The early geochemical cycles on the terrestrial planets differed from their modern counterparts—Prior to about 3.5 billion years ago, the atmospheres of Earth, Mars and Venus were chemically neutral or mildly reducing and all three planets lacked significant biological activity. One of my primary interests is in exploring how differences in oxidation state and biological activity, among others, affected cycling of the elements on a global scale and how the surface environments of these planets evolved from their early state to the present-day.

One example of the possible effect of such differences is in the behavior of the sulfur cycle and its coupling to the carbon cycle. Sulfur dioxide (SO₂) is one of the most abundant gases in volcanic volatiles. Like CO₂, it is a greenhouse gas and weak diprotic acid (H₂SO₃ or sulfurous acid) whose conjugate base (sulfite) can precipitate to form a mineral. In atmospheres rich in CO₂ and poor in O₂, such as those that existed early in the history of Earth, Mars and Venus, the lifetime of SO₂ to photolysis and oxidation is substantially lengthend. If the aqueous reservoir is relatively small, as it likely was on early Mars, small aqueous sinks together with the long atmospheric lifetime could lead to accumulation of SO₂ to relatively high concentrations. This raises the possibility that the climate of Mars during parts of the Noachian epoch (prior to 3.8 billion ago) was regulated by a negative feedback between the atmospheric abundance of SO₂ and the rate of chemical weathering of silicate minerals, instead of the analogous CO₂-silicate weathering feedback active on the modern Earth [Halevy et al., Science, 2007].

Considering the differences from the modern sulfur cycle is important also for understanding the record of mass-independent fractionation of sulfur (MIF S) isotopes. The processes responsible for production of MIF S are topics of ongoing study, but it is generally accepted that oxygen levels must have been extremely low for MIF S to be preserved in the geologic record. Research in this field has centered on enriching the observational record and on investigating the fractionation mechanism, but not much attention has been paid to biogeochemical processing of the anomalous material after deposition from the atmosphere and until its burial and preservation. Accounting for this processing, several features of the Archean mass-independent sulfur isotope record can be linked to variability in the speciation of volcanic sulfur-bearing gases, and used to constrain the relative importance of biological sulfur cycling [Halevy et al., Science, 2010].