COLD SNAPS IN THE GREENHOUSE WORLD: EUSTASY NOT TECTONICS

Geologists have incorrectly assumed that Greenhouse worlds are uniformly typified by high (>>2x modern) atmospheric CO₂, extreme polar warmth, and ice free conditions. Backstripped eustatic estimates from New Jersey and Russia show large (>25 m) and rapid (<1 Myr) sea-level changes in the Jurassic to early Eocene (200-49 Ma) Greenhouse world. Though many mechanisms can cause regional sea-level changes (e.g., Lovell, 2010), we document that many of these events are global. We illustrate examples of major global sea-level falls associated with cooling (cold snaps) and ice volume increases: 1) the late Pliensbachian is globally documented and correlates with glendonites and other evidence of Siberian glaciation; 2) the late Cenomanian–early Turonian was the warmest interval of the past 200 my, yet it was bracketed by two inferred eustatic falls of ~25 m that were associated with large (>0.75‰) deep-sea δ¹⁸O increases (92–93 Ma, mid-Turonian; 96 Ma, mid- Cenomanian); 3) the early/middle Eocene boundary was a major global fall associated with a δ¹⁸O increase and the slide into the Icehouse. Glacioeustasy is the only known mechanism that can account for these rapid global changes because other hypothesized mechanisms are too slow or too small. We reconcile records of warm high latitudes with glacioeustasy by proposing that Greenhouse ice sheets generally reached maximum volumes of 8-12 million cubic km (20-30 m glacioeustatic equivalent) in Antarctica during the Late Cretaceous to Eocene and Siberia during the Jurassic to Early Cretaceous, but did not reach the coast. Icehouse sequences of the past 34 Myr correlate well with ~1.2 myr obliquity cycles and isotopic changes, highlighting the link among sea-level lowerings, sequence boundaries, and obliquity nodes. Mesozoic Greenhouse sequences correlate with the ~2.4 myr eccentricity cycle, suggesting that orbital forcing of Greenhouse sea level was controlled by precession modulated by eccentricity and may be linked to precipitation-evaporation changes and not direct high latitude forcing. Unlike the Icehouse world, these ice sheets only existed during short intervals of peak Milankovitch insolation, leaving the Earth ice-free during much of the Greenhouse. This highlights the need to re-evaluate the paradigm that polar ice sheets did not exist during the Greenhouse .