{'fraction_non_alphanumeric': 0.018518518518518517, 'fraction_numerical': 0.005555555555555556, 'mean_word_length': 5.431547619047619, 'pattern_counts': {'":': 0, '<': 0, '<?xml version=': 0, '>': 0, 'https://': 0, 'lorem ipsum': 0, 'www.': 0, 'xml': 0}, 'pii_count': 0, 'substrings_counts': 0, 'word_list_counts': {'cursed_substrings.json': 0, 'profanity_word_list.json': 0, 'sexual_word_list.json': 0, 'zh_pornsignals.json': 0}}
Hypothesized link between Neoproterozoic greening of the land surface and the establishment of an oxygen-rich atmosphere

Significance The evolution of the biota and of atmospheric composition have been tightly coupled throughout Earth history. Of key importance has been the establishment of an oxygen-rich atmosphere, and this has apparently occurred in two major steps. The second step occurred during the Neoproterozoic, at the time when various lines of evidence suggest a significant expansion of terrestrial life and soil development. The present paper links these two events in a unique way, not through increased oxygen production on land, but through increased consumption in biotic soils that until compensated by higher atmospheric oxygen levels, restricted the supply of oxygen to the weathering environment. Considerable geological, geochemical, paleontological, and isotopic evidence exists to support the hypothesis that the atmospheric oxygen level rose from an Archean baseline of essentially zero to modern values in two steps roughly 2.3 billion and 0.8–0.6 billion years ago (Ga). The first step in oxygen content, the Great Oxidation Event, was likely a threshold response to diminishing reductant input from Earth’s interior. Here I provide an alternative to previous suggestions that the second step was the result of the establishment of the first terrestrial fungal–lichen ecosystems. The consumption of oxygen by aerobes respiring this new source of organic matter in soils would have necessitated an increase in the atmospheric oxygen content to compensate for the reduced delivery of oxygen to the weathering environment below the organic-rich upper soil layer. Support for this hypothesis comes from the observed spread toward more negative carbon isotope compositions in Neoproterozoic (1.0–0.542 Ga) and younger limestones altered under the influence of ground waters, and the positive correlation between the carbon isotope composition and oxygen content of modern ground waters in contact with limestones. Thus, the greening of the planet’s land surfaces forced the atmospheric oxygen level to a new, higher equilibrium state.