Using a terrestrial paleosol sequence to evaluate a weathering origin for clays on Mars

Abstract:

Soils provide abundant geochemical sources of energy for life, and when soils are buried as paleosols, their mineralogy and geochemistry can be used to reconstruct ancient surface environments and paleoclimates. Here we analyze how the clay mineralogy and spectral properties of the paleosol sequence preserved in the John Day Fossil Beds National Monument in eastern Oregon relate to environmental and climatic conditions at the time of soil formation. This 440-meter thick volcaniclastic deposit contains over 500 individual clay-rich (30-90%) paleosols. The deposits span 15 My around the Eocene-Oligocene boundary, a period of global cooling. The paleosol mineralogy reflects this climatic shift by transitioning from kaolinites, to smectites, to poorly-crystalline phases, with local variations due to environmental variables like water saturation, pH, salinity, and organic content.

Analog studies of this and other paleosol sequences are helping us to better understand surface environments and climates on ancient Mars. Possible paleosol sequences on Mars are best exemplified by the regionally extensive clay-rich deposits in the region of Mawrth Vallis. Using our terrestrial analogs, we can interpret the mineralogy and physical stratigraphy at Mawrth. Under a paleosol framework, the Mawrth clays are consistent with regional paleosols formed under a temperate climate, with local variations due to chemical gradients in a wetlands-like surface environment, providing many energy pathways for life and high preservation potential.