The Eocene Falkland fossil flora, Okanagan Highlands, British Columbia : paleoclimate and plant community dynamics during the Early Eocene Climatic Optimum
Smith, Robin Yvonne
The fossil flora and depositional setting of the early Eocene Falkland site in the southern interior of British Columbia, Canada is reported here in detail for the first time. Falkland is part of a series of fossil localities that occur in a region known as the Okanagan Highlands. These sites represent relatively cool upland environments in the greenhouse world of the early Eocene. Macrofossil collections were obtained from Falkland using an unbiased census approach with systematic sampling through three informal units in the exposed outcrop. A stratigraphic log reveals a lacustrine sequence dominated by finely laminated mudstone or shale with periodic influx of coarser material, punctuated by thin volcanic ash layers. Paleoelevation of the site is estimated based on paleobotanical evidence to have been similar to or slightly higher than modern levels (¡Ý1.3 km) during the early Eocene. Paleoclimate is assessed using both physiognomic and floristic approaches as applied to the Falkland flora. Physiognomic approaches correlate aspects of leaf morphology with climate, while floristic approaches use the tolerances of modern nearest living relatives to infer a climate envelope for the fossil flora. Overall, the different methods give broadly consistent results, with an identifiable zone of overlap in the estimates for mean annual temperature at ~10.5¡ãC, cold month mean temperature at 2.3¨C6.3¡ãC, warm month mean temperature at 20.2¨C23.7¡ãC, and a minimum mean annual precipitation of 82¨C120 cm/yr. Assessment of paleoclimate for the three individual units indicates a cooling trend over time, consistent with a radiometric date of 50.61¡À0.16 Ma that places the site in the waning phase of the Early Eocene Climatic Optimum (EECO). The stomatal frequency of fossil Ginkgo adiantoides from Falkland is used to estimate paleoatmospheric carbon dioxide (pCO2). Results from Falkland indicate that pCO2 was significantly higher than modern (>2x) in the early Eocene, although the upper limit of the estimate is unconstrained due to limitations with modern calibration datasets. Analysis of specimens from the three units indicates that climate and pCO2 were coupled during the EECO. Examination of modern Ginkgo biloba leaves suggests that stomatal density is more likely to be accurately measured than stomatal index. In addition, there is a significant difference between stomatal frequencies of long- and short-shoot leaves, suggesting that this factor needs to be taken into account in modern calibration datasets. The Falkland flora was described in two phases. In the first phase, specimens were assigned to morphotypes, informal categories that ideally correspond to species-level organization. In total, 1561 specimens were assigned to 138 morphotypes encompassing foliage and reproductive structures. The taxonomic literature was then investigated and morphotypes were assigned to formal taxa wherever possible. Gymnosperms are dominated by taxa in Cupressaceae, Pinaceae, and Ginkgoaceae, and there is a diverse angiosperm flora particularly rich in taxa belonging to Rosaceae, Betulaceae, and Sapindaceae. Rarefaction analysis shows Falkland as having diversity comparable to that of the hyper-diverse Laguna del Hunco site in Argentina. These data are consistent with an emerging understanding of high diversity in early Eocene forest communities associated with mild but equable climates. The Falkland flora retains a foundation of common taxa through all three units, including Metasequoia, Ginkgo, and Alnus; however, there is a distinct plant community in the upper unit as angiosperms become more abundant and the assemblage more diverse. Patterns in plant diversity are assessed within a context of changing climate and an active disturbance regime at the Falkland site.
DegreeDoctor of Philosophy (Ph.D.)
SupervisorBasinger, James; Greenwood, David
CommitteeMangano, Gabriela; Renaut, Robin; Davis, Art; Merriam, James; Pigg, Kathleen
Copyright DateJanuary 2011