Jenny White McKee is underwater and she can’t see thing.
The air bubbles from her scuba rig rise from a cloud of mud and dead plant matter stirred up from the bottom of the lake. A large stump looms in front of her, rising to the surface.
Then there’s a sound, like the creak of an ancient door hinge: the eerie distorted twist of metal into wood.
The noise carries through the water, makes your skin crawl.
United States Geological Survey divers White McKee and Pete Dal Ferro are twisting a T-shaped tool called an increment borer into the submerged tree to remove a straw-sized plug of wood.
“That was brutal,” Dal Ferro says, coming to the surface with the tree core sample. His face is covered in mud.
The diving conditions are tough, but the payoff should be satisfying. The tube of extracted wood will help USGS researchers figure out when this tree died, and in the process when a massive earthquake shifted the ground on the Olympic Peninsula and formed Price Lake.
Earthquake risk under our feet
In the Pacific Northwest, there are dozens of earthquake faults. Most are far shallower and closer to where people live than the more-well-known Cascadia subduction zone offshore, which roughly parallels our coastline.
“They’re not as big as Cascadia. If Cascadia ruptures — say a magnitude eight or nine - that would affect the entire West Coast. We’d feel it for sure inland,” said geologist Steve Angster, who’s with the USGS Earthquake Science Center in Seattle.
“But the amount of shaking here wouldn’t be as intense as if a rupture on the Seattle fault occurred, mostly because we’re so close to the Seattle fault,” he said.
Despite the proximity of these shallow inland faults to communities, there’s still a huge amount we don’t know about their extent and how active they are.
Some of these answers can be found in the underwater ghost forest of Price Lake. The lake was created when the ground shifted along a fault during an earthquake, creating a natural dam across an ancient stream. The water backed up and flooded the forest, which remains to this day.
On this day, USGS dendrochronologist Jessie Pearl (who has since gone to work for The Nature Conservancy) is leading the expedition to collect samples from the earthquake-drowned forest of Douglas fir. For her, the trees are the key.
“They record, within the chemical and physical composition of their wood, everything that happens to them. Whether it’s a big storm, whether it’s a decade of drought, whether it’s a big severe fire that came through,” she said.
Tree ring growth is heavily influenced by climate. That means trees of the same species growing in the same region will put on similar patterns of rings.
“And so tree ring scientists like myself can use all these different clues that are within the trees to tell a story about what has happened to this landscape in the past,” Pearl said.
Dendrochronologists create tree ring timelines — or chronologies — by matching up these patterns on older and older trees.
But Pearl has evidence to suggest the trees in Price Lake are around 1,100 years old, and the known Douglas fir chronology doesn’t go back that far.
So she will need to use another tool — radiocarbon — and a phenomenon that’s only been understood for about a decade.
“We’ve discovered that the radiocarbon in our atmosphere sometimes has these large events — what we call radiocarbon excursion events — and they’re global.”
These are surges in detectable Carbon 14 radiation, likely triggered by solar storms.
Every tree on the planet alive during one of these events will have a radiocarbon spike in that year’s ring. Over the past 2000 years, there have been at least two of these major events. One happened around the winter of 774-75 and another in 993-94.
For Price Lake, Pearl and University of Arizona dendrochronologist Bryan Black will look for the tree ring with the tell-tale radiocarbon spike — in this case from the pulse that occurred in 774-75. Then they’ll count forward until they hit the outermost ring.
Using this technique for multiple samples, they’ll be able to figure out the exact six-month period, more than 1,000 years ago, that this submerged forest died.
“With the tree ring analysis, we can… determine whether these trees all died at exactly the same time, which would suggest a very large earthquake. Or whether these faults all slipped in rapid succession, maybe a few years apart. A mega quake versus a decade of terror kind of scenario,” Black said.
It would give emergency planners a better idea of what to expect when the next earthquake hits the Puget Sound region.
Back out on the water, the research crew is having a problem with the core samples extracted by the divers with the increment borer. A thousand years underwater and the wood has turned to mush.
“It’s because it’s so extremely soft, that the spoon is just ripping through it,” Pearl says as she tries to remove the wood from the boring tube.
To solve this, they’ll have to deploy some bigger tools: a 350-pound hydraulic motor that will power an underwater chainsaw.
On a makeshift canoe-and-plywood catamaran, the team follows the divers to another cluster of underwater trees and find their target.
“We’re gonna buck this tree right here because it is pretty close to the surface and uh the water is clear. A tree this size should have quite a few rings,” Pearl explains.
And then it’s go time. The engine rumbles to life and Dal Ferro swims the chainsaw over to the log.
Swimming in open water, he digs the chainsaw into a massive dead tree — the base of which is at least 4-feet wide.
Sawdust flies out of the cut in watery slow-motion. It swirls around, filling the lake with woody snow.
Dal Ferro surfaces, breathing hard and motions for the canoe crew to cut the engine.
“That looks hard. You’re really having to push on it, huh?” White McKee asks as she emerges beside him.
“Yeah, it’s pretty hard!” he answers. “I sort of have to lever way harder than you do in real life.”
The physics of normal chainsaw use don’t really apply underwater — it’s a different experience.
“I’m going to aim as deep as I can. I’m going to try to get all the rings,” he says
Dal Farro dips back into the cloud of wood chips, digging in once again with the chainsaw.
Nearly 20 minutes pass. Then, a huge wedge of tree floats to the surface.
The smell of cut Christmas trees fills the air.
Cheers erupt from the boat.
The wedge is so heavy, it takes three people to get it out of the water and up onto the floating platform.
As the day moves on and more wood is pulled onto the platform, the researchers feel they’ll get the answers they came for.
“This lake was holding its secrets, but this is the piece that will help crack it,” says Black, running his hands along a particularly old sample.”
He points to the outermost tree ring preserved under a piece of ancient bark.
“This last ring right here is the year the tree died and it’s gonna tell us the year of the earthquake.”
And while uncovering the Pacific Northwest’s seismic past is the immediate aim of Pearl’s research, she’s also working towards a far grander goal: finding the trees to fill in the gaps in the Douglas fir tree ring chronology.
“If at the end of this project, we also come up with a 2,000-year long tree ring record, which you could utilize for any other trees you might find at other ghost forest sites, to figure out when they died,” she said. “That would be something that will be used for generations of scientists to come.”