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Yellowstone Caldera Chronicles is a weekly column written by scientists and collaborators of the Yellowstone Volcano Observatory. This week's contribution is from Christopher Schiller, postdoctoral scholar with the Department of Biology at the University of Washington, and Cathy Whitlock, Professor and Director of the Paleoecology Laboratory at Montana State University.

Lower Geyser Basin is the largest thermal basin in Yellowstone National Park (~14 km² [5.5 mi²] of actively hot area) and includes some of the Park’s most famous hydrothermal features, like Fountain Paint Pots, Firehole Lake, and Great Fountain Geyser. It’s a landscape with a complicated past, sculpted by rhyolite lava flows, glaciers, hydrothermal activity, and climate change. Today, visitors see a flat, grassy landscape, where lodgepole pine cannot grow due to the hot ground.

In 2018, a research team from Montana State University collected an 8.08 m (26.5 ft) sediment core from Goose Lake, in Lower Geyser Basin, to study how the ecosystem of the area was impacted by hydrothermal activity. The scientists studied a variety of fossils, as well as the lithology and geochemistry of the cores to reconstruct the environmental history. Pollen and charcoal were examined to reconstruct the vegetation and fire history, and diatoms (siliceous algae) documented changes in lake biology. The physical and chemical properties of the cores also provided information on past hydrothermal activity. 

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The Goose Lake record shows two distinct periods in the history of Lower Geyser Basin. In the earlier period (10,300 to 3,800 years ago), pollen and charcoal data suggest the presence of a lodgepole forest that became gradually denser, with more fire over time. This trend is repeated in many records around Yellowstone, and it is probably the result of the climate becoming cooler and wetter (especially in the summer). At the same time, there is evidence for hydrothermal activity from this period at Goose Lake—diatoms adapted to very warm water and wide alkalinity tolerances thrived in Goose Lake during this period. Sediments also contain abundant arsenic, known to be concentrated in thermal waters, and deposits of fluorite, a mineral produced by hydrothermal activity. Together, this tells us that Lower Geyser Basin was forested from 10,300 to 3,800 years ago, and that hydrothermal features were active near or within Goose Lake (there is no hydrothermal activity at Goose Lake today).

Later in the record (after 3,800 years ago), the landscape changed dramatically. First, pollen and charcoal data suggest an abrupt shift to a landscape with open grassland or steppe—much more similar to Lower Geyser Basin today. This open landscape is maintained by soils that are heated by hydrothermal activity below the surface (the grasses and herbs that thrive in Lower Geyser Basin today can take the heat, but lodgepole pine cannot!), suggesting that hydrothermal activity became widespread in Lower Geyser Basin about 3,800 years ago. At the same time, Goose Lake sediments ceased to be arsenic-rich and heat- and alkalinity-adapted diatoms declined, suggesting that hydrothermal activity at Goose Lake itself ceased.

It seems, then, that Lower Geyser Basin changed dramatically around 3,800 years ago, with hydrothermal activity stopping in some locations, like Goose Lake, but becoming widespread throughout the basin. Hydrothermal areas change due to a variety of factors. The 1959 Hebgen Lake earthquake, for example, disrupted hydrothermal features throughout the Yellowstone, including within Lower Geyser Basin. Periods of drought also impact hydrothermal features, such as a period around 650 to 800 years ago when eruptions stopped at Old Faithful Geyser during a severe, sustained drought. An earthquake or severe drought are both possible causes of the reorganization of Lower Geyser Basin around 3,800 years ago.

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Yellowstone has been called a geo-ecosystem, since it is strongly influenced by geological processes, like volcanism, hydrothermal activity, faulting, and tectonic uplift, as well as changes in climate and hydrology. The Goose Lake data show how closely tied geological, climatic, and ecological forces were in the creation of Yellowstone’s iconic thermal areas. If you would like to read more about this work into the ecosystem changes at Lower Geyser Basin over time, see the recent research article in the journal Quaternary Research at https://doi.org/10.1017/qua.2021.42.