Earth’s atmosphere currently contains approximately 80% molecular nitrogen (N2), 20% molecular oxygen (O2), and 400 parts per million---or 0.4%---carbon dioxide (CO2), along with other trace constituents. However, this has not always been the case. We know that Earth’s earliest atmosphere was devoid of molecular oxygen, instead being completely anoxic. Atmospheric oxygen levels subsequently increased in two stages---first, during the so-called “Great Oxygenation Event” around 2.3 billion years ago, and second, during the so-called “Neoproterozoic Oxygenation Event” between around 800 and 600 million years ago.
While geologists have garnered a good understanding of the timing and, to a lesser degree, the magnitude of these oxygenation events, the exact combination of geological, chemical, and biological mechanisms that allowed them to occur remain less constrained. While (nearly) all molecular oxygen on Earth is originally produced by photosynthesizing organisms, subsequent reactions such as respiration consume this O2 and prevent it from accumulating. Thus, the very existence of an oxygenated atmosphere requires that photosynthesis outpaces respiration, and that this was especially true during Earth’s two oxygenation events. Still, the exact reason(s) for this remain poorly known, largely due to a lack of quantitative constraints on photosynthesis and respiration through geologic time.
One such respiration mechanism is the conversion of the iron-sulfide mineral pyrite (FeS2) into sulfate (SO42-) during weathering of rocks on land, which simultaneously consumes molecular oxygen and releases carbon dioxide. Pyrite oxidation thus links the global carbon, sulfur, and oxygen cycles. The objective of this project is to develop chemical and isotopic tools to constrain and quantify pyrite oxidation, both on the modern Earth and throughout Earth’s history. We are specifically addressing this by combining well-constrained pyrite weathering experiments in the lab to understand isotope signatures with field studies of modern weathering environments (i.e. river basins) to determine controlling mechanisms and with geologic studies to reconstruct this process through time.
Beyond understanding the history of Earth’s atmospheric oxygen levels, this project is important for society in several ways: First, pyrite oxidation is a major natural process by which atmospheric carbon dioxide is produced, and better understanding the mechanisms that control this will yield insight into ongoing anthropogenic climate change. Second, by developing isotopic fingerprints for sulfur-cycle processes, we are developing a new tool that will aid in paleoclimate reconstructions---thus placing our modern climate perturbations within a historical context. Finally, the results of this work will develop a framework to analyze and interpret sulfur cycling in non-terrestrial materials, potentially including reconstructing traces of atmospheric O2 in other celestial bodies.
