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For almost 15 years, a panel of scholars has been chewing over a big question: Has our species transformed the planet so much that we have plunged it into a new interval of geologic time?

On Tuesday, the panel announced a key part of its case for declaring that we had. The group said it had chosen a lake in an Ontario conservation area to represent the start of Anthropocene epoch, a potential new chapter in Earth’s 4.6-billion-year history that could soon sit alongside the Cambrian, the Jurassic and the Cretaceous in marking periods of momentous planetary change.

The scientists picked Crawford Lake over 11 other candidate sites because it contained the clearest and most pronounced evidence of humankind’s influence on the global geologic record, representatives for the group said at a news briefing in Lille, France. This evidence includes sharp changes in plutonium and radiocarbon from nuclear detonations, and in fly ash from accelerated burning of fossil fuels.

“It is a reflection of that tipping point in Earth history when the Earth system ceased to behave the way it had for 11,700 years,” said Francine M.G. McCarthy, a micropaleontologist at Brock University in St. Catharines, Ontario, and a member of the Anthropocene Working Group, the panel that since 2009 has been discussing whether, and how, to grant the human age a place on the official geologic timeline.

Canonizing the Anthropocene would affirm that humans have changed Earth so significantly in recent decades that our current geologic epoch, the Holocene, which began 11,700 years ago and fostered the conditions for complex human civilizations to emerge, has come to a definitive end.

Scientists’ final ruling on the Anthropocene will dictate nomenclature used in academic studies, textbooks and museums for generations to come, and help shape humanity’s understanding of its place on Earth.

Preparing for such a monumental declaration has been anything but straightforward. And there’s still a ways to go before it’s official.

Once the working group writes up its formal proposal for recognizing the Anthropocene epoch based on the site in Canada, three more committees of geologists will vote on it, a process that could start this fall. Sixty percent of each committee will need to approve the proposal for it to advance to the next one. Ratification by any is far from guaranteed.

In fact, two of the Anthropocene Working Group’s roughly three dozen members resigned recently because they disagreed with the panel’s approach. Geologists on the other voting committees could prove similarly hesitant to enshrine a period that is still a mere infant by the standards of Earth time, no matter how consequential it has been for the planet.

“It’s going to be a bumpy ride, indeed,” said Jan A. Zalasiewicz, a geologist at the University of Leicester in England and a member of the working group. “I’m not sanguine about the chances. But it’s an important bit of science.”

In 2019, the panel agreed, after a decade of debate, to recommend that the new epoch began in the mid-20th century, which is when globalization, industrialization and energy consumption started accelerating. Late last year, the group’s members began voting on a physical site, known as a “golden spike,” where the rock record clearly divides the Anthropocene from the Holocene before it.

Nearly all geologic time units have golden spikes, and they aren’t just symbolic. Each one has to contain geochemical markers so distinctive that, when scientists come across unfamiliar rocks in other places, they can match the markers up to determine roughly how old they are.

It took three rounds of voting, from last fall through the spring, for the Anthropocene panel to pick Crawford Lake, whose waters are so deep that whatever falls to the bottom is preserved in the mud, accumulating over time into a tree-ring-like record of planetary change. The other finalists were Sihailongwan, a volcanic-crater lake in China, and Beppu Bay, off Kyushu in Japan.

“It was an extremely close call,” said Colin N. Waters, the working group’s chairman. “There was a lot of careful thought about this.”

In the coming months, the panel will also select auxiliary sites that could help geologists locate the boundary between the Holocene and the Anthropocene in other environments, not just lake beds. Like corals, for instance. Or peatlands.

But to Philip L. Gibbard, a geologist at the University of Cambridge, it feels as if the group is doing all this careful work to craft a definition of the Anthropocene that will be irrelevant on arrival to many people.

Dr. Gibbard joined the panel at its start in 2009. But for the last several years, he has felt his views drifting away from those of the rest of the group, he said. Finally, he and another member — Matt Edgeworth, an archaeologist — resigned this year “in exasperation,” he said.

As a term, “Anthropocene” long ago diffused out of the realm of natural science, and the archaeologists, anthropologists and artists who use it aren’t likely to listen to geologists who insist that it apply only to the world post-World War II, Dr. Gibbard said. “We are not policemen,” he said. “We can’t tell colleagues in social sciences what to do.”

The strict rules of the geological timeline also require that the new epoch have a fixed starting point, which Dr. Gibbard believes would do a disservice to the sprawling story of humankind’s transformation of the planet.

Dr. Waters, the working group’s chair, said that he had known Dr. Gibbard for two decades, and that they had always gotten along well. Now, after their split over the Anthropocene, and with the tone of their emails turning increasingly sour, he wonders whether Dr. Gibbard will even speak to him next time they’re at the same scientific conference, Dr. Waters said.

“There’s a sense of emotion to this that is strange,” Dr. Waters said.

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