Abstract
The Great Oxygenation Event (GOE), ca. 2.4 billion years ago, transformed life and environments on Earth. Its causes, however, are debated. We mathematically analyze the GOE in terms of ecological dynamics coupled with a changing Earth. Anoxygenic photosynthetic bacteria initially dominate over cyanobacteria, but their success depends on the availability of suitable electron donors that are vulnerable to oxidation. The GOE is triggered when the difference between the influxes of relevant reductants and phosphate falls below a critical value that is an increasing function of the reproductive rate of cyanobacteria. The transition can be either gradual and reversible or sudden and irreversible, depending on sources and sinks of oxygen. Increasing sources and decreasing sinks of oxygen can also trigger the GOE, but this possibility depends strongly on migration of cyanobacteria from privileged sites. Our model links ecological dynamics to planetary change, with geophysical evolution determining the relevant time scales.
Highlights
The Great Oxygenation Event (GOE), ca. 2.4 billion years ago, transformed life and environments on Earth
P to represent the environment, which is similar to the H2 and P employed in other studies[48,49]. The logic of this choice is that in Archean oceans, Fe2+ is thought to have been the principal electron donor for anoxygenic photosynthesis[50,51], whereas P governed total rates of photosynthesis. (Kasting[14] argued that H2 was key to photosynthesis on the early Earth, a view supported by low iron concentrations in some early Archean stromatolites52.)
Phosphate is used up during the growth of anoxygenic photosynthetic bacteria (APB) and cyanobacteria. (We investigate extensions of the model that incorporate bounded bacterial growth rates and organic carbon in Supplementary Note 2 and Supplementary Note 3, respectively.) We posit iron(II) as the primary electron donor for anoxygenic photosynthesis, and for simplicity of presentation, we refer to y1 and f1 in this context
Summary
The Great Oxygenation Event (GOE), ca. 2.4 billion years ago, transformed life and environments on Earth. While pO2 may have fluctuated during the GOE1,2, its results are clear: multiple lines of geological and geochemical evidence document the initial rise of O2 to permanent prominence in the atmosphere and surface ocean[3] These include (1) a sharp drop in iron formation deposition[4], (2) the appearance of red beds in continental sedimentary successions[5] and calcium sulfates in marine environments[6], (3) the retention of iron in ancient weathering horizons[7], (4) the loss of detrital uraninite and other redox-sensitive minerals from fluvial and deltaic sandstones[8], and (5) the loss of a mass-independent sulfur-isotopic signature best explained in terms of photochemical reactions in an essentially oxygen-free atmosphere[9,10]. Goldblatt et at.[26] considered that oxygenic photosynthesis was well established prior to the GOE but that atmospheric methane oxidation suppressed oxygen levels They modeled a low-level steady state of oxygen and a high-level steady state—the latter being characterized by an ozone layer that shields the troposphere from ultraviolet radiation, limiting the rate of methane oxidation. Related to methane oxidation, Konhauser et al.[27] proposed a coupled biological-geological driver for the GOE, concluding that a reduced flux of nickel to the oceans in the late Archean limited methanogen activity, thereby capping the supply of biogenic methane
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