PALYNOSTRATIGRAPHIC CONTROL ON SINTER DEPOSITION IN UPPER GEYSER BASIN, YELLOWSTONE NATIONAL PARK

The large sinter deposits of Upper Geyser Basin of Yellowstone National Park are evidence of persistent hydrothermal activity, but the timing of their development has been poorly understood. We examined pollen assemblages in sinter to determine the age of the deposit and to characterize hydrothermal activity through time. Standard radiocarbon dating of these deposits has been problematic, given the lack of datable material and the presence of old carbon in many samples. Drawing on the pollen-based vegetation history of the region, we were able to contribute biostratigraphic age constraints. Sinter was collected from multiple stratigraphic sections in the basin, and pollen was chemically extracted using standard pollen processing procedures. Residual pollen had highly variable preservation, suggesting different subaerial exposure times prior to burial. Levels of pollen preservation were used to infer episodes of slow sinter deposition (sparse, highly degraded pollen) from episodes of more rapid deposition (abundant, well-preserved pollen). All pollen samples were dominated by Pinus contorta-type, with relatively minor amounts of Artemisia and Poaceae. Notable was the abundance of Cystopteris/Woodsia-type spores and the rarity of extralocal taxa (eg. Abies, Picea, Sarcobatus, Amaranthaceae), indicating a source area localized to the geyser basin vicinity. Comparison of these pollen spectra with postglacial lake-sediment records suggests deposition following the regional establishment of Pinus contorta forests (ca. 11,000 cal yr BP). Therefore, we contend a Holocene age for the sinter formations sampled. This preliminary application of palynology to date sinter deposits, identify temporal variations in sinter deposition, and infer local vegetation changes offers a new opportunity to reconstruct the history of hydrothermal basins.