Researchers move closer to defining the Anthropocene

Tens of thousands of years ago, small bands of foraging humans left few long-lasting impacts on their environment. Today, we carve huge and indelible marks on the landscape, including megacities, large-scale agriculture, and widespread deforestation. Humans have even changed the chemistry of Earth’s sediments with industrial pollution, radioactive fallout from nuclear tests, and rising levels of nitrogen from fertilizers. Welcome to the Anthropocene—the Epoch of Humans. Or, at least, that’s the label some anthropologists have been lobbying for.

The dawn of the atomic bomb is arguably among the most salient signals of the Anthropocene’s start. The Trinity test, in July of 1945 near Socorro, New Mexico—here shown 16 seconds after detonation—was the world’s first nuclear explosion. Image credit: Science Source; https://www.sciencesource.com/1773357-trinity-test-16-seconds.html.

But even as researchers wrangle over whether science needs the Anthropocene, a parallel debate has raged about how we might determine when it began [see “Are We in the Anthropocene?”(1)]. That debate is now one step closer to a resolution. After more than a decade of work, geologists say they have identified a site at Crawford Lake in Ontario, Canada, that they contend has the best combination of characteristics needed to define the onset of the Anthropocene.

Pinpointing that site marks a key milestone in the effort to officially recognize the epochal impacts that humans have written into the geological record. But the site still faces an uphill struggle for approval by some scientific committees, and the researchers who selected Crawford Lake acknowledge that the toughest part of their job may yet lie ahead. “We still need to convince geologists that it needs to be adopted at all,” says Colin Waters, a geologist at the University of Leicester in England, who is the chair of the working group that singled out the site.

There are 12 candidate reference sites being considered as part of efforts to codify the Anthropocene. Each provides records of human development. Image credit: Reproduced from ref. 6.

Setting Boundaries

Scientists conventionally divvy up geological time into four long intervals called eons, some of which are sliced into smaller and smaller slivers—eras, periods, epochs, and ages. These divisions help stratigraphers and other scientists to establish the ages of rock formations around the world based on their geochemical characteristics and the fossils within them. But defining one of these intervals takes a lot more than identifying a distinctive layer of rock, writing up a journal paper, and making a declaration. To formalize this determination, there’s a lengthy, multilayered, and sometimes contentious process, Waters explains.

The overarching group of scientists in charge of such efforts is the International Union of Geological Sciences (IUGS), a nongovernmental organization based in Beijing. The largest group within the IUGS is the International Commission on Stratigraphy (ICS), which curates the worldwide geological time scale. The lower boundaries for many of the ages in this time scale are defined by Global Boundary Stratotype Sections and Points (GSSPs). Each of these points is located at a specific site where rock strata contain an observable and unambiguous change in physical properties or fossil content that can be readily dated. Ideally, a GSSP site should contain several of these signifiers, making it easier to correlate these markers with other locations around the world that may only bear one or two of the telltale traces.

Once identified, such sites are tagged with a physical marker—a so-called “golden spike”—to identify the precise boundary for future researchers. According to the current ICS scheme, we are living in the Quaternary Period (which began about 2.58 million years ago) and the Holocene Epoch (which began about 11,700 years ago, around the end of the last ice age).

But in 2002, atmospheric chemist Paul Crutzen proposed that we were no longer in the Holocene (2). Instead, he suggested, a new epoch called the Anthropocene may have begun in the late 18th century, around the beginning of the Industrial Revolution in Europe. After much discussion, the ICS’s Subcommittee on Quaternary Stratigraphy established the Anthropocene Working Group (AWG) in 2009 and asked it to find the evidence needed to formally define the Anthropocene. Some members of the group initially agreed with Crutzen that the Industrial Revolution marked the start of the Anthropocene, whereas others thought that the new epoch began the moment that the United States detonated the first-ever nuclear bomb—a test conducted at the Trinity site near Alamogordo, New Mexico, on the morning of July 16, 1945.

Following a decade of work scouring the landscape and analyzing sediments worldwide, the AWG eventually decided that a time somewhere in the mid-20th century would be most appropriate (3). Specifically, they found that the geological record chronicles a number of distinct signals in the post-World War II era, such as the presence of industrial soot, toxic substances, and traces of microplastics. Many researchers refer to this postwar period as “the Great Acceleration,” marked by a rapid surge in population growth, energy consumption, industrialization, and global economic interdependence.

For example, about 60% of global energy consumption during the Holocene has occurred since 1950, Waters and his colleagues report (4). Furthermore, rates of erosion and sediment transport—largely driven by urbanization and agriculture—have jumped more than an order of magnitude since then. Large-scale agriculture has markedly accelerated the pace of the carbon, nitrogen, and phosphorus cycles, which have, in turn, boosted the size of oceanic dead zones that have little or no dissolved oxygen. Habitat loss has increased the numbers of species going extinct, and global transportation has enabled the proliferation of invasive species. Concrete, plastics, and radioactive materials generated by above-ground nuclear tests join a rapidly growing set of modern-day “technofossils” (5). “However far forward we go, when we look back, it’ll be clear that major change occurred in the mid-20th century,” says Martin Head, a geologist at Brock University in St. Catharines, Ontario, who is part of the AWG.

