Seafloor hydrothermal activity and spreading rates: the eocene carbon dioxide greenhouse revisited

A suggestion has been made that enhanced rates of hydrothermal activity during the Eocene could have caused a global warming by adding calcium to the ocean and pumping CO₂ into the atmosphere (Owen and Rea, 1984). This phenomenon was purported to be consistent with the predictions of the CO₂ geochemical cycle model of Berner, Lasaga and Garrels (1983) (henceforth BLAG). In fact, however, the BLAG model predicts only a weak connection between hydrothermal activity and atmospheric CO₂ levels. By contrast, it predicts a strong correlation between seafloor spreading rates and pCO₂, since the release rate of CO₂ from carbonate metamorphism is assumed to be proportional to the mean spreading rate. The Eocene warming can be conveniently explained if the BLAG model is extended by assuming that the rate of carbonate metamorphism is also proportional to the total length of the midocean ridges from which the spreading originates.