DESIGNING AN INTEGRATED THEORY OF EARTH SYSTEM OXYGENATION (Invited Presentation)

The Great Oxidation Event (GOE) represents one of the most fundamental transformations in the history of life and its environment, but its cause is still debated. Most agree that the evolution of O₂-producing photosynthesis was a necessary condition, but not a sufficient one because of the complex connections between biogeochemical evoluton and solid Earth processes. This view has strengthed over the past decade, as multiple lines of evidence - notably from inorganic paleoredox proxies - suggested that biological O₂ production began hundreds of millions of years before the GOE. Hence, a variety of hypotheses were advanced that link the GOE to changes in solid Earth processes, such as in the flux or composition of volcanic gases, in crustal composition, or in tectonic evolution.

We propose that these hypotheses are not mutually exclusive, but are best understood as complementary consequences of Earth's thermochemical evolution as it gradually cooled, and that the next step of GOE research requires that we develop a quantitative, predictive “theory” of the abiotic Earth system that describes the mechanistic linkages between the interior and the surface, and their change with time, as a function of planetary mass, bulk composition, and cooling history. Ideally, this theory would employ minimal parameterization so that it could extend to Earth-like exoplanets, providing a first-principles boundary condition for understanding life’s emergence, evolution, and impact on Earth and Earth-like planets.

At the extremes, the framework of such a theory is straightforward: On a very hot planet, mixing between the surface and the interior is very rapid, so that any O₂ produced biologically (or non-biologically, such as by UV photolysis or H-escape) will be overwhelmed by internally derived reductants. O₂ accumulation is impossible on such a world even if O₂ is rapidly produced. In contrast, on a cold planet, mixing is very slow, so that surface redox is unaffected by internal reductants. O₂ accumulation is easy on such a world, so long as there is any O₂ production. Between these extremes lies an immense, complex generational “grand challenge” requiring fundamental advances and collaboration across subdisciplines that rarely interact. The hurdles are both scientific and sociological, but if overcome would transform the geosciences.