Geologists could soon decide which spot on Earth marks the first clear evidence of the Anthropocene — which many of them think is a new geological epoch that began when humans started altering the planet with various forms of industrial and radioactive materials in the 1950s. They have so far whittled their choices down to nine candidate sites worldwide (see ‘Defining the Anthropocene’), each being considered for how reliably its layers of mud, ice or other matter tell the story of people’s influence on a timeline that extends billions of years into the past.

If the nearly two dozen voting members of the Anthropocene Working Group (AWG), a committee of scientists formed by the International Commission on Stratigraphy (ICS), agree on a site, the decision could usher in the end of the roughly 12,000-year-old Holocene epoch. And it would officially acknowledge that humans have had a profound influence on Earth.

“We’re pointing to something in the rock record that shows we’ve changed the planet,” says Kristine DeLong, a palaeoclimatologist at Louisiana State University in Baton Rouge who studies the West Flower Garden Bank, a candidate site in the Gulf of Mexico.

The Anthropocene site will join 79 others that physically define stages of Earth’s geological timescale — that is, if it’s approved. Even if the AWG agrees on a final candidate, several other committees of geologists must vote on the selection before it is made official. And not all scientists agree that it should be.

Here, Nature examines what it will take to formally define the Anthropocene epoch.

Why do some geologists want an Anthropocene marker?

Scientists coined the term Anthropocene in 2000, and researchers from several fields now use it informally to refer to the current geological time interval, in which human activity is driving Earth’s conditions and processes. Formalizing the Anthropocene would unite efforts to study people’s influence on Earth’s systems, in fields including climatology and geology, researchers say. Transitioning to a new epoch might also coax policymakers to take into account the impact of humans on the environment during decision-making.

Coral growing on oil rig, Flower Garden Banks National Marine Sanctuary, Texas

Coral grows on an oil rig in Flower Garden Banks National Marine Sanctuary, in the Gulf of Mexico.Credit: Flip Nicklin/Minden Pictures/Alamy

“It’s a label,” says Colin Waters, who chairs the AWG and is a geologist at the University of Leicester, UK. “It’s a great way of summarizing a lot of concepts into one word.”

Mentioning the Jurassic period, for instance, helps scientists to picture plants and animals that were alive during that time, he says. “The Anthropocene represents an umbrella for all of these different changes that humans have made to the planet,” he adds.

How do scientists usually choose sites that define the geological timeline?

Typically, researchers will agree that a specific change in Earth’s geology must be captured in the official timeline. The ICS will then determine which set of rock layers, called strata, best illustrates that change, and it will choose which layer marks its lower boundary. This is called the Global Stratotype Section and Point (GSSP), and it is defined by a signal, such as the first appearance of a fossil species, trapped in the rock, mud or other material. One location is chosen to represent the boundary, and researchers mark this site physically with a golden spike, to commemorate it.

Defining the Anthropocene: nine sites are in the running to be given the ‘golden spike’ designation

Site

Location

Material type

How it captures signs of human activity

Beppu Bay

Kyushu Island, Japan

Marine sediment

Sediment fell to the sea floor, where the oxygen-deprived water limits disturbance by animals.

Crawford Lake

Ontario, Canada

Lake mud

Particles accumulated at the steep-sloped lake bottom, where water layers don’t mix.

Flinders Reef

Coral Sea, Australia

Coral

The corals’ growing exoskeletons have trapped chemicals and particles.

Gotland Basin

Baltic Sea

Marine sediment

Sediment fell to the sea floor, where the oxygen-deprived water limits disturbance by animals.

Palmer ice core

Antarctic Peninsula

Ice

Annual layers of snowfall captured particles and chemicals from the air.

Searsville Lake

California

Lake mud

Silt layers accumulated at the bottom after storm events.

Sihailongwan Lake

Jilin province, China

Lake mud

Particles fell to the bottom of this lake, which has an oxygen-deprived lower layer that limits disturbance.

Śnieżka peat bog

Sudetes Mountains, Poland

Peat layer

Peat in this high-altitude bog has captured chemicals and particles from the air.

