Modeling novel isotopic proxies for the oxygenation of the earth's surface

by Domagal-Goldman, Shawn D.

iii Tracking the evolution of the oxidation state of the Earth?s surface environment has increased understanding of the biological, atmospheric, oceanic, and geological evolution of the Earth, and may allow us to broaden the search for life on extrasolar planets. In this thesis, two relatively new proxies for the evolution of the Earth?s surface oxidation state are examined. Both proxies use stable isotope measurements to identify a permanent oxidation of the surface that occurred between ~2.4 and ~1.8 billion years ago (Ga). Measurements of the stable isotopes of Fe (54Fe, 56Fe, and 57Fe) in sediments demonstrate an increase in maximum 56Fe/54Fe prior to ~1.8 billion years ago (Ga) and an increase in the maximum and decrease in the minimum 56Fe/54Fe prior to ~2.3 Ga. These data have been interpreted as being the result of stepwise changes to the oxidation state of the Earth?s oceans. However, the measurement of Fe isotopes has also been proposed as a way to identify a history of life in a sample, as Fe isotopes have been shown to fractionate during metabolic processes and upon complexation with organic acids. In the first two chapters of the thesis, the fractionation associated with complexation of Fe with organic ligands is modeled. Equilibrium constants are predicted for equilibrium isotope exchange for redox and ligand exchange reactions. These predictions allow comparison of these two types of fractionation and place the two proposed uses of Fe isotopes in better theoretical context. Another novel tool for tracking the redox history of the Earth is the measurement of multiple S isotopes. In sediments younger than ~2.3 Ga, the fractionation of the stable iv S isotopes (32S, 33S, 34S, and 36S) follows specific trends that depend on the mass of the isotopes. This behavior is classified as mass-dependent fractionation, and is witnessed for almost all known kinetic, equilibrium, and biological fractionation processes. However, sediments older than ~2.45 Ga do not follow this trend. As such they are said to exhibit mass-independent fractionation of Sulfur isotopes (S-MIF), a process that has been recreated in the laboratory using photolysis of SO2 using UV light. The presence of S- MIF in these older rocks is commonly accepted as evidence that atmospheric O2 concentrations permanently rose at ~2.4 Ga, establishing an ozone shield that shielded SO2 from UV radiation and prevented the creation of S-MIF in the lower atmosphere. Subsequent analyses have uncovered secondary features in the S-MIF record. The most notable excursion is a decline in the magnitude of S-MIF between ~3.2 and ~2.7 Ga that has been used to invalidate the aforementioned use of S-MIF to date the rise of atmospheric O2.