DECOUPLING THE ECOLOGICAL RESPONSE TO CHANGES IN CLIMATE AND HYDROTHERMAL ACTIVITY IN LOWER GEYSER BASIN OF YELLOWSTONE NATIONAL PARK

The postglacial vegetation history of Yellowstone National Park was driven by changes in regional climate and fire activity and the influence of different substrate types. In the central part of Yellowstone, vegetation dynamics have also been also affected by past hydrothermal activity, including surface discharge of hydrothermal fluids and large hydrothermal explosions. The pollen, charcoal, and lithology of a 10,000-year-old core from Goose Lake (44° 32’ 29” N, 110° 50’ 33” W, 2200 masl) in Lower Geyser Basin were analyzed to identify the influence of climate, fire, and hydrothermal activity on Holocene vegetation and landscape development. From 10,000 to 3500 cal yr BP, local hydrothermal fluid discharged into the basin, as evidenced by deposition of laminated organic sediment enriched in silica (diatoms) and heavy metals (e.g., As, Mo) and by relatively heavy sediment δ¹³C values (-18.43 to -9.7 ‰ VPDB). From 3500 cal yr BP to the present, Goose Lake sediments consist of sapropel that was relatively depleted in silica, As, and Mo and had more negative δ¹³C values than before (-25.3 to -22.08 ‰ VPDB). This shift likely marks cessation of proximal hydrothermal activity. The pollen data indicate the presence of a relatively open Pinus contorta forest at 10,000 cal yr BP, which became more closed at 6200 cal yr BP. Other vegetation records in central Yellowstone show a similar transition around 6000 cal yr BP, suggesting the onset of cooler, effectively wetter conditions in the middle Holocene. Charcoal accumulation rates were high from 10,000 to 3500 cal yr BP, implying a prolonged period of large stand-replacing fires, potentially even within the hydrothermal wetland. After 3500 cal yr BP, Pinus contorta forest became more open and fire activity greatly declined. Given the lack of synchronous changes in other pollen and charcoal records, we suggest that a local decline in hydrothermal activity, rather than a regional climate change at 3500 cal yr BP, led to the establishment of a less-dense forest and more surface fires at Goose Lake. This record provides the first opportunity to separate the influence of hydrothermal activity and climate on the ecosystem dynamics of central Yellowstone.