West Flower Garden Bank

Gulf of Mexico

Coral

The corals’ growing exoskeletons have trapped chemicals and particles.

But the Anthropocene has posed problems. Geologists want to capture it in the timeline, but its beginning isn’t obvious in Earth’s strata, and signs of human activity have never before been part of the defining process. The AWG was established in 2009 to explore whether the Anthropocene should enter the geological timescale and, if so, how to define its start.

“We were starting from scratch,” says Jan Zalasiewicz, a geologist at the University of Leicester who formerly chaired the AWG and remains a voting member. “We had a vague idea about what it might be, [but] we didn’t know what kind of hard evidence would go into it.”

Years of debate among the group’s multidisciplinary members led them to identify a host of signals — radioactive isotopes from nuclear-bomb tests, ash from fossil-fuel combustion, microplastics, pesticides — that would be trapped in the strata of an Anthropocene-defining site. These began to appear in the early 1950s, when a booming human population started consuming materials and creating new ones faster than ever.

Cryogenian-Ediacaran geological boundary in rock strata marked by a brass plate, Flinders Ranges, South Australia

This golden spike in the Flinders Ranges of South Australia was approved by geologists in 2004, to mark strata exemplifying the Ediacaran period.Credit: James St. John (CC BY 2.0)

During a review that took place a few months ago, the AWG narrowed its list from 12 to 9 candidate sites, tossing out certain locations because their layers weren’t ideal. Among the sites remaining is Crawford Lake in Ontario, Canada, which is described as a sinkhole by Francine McCarthy, a geologist at Brock University in St Catharines, Canada, who studies the location. “The lake itself isn’t very big in area, but it’s very, very deep,” she says. Particles that fall into the lake settle at the bottom and accumulate into undisturbed layers.

Another site on the shortlist is West Flower Garden Bank. Corals here could become a living golden spike because they constantly build new exoskeletons that capture chemicals and particles from the water, DeLong says. “The skeleton has layers in it, kind of like tree rings,” she adds.

Why do some geologists oppose the Anthropocene as a new epoch?

“It misrepresents what we do” in the ICS, says Stanley Finney, a stratigrapher at California State University, Long Beach, and secretary-general for the International Union of Geological Sciences (IUGS). The AWG is working backwards, Finney says: normally, geologists identify strata that should enter the geological timescale before considering a golden spike; in this case, they’re seeking out the lower boundary of an undefined set of geological layers.

Lucy Edwards, a palaeontologist who retired in 2008 from the Florence Bascom Geoscience Center in Reston, Virginia, agrees. For her, the strata that might define the Anthropocene do not yet exist because the proposed epoch is so young. “There is no geologic record of tomorrow,” she says.

Edwards, Finney and other researchers have instead proposed calling the Anthropocene a geological ‘event’, a flexible term that can stretch in time, depending on human impact. “It’s all-encompassing,” Edwards says.

Zalasiewicz disagrees. “The word ‘event’ has been used and stretched to mean all kinds of things,” he says. “So simply calling something an event doesn’t give it any wider meaning.”

What happens next?

In a recent Perspective article in Science, Waters and AWG secretary Simon Turner at University College London wrote that the committee would vote to choose a single site by the end of this year1. But 60% of the group’s voting members must agree on a final candidate — and, with several sites under consideration, Waters isn’t sure that a consensus can be reached anytime soon. If no clear winner emerges this month, more voting will be needed to narrow the candidate list, delaying a decision possibly until May 2023.

And that’s not the end of the process. After selecting a finalist, the AWG will present its findings to the ICS’s Subcommission on Quaternary Stratigraphy. Favourable votes from this group would move the proposal to another ICS committee, and subsequent approval would push it to the final stage: ratification by the IUGS.

But the motion could fail at any of those points. And if it does, the AWG will have to revamp its proposal before it can try again — and possibly nominate a new golden-spike site.

Regardless of the outcome, Zalasiewicz thinks that the AWG’s work to define the Anthropocene has been useful. What everybody wants to know is how humans are changing the planet’s geology, he says. “That is the underlying reality that we’re trying to describe.”