instruments. Furthermore, the study of terrestrial paleosols is complicated by diagenetic 45 additions of organic carbon, which can confound interpretations of their organic preservation 46 potential. The objectives of this study were a) to determine whether organic carbon in ~30-million47 year-old Mars-analog paleosols can be detected with thermal and evolved gas analysis, and b) 48 constrain the age of organic carbon using radiocarbon (14C) dating to identify late diagenetic 49 additions of carbon. Al/ Fe smectite-rich paleosols from the Early Oligocene (33 Ma) John Day 50 Formation in eastern Oregon were examined with a thermal and evolved gas analyzer configured to 51 operate similarly to the Sample Analysis at Mars Evolved Gas Analysis (SAM-EGA) instrument 52 onboard the Mars Science Laboratory Curiosity rover. All samples evolved CO2 with peaks at ~400 53 °C and ~700° C from the thermal decomposition of refractory organic carbon and small amounts of 54 calcium carbonate, respectively. Evolutions of organic fragments co-occurred with evolutions of 55 CO2 from organic carbon decomposition. Total organic carbon (TOC) ranged from 0.002 0.032 ± 56 This paper is a non-peer reviewed preprint submitted to EarthArXiv 0.006 wt. %. Like modern soils, the near-surface horizons of all paleosols had significantly higher 57 TOC relative to subsurface layers. Radiocarbon dating of four samples revealed an organic carbon 58 age ranging between ~6,200 – 14,500 years before present, suggesting there had been inputs of 59 exogenous organic carbon during diagenesis. By contrast, refractory carbon detected with EGA 60 and enrichment of TOC in near-surface horizons of all three buried profiles were consistent with 61 the preservation of trace amounts of endogenous organic carbon. This work demonstrates that 62 near-surface horizons of putative martian paleosols should be considered high priority locations for 63 in-situ biosignature investigation and reveals challenges for examining organic matter preservation 64 in terrestrial paleosols. 65