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Crawford Lake in Ontario, Canada, is giving up its secrets to geologists.

There is always something new to learn from the ancient record if we know how to read it. Case in point: the unusual characteristics of a deep lake in Canada are helping geologists understand a bit more about today’s rapid changes to the planet.

Sarah Kaplan, Simon Ducroquet, Bonnie Jo Mount, Frank Hulley-Jones, and Emily Wright each contributed to the story at the Washington Post.

“This summer, researchers will determine whether Crawford Lake should be named the official starting point for [the current] geologic chapter, with pollution-laden sediments from the 1950s marking the transition from the dependable environment of the past to the uncertain new reality humans have created.

“In just seven decades, the scientists say, humans have brought about greater changes than they did in more than seven millennia. Never in Earth’s history has the world changed this much, this fast. Never has a single species had the capacity to wreak so much damage — or the chance to prevent so much harm.

“ ‘It’s a line in the sand,’”’ said Francine McCarthy, a professor of Earth sciences at Brock University in Ontario, who has led research on Crawford Lake. …

“Every new phase of Earth’s history begins with a ‘golden spike‘ — a spot in the geologic record where proof of a global transformation is perfectly preserved.

“An exposed Tunisian cliff face bearing traces of an ancient asteroid impact marks the transition from the age of the dinosaurs to the Cenozoic era. Hydrogen molecules uncovered in Greenland’s ice denote the start of the Holocene — the 11,700-year stretch of stable temperatures that encompasses all of human civilization, up to and including the present day.

“These spikes are like exclamation points in the story of the planet, punctuating a tale of shifting continents, evolving species and temperatures that rose and fell as carbon levels fluctuated in the atmosphere. They mark the starts of epochs — small segments of geologic time. And they have helped scientists interpret the forces that shaped Earth’s past climates, which in turn allows them to forecast the effects of modern warming.

“In 2009, the International Commission on Stratigraphy — an obscure scientific body responsible for defining the phases of Earth’s past — created a new working group to investigate the evidence for the Anthropocene. The group’s mission: to identify a potential ‘golden spike’ site that might convince fellow scientists of the new epoch’s validity.

“Their search spanned from mountain summits to the depths of the ocean, from the Antarctic ice sheet to tropical coral reefs. And, in 2018, it led them to McCarthy’s office door.

“Before that moment, few beyond her field knew of McCarthy’s research studying lake sediments for signs of past climate change. Her outreach work was meaningful, but largely local: advocating for conservation of the Great Lakes, teaching geology to students at her midsize public university.

“Crawford Lake was similarly modest. … Yet McCarthy’s colleague Martin Head, a geologist at Brock who had been involved with the Anthropocene Working Group, was intrigued by the rare chemistry uncovered at Crawford.

“Crawford Lake developed thousands of years ago, as water filled a sinkhole in the limestone cliffs of Southern Ontario. Though tiny, the lake is exceptionally deep — so deep its waters are separated into two distinct layers.

“The upper waters are warmed by the sun and mixed by the wind. The layer below is cold and dark, with barely any life to disturb the sediments that accumulate at the bottom. All year long, a constant stream of dead microbes, animal droppings and other organic debris drifts through the Crawford’s waters to settle on the lake bed.

“But during summer, when the the temperature and acidity levels are just right, the water also produces minerals of a white color called calcite that falls to the lake bed forming a thin white cap. Each annual pair of dark and light sediments is also laced with material from outside the lake — pollen grains, pollution particles — that can serve as indicators of the changing environment.

“No other water body is known to possess this particular combination of attributes, making Crawford Lake a unique bellwether of global change. …

“As she considered her colleague’s proposal, McCarthy thought about the decades she’d spent studying prior planetary upheavals. Her work on lake sediments from the past several million years had shown her how dramatic swings in temperature destabilized ecosystems and drove species to extinction.

“Without drastic action to stave off modern climate change, she said, that history could repeat. …

“First, researchers had to tether a wooden raft in the deepest part of the lake, right over the spot they wanted to sample. To extract the lake’s layered sediments, the team used a tool called a ‘freeze corer.’ … The long aluminum wedge was filled with a mixture of alcohol and dry ice, making it much colder than the surrounding water, soil and air.

“They suspended the freeze corer from a tripod and lowered it through a hole in the raft. Down, down it went, through 75 feet of water, until finally it sank into the squishy mud on the lake bottom. Then they waited. It would take about 40 minutes for the lake sediments to freeze onto the corer’s chilly surface.

“Finally, it was time to pull the corer back up. Clinging to its face was a five-foot slice of mud, cut from the lake bottom like a piece from the center of a cake.

“Back on shore, McCarthy traced a gloved finger over the core’s delicate brown and white stripes — sharper than any other sample she’d seen. … Each sample, she knew, would give her a glimpse into a thousand years of the lake’s history, revealing its deepest responses to the changing world above. Each was like a new page from the diary of the Earth. …

“The archive inside Crawford Lake’s cores shows how human pressures on the lake built up over the centuries like steam inside a kettle, until finally the kettle boiled over.

“But humanity’s influence hasn’t always been so destructive.The first people to make their mark on the lake were Native villagers who built longhouses near the lakeshore. Researchers have counted more than two centuries’ worth of sediments from the lake’s ‘Indigenous period’ containing crop pollen and other evidence of human habitation alongside ancient goose droppings and traces of trees.

“Around the start of the 16th century, all signs of the settlement vanished for reasons still unknown. … Sediments from subsequent eras showed Europeans’ growing influence on the landscape. White pine pollen counts dwindled as people cut down trees. Traces of ragweed marked how different species flourished in the cleared land.

“The impacts piled up throughout the 19th and 20th centuries. Tiny black bits of fly ash — a byproduct of burning coal and oil — drifted into the lake from rapidly industrializing cities. Heavy metals like copper and lead increased in the mud.

“And then, around 1950, the world reached a tipping point.

“ ‘This is when humans essentially overwhelmed the Earth as a functioning system,’ said Head, McCarthy’s collaborator. Crawford Lake — and the rest of the planet — were fundamentally, irrevocably transformed.

“The sharpest sign of change was a surge in radioactive plutonium that started in Crawford Lake’s mud around 1950. … A lighter form of nitrogen — a molecular signature of burning fossil fuels — proliferated. The amount of fly ash increased eightfold in less than five years.”

More at the Post, here. If you have a subscription, you can see very cool graphics showing odds and ends floating downward through water.

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