COUPLING THE GEOCHEMICAL CYCLES OF C, P, FE, AND S - THE EFFECT ON ATMOSPHERIC O-2 AND THE ISOTOPIC RECORDS OF CARBON AND SULFUR

A model that tracks the coupled cycling of carbon, phosphorus, iron, s ulfur, and oxygen as well as carbon and sulfur isotope ratios through surficial reservoirs on multi-million year time scales has been constr ucted. Phosphorus-limited marine productivity, in which surface ocean P availability is coupled to the degree of anoxia in ocean bottom wate rs, is employed to enhance Po-2 stability. In separate trials, paramet ers controlling continental weathering fluxes, ocean vertical mixing r ates, and burial of terrigenous organic matter are adjusted to examine the effects on the model in terms of reservoir masses, fluxes, and is otopic compositions. As expected, the system responds to imposed pertu rbations with significant changes in the rates of both burial and weat hering of organic carbon and pyrite. These changes in turn influence t he isotope ratios of carbon and sulfur reservoirs and the mass of atmo spheric oxygen. This paper explores the range of parameter values that concurrently generate equable Poe and realistic reservoir masses and isotopic compositions. The perturbations applied in this model generat e isotope variations of up to +/-4 permil for dissolved inorganic carb on and up to +/-3 permil for dissolved sulfate, generated under Pot wi thin a factor of 2 of the present atmospheric level. These isotope exc ursions last a minimum of 30 my for carbon isotope and significantly l onger than model run time (150 my) for sulfur isotopes. Thus in both m agnitude and duration, these isotope shifts begin to approach those ob served in the geologic record of carbonate delta(13)C and gypsum delta (34)S without requiring catastrophic O-2 variations. Recognition that geologically realistic isotope excursions can be modelled with concurr ent equable Po-2 reaffirms the potential of the geologic records of de lta(13)C and delta(34)S as useful tools to construct a history of Phan erozoic oxygen.