I have seen several graphs showing the prehistoric temperatures and CO2 concentrations derived from ice-core data. My understanding is that CO2 and temperature correlate. I assume that Milankovich Cycles are what caused the variations observed in temperature in the ice record, but what specifically caused the CO2 to fluctuate?
This article investigates the ocean's dissolved carbon content (ocean reservoir) and the seafloor's methane hydrates ("methane ice") as a source of CO2 fluctuations in ice cores. They discuss three ways the ocean may release CO2.
through physics (changes in temperature and ocean currents)
biologically or chemically
through general climate changes
Hope this helps.