Having concluded that these strands of evidence would collectively herald the start of the Anthropocene, geologists then faced a fresh challenge: finding a site that contained as many of these signals as possible.

Where, Oh Where?

In 2019, Waters and his colleagues began hunting for a location to host the Anthropocene’s golden spike. During the next 3 years, they identified a dozen potential sites scattered across five continents and located in diverse environments (6).

These sites were chosen because they featured continuous accumulations of material in layers that could be dated on a year-by-year, or even season-by-season, basis. They include anoxic lakes and ocean basins, where there are no sediment-dwelling organisms to disturb sediments once they accumulate; cave formations that display growth features similar to tree rings, which can betray wet and dry seasons; and corals or shell deposits that can also chronicle annual or seasonal growth. The sites are also readily accessible to researchers, yet protected from development so that the geological record can be preserved indefinitely, Waters says.

“It’s unusual to be defining a geological age while you’re living in it.”—Jacquelyn Gill

When the AWG voted on these options earlier this year, 14 of the 23 members decided that a site at Crawford Lake, about 30 kilometers north of Hamilton, Ontario, would best represent the onset of the Anthropocene. That result, announced at conference hosted by the Max Planck Society in Berlin on July 11, barely met the 60% supermajority required for approval, Waters says.

Crawford Lake is a small body of water that formed in a natural sinkhole in limestone terrain, and its deepest reaches are slightly saline and highly alkaline. This chemistry prevents the survival of sediment-dwelling creatures that could churn up the layers of sediment accumulating on the lake bottom, Head and his colleagues reported in February (7). Those undisturbed layers chronicle numerous signs of human activity. For example, the sediments feature a sharp increase in plutonium that appeared in the late 1940s as fallout from nuclear testing. There are also so-called spherical carbonaceous particles (SCPs), a distinct form of black carbon generated by burning fossil fuels at extremely high temperatures in energy production and heavy industry. A spike in these SCPs appears in lake sediments deposited in the 1950s, reflecting the global increase in fossil fuel consumption at that time (8).

Researchers have agreed that a sample of Crawford Lake’s sediment archive will be held in cold storage in a national cryobank, Waters says. Analyses of material from that sample will be limited, he notes, but scientists will always have the option of drilling more samples from the lake bottom for more extensive study.

In the next few months, the AWG’s recommendation will be referred up to the Subcommittee on Quaternary Stratigraphy, whose endorsement also requires approval by 60% of its members. If that passes, Waters says, the site selection must then be discussed during an indeterminate and possibly lengthy period that he hopes will be completed in time for a major meeting of the International Geological Congress in August 2024. Approval at that level also requires 60% of the voting members of the ICS. Final approval of the site would come from the members of the Executive Committee of the IUGS. “None of these votes are guaranteed to pass,” Waters says.

When, Oh When?

Despite the promise of the Crawford Lake site, some researchers question whether any of the dozen options assembled by the AWG are reliable long-term boundary markers for the Anthropocene. In order to capture the beginning of that epoch, they all feature materials laid down since the 1940s. Yet, erosion can readily remobilize sediments that haven’t yet consolidated into rock, says Lucy Edwards, a scientist emeritus at the US Geological Survey in Reston, Virginia. In the coming decades and centuries, lakes and bogs could dry up and disappear, coastlines and ocean currents could shift, and earthquakes might jiggle nicely layered sediments into a blurry mess. “Pure and simple, we don’t know what the future’s going to bring,” she notes.

Jacquelyn Gill, a paleoecologist at the University of Maine in Orono, agrees. “It’s unusual to be defining a geological age while you’re living in it,” she says. Gill also questions the AGW’s decision to put the beginning of the Anthropocene in the mid-20th century, noting that humans have been affecting the landscape for much longer than 75 years. Indeed, she and her colleagues argue that human activity has shaped large parts of Earth’s terrain for more than 12,000 years.

In 10,000 BCE, for example, almost three-quarters of the landscape was inhabited either by hunter-gatherers or by early agricultural societies, according to research by Erle Ellis, a landscape ecologist at the University of Maryland Baltimore County in Baltimore (9). Their activities included burning grasslands, cultivating plants, and domesticating animals, all of which modified their surroundings to some degree. Evidence for these activities includes the remains of domesticated plants and animals in archaeological middens, as well as smaller clues like pollen and ash in lake sediments [see “Unearthing the Origins of Agriculture” (10)]. “It never ceases to amaze me what persists in the fossil and archaeological record,” says Torben Rick, an archaeologist at the Smithsonian Institution’s National Museum of Natural History in Washington, DC.

Ellis says that by placing the start of the Anthropocene around 1950, the AWG ignores all of this evidence that humans have been influencing the landscape for millennia. Although archaeological evidence, such as human artifacts, had not previously been used to define a geological age, Rick says that it could play a vital role in characterizing the Anthropocene because much of the evidence stems from human activity. “Defining the Age of Humans is not just a geological question,” he says.

“It’s challenging to agree on a start date for the Anthropocene,” agrees Gill. “Any one choice almost seems arbitrary.” The debate, it appears, is far from over. But with major votes occurring in the next couple of years, the community might finally reach some consensus on what geology and archeology say about the impact of the scars humankind has left on the planet.