UC Riverside

Oxygen availability in the oceans and atmosphere has been one of the primary controls on evolutionary advances on Earth, but our inability to measure oxygen concentrations directly requires the use of proxies that have been calibrated in low-oxygen settings. The biogeochemical cycles of sulfur and carbon, among many other elemental cycles, have been closely linked to oxygen concentrations through time, and many proxies for oxygen have arisen from observations of their linked cycles. This dissertation applies a number of geochemical proxies in two different types of environments to constrain the role of sulfate reduction and its relationships to organic carbon burial.

First, I explore the role of hydrocarbon seep environments as a source of methane and reduced sulfur to the oceans, atmosphere, and biosphere using a system of Cretaceous-aged seeps in Colorado, U.S.A. The connection between sulfur and carbon cycling in seep settings through coupled sulfate reduction and anaerobic methane oxidation suggests that sulfur isotope compositions of carbonate-bound sulfate can be used to track paleo-sulfate reduction rates and methane concentrations.

The next chapters focus on reconstructing local and global redox conditions in the Miocene Monterey Formation. The Miocene was a time of elevated global temperatures, and the Monterey Formation's deposition into multiple sub-basins provides an ideal opportunity to study how preservation of geochemical signatures of basin redox conditions vary within and among basins as a function of local and global climatic and oceanic conditions. The first study presents commonly employed redox indicators, including iron speciation and redox-sensitive trace metal concentrations, to constrain paleoredox conditions from three basins with different hydrographic conditions during deposition, including degrees of restriction and intensities of upwelling. A second study builds on these interpretations, applying Mo and Tl isotope proxies to determine whether the low-oxygen conditions achieved in these basins during the Miocene were local or reflect more widespread low-O2 conditions in the global ocean. While collective data suggest that the California Margin was more reducing in the Miocene than it is today, Mo and Tl isotope results suggest this was likely localized and does not represent global-scale deoxygenation.