By Diana Setterberg. MSU News Service
BOZEMAN – A study led by a Montana State University research professor reveals how climate and hydrothermal activity have shaped the vegetation, wildfire and aquatic ecosystem history of the Yellowstone Plateau over the past 15,000 years.
The results of the National Science Foundation-funded project were published this week in the journal Proceedings of the National Academy of Sciences .
Cathy Whitlock, MSU Regents Professor emerita of earth sciences in the College of Letters and Science and the first scientist to be elected to the National Academy of Sciences from a Montana university , has spent decades working in the Greater Yellowstone Ecosystem. Her interest in Yellowstone National Park's Lower Geyser Basin was piqued in 2020 after she and co-author Chris Schiller took sediment cores from the bottom of one of the basin's small lakes. They noticed that its contents differed from those in lakebeds in other parts of the park, suggesting that distinct geologic factors may have influenced the lake's evolution. At about the same time, she read a paper speculating that a lack of water in the geyser system caused Old Faithful to stop erupting in the early 13th century for about 100 years.
As founder and past director of MSU's 22-year-old Paleoecology Laboratory , which studies how past ecosystems responded to changing climates, Whitlock said the opportunity to learn more about Yellowstone's geyser basins was irresistible. but also required the expertise of many scientific disciplines.
"I thought, 'Well, there have been long, dry periods in Yellowstone in the past, some of them lasting for several centuries. I wonder if we could document how periods of sustained drought affected ancient geyser activity,'" she said.
Collaborators on the project included MSU professor of earth sciences Dave McWethy, two MSU post-doctoral researchers, two MSU students, and scientists from the U.S. Geological Survey, Oregon State University, Manchester University and Colorado State University.
Throughout her career, Whitlock has reconstructed the histories of ecosystems from tiny clues deposited over time in the layers of lake sediments. By examining sediment cores, scientists can pinpoint the age of the layers through radiocarbon dating or from the buried evidence of known, natural events, such as past volcanic eruptions. Pollen assemblages tell scientists what type of vegetation was present at particular times. Layers with abundant charcoal identify past wildfire events. The concentration of arsenic and cesium and the composition of the diatoms in the sediments provide information on changes in hydrothermal activity, lake chemistry and water depth.
In this study, the team collected core samples from small lakes of different ages in the Lower Geyser Basin, which is Yellowstone's largest geyser system. The lakes are closed, meaning that they have no inflowing or outflowing streams. Whitlock said they likely developed in depressions left by hydrothermal explosions that occurred during wet climate periods. Two of the lakes, in fact, formed shortly after the glaciers that once covered the region melted; removal of ice released overlying pressure that probably led to lake-forming, hydrothermal eruptions.
Pollen records from the lakes indicate that after the ice receded, a grassy steppe ecosystem developed on the rhyolite volcanic soils. Steppe was replaced by a lodgepole pine forest that established between 12,800 and 11,000 years ago. Despite subsequent changes in climate, these pine forests have changed little in composition, leading the authors to suggest that lodgepole pine will continue to dominate the vegetation on the plateau even as the climate continues to warm in the future.
"The persistence of lodgepole pine for thousands of years is explained by the infertile soils on the rhyolite volcanic plateau and lodgepole's adaptation to fire. It's been very hard for anything else to get established in that area given limited nutrients and well-drained substrates," Whitlock said. "While the vegetation has been remarkably insensitive to past climate change, fire activity on the plateau has responded dramatically."
The team realized the importance of both geology and climate in explaining the environmental history of the geyser basin when the lake records were compared with high-resolution paleoclimate model results for Yellowstone. For example, the model showed that summers in the geyser basin were warmer and drier than today's between 12,000 and 6,000 years ago, an observation that matched the charcoal evidence for more fires and the diatom records for lower lake levels then.
Whitlock said reconstructing a long-term fire history is extraordinarily complex, and the project wouldn't have been possible without the baseline research her group conducted for 10 years after the Yellowstone fires of 1988.
"As soon as the fires were over, we started studying charcoal deposition into lakes to determine how far charcoal particles traveled during a fire, how long they took to get buried in the mud, that kind of thing," she said. "The charcoal analytical techniques we developed for Yellowstone are now used in fire history studies on every continent. We have a much better understanding of fire as a global phenomenon because of the work we did following the 1988 fires."
Sediments in the Lower Geyser Basin lakes reveal that Yellowstone's hydrothermal systems were more active in the past during wet periods than dry ones, suggesting that future warming in Yellowstone may result in reduced hydrothermal activity, as well as increased wildfire incidence.
Research geophysicist Michael Poland, scientist-in-charge of the USGS Yellowstone Volcano Observatory, said such changes likely won't be noticeable on the timescale of a human lifetime, though the study does point to interesting possibilities for near-term changes in the timing, force and frequency of geysers in Yellowstone's hydrothermal areas.
"This study and ones like it help to give us a sense of what might be expected in the future for hydrothermal activity given current trends in climate," he said. "That's always been a key aspect of geology, and these sorts of studies show the past is the key to the present, which is then the key to the future. The more we understand past conditions and what drove them, the more we understand what's likely to occur in the future."
