Eccentricity pacing and rapid termination of the early Antarctic ice ages

The intricate rhythms of changes in Earth’s axial tilt (obliquity) and orbit (eccentricity) are strongly imprinted on records of past climate. Some of our best-dated records of astronomically paced changes in climate and continental glaciation come from deep-sea benthic foraminiferal oxygen isotope records (δ¹⁸O_b). However, even these data present major questions about the mechanisms linking Earth’s climate to its astronomical configuration, particularly the importance of eccentricity- and obliquity-paced changes in climate. We studied striking site-to-site disagreement over the frequency of change in δ¹⁸O_b, which violates the first principles of oxygen isotope systematics, and inferred Antarctic ice volume for the late Oligocene and early Miocene (Oligo-Miocene) interval.

We present a new, finely resolved δ¹⁸O_b record for ~26.4 to 21.8 million years ago from International Ocean Drilling Program (IODP) Site U1406 in the northwest Atlantic Ocean. Our new record shows clear variability at both obliquity and eccentricity frequencies, but not in equal measures. A comparison of our record to other δ¹⁸O_b records for the time interval shows a remarkably consistent global imprint of eccentricity on δ¹⁸O_b whereas the obliquity signal is inconsistent between sites, indicating that eccentricity was the primary pacemaker of land ice volume. Our results also show that the larger eccentricity-paced early Antarctic ice ages were vulnerable to rapid termination. These findings imply that the self-stabilizing hysteresis effects of large land-based early Antarctic ice sheets were strong enough to maintain ice growth despite consecutive insolation-induced polar warming episodes. However, rapid ice age terminations indicate resistance to melting was weaker than simulated by numerical models and regularly overpowered, sometimes abruptly.