Why do the frequencies of glaciers in the past 2.5 million years change?

In the pass 2.5 million years, the frequencies of glaciers changed. The last 700,000 years were marked with oscillations of 100,000 years, but prior to 700,000 years ago, the oscillations are much weaker and show frequencies of 41,000. What happened? Did oceans play some roles in this process?

What is the ocean circulation like in the Cretaceous?

The middle Cretaceous (120-90 million years ago) is substantially warmer than the present. The deep ocean temperature can reach up to 10-20 C. Therefore, the circulation could be totally different from the present state. Furthermore, the carbon cycle and atmospheric circulation could also be different. Any works on these issues?

Evidences show the increasing CO2 happened first due to the intensive volcanism eruption, then the temperature will rise for the greenhouse gas effect.       

One thing needs to be noted here is the bathymetry then is totally different from now.