LAKE SEDIMENT RECORD OF HYDROTHERMAL AND ECOLOGICAL CHANGE IN LOWER GEYSER BASIN, YELLOWSTONE NATIONAL PARK

Lower Geyser Basin (LGB) is the largest of the iconic thermal areas of Yellowstone National Park. Today, hydrothermal activity is pervasive in LGB, producing vast, treeless flats of sinter, hydrothermal mud, and diatom ooze. Pollen, charcoal, diatoms, and lithology from a 10,250-year-old core recovered from Goose Lake (44°32′29″ N, 110°50′33″ W, 2200 masl) in LGB were analyzed to reconstruct past hydrothermal activity and its impact on terrestrial and lacustrine ecosystems. From 10,250 to 7750 cal yr BP, hydrothermal activity occurred at Goose Lake, inferred from the deposition of heavy metal-rich sediments (As, Mo) and heavy δ¹³C_sediment values. The basin was covered by Pinus contorta forest and experienced more fire activity than today. The diatom community within Goose Lake included taxa adapted to temperatures above 10-15°C (Diatoma moniliformis) and a range of pH tolerances (Achnanthidium minutissiumum, acidic and Navicula wildii, alkaline), presumably produced by hydrothermal discharge of varying alkalinity. Measured biogenic silica abundance was low in this interval, although diatoms were abundant, possibly suggesting diagenetic alteration of biogenic silica. From 7750 to 3860 cal yr BP, P. contorta became increasingly abundant and fire activity continued to increase as wetter summer conditions increased available fuels. Diatom communities at this time lack planktic types, possibly indicating a change in lake level. After 3860 cal yr BP, local hydrothermal activity ceased at Goose Lake, as evidenced by the depletion of heavy metals, a shift to lighter δ¹³C_sediment values, and a decrease in diatom species suited to warm (D. moniliformis), alkaline (N. wildii) conditions. Synchronously, P. contorta abundance was reduced and fire activity declined, indicating a more treeless environment with less fuel biomass. The combined proxies suggest a reorganization of basin-wide hydrothermal activity and development of the modern, treeless LGB flats. This record of the dynamic history of LGB demonstrates the long-term influence of hydrothermal activity on local terrestrial and limnic ecosystems.