The Sulfur-Isotopic Composition of Cenozoic Seawater Sulfate: Implications for Pyrite Burial and Atmospheric Oxygen

Abstract

The availability of high-temporal-resolution C- and S-isotope curves for the Cenozoic permit for the first time modeling of the influence of the C and S cycles on the partial pressure of atmospheric O2 on comparable time scales. A simple isotope mass-balance model was used to calculate atmospheric O2 levels from the burial rates of organic C and pyrite S. Burial rates were derived from the C- and S-isotope records of seawater-dissolved inorganic C and sulfate. Results indicate that in the early Cenozoic atmospheric O2 levels were about 16% higher than current levels. Extension of the model to Phanerozoic time scales yields atmospheric O2 levels that are inconsistent with geological evidence that suggests that the mass of atmospheric O2 has not changed by more than a factor of two from the present atmospheric level since the Cambrian (Berkner and Marshall, 1974; Watson et al., 1978; Jones and Chaloner, 1991). These results indicate that either our knowledge of the parameters controlling atmospheric O2 is incomplete, or that the assumptions used in such models inadequately represent the complexity of the natural systems. Here we critically examine the assumptions inherent in isotope mass-balance models to determine whether they may be the source of the model-data discord. A major problem with these models is the extreme sensitivity of the mass of atmospheric O2 to very small changes in the much larger masses of oxidized and reduced C and S reservoirs. For example, small variations in continental weathering fluxes and the associated isotope ratios of river input have profound effects on calculated O2 levels and need to be accounted for. Similarly, variations in the isotopic composition of pyrite and organic C buried in sediments, which are strongly influenced by changes in isotopic fractionation, dramatically influence calculated O2 concentrations. Thus, constant fractionation factors should not be applied in such models. In addition, the assumption that the isotopic composition of dissolved inorganic C is controlled only by the relative amounts of reduced and oxidized C buried in sediments and their respective isotope ratios is questionable when relatively short time scales are considered. Isotope mass-balance models do not adequately encompass and simulate the actual processes being modeled because of the simplifications and assumptions made. More “realistic” models are required to achieve stabilization of atmospheric O2 over geological time.