At the bottom of Crawford Lake in Ontario, Canada, pristine layers of fine sediment accumulate year after year. Because the lake is small and tranquil, bottom currents do not jostle the sediment, nor does lacustrine life chew through the laminations. Each layer, called a varve, represents an annual record of any hubbub (or lack thereof) affecting the lake waters, going back almost 1,000 years.
In sediment cores carefully extracted from the lake, scientists can see traces of Iroquois horticultural activity dating back at least 750 years, and evidence of invading European settlers in the mid-19th century, said University of Leicester paleobiologist Jan Zalasiewicz. In the topmost layers curated by the lake, scientists can systematically search for signs of plutonium, cesium, radiocarbon, fly ash, and microplastics. Such signatures might indicate the beginning of the Anthropocene, a proposed addition to the geologic timescale governed by the International Commission on Stratigraphy (ICS).
The idea of the Anthropocene, introduced by late atmospheric scientist Paul Crutzen at a conference in Mexico in 2000, expresses humanity’s profound impact on the planet and signals an end to the Holocene epoch, which began 11,700 years ago. Because the Anthropocene results from the intersection of the natural world and human society, the term has been adopted by numerous scholarly communities including archaeology, philosophy, and even international law.
In a recent paper, Zalasiewicz, chair of the ICS Subcommission on Quaternary Stratigraphy (SQS), and a corps of colleagues in fields ranging from history to soil science explored what the Anthropocene means to the various groups who use the term. “One person’s Anthropocene is not another person’s Anthropocene,” said Zalasiewicz, “and if the difference between the meaning stretches too widely, then there’s always a possibility of confusion.”
Identifying Ground Zero
To define a new unit of geologic time, scientists must find a synchronous, globally identifiable signal. For example, at the end of the Cretaceous, an asteroid crashed into what is now the Gulf of Mexico’s Yucatan Peninsula, decimating nonavian dinosaurs and countless other species. The extraterrestrial impactor vaulted gas and ash into the atmosphere and left behind a globally distributed, iridium-rich layer that today formally defines the Cretaceous–Paleogene boundary.
However, finding the base of the Anthropocene has been “a long and fiddly process,” said Zalasiewicz, in part because the SQS Anthropocene Working Group originally focused on a red herring: the beginning of the Industrial Revolution, around 1800. More recently, the group has found promise in the Great Acceleration of the 1950s, defined by a surge in numerous measures of human activity. The newest veneers of the geologic record contain various human-sourced signals like radionuclides produced from aboveground nuclear tests, plastics intermingled with sediment, and fine particles of coal combustion called fly ash. Although not traditionally geological, these features can be treated as geological ingredients of strata, said Zalasiewicz.
To find the best possible candidate for the fabled golden spike that will serve as the type locality should the Anthropocene become formally defined, “there is quite a long initial sift,” explained Zalasiewicz. The SQS group is studying a dozen or so candidates, including coral skeletons, Antarctic ice cores, peat bogs, and, of course, lake sediments. Layer by layer, the search for plutonium content, radiocarbon signals, and other signatures will form the bricks for constructing descriptions of each candidate. The most robust site with the strongest foundation of data will then be recommended as the type locality of the shift from the Holocene to the Anthropocene, said Zalasiewicz.
Carrying the Burden
Some geologists argue that the Anthropocene should remain an informal term, said Dipesh Chakrabarty, a historian at the University of Chicago who was not involved in the recent review. For example, hominin management of fire marks the first time that a biological species had more energy available than it could produce on its own. Alternatively, the transition from hunting and gathering to agrarian food production denotes a striking change in societal litter.
Besides these examples, some scholars wish to “connect the beginning of [the Anthropocene] to European expansion and [the] death of hundreds of thousands of people in Latin America,” Chakrabarty said, because “they want to find a political origin.” Proponents of these political definitions suggest various portmanteaus that reflect colonialism’s role in industrialization, including terms like Capitalocene, Econocene, or even Plantationocene. Anthropo- implies that all humans have been equally responsible for greenhouse gas emissions, but “privileged people emit more emissions, [and] the bad impact of climate change will fall unevenly in the world,” explained Chakrabarty, possibly making the term misleading, depending on the context.
In addition, nongeologists may not recognize that “the name of a geological time period does not have to account for why that period came about,” Chakrabarty said. “That term ‘Anthropocene’ doesn’t have to carry the burden of pointing to [its] causal factors.”
Indeed, Zalasiewicz and his colleagues, several of whom are nongeologists, want to “encourage the interchange between different disciplines while making the clarity of the communication as precise as possible.” They suggest that should the Anthropocene become a formal unit of geologic time, other groups could use alternate terms, similar to how different communities use the terms Pleistocene and Paleolithic for a roughly contemporaneous period of time. (The former denotes a geological time span corresponding to the most recent episode of glaciations, whereas the latter denotes an anthropological time span corresponding to the human use of stone tools.)
“If [the Anthropocene] does get ratified, said Chakrabarty, “that would be great, but for somebody who is trying to think through human history…I’m deeply interested in the predicament.” That predicament is the one recorded by threatened corals, warming ice, and the sediments of Lake Crawford, laden with evidence of humanity’s excesses.
—Alka Tripathy-Lang (@DrAlkaTrip), Science Writer