100-kyr climate cycles caused by 2.4-Myr eccentricity-modulated carbon cycles

Earth’s climate has been dominated by ~100-kyr glacial cycles over the past ~800 ka, yet the mechanism remains debated. Here, we present correlation analyses of spectral power ratios of global records spanning the past 2.7 Ma, revealing a persistent anticorrelation between ~21-kyr and ~100-kyr power ratios, but no significant relationship between ~41-kyr and ~100-kyr power ratios. This suggests that ~100-kyr climate cycles are more related to eccentricity-modulated precession than to obliquity. Phase analyses of benthic δ18O/ice volume and δ13C (carbon cycle) since Antarctic glaciation onset (~34 Ma) show that strong ~100-kyr cycles emerged only when these proxies were phase-coupled. Such coupling recurred at ~2.4-Myr eccentricity maxima during the unipolar regime (before 7.5 Ma) and minima during the bipolar regime (after 4 Ma), explaining the persistent ~21-kyr/~100-kyr anticorrelation because eccentricity modulates precession amplitude. We propose that internal carbon cycle dynamics and ~2.4-Myr eccentricity-modulated δ¹⁸O/ice volume–δ¹³C coupling amplified ~100-kyr climate cycles not only over the past ~800 ka but since 34 Ma. Given that eccentricity will remain low for the next 400 kyr, ~100-kyr periodicities may continue to dominate future climate variability, assuming Earth remains in a bipolar regime.