Abstract
Geological records of atmospheric oxygen suggest that pO2 was less than 0.001% of present atmospheric levels (PAL) during the Archean, increasing abruptly to a Proterozoic value between 0.1% and 10% PAL, and rising quickly to modern levels in the Phanerozoic. Using a simple model of the biogeochemical cycles of carbon, oxygen, sulfur, hydrogen, iron, and phosphorous, we demonstrate that there are three stable states for atmospheric oxygen, roughly corresponding to levels observed in the geological record. These stable states arise from a series of specific positive and negative feedbacks, requiring a large geochemical perturbation to the redox state to transition from one to another. In particular, we show that a very low oxygen level in the Archean (i.e., 10-7 PAL) is consistent with the presence of oxygenic photosynthesis and a robust organic carbon cycle. We show that the Snowball Earth glaciations, which immediately precede both transitions, provide an appropriate transient increase in atmospheric oxygen to drive the atmosphere either from its Archean state to its Proterozoic state, or from its Proterozoic state to its Phanerozoic state. This hypothesis provides a mechanistic explanation for the apparent synchronicity of the Proterozoic Snowball Earth events with both the Great Oxidation Event, and the Neoproterozoic oxidation.
Highlights
Though pO2 may have varied between these bounds through the Proterozoic, the appearance of large multicellular animals and large colonial algae supports a second rise in O2 near the end of the Neoproterozoic (Anbar & Knoll 2002), and the charcoal record requires at least 60% present atmospheric levels (PAL) since the Silurian (Scott & Glasspool 2006)
We identify one possible mechanism for this feedback involving more e cient nutrient scavenging by iron under more reduced conditions
Most proxies suggest that atmospheric oxygen has been roughly constant near modern levels since the Devonian, but was systematically lower before the Neoproterozoic (Kump 2008), most likely between 1 and 10% present atmospheric levels (PAL) (Holland 1984). pO2 may have varied during the Proterozoic, but evidence from paleosols, detrital pyrite and uraninite, sulfur isotopes and iron speciation all point to an atmosphere and ocean with oxygen levels well below Phanerozoic levels on long time scales
Summary
As much as 5 times the bioavailable fraction exists as phosphate ions adsorbed to suspended solids and sediment, with iron playing an important role (Delaney 1998) This phosphorus is not directly available to organisms, but does exchange with the dissolved reservoir at a rate determined by complex kinetic laws that depend on redox conditions, particle mineralogy, and solution chemistry (Froelich 1988). In 1968, Preston Cloud argued that oxygen must have been scarce in the atmosphere before about 2 billion years ago, due to the presence of detrital uranite and pyrite grains, and the absence of oxidized iron “red beds, in sedimentary rocks of that age This argument was given a widely-accepted quantitative bound 30 years later, when James Farquhar et al (2000) reported mass-independent fractionation of S isotopes in sulfur minerals older than 2.4 Ga. Mass-independent fractionation (MIF) is generated during photolysis of sulfur compounds in the atmosphere, but in a su ciently oxidized surface environment this signal is homogenized by rapid cycling of sulfur between its oxidized and reduced reservoirs Local enrichments of redox-sensitive trace metals such as Mo and Re at 2.5 Ga have been used to argue for transient oxidative processes during the latest Archean, but Mo mobilization does not require oxygen levels above 10 5 PAL (Anbar et al 2007)
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