Air density 2.7 billion years ago limited to less than twice modern levels by fossil raindrop imprints
Abstract
Until recently, most numerical models have investigated the Archean atmosphere assuming a total atmospheric pressure of ~1 atm. However, there are good reasons why barometric pressure may have been different, owing to the negligible presence of oxygen in the early atmosphere, and the consequent changes in the redox-sensitive cycling of nitrogen. To investigate this issue, we have developed a new method of paleobarometry that uses raindrop imprint dimensions as a proxy for atmospheric density. The size of the raindrop imprints are controlled ultimately by the terminal velocity of the impacting drop, which is dependent on air density. However, the maximum size a raindrop can reach is independent of air density. We experimentally determine the relationship between imprint volume and area as a function of raindrop size in volcanic ash from the 2010 Eyjafjallajokull event (Iceland) and Pahala (Hawaii). These are modern analogs, in terms of grain size and approximate composition, of tuffs that preserve 2.7 billion year old raindrop imprints in the Ventersdorp Supergroup of South Africa. Using the experimental relationship between imprint dimension and raindrop size, and with precise topographical measurements of the Ventersdorp imprints, we can then place an upper bound on the late Archaean atmospheric density by assuming that the largest imprints were formed by the largest possible raindrop size. But in reality this maximum drop size is very rarely attained and using more normal raindrop size distributions appropriate for modern climates analogous to the inferred paleoclimate of the Ventersdorp tuffs allows additional constraints to be made.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2012
- Bibcode:
- 2012AGUFM.P11G..04S
- Keywords:
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- 0325 ATMOSPHERIC COMPOSITION AND STRUCTURE / Evolution of the atmosphere;
- 0350 ATMOSPHERIC COMPOSITION AND STRUCTURE / Pressure;
- density;
- and temperature;
- 5210 PLANETARY SCIENCES: ASTROBIOLOGY / Planetary atmospheres;
- clouds;
- and hazes;
- 5405 PLANETARY SCIENCES: SOLID SURFACE PLANETS / Atmospheres