Simulating the Cenozoic Carbon Cycle
Over the course of the Cenozoic – the last 65 million years – Earth’s climate transitioned from greenhouse conditions to the recurring ice ages during which human civilization was born. To our best knowledge secular global cooling was caused by progressively declining levels of atmospheric CO2, which is commonly attributed to declining levels of volcanism and/or increasing weatherability of continental rocks. The consequences for and the role of ocean chemistry in this global carbon cycle transition is poorly understood and often ignored. Reconstructed changes in the major ion composition of seawater, especially its calcium concentration, imply drastic changes in seawater acid/base chemistry.
Relative to modern seawater (left) a high-calcium ocean would yield higher atmospheric CO2 (y-axis) and lower seawater pH (contours) for any given combination of carbonate saturation state (x-axis) and seawater carbon concentration (colored contours). That is to say, atmospheric CO2 was higher not because there was more carbon but because the pH of the ocean was lower.
The source code for MyAMI, the model used to calculate the equilibrium constants, can be downloaded from my MyAMI GitHub page.
- Hain, M.P., Sigman, D.M., Higgins, J.A., and Haug, G.H. (2015) The effects of secular calcium and magnesium concentration changes on the thermodynamics of seawater acid/base chemistry: Implications for Eocene and Cretaceous ocean carbon chemistry and buffering, Global Biogeochemical Cycles, doi: 10.1002/2014GB004986