3GEOCARB-II - A REVISED MODEL OF ATMOSPHERIC CO2 OVER PHANEROZOIC TIME

Revisions of the parameters in the model of Berner (1991) (henceforth GEOCARB I) for the long term geochemical carbon cycle have been made. Principal changes are: (1) inclusion of the effects of the variation of solar radiation in the feedback functions for continental weathering; (2) distinction between the feedback function for silicate weathering and that for carbonate weathering; (3) exclusion of land area from the expression for Ca-Mg silicate weathering and its replacement by an uplift factor calculated from changes in the strontium isotopic composition of the oceans; (4) consideration in the expression for carbonate weathering of changes in land area underlain by carbonate rocks; (5) use of new data on the rate of seafloor subduction and spreading over the past 150 my; (6) use of new data on the temperature coefficient of silicate dissolution and the relation between river runoff and rates of silicate weathering; (7) application of a new parameterization of global runoff over time; (8) consideration of an alternate model where total crustal carbon (organic C plus carbonate C) is allowed to vary with time. Results indicate that: (1) increase in the solar constant over the past 570 my has brought about a gradual drop in atmospheric CO2 from values much higher during the early Paleozoic than today. Superimposed on this are large fluctuations which in: general are similar to those calculated by the previous model; (2) high CO2 values during the Mesozoic and a decrease in CO2 during the Cenozoic are due largely to low Mesozoic relief and Cenozoic mountain uplift combined with decreasing metamorphic/volcanic degassing of CO2 during the Cenozoic; (3) changes in river runoff and carbonate land area are relatively unimportant, compared to other factors, as major controls on CO2; (4) variable degassing, due to changes in seafloor spreading rate, was not a major control on atmospheric CO2 during the Paleozoic; (5) calculated Phanerozoic CO2 is sensitive to the proportion of carbonate deposition in shallow water versus the deep sea as this affects the probability that the carbonate will be deeply buried and thermally degassed; (6) letting total crustal carbon vary by not tracking the masses of organic C and carbonate C with time has little effect on results. Over Phanerozoic time a major control on global climate has been the CO2 greenhouse effect, and changes in CO2 have been a consequence of a combination of geological, biological, and astronomical factors.