2014 GSA Annual Meeting in Vancouver, British Columbia (19–22 October 2014)

Paper No. 199-4
Presentation Time: 8:55 AM


GUSHULAK, Cale A.C., Dept. of Earth Sciences, University of Western Ontario, Biological & Geological Sciences Building, Room 1026, 1151 Richmond Street N, London, ON N6A 5B7, Canada, ARCHIBALD, S. Bruce, Biological Sciences, Simon Fraser University, 8888 University Drive, Burnaby, BC V5A 1S6, Canada and GREENWOOD, David R., Dept. of Biology, Brandon University, J.R. Brodie Science Centre, 270-18th Street, Brandon, MB R7A 6A9, Canada, calegushulak@gmail.com

Early Eocene fossil floras from British Columbia are a rich resource for reconstructing western North American early Cenozoic climate. These floras reflect cooler (MAT ≤ 15°C) upland forest communities in contrast to coeval mesothermal (MAT ≥ 18°C) forests in lowland western North American sites. Of particular interest is whether Early Eocene Climatic Optimum climates were monsoonal, i.e. markedly seasonal precipitation. The McAbee site is a ~0.7km outcrop of bedded lacustrine shale radiometrically dated to ~51 Ma, near the middle of the EECO. The flora has been reported on by several workers based on collections of unknown provenance. In this report 2 collections made independently at different stratigraphic levels and/or laterally separated by ~300m in the 1980s (USask) and 2000s (SFU/BU) are investigated to: (1) assess whether they represent the same leaf population, (2) reconstruct paleoclimate, and (3) assess whether a combined collection yields more precise climate estimates. Combined, the 2 samples yielded 43 different dicot leaf morphotypes; 29 unique to SFU/BU sample, 3 unique to the USask sample, and 11 in both collections. Analysis using ANOVA and t-test of leaf size distribution showed no difference between them and so they were combined for climate analysis. The collections are typical for reports of the macroflora, with Alnus, Betula, Fagus, and Ulmus common; however, Sassafras was also relatively common. Rare Acer were also present, as were Cercidiphyllum, and Comptonia, also consistent with other reports. A large-leaf morphotype and seeds are tentatively identified as Bignoniaceae (cf. Catalpa). MAT is estimated as 11.7°C ± 2.5° using leaf margin analysis. Mean annual precipitation estimated using leaf area analysis was 117 cm/yr -35, +51cm; taphonomic bias towards smaller leaves in lake sediments indicates the upper bound of the error for the estimate based on leaf area should apply. Using CLAMP (completeness statistic 81%), MAT was 12.9°C ± 2.1°, Warm Month Mean Temperature 20.5°C ± 2.5°, Cold Month Mean Temperature 5.1°C ± 3.4°, Growing Season Precipitation 130cm ± 32cm (growing season 7.6 months ± 1.1 mo), 3-wet months precipitation 62 ± 23cm, 3-dry months precipitation 30 ± 6cm, and relative humidity 84.4 ± 8.6%. These data indicate a mild summer-wet climate, but not a monsoonal climate.