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An Even Longer Term Effect
Notes:
When 41K integration is applied, followed by a second 96K integration pass, there appears to be another correlation peak at about 400K-500K years. This corresponds to a known second order variability in the Earth's orbital eccentricity which has about a 400K year period. The peak at 800K years is non deterministic since there are so few samples that are 800K years apart. However, the Vostok data has a similar non deterministic peak at 400K years, which turned out to be real. The 800K year peak could be related to the phase alignment of the 21K and 42K effects.
The peak effect of the 400K-500K cycle seems to have been about 450K years ago, coincidentally aligning with slightly separated peaks of the 96K and 41K effects. This led to the most persistently warm interglacial period in the record. It's minimums corresponding to the shortest interglacial period 250K years ago and the longest ice age about 650K years ago. It takes millions years for all of these forcing effects to cycle through a complete set of relative phase relationships.