Petrified wood suggests Old Faithful’s regular eruptions went quiet 800 years ago.

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Old Faithful, it turns out, wasn’t always so faithful. The geyser, in Wyoming’s Yellowstone National Park, is famous because it blasts hot water tens of meters into the air at regular intervals—every 90-94 minutes, on average. Now, geologists examining petrified wood from the park have found evidence that 800 years ago, Old Faithful stopped erupting entirely for several decades, in response to a severe drought. With climate change making drought more common across the western United States, the researchers say a similar shut down might happen again.

The world’s 1000 or so geysers typically occur in areas that are volcanically active, like Yellowstone. Water percolating down through the ground reaches the boiling point as it approaches the heat of a magma chamber. But because the water is deep underground it is also at a high pressure that prevents it from becoming steam. Eventually the superheated water becomes hot enough to vaporize, triggering an explosive eruption of water and steam at the geyser’s vent.

Many geysers erupt randomly, but when Henry Washburn and his fellow explorers travelled through Yellowstone in 1870, they noted the regularity of Old Faithful, and named it to reflect its predictability.

Shaul Hurwitz, a geologist at the U.S. Geological Survey, and his colleagues wondered if it was always so dependable. Today, the area around the geyser is barren because vegetation can’t survive the blasts of hot, alkaline water. Back in the 1950s, however, a researcher had found chunks of ancient wood near the geyser, mineralized and preserved by Old Faithful’s silica-rich waters. The wood suggested there was a time when trees could grow because there were no eruptions. To investigate, Hurwitz and his colleagues persuaded the National Park Service to let them date 13 such specimens.

“When I submitted the samples for radiocarbon dating I didn’t know whether they would be hundreds or thousands of years old,” Hurwitz says. “It was an ‘Aha!’ moment when they all clustered within a hundred-year period in the 13th and 14th centuries.”

Three of the wood fragments were lodgepole pines, still common in Yellowstone today, the team reports on 7 October in Geophysical Research Letters. One specimen—a 2.4-meter-long section of trunk—came from a tree that the researchers estimate grew for 80 years, which suggests Old Faithful failed to erupt for almost a century.

The researchers turned to the scientific literature in search of an explanation for the eruption hiatus. They found that all their samples fell at the end of the Medieval Climate Anomaly, a warm and dry period felt across the northern Hemisphere thought to have been caused in part by changes in ocean circulation. It began in A.D. 950 and in the western U.S. was still causing droughts in the late 1200s. The researchers argue that a lengthy drought would have starved Old Faithful of the water it needs to erupt, paradoxically allowing some drought-tolerant trees to grow.

Jamie Farrell, a geophysicist at the University of Utah who studies Yellowstone and who wasn’t involved in the study, says the analysis makes sense. “If you have prolonged drought and there isn’t enough water to feed these systems, then features like Old Faithful might sometimes stop erupting,” he says.

Broxton Bird, a geologist at Indiana University–Purdue University Indianapolis who specializes in climate change, agrees. He says that the Medieval Climate Anomaly was severe enough to make it happen. “Water tables would be getting lower and lower, and right at the end of that Old Faithful shuts down,” he says. He adds that the droughts would have made farming in the western U.S. difficult. This may help explain why the Pueblo dwellings of ancestral Native Americans in the U.S. southwest were abandoned at the time.

Hurwitz and his colleagues say that, with climate change making megadroughts more likely across the western United States, Old Faithful might erupt less frequently in future–and might even stop altogether.

Maxwell Rudolph, a geophysicist at the University of California, Davis, says that would be tragic for the millions of people who visit Old Faithful. “The extinction of this natural treasure would be a profound loss.”

source: sciencemag.org

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