Modeling isotopic proxies for the oxygenation of the earths surface
Open Access
- Author:
- Domagal-Goldman, Shawn David
- Graduate Program:
- Geosciences
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- May 14, 2007
- Committee Members:
- James David Kubicki, Committee Chair/Co-Chair
James Kasting, Committee Chair/Co-Chair
Michael Allan Arthur, Committee Member
Jorge Osvaldo Sofo, Committee Member - Keywords:
- Archean
Isotope Geochemistry
Atmospheric Models
Molecular Models - Abstract:
- 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, I examine two relatively new proxies for the evolution of the Earth’s surface oxidation state, both of which use stable isotope measurements to identify a permanent oxidations of the surface that occurred between ~2.4 and ~1.8 billion years ago (Ga). Stable isotope measurements of Fe in sediments demonstrate an increase in maximum 56Fe prior to ~1.8 billion years ago (Ga) and in the magnitude of 56Fe 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, Fe isotope measurement has also been proposed as a biomarker, 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, I model the fractionation associated with complexation of Fe with organic ligands. 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. A large spread in 33S values in sediments older than ~2.45 Ga and the complete absence of significant 33S in sediments younger than ~2.3 Ga is commonly accepted as evidence that atmospher O2 concentrations permanently rose at ~2.4 Ga. Subsequent analyses have uncovered uncover secondary features in the 33S record. The most notable excursion is a decline in the magnitude of 33S between ~3.2 and ~2.7 Ga that has been used to invalidate the aforementioned use of 33S to date the rise of atmospheric O2. In Chapter 3, I propose a new control on 33S – namely an organic haze that could have limited production of 33S in an anoxic atmosphere. I use a 1-dimensional photochemical model to predict the reaction rates for photolysis of SO2, a reaction thought to be required for 33S production. Finally, in Chapter 4, I examine the climatic implications of the haze and of other hydrocarbon species that would have been present in the Archean atmosphere, and demonstrate how “Gaian”-like feedbacks should have existed in the Archean that stabilized climate and atmospheric composition.