What caused the mid-Pleistocene transition, when glacial cycles shifted from a roughly 41,000 year rhythm to the roughly 100,000 year rhythm still running today?
Milankovitch cycles predict a strong 41,000 year obliquity signal throughout, yet ice core and marine sediment records show glacial cycling shifted toward a dominant 100,000 year period around one million years ago, a change the orbital forcing itself does not obviously explain, since the eccentricity cycle it would need to track is a comparatively weak driver of incoming solar radiation on its own.
What would resolve this A mechanism, most proposed candidates involve declining atmospheric carbon dioxide, thinning regolith on the continents changing how ice sheets respond, or some combination, that can be tested against the same ice core and sediment record well enough to be preferred over its rivals rather than merely consistent with the data.
Climatology, GeochronologyGeologic Time Scale (Wikipedia)