Finding answers to Yellowstone's geothermal history

"It was a big surprise to find this," Cathy Whitlock,  professor in the Department of Earth Sciences at Montana State University.

When the MSU scientists rowed rafts onto Goose Lake and extracted a core of mud and silt nearly 30 feet long from the lake bottom, they thought they might fill in gaps in their knowledge of the area's prehistoric climate and vegetation. Whitlock has studied Yellowstone lake sediments for four decades, examining buried layers of pollen and charcoal to reconstruct past patterns of vegetation, fire and climate change.

What was most notable about the Goose Lake samples was a shift in the composition of the sediments. In the layers below the transition, pollen from lodgepole pine was dominant; above, the pollen was mostly from sagebrush.

The shift couldn't be explained by the regional and global climate changes posited by previous studies by Whitlock and others. The change in the composition of the core, suggested that a shift in geothermal activity — the geysers and mudpots visible today.

"We don't know exactly what happened, maybe an earthquake or some kind of geological uplift, but it was like a switch turning on," Christopher Schiller, lead author of the study, said.

The pollen suggests that before about 3,800 years ago, the roughly 18-square-mile Lower Geyser Basin would have been more like Norris Geyser Basin, where hot pools are surrounded by dense lodgepole forest. A sudden influx of the shallow, widespread geothermal activity would have heated the soils, likely causing trees to die and meadows to expand in their place, according to Schiller, who conducted the research while earning his doctorate at MSU.

"By the time you get to about 3,000 years ago, you might recognize Lower Geyser Basin more or less as it is today — basically treeless, steaming ground," Schiller said. Today, the area remains one of the most hydrothermally active in the national park and on Earth, with geysers and other thermal features discharging thousands of gallons of hot water each minute.

The study raises questions about how extensive environmental change might have been around other Yellowstone geothermal features. "Clearly these landscapes are very dynamic, and it's something that we can all appreciate," Whitlock said. "Change is the order of the day and that these geyser basins probably looked completely different even in the relatively recent past. Yellowstone is a very unique place in that respect."

The Goose Lake study was part of a $2.7 million project led by Woods Hole Oceanographic Institution, MSU and other institutions. About half a dozen MSU undergraduates were involved in doing fieldwork and lab analysis.