Question

Three scientists discuss the cause of the Great Oxidation Event (GOE) approximately 2.4 billion years ago, during which atmospheric oxygen ($O_2$) levels rose from virtually zero to significant fractions of modern levels.

Scientist 1
The GOE was driven entirely by the biological evolution of oxygenic photosynthesis in cyanobacteria. Prior to the GOE, cyanobacteria produced $O_2$, but it was immediately consumed by abundant reducing agents, primarily dissolved ferrous iron ($Fe^{2+}$) and volcanic gases. The GOE occurred when these local chemical sinks were finally saturated. The timing and rate of the $O_2$ rise were controlled strictly by the burial rate of organic carbon. The burial of organic matter prevented it from reacting with $O_2$ to reform $CO_2$, thereby leaving a net surplus of $O_2$ in the atmosphere.

Scientist 2
Biological production of $O_2$ was necessary but not sufficient for the GOE. Cyanobacteria evolved hundreds of millions of years before the GOE, but atmospheric $O_2$ could not accumulate due to the continuous input of highly reduced volcanic gases ($H_2$ and $CO$) from submarine volcanoes. The GOE was triggered by a major tectonic shift: a transition from predominantly submarine volcanism to subaerial (land-based) volcanism. Subaerial volcanoes release more oxidized gases ($CO_2$ and $SO_2$) because they erupt at lower pressures and react with the atmosphere. This tectonic transition reduced the planetary volcanic sink for $O_2$, allowing $O_2$ to accumulate without requiring any change in the rate of organic carbon burial.

Scientist 3
The GOE was caused by a permanent change in Earth's overall redox state driven by hydrogen escape to space. Early Earth had an atmosphere rich in methane ($CH_4$) produced by methanotrophic and methanogenic archaea. When cyanobacteria produced $O_2$, it reacted with methane, but solar ultraviolet radiation also photolyzed methane in the upper atmosphere. The resulting hydrogen gas ($H_2$), being extremely light, escaped Earth's gravity into space. This loss of hydrogen represents a permanent oxidation of the planet. Cyanobacteria and carbon burial rates were stable; the GOE occurred only when the cumulative loss of hydrogen reduced the abundance of reducing agents to the point that $O_2$ could persist.

Match each of the following statements with the scientist whose viewpoint it represents.