Using a carbon cycle model, I show that the 405 kyr periodicity found in marine δ¹³C during the last 5 million years and the offset in atmospheric δ¹³CO₂ between the Last Glacial Maximum and the Penultimate Glacial Maximum are probably related to each other. They can be explained by variations in the δ¹³C signature of weathered carbonate rock or of volcanically degassed CO₂, which vary mainly with obliquity (41 kyr), suggesting that Northern Hemispheric land ice sheets are their ultimate drivers.