UC Santa Barbara geologist correlates patterns in sediment cores to climate cycles, particularly glaciation periods........

 

http://www.sciencedaily.com/releases/2010/04/100406133707.htm

It is known that Earth's orbit around the sun changes shape every 100,000 years. The orbit becomes either more round or more elliptical at these intervals.

 

Makes it sound like the eccentricity change happens all at once, rather than a slowly-varying change with a dominant fourier component at that period.