Oxidizing atmosphere and life on land during the late Paleoproterozoic outset of the “boring billion”

Abstract

The so called “boring billion” (1800–800 Ma) of Late Paleoproterozoic and early Mesoproterozoic time can now be reassessed from the evidence of diverse paleosols in the Ruyang Group (1749–1561 Ma) in Henan Province, China. Widespread marine sulfidic facies is the reason for the term “boring billion”, but terrestrial facies and paleosols were far from uniform or anoxic at that time. The Ruyang Group in Henan has 12 distinct kinds of paleosols, with 5 distinct pedotypes in alluvial fan facies of the Bingmagou Formation, 6 in fluvial facies of the Yunmengshan Formation, 3 in coastal plain facies of the Baicaoping Formation, and 4 in the coastal to intertidal Beidajian Formation. These paleosols are evidence of hyperarid climate that was cool temperate despite proximity to the paleoequator. These ancient soils were alive with iron- and manganese-fixing bacteria that created desert varnish and vesicular structure, as well as sulfur-oxidizing bacteria creating desert roses of gypsum. The paleosols may also have supported cyanobacteria, algae and fungi, known from palynomorphs in shales of the Baicaoping and Beidajian formations. Consumption of alkali and alkaline earth elements in the paleosols are evidence of 1700–20,600 ppm (6–73 times preindustrial level or PAL) of atmospheric CO2, consistent with the amount needed for a greenhouse effect to offset a faint young sun. A comparable model based on iron and manganese oxidation in the paleosols indicates 318–16,623 ppm (0.002–0.08 PAL) O2. This was enough to support animals (0.005 PAL O2), considering that these are minimal estimates of O2 diluted by some soil respiration. Animals had not yet evolved, but that evolutionary process may not have been limited by atmospheric composition.