Coupling the geochemical cycles of C, P, Fe, and S: The effect on atmospheric O2 and the isotopic records of carbon and sulfur

Abstract

A model that tracks the coupled cycling of carbon, phosphorus, iron, sulfur, and oxygen as well as carbon and sulfur isotope ratios through surficial reservoirs on multi-million year time scales has been constructed. Phosphorus-limited marine productivity, in which surface ocean P availability is coupled to the degree of anoxia in ocean bottom waters, is employed to enhance Po2 stability. In separate trials, parameters controlling continental weathering fluxes, ocean vertical mixing rates, and burial of terrigenous organic matter are adjusted to examine the effects on the model in terms of reservoir masses, fluxes, and isotopic compositions. As expected, the system responds to imposed perturbations with significant changes in the rates of both burial and weathering of organic carbon and pyrite. These changes in turn influence the isotope ratios of carbon and sulfur reservoirs and the mass of atmospheric oxygen. This paper explores the range of parameter values that concurrently generate equable Po2 and realistic reservoir masses and isotopic compositions. The perturbations applied in this model generate isotope variations of up to ±4 permil for dissolved inorganic carbon and up to ±3 permil for dissolved sulfate, generated under Po2 within a factor of 2 of the present atmospheric level. These isotope excursions last a minimum of 30 my for carbon isotope and significantly longer than model run time (150 my) for sulfur isotopes. Thus in both magnitude and duration, these isotope shifts begin to approach those observed in the geologic record of carbonate δ13C and gypsum δ34S, without requiring catastrophic O2 variations. Recognition that geologically realistic isotope excursions can be modelled with concurrent equable Po2 reaffirms the potential of the geologic records of δ13C and δ34S as useful tools to construct a history of Phanerozoic oxygen.