Stepwise oxygenation of Earth is an inherent property of global biogeochemical cycling

Geological studies of ancient Earth have indicated that, when the planet began 4.54 billion years ago (BYA), it was basically anaerobic (i.e. very low levels of atmospheric O2). Then, the Great Oxidation Event (GOE) occurred between ~2.5 and 2.0 BYA, and scientists continue to argue about the cause. In mid-Dec 2019, these GEITP pages shared an article, “The Great Oxidation Event and the Lomagundi Event might have both occurred by tectonic plate movements about 2.5 billion years ago” — which suggested the entire oxygenation of Earth’s surface environment might be explained by one big event, tectonic plate transitions.

Others postulate the GOE might have occurred across three broad steps [see attached article]: [a] From 2.4 to 2.2 BYA, atmospheric O2 rose from trace levels to more than 10–5 of the present atmospheric level (PAL); [b] The next ~1.7 billion years of the Proterozoic Eon likely sustained atmospheric O2 levels of ~10–3 to 10–1 PAL; [c] The Paleozoic Oxygenation Event (POE) ~450 to 400 million years ago (MYA) appears to have elevated atmospheric O2 to present-day levels and established a dominantly oxygenated deep ocean, which persisted throughout the Mesozoic and Cenozoic Eras [these major oxygenation steps are entwined with evolution of progressively more complex life-forms, the first eukaryotes (organisms having pairs of chromosomes and nuclear membrane) evolved either after the GOE, or during the run-up to the GOE when O2 began to rise], whereas the Neoproterozoic Oxygenation Event (NOE) occurred between ~800 and 540 MYA — and was coincident with major eukaryote diversification and evolution of the first animals, followed by the Cambrian explosion, during which animals began to dominate marine ecosystems.

Tectonic evolution has also been considered as a potential driver of the stepwise transitions in Earth surface oxygenation. Changes in plate tectonics have been linked to the GOE through (e.g. a change in the fraction of subaerial volcanism or composition of the

crust). Some, but not all, supercontinent formation times correspond to oxygenation events, as do emplacement times of some large igneous provinces (LIPs), which are proposed to have driven ocean oxygenation through delivery of the limiting nutrient, phosphate (PO4—). However, the geologically rapid yet ultimately rare nature of Earth’s oxygenation events does not clearly correspond to either tectonic or evolutionary processes; for example, mantle dynamics and the supercontinent cycle are unlikely to produce large-scale changes on time-scales on the order of less than ~100 million years, whereas LIP emplacements are far more common than major rises in O2. Looking to biological innovations, the time scale between origination of a domain or kingdom of life (e.g. Eukarya) and its rise to global ecological dominance may also be hundreds of million-years. Furthermore, oscillations in ocean redox (reduction–oxidation) — that are apparent during the NOE — are difficult to explain through a sequence of tectonic or biological “switches” acting on the system.

It is therefore possible that Earth’s stepwise-oxygenation was not the product of individual trigger events and may instead be explained by some inherent gradient of global biogeochemical feedbacks. This hypothesis has wide implications for the evolution of life on Earth and other planets; therefore, there have been a number of attempts to explain the known stepwise O2 trajectory as a feature of Earth’s internal dynamics (e.g. it has been shown that atmospheric feedbacks might have promoted the GOE). However, no study has provided a sound theoretical basis that can explain the trajectory and timing of marine and atmospheric oxygenation over Earth’s history — without relying on either external trigger events or arbitrary switches in the model itself (e.g. assuming a transition to greater nutrient availability when O2 crosses a threshold).

Authors [see attached article] identified a set of feedbacks that exist — between the global P, C, and O cycles — which are capable of driving rapid shifts in oceanic and atmospheric O2 levels, without requiring any stepwise change in either tectonics

or evolution of the biosphere. These feedbacks replicate the observed three-step oxygenation pattern, when driven solely by a

gradual shift from reducing to oxidizing surface environmental conditions over time. Phosphorus (P) is generally considered the

ultimate limiting nutrient for marine productivity over geological time-scales, and P bioavailability exerts a key control on the

long-term rate of O2 production through oxygenic photosynthesis and organic carbon (Corg) burial. Using a theoretical model — that the observed oxygenation steps are a simple consequence of internal feedbacks in the long-term biogeochemical cycles of P, C, and O — authors show there is no requirement for a specific stepwise external forcing to explain the course of Earth surface oxygenation. They conclude that Earth’s oxygenation events are entirely consistent with gradual oxygenation of the planetary surface — after the evolution of oxygenic photosynthesis. To some of us, this makes the most sense. 😊

DwN

Science 13 Dec 2019; 366: 1333-1337

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