Smith Lecture - Kristin Bergmann, MIT

With this talk, I explore pre-Cenozoic glaciations to answer the following questions: "What is the record of ice volume and extent through time, and can this be reconciled with the oxygen isotope record?". Utilizing carbonate-clumped isotope thermometry and integrating observations from the geologic record, I will first examine the end-Ordovician glaciation—a test case that exhibits both similarities to and differences from Cenozoic glaciation. Prior to glaciation, a long-term gradual cooling trend is mirrored by Ordovician radiation in biological diversity, consistent with temperature-dependent oxygen solubility and metabolism as primary controls. Evidence for significant ice growth is constrained to less than 2 Myr of the Hirnantian Stage, underscoring the high sensitivity of ice growth to pCO₂ and temperature. Our independent estimates for ice volume, area, and sea-level change during the Hirnantian glacial maximum are internally consistent and comparable to those of the Last Glacial Maximum. In addition to our work documenting cooling in the lead-up to the end-Ordovician Hirnantian glaciation, our targeted geographic studies have also allowed us to reconstruct progressive and protracted cooling before the Sturtian Snowball Earth glaciation and the Late Paleozoic Ice Age. These three case studies suggest that a cool, low pCO₂ climate is a prerequisite for glaciation, even Snowball Earth, and highlight the importance of identifying and constraining meteoric alteration associated with large amplitude sea level change before interpreting isotope records. The implications of these results extend to refining current paleoclimate models, especially in understanding transitions from greenhouse to icehouse conditions.