RADIOCARBON DATING OF MARS-ANALOG PALEOSOLS REVEALS CONTAMINATION WITH EXOGENOUS ORGANIC CARBON

Three- to four-billion-year-old paleosols (ancient, buried soils) on Mars are a high priority target for biosignature investigation and Mars Sample Return. However, little is known about the ability of paleosols to preserve organic carbon over geological time scales. Here on Earth, examining organic carbon preservation in paleosols is complicated by introductions of exogenous recent and/or modern organic carbon from plants and microbes. Here we perform the first radiocarbon dating of Oligocene (33 Ma) Mars-analog paleosols from Eastern Oregon, USA. Samples were collected by trenching to 50 cm and selecting lithified, brick-like samples from three outcrops at 10 cm intervals. We find evidence for persistent contamination with exogenous recent/modern organic carbon across all samples. Radiocarbon ages ranged from 6265 ± 25 to 14560 ± 125 years BP and may represent small amounts of recent/modern organic carbon that have mixed with larger amounts of ¹⁴C-free endogenous organic carbon. Samples collected from the shallowest depths (10 cm) were significantly younger (~6000 years BP) than those from deeper depths (40 cm, ~14000 years BP) suggesting there is an age-depth relationship at the site. Thus, there may be a sampling depth (e.g. 100 cm) that minimizes or eliminates contributions from exogenous organic carbon. Results from this work challenge the view that the entire organic fraction of paleosols has been “preserved” over geological time scales.