Low But Persistent Organic Carbon Content of Hyperarid River Deposits and Implications for Ancient Mars.

Mars has many well‐exposed fluvial ridges and fluvio‐deltaic basins; in two of these locations, the Curiosity and Perseverance rovers are currently searching for signs of habitability. The distribution of organic carbon that might persist in ancient fluvial deposits present on Mars is not well understood. In this study, we set out to assess the preservation potential of organic carbon in a hyperarid fluvial environment with observations and analyses of the Amargosa River in Death Valley, California (United States). The lower reaches of the Amargosa River in Badwater Basin are nearly devoid of plants and contain low gradient, meandering channels, making them a valuable terrestrial analog for early martian fluvial systems. We analyzed sediment taken from fluvial deposits exposed in cutbanks of two bends of a meandering channel. We found total organic carbon abundances that were on average 0.15% up to a meter below the surface. X‐ray diffraction and electron microscopy analyses revealed a suite of high redox potential mineral phases (including iron and manganese oxides) mixed with detrital and authigenic silicates, carbonate, and sulfate salts at or close to redox equilibrium with pore fluids in contact with the atmosphere. This finding highlighted that organic carbon can persist in fluvial deposits at low abundance despite oxidizing conditions and saturated sediments and suggested that ancient fluvial deposits on Mars may retain traces of organics in fine‐grained deposits if they are present during deposition. Plain Language Summary: The Curiosity and Perseverance rovers are currently searching for signs of past life on Mars in two ancient river deposits. Since carbon on the surface of Mars could originate from meteorites and chemical reactions in addition to potential Martian life forms, it is valuable to identify sedimentary environments that best preserve biospheric organic carbon. The distribution of organic carbon that might persist in ancient river deposits on Mars is not well understood, yet this understanding is crucial to detecting evidence of past life on Mars. In this study, we set out to determine the preservation potential of organic carbon in an arid, fluvial environment. The Amargosa River (Death Valley, California, United States) hosts a low gradient single threaded river, running through a catchment with little vegetation and making it a valuable terrestrial analog for early Martian fluvial systems. We found that organic carbon can persist in fine‐grained river sediments at low abundance despite oxidizing conditions in the subsurface. Our results suggested that fine‐grained ancient river deposits on Mars are the most promising sampling targets for the Perseverance rover to detect traces of potential life on Mars. Key Points: The hyper‐arid deposits of the ephemeral, alluvial Amargosa River are a useful process analog for ancient Martian river depositsX‐ray diffraction and electron microscopy analyses revealed that the subsurface remains highly oxidizing despite saturated sedimentsFluvial deposits had low abundances of organic carbon in fine‐grained sediment; mudstones are the most promising targets for sampling on Mars [ABSTRACT FROM AUTHOR]