HIGH ARCTIC FOREST DIVERSITY AND FLORISTIC CHANGE DURING EARLY EOCENE HYPERTHERMAL EVENTS

Understanding the causal mechanisms of the modern latitudinal diversity gradient (LDG) is a long-established problem in ecology. Temperature has been proposed as the primary driver of the modern LDG, although other hypotheses (e.g. precipitation, insolation, seasonality, biogeographical history, and biological interactions), have been suggested as constraints or drivers of diversity in the extra-tropics (i.e. the mid- and high-latitudes). The modern Arctic is characterized by very low floral diversity and a cold dry climate; however, the early Eocene Arctic was much warmer and wetter, as evidenced from paleobotanical climate reconstructions (e.g. MAT ≈ 8.5–12.7 ºC and MAP >150 cm/yr), and the presence of thermophilic flora and fauna (e.g. palm or palm-like palynomorphs and alligators). Nevertheless, forest diversity for Arctic Eocene ecosystems remains relatively untested and is typically described as low and homogenous. Reported here are the first quantitative megafloral diversity estimates from Stenkul Fiord, Ellesmere Island, Canada, utilizing two purpose-made census-sampled fossil leaf collections coupled with horizon-specific palynological analyses. Recent U-Pb zircon geochronology, and new geological mapping at Stenkul Fiord, place the fossil sites stratigraphically near the PETM and ETM2 hyperthermal events of the early Eocene, a time when warm equable climates allowed temperate and tropical plant taxa to survive at high northern latitudes. Diversity was assessed using coverage-based interpolation- and extrapolation-based rarefaction, a method that reconstructs richness with high accuracy. The results show that the early Eocene paleoarctic forests supported diverse forest ecosystems with floral diversity similar to modern mid-latitude mesic-mesothermal broadleaf forests from North America, but overall floral diversity may have been restricted as a result of photic seasonality. Furthermore, these ecosystems experienced floristic change probably related to the transient hyperthermal events.