Eolianite Grain Size Distributions as a Proxy for Large Changes in Planetary Atmospheric Density

Plain Language Summary Atmospheres are dynamic over geologic timescales, making large changes in planetary air density possible. For the Earth, geological proxies suggest that air density in the Neoarchean was similar to or lower than today. This air density variation possibly affected eolian dune grain sizes by controlling the trajectories of grains though the air. Balancing the fall velocity and threshold friction velocity, a metric separating saltation and suspension transport, suggests that a lower air density could increase the mean grain size of dunes because decreased air drag extends the size range of grains in modified saltation and incipient suspension regimes. Consequently, the dune-forming sand left behind in pure saltation, the dominant dune-forming transport mode, could have coarser grains. We analyzed size distributions of two eolianites from 2.64 and 1.5Ga (billion years ago) for deviations from modern sand dunes emplaced at sea level, which globally exhibit similar mean grain sizes. Both aeolianites have mean grain sizes within one standard deviation of the modern mean and are not statistically separable at 95% confidence. Overall, this suggests that while air density is important in eolian physics, a factor of 2 to 4 change in density is insufficient to produce an unambiguous grain size signal. This suggests that while eolian dune grain sizes have not significantly changed over the range of Earth's atmospheric conditions, they could be useful when investigating the order of magnitude changes thought to have occurred on Mars. The atmospheres of planets can change dramatically through time, and 2.7 billion years ago Earth's atmosphere may have been thinner than it is today. One geological process that might record this change is windblown sand dunes. A thinner atmosphere could shift the size ranges of sand in windblown transport and would overall increase the grain sizes in wind-formed sand dunes because there is less air drag when sand grains travel through the wind. We looked at two ancient windblown sandstones to see if there was any significant change in grain sizes. Neither showed a difference from modern sand dunes, and it is probable that larger air pressure changes than available on the Earth (past or present) are needed to see a difference in grain sizes. As a result, this method may only work on planets where air density fluctuations are several orders of magnitude, such as Mars.