nai

Project Progress

In the last year, our group made significant progress in understanding the environmental causes of the Late Ordovician glaciation and in understanding the evolutionary consequences of the Late Ordovician mass extinction. Using atmosphere and ocean general circulation models and a dynamic ice-sheet model, we found that pCO₂ must have been no higher than 8 times pre-industrial atmospheric levels in order to initiate ice-sheet growth in the Late Ordovician. Furthermore, paleogeographic changes, sea level, and poleward ocean heat transport played an important role in priming the Late Ordovician climate system for glaciation. Most significantly, we found that when obliquity forcing was added to the dynamic ice-sheet model to simulate Late Ordovician glaciation, pCO₂ must drop to 8 times pre-industrial levels to initiate glaciation, but then CO₂ must rise to above 10 times pre-industrial levels in order to melt the ice sheets. Such hysteresis effects may have played an important role in other glaciations in Earth history, such as the Snowball Earth. In investigating geographic variability in the Late Ordovician mass extinction, we performed a sample-standardized analysis (diversity estimates based on equal sized samples) of Late Ordovician through the Early Silurian diversity for the Laurentian paleocontinent. We found that diversity on Laurentia rebounded to pre-extinction levels within 5 million years, approximately 10 million years faster than global diversity rebounded. This suggests that the rebounds of regional and global diversity were decoupled following the Late Ordovician mass extinction, but it also raises the question whether sample standardization at the global level would show that global diversity also rebounded to pre-extinction levels in 5 million years.