ICE VOLUME CHANGES IN A GREENHOUSE WORLD

Backstripped eustatic estimates from New Jersey and the Russian platform show large (>25 m) and rapid (<1 m.y.) sea-level changes in the Late Cretaceous to early Eocene (99-49 Ma). Glacioeustasy is the only known mechanism that can account for these changes because other hypothesized mechanisms (steric effects, water storage in lakes, deep-water changes, groundwater, or sea ice) cannot explain the observed 25-40 m changes in <1 m.y. In contrast to this evidence for glacioeustasy, ample geological evidence points to warm high-latitude temperatures at this time. The late Cenomanian-early Turonian oxygen isotopic record highlights the enigma. This was the warmest interval of the past 200 m.y., yet it was bracketed by two inferred eustatic falls of ~25 m that were associated with two large (>0.75‰) deep-sea oxygen isotopic record increases (92-93 Ma, mid-Turonian and 96 Ma, mid-Cenomanian). The largest of the Cretaceous-early Eocene oxygen isotopic record increases occurred at the Campanian/Maastrichtian boundary (71.5 Ma); we infer that ice growth and attendant sea-level lowering were as great as many Oligocene and younger events (~40 m sea-level change or ~60% of the modern East Antarctic ice sheets). We reconcile records of warm high latitudes with glacioeustasy by proposing that Late Cretaceous-early Eocene ice sheets generally reached maximum volumes of 8-12 x 10⁶ km³ (20-30 m glacioeustatic equivalent), but did not reach the Antarctic coast; hence, coastal Antarctica (hence deep water) remained relatively warm even though there were significant changes in sea level as the result of glaciation. Unlike the Oligocene and younger icehouse world, these ice sheets only existed during short intervals of peak Milankovitch forcing, leaving Antarctica ice-free during much of the greenhouse Late Cretaceous to middle Eocene. These results highlight the need to re-evaluate the paradigm that polar ice sheets did not exist during times of warm high-latitude climates.