Here, we calculate absolute Archaean barometric pressure using the size distribution of gas bubbles in basaltic lava flows that solidified at sea level ∼2.7 Gyr in the Pilbara Craton, Australia. Our data indicate a surprisingly low surface atmospheric pressure of Patm = 0.23 ± 0.23 (2σ) bar, and combined with previous studies suggests ∼0.5 bar as an upper limit to late Archaean Patm. The result implies that the thin atmosphere was rich in auxiliary greenhouse gases and that Patm fluctuated over geologic time to a previously unrecognized extent.
We present a new dynamic thresholding method for computationally separating amygdules from their basaltic matrix in X-ray images that is based on a technique used in seismology. The technique is sensitive to the gradient of the gray-scale value, rather than an absolute threshold value often applied to an entire set of X-ray images. Additionally, we present statistical methods for extrapolating the volumetric measurement mean and standard deviation of amygdules in the measured samples to the entire population in the flow. To do so, we create additional amygdule sample sets from the original sample set in the process of ‘bootstrap’ resampling, and use the Central Limit Theorem to calculate the mean and standard deviation of the amygdule population from these sample sets. This suite of methods allows the extension of bubble-size distribution studies typically done on modern flows to the ancient rock record and potentially has many other uses in geosciences where quantitative discrimination between materials with a range of densities is required.
Here we show that raindrop imprints in tuffs of the Ventersdorp Supergroup, South Africa, constrain surface air density 2.7 billion years ago to less than twice modern levels. We interpret the raindrop fossils using experiments in which water droplets of known size fall at terminal velocity into fresh and weathered volcanic ash, thus defining a relationship between imprint size and raindrop impact momentum. Fragmentation following raindrop flattening limits raindrop size to a maximum value independent of air density, whereas raindrop terminal velocity varies as the inverse of the square root of air density. If the Archaean raindrops reached the modern maximum measured size, air density must have been less than 2.3 kg m(-3), compared to today’s 1.2 kg m(-3), but because such drops rarely occur, air density was more probably below 1.3 kg m(-3). The upper estimate for air density renders the pressure broadening explanation possible, but it is improbable under the likely lower estimates. Our results also disallow the extreme CO(2) levels required for hot Archaean climates.