GEOCHRONOLOGY CONSTRAINING EVOLUTIONARY AND ECOLOGICAL PROCESSES AT THE PALEOCENE-EOCENE BOUNDARY

The Paleocene-Eocene boundary thermal maximum (PETM) is an episode of profound greenhouse warming driven by the sudden release to up to 4000 gigatons of C to the earth surface system. It included a brief (~20 ka) interval of rapid warming, a longer (~100 ka) interval of intense global warmth, and a recovery of uncertain length. Earth surface mean annual temperatures rose by >5°C at the tropics and poles. Carbon cycle perturbations at the PETM created a negative carbon isotope excursion (CIE) that serves as a precise marker of the event in terrestrial and marine sections. Geochronological constraints supplied by isotope stratigraphy and other methods have illuminated the ecological and evolutionary responses of organisms to dramatic global change.

Dramatic poleward shifts in geographic range in marine and terrestrial plants and animals at the onset of warming occurred at rates as great as, or greater than, those associated with Pleistocene glacial-interglacial transitions. In the northern hemisphere, faunal exchange over high-latitude land bridges ended the endemism of the Paleocene. The direction of migration can sometimes be inferred by examining the timing of first appearance of taxa relative to the CIE. On continents and in the surface oceans extinctions are minor, even at the poles. Instead, ecosystems contain novel "excursion taxa" known only from the period of dramatic warmth. On northern continents, the event is marked by the first appearances of new taxa (Primates, Artiodactyla, Perissodactyla) that subsequently diversify and dominate Cenozoic faunas. Overall, the interval is marked by high diversity. Rates of morphological change within lineages at the start and end of the PETM are rapid, but similar to evolutionary rates observed in modern field and laboratory studies. Finally, some lineages show rapid speciation within the event, with the development of new species taking place during the first 20 to 30 ka of the event.

The high-resolution geochronology provided by isotope stratigraphy reveals the tremendous abilities of ecosystems to adapt to dramatic environmental change through both ecological and evolutionary mechanisms. Especially on land, the PETM is a major event within Cenozoic evolutionary history, demonstrating that episodic, short-term events can lead to dramatic and permanent changes in ecosystem state.