Ph.D Defense Seminar


Mojtaba Fakhraee
Ph.D. Candidate, Water Resources Science

July 26, 2018


A New Insight into the Geochemistry of Sulfur in Low Sulfate Environments

As an essential element for life, sulfur plays an important role in biosphere, hydrosphere, atmosphere and lithosphere. Study of sulfur cycling has been traditionally concentrated on modern marine environments with 28mM of sulfate, yet the importance of sulfur cycling in low sulfate environments of the present such as large freshwater systems as well as oceans of the past (>0.5 billion years ago) cannot be neglected. This thesis, through modeling and theoretical approach, aims to provide a new insight into several aspects of sulfur cycling in low sulfate environments. For example, it is widely assumed that water-column sulfate is the main sulfur source to fuel microbial sulfate reaction in sediments. While this assumption may be justified in high-sulfate environments such as modern seawater, I show that in low-sulfate environments, mineralization of organic sulfur compounds can be an important source of sulfate and sedimentary sulfide. The results in this thesis indicate that in low sulfate environments (<500 µM) the in-sediment production of sulfate can support a substantial portion (>50%) of sulfate reduction. Extrapolating the results to primordial oceans with tens of µM of sulfate, modeling results reveal that mineralization of organic sulfur generated sulfite, which in the absence of ambient sulfate fueled microbial S reduction, and hydrogen sulfide, which provided a pathway to pyrite that bypassed the microbial reduction of sulfate or sulfite. Reproducing isotopic records in sedimentary sulfides from the rock record, modeling results show that in the low sulfate (<10 µM) environment of Archean oceans (2.5-4 billion years ago), oxygen could have accumulated to up to 25 µM, while being consistent with the sulfur isotopic composition in Neoarchean rocks. A mass balance model coupled to sediment diagenesis model suggests that seawater sulfate concentrations during the Proterozoic Eon (0.5-2.4 billion years ago) remained below 1.5% of modern values (<500 µM), and possibly as low as 100 µM. Using exploratory modeling of sulfur cycling, I also constrain the geochemical factor that control on the fluxes of methylmercury from modern freshwater sediments. Modeling results identify oxygen, sulfate, and organic matter as leading geochemical parameters. They also suggest a critical level of oxygen at the sediment water interface, below which methylation rate dominates demethylation rate, resulting in an efflux of methylmercury into the overlying water.