QUANTIFYING LIMITS ON EXTINCTION AND DIVERSIFICATION RATES DURING RAPID CLIMATE CHANGE EVENTS: CALCAREOUS NANNOPLANKTON AT THE PALEOCENE-EOCENE THERMAL MAXIMUM

There is little doubt that long-term climatic change is a major factor controlling patterns of plankton evolution and biodiversity. However, the precise impact of transient climate changes on short and long-term evolution is unclear; this knowledge has direct implications for modern global warming. The geological record from the Paleogene provides us with a direct analogue to the modern scenario in the form of an abrupt warming event, the Paleocene-Eocene Thermal Maximum (PETM, ~55 Ma). The PETM is thought to have resulted from the rapid release and oxidation of fossil fuel in the form of gas hydrate and/or thermogenic methane from the marine sediment reservoir, a process directly analogous to the modern carbon cycle perturbation. We present calcareous nannofossil data from a number of well-preserved PETM sections in New Jersey, Tanzania, and the paleo-equatorial Atlantic. Using assemblage data and current time scales we have calculated absolute evolutionary rates allowing us to compare pre-event, event onset and peak, and recovery interval rates - the first quantitative estimates for past abrupt climate change. A globally consistent spike in evolutionary rates at the event onset and peak amounts to a tripling to quadrupling of speciation and extinction rates compared to pre-event or event recovery rates. It is therefore clear that the abrupt climate change at the onset of the PETM had a huge impact on evolutionary rates, and that the rate of environmental change, rather than the absolute environmental limits, was the controlling factor. However, there is no obvious lasting effect on evolutionary rate patterns, and taxonomically, the PETM had little long-term evolutionary significance.