A MODEL FOR ATMOSPHERIC CO2 OVER PHANEROZOIC TIME

A new model has been constructed for calculating the level of atmospheric CO2 over Phanerozoic time which is much simpler mathematically than the BLAG model, but more complex geologically and biologically. Mathematical simplification comes about by following the cycle of carbon only, lumping all carbonate minerals together, combining the ocean and atmosphere into one reservoir, and calculating atmospheric CO2 level as a series of successive ocean-atmosphere steady states. The isotope mass balance model of Garrels and Lerman (1984) is expanded to include the effect of changing area, elevation, and position of the continents as well as the evolution of land plants as they affect weathering rate, and changes in seafloor area generation rate and the relative importance of deep sea versus shallow platform carbonate deposition as they affect global degassing. Atmospheric CO2 level is calculated from a weathering feedback function for silicates which varies with time as vascular land plants arise and evolve. Results suggest that there has been a notable pattern of varying atmospheric CO2 level over the past 570 my with high levels during the Mesozoic and early Paleozoic and low levels during the Permo-Carboniferous and late Cenozoic. Sensitivity analysis shows that, within reasonable limits of the principal governing parameters, this qualitative trend of varying CO2 is relatively insensitive to the actual values chosen for these parameters. Causes of the CO2 variation are multiple, and no single geological or biological process can be called upon to explain all CO2 variation for all time. The calculated trend of CO2 over time agrees well with independent estimations of paleoclimates. Thus, the greenhouse theory of paleoclimate on a long geological time scale is supported by the results of the present study.