Abstract:
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 1.5 Ga (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.
