ATMOSPHERIC PCO2 SINCE 60 MA FROM RECORDS OF SEAWATER PH, CALCIUM, AND PRIMARY CARBONATE MINERALOGY

A 60-m.y.-long record of atmospheric pCO₂ has been refined from knowledge of (1) secular changes in the major ion composition of seawater (particularly Ca and Mg) measured from fluid inclusions from marine halite deposits and (2) oscillations in the mineralogy of primary oceanic carbonate sediments. Both factors have had a significant impact on the chemistry of the ocean carbonate buffer system. At equilibrium between surface seawater and a crystalline CaCO₃ phase at constant temperature, the [Ca²⁺] and [CO₃^2–] ion concentrations in seawater are governed by the solubility-product constant ([Ca²⁺][CO₃^2–]) for the particular carbonate phase under consideration. Once seawater pH and [CO₃^2–] are known, the pCO₂ of the atmosphere in equilibrium with surface seawater can be calculated from the relationship: pCO₂=[H+]2[CO₃^2–]/K₁K₂K_H, where K_H is the Henry’s law constant for CO_2(g), and K₁ and K₂ are the first and second dissociation constants for carbonic acid. Calculated atmospheric pCO₂ oscillated between values of 100–300 ppm and up to maxima of 1200–2500 ppm from 60 to 40 Ma and varied between 100 and 300 ppm from 25 Ma to the present. The refined pCO₂ values are significantly lower than previous estimates made from seawater pH data where total dissolved inorganic carbon (TDIC) was assumed constant and more in line with modeling and stomatal index estimations of atmospheric pCO2 for the Tertiary.