Carbon Dioxide levels appear to be linked to the Milankovitch cycles according to Ice Core data but what is the mechanism?

Question

As in ice core data shows that $CO_2 $ levels have fluctuated on levels correlating to the temperature and ice levels, but I couldn't think of a mechanism for what might be the link between the release of carbon dioxide and other gases, and presumably orbital geometry?

Is it to do with gases dissolved in the oceans? Or the melting of permafrost?

Answer 1

The link between orbital parameters and CO₂ levels is, as you wrote, the temperature.

Orbital parameters may change, via feedback mechanisms, the atmospheric temperature. To my knowledge the most important feedback mechanism with respect to the Milankovich cycles is the ice-albedo feedback.

The temperature, however, is an important player in geochemical feedbacks which influence the concentration of gases like CO₂ or methane. For CO₂, again to my knowledge, you named the most important feedback; the exchange of CO₂ between atmosphere and ocean.

The exchange of CO₂ between ocean and atmosphere in turn depends on the partial pressure of CO₂ in the atmosphere and the surface layer of the ocean; there is uptake, release according to the pressure gradient. Now, the partial pressure of CO₂ in the surface layer depends also on the amount of carbon transported or received from the deep sea layers. There is way more CO₂ stored in the cold water of the deep sea. So the exchange of CO₂ does depend on the vertical circulation pattern of the ocean.

There are, to my knowledge, three major mechanisms that transport carbon between the surface and the deep sea and they all depend on temperature.

Solubility pump: Cold water can store more CO₂ than warm water. Therefore at the poles, where cold water is sinking into the deep sea, you have CO₂ rich water. The CO₂ stored in the deep sea can, as far as I know, remain there for hundreds of years, before it comes up again in the warmer upwelling regions at the equator. The temperature dependence of that mechanism is obvious.

Biological pump: In the surface layer you have photosynthesis of algeas etc., so carbon is withdrawn from the layer and stored in biomass. A portion of dead plants is not reused (eaten) but sinks into the deep sea. The process is temperature dependend since biological activity depends on temperature.

Carbonate pump: The amount of carbonate and CO₂ are linked to each other via marine chemistry. Animals building shells etc. need carbonate thereby also influencing the amount of CO₂ present in the water. Again, a portion of this carbonate is withdrawn from the short term cycles once the animals die and sink to the deep sea. Again this mechanism is temperature dependend since biological activity is.