Paper No. 14
Presentation Time: 4:30 PM


STRICKLAND, Laura E.1, BAKER, Richard G.2, THOMPSON, Robert S.1 and MILLER, Dane3, (1)U.S. Geological Survey, Geoscience and Environmental Change Science Center, Box 25046, MS 980, Denver Federal Center, Denver, CO 80225, (2)Department of Geoscience, Univ of Iowa, Iowa City, IA 52242, (3)Botany, University of Wyoming, 1000 East University Ave, Laramie, WY 82072,

Plant macrofossils preserved in lake and wetland sediments of Ziegler Reservoir provide a record of high-elevation environmental change in the Rocky Mountains during the Last Interglacial MIS5 (~138 ka to ~77 ka). Present-day vegetation surrounding the site is montane aspen forest at an elevation (2705 m) where higher elevation species (Picea spp., Abies lasiocarpa) intermix with plants from intermediate and lower elevations (Pseudotsuga menziesii, Quercus gambelii, Artemisia spp.). The basal stratigraphic unit is Bull Lake (MIS6) glacial till overlain by lacustrine silts and debris flows (Units 4-8) of warm stage 5e, which contain a diverse plant assemblage of trees, shrubs, and herbs from montane forest, wetland, and aquatic environments. Many of the MIS5e fossil taxa are currently found in the modern flora including deciduous elements (Quercus gambelii type, Crataegus erythropoda type, Prunus virginiana) and conifers (Picea spp., Pseudotsuga menziesii). Interestingly, the MIS5e assemblage contains taxa not presently found in the modern flora, but occurring at lower elevations within Colorado (Abies concolor, Verbena hastata, Lycopus americanus and others), as well as plants not known to be present in the state (Najas flexilis and Pilea pumila).

The flora is similar but less diverse in MIS5d and MIS5c (Units 10-13). A significant change is observed in Unit 14 where trees are absent and only a few herbs, wetland, and aquatic plants are present, which may indicate colder conditions during MIS5b. This macrofossil-poor interval is overlain by a peat deposit (Units 15-16) equating to warm stage 5a, rich in the remains of mosses, conifers, and aquatics, but lacking deciduous and herb taxa.

The macrofossils (90 taxa from 36 families) suggest that montane forest, compositionally similar to the modern flora, was dominant during MIS5e-5c. Explorations of the modern climatic ranges of the fossil taxa suggest that summer temperatures were warmer and conditions were wetter than the present, especially during MIS5e when a mixture of local and extra-local taxa were present. This warmer than present time was followed by a cooler interval (MIS5b) where trees appear to be absent and subsequently, relatively warm conditions returned during MIS5a.