Using raindrops to constrain past atmospheric density

- The Som method uses lithified raindrop imprints to calculate atmospheric density.
- This predicts an upper limit on modern density lower than the known density.
- The upper limit on Archean density should be revised from 2.3 to 11.1 kgm−3.
- Atmospheric density is a second-order effect compared to rainfall rate.

There exists a dearth of constraints on the physical properties of the early Earth atmosphere. The Som palaeopycnometry method estimates an upper limit on ancient atmospheric density based on the size of lithified raindrop imprints preserved in ancient strata, with the assumption that the largest imprint was made by the largest possible raindrop. Using this technique Som et al. (2012) proposed a constraint on Archean atmospheric density of less than 2.3 kgm−3. Applying this method to modern raindrop imprints, the upper bound on surface density produced is 0.9 kgm−3, lower than the actual value of 1.2 kgm−3, refuting the method. We propose several changes to the method, the most important of which is increasing the maximum possible drop size from 6.8 to 10 mm to be consistent with new large datasets of raindrop observations. With these changes, our upper bound on modern surface density becomes 2.7 kgm−3, a valid limit. The upper bound on Archean atmospheric density is then revised to 11.1 kgm−3. In general, we find that raindrop imprint size distribution depends much more strongly on rainfall rate than atmospheric density, which translates into large errors. At best, the precision of raindrop palaeopycnometry will be a factor of a few to an order of magnitude.