Simulating the Cenozoic Carbon Cycle

Changing seawater composition over the last 120 million years, highlighting changes in calcium and magnesium.
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.

Relationships between pH, DIC, calcium carbonate saturation and atmospheric CO2

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.

Decline in seawater buffering caused by increasing ocean calcium concentration
The scenario for early Cenozoic seawater chemistry outlined above has another severe consequence, relevant to the stability of the climate system and the evolution of marine organisms. If indeed the pH of the ocean was much lower in the early Cenozoic (and the Cretaceous period) the buffering of seawater must have been substantially lower, thereby making seawater pH, CO2 and carbonate saturation much more sensitive to perturbation. Organisms that manipulate their internal chemistry would find it easier to do so in a high-calcium, low-pH ocean. The secular increase in the buffering of seawater over the course of the Cenozoic likely was a powerful driver for evolution by selecting for organisms with ever more refined mechanisms regulating their internal chemistry.

The source code for
MyAMI, the model used to calculate the equilibrium constants, can be downloaded from my MyAMI GitHub page.

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