Carbon Dioxide in the Atmosphere

It is not unlikely that atmospheric carbon dioxide concentrations have varied greatly over Phanerozoic time. While obviously it is impossible to directly measure this, there are a number of clever indirect methods that researchers can use.

One involves the ratio of aragonite to calcite in rock deposits. Aragonite is a form of calcium carbonate, like calcite. It will dissolve readily if atmospheric pCO₂ levels are high. When the line falls below the aragonite threshold (hatched area on the diagram below) it implies lower pCO₂ levels in the atmosphere.

If you look at the diagram above you can see that for a number of periods, such as the early and middle Paleozoic and the Mesozoic and early Cenozoic there is virtually no aragonite, but only calcite. Glaciations are known from the late Proterozoic, the Permo-Carboniferous and of course the Quaternary; these were periods of low CO₂ (indicated by aragonite deposits). However the end Ordovician glacial event does not match this cycle. It can be presumed that a number of factors are at work in the regulation of the icehouse-greenhouse cycle.

Links

Links:  Geology 150 - Climate Changes