What caused the Carbon Dioxide Variations observed in the 800,000-year polar ice record?
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.