An unlikely discovery in a cave on Prince of Wales Island could help scientists understand Earth’s climate history. A small pile of sand and stones tells the story of formidable glacial meltwater floods that washed through Southeast Alaska during the last Ice Age.
Paul Wilcox is a paleoclimate researcher at the University of Lapland in Finland. While looking for stalactites and stalagmites on Prince of Wales Island in 2022, he stumbled across something odd in Devil’s Canopy Cave.
“I found this very impressive pile of sediment in one of the cave rooms,” he said.
The pile is made of rounded cobbles and pebbles on top of sand, which is unlike what he usually finds in most caves.
“It often is either like a goopy mud or the rocks are very sharp-edged,” Wilcox said.
The rounded and weathered rocks looked like they traveled a vast distance over land, and the layers gave Wilcox a clue that the sediments could have been transported by multiple floods.
He and his colleagues dated them using a method called optically stimulated luminescence that traces when quartz crystals were last exposed to sunlight. They found that the sediments were preserved in the cave like a time capsule between 17,000 and 20,000 years ago.
The researchers say the pile is evidence of enormous glacial meltwater floods coming from the Cordilleran Ice Sheet that covered much of British Columbia and parts of Alaska, including Southeast, South Central and the Aleutian Islands, during the last Ice Age.
“These meltwater events were a common occurrence during the height of the last Ice Age,” Wilcox said.
The finding could help researchers unravel the mystery of how Earth’s climate rapidly fluctuated thousands of years ago.
Maureen Walczak, a researcher at the University of Washington, named these prehistoric floods Siku events, an Iñupiaq word for ice.
She said this pile is significant because Siku sediments are a rare find on land. The glacial ice usually destroys everything in its path when it advances forward.
Walczak didn’t work on this cave study, but she and her team discovered evidence for Siku events in the Gulf of Alaska in 2020.
“The ice basically picks up the rock, and it gets mixed up with the ice, and then the icebergs float out to the middle of the ocean and drop it,” she said.
Along with boulders at the bottom of the ocean, she found glacial silt fanned far out from the coast of Alaska. Radiocarbon dating revealed these floods started around 40,000 years ago.
Walczak said that the timing is key. It could shift how scientists understand the chain reaction that led to prehistoric climate swings.
The same sort of glacial calving and flooding events happened on the other side of the globe, in the North Atlantic Ocean. They’re named Heinrich events after the scientist who discovered them in the late 1980s.
The tremendous influx of cold freshwater into the North Atlantic is thought to have changed the ocean’s circulation. Ocean circulation affects climate, so many climate scientists think that Heinrich events could have played a big role in abrupt climate fluctuations during the last Ice Age.
But Heinrich events started after the more recently discovered Siku events.
Walczak suggests that Siku events may have even triggered Heinrich events, or they just responded to the same trigger earlier. What could have set them off is still a mystery. But once they were in motion, researchers say they could have created a feedback loop that further destabilized Earth’s climate.
“We’re talking about shifting a paradigm here,” Walczak said. “This is right in the beginning of changing how we’re thinking about the role of the Cordilleran [Ice Sheet] in the global climate system.”
Historically, the Pacific Ocean was seen as a relatively passive entity in climate systems. When it came to driving global climate, the North Atlantic got the most research attention.
“Probably because it’s the easiest thing to study, and it’s ringed by a bunch of big, illustrious academic institutions,” Walczak said.
But she said that’s changing, especially since scientists understand the Pacific and Southern Oceans play important roles in shifting global patterns now.
Walczak said that understanding climate changes in Earth’s history can help us prepare for the effects of future warming now.
“We don’t have in historic times any kind of analog. So we have to go back, you know, over 10,000 years ago to understand what this might look like,” she said.
Sometimes, to understand Earth’s history, digging around in Alaska’s caves can help.