MILLENNIAL-SCALE PLANT COMMUNITY AND CLIMATE DYNAMICS AT THE ONSET OF THE EARLY EOCENE CLIMATIC OPTIMUM, MCABEE FOSSIL BEDS, BRITISH COLUMBIA, CANADA

The early Eocene McAbee fossil beds in southern British Columbia host a diverse upland megaflora of predominately warm-temperate vegetation. Fossils occur within lacustrine shales interbedded with volcanic ash dated at 52.9 ± 0.83 Ma, coinciding with recent estimates of the onset of the Early Eocene Climatic Optimum (EECO). Prior work at McAbee amalgamated fossil collections from several stratigraphically unconstrained exposures, producing a ~10³ - 10⁵ year time-averaged perspective on the local plant community composition and climate. This study assessed the parataxonomic composition and leaf physiognomy of two stratigraphically separated and purpose-made megaflora collections to document the millennial-scale ecological response of local forests to both climate change associated with the onset of the EECO and volcanic disturbance events. Current work demonstrates a shift from an Ulmus (elm)-dominated forest of lower dicot diversity and a high-abundance of conifers, including Metasequoia (dawn redwood), Abies (fir), and Picea (spruce), to an Alnus (alder)-dominated forest of higher dicot diversity and a much lower abundance of conifers. Only ~22% of the dicot morphotypes are shared between the collections. Rarefaction analysis of the combined collections, containing ≥ 51 morphotypes, demonstrates dicot diversity comparable to fossil localities of similar sample sizes from the hyperdiverse early and middle Eocene sites of Falkland BC, and Laguna del Hunco and Río Pichileufú of Patagonia. CLAMP generated estimates of climate are similar between horizons and largely consistent with previous work, suggesting a warm (MAT = 9.7 ± 2.1 °C), mesic (GSP ≈ 93 cm/yr), and seasonally mild climate albeit with some frost (CMMT = -2 ± 3.4 °C). Given the seemingly static climatic conditions and the presence of numerous interbeds of volcanic ash between the two horizons, we interpret the observed shift in local plant community composition as a response to ecological disturbance induced by volcanic eruptions that mantled the landscape in ash.