Geologic time is vast and incomplete. Scientists can look at the stratigraphy of rock record and ascertain a great wealth of information about the history of our planet, but any single outcrop or section of layers will not tell the complete story. Subtle nuances and great unconformities exist between the rocks from one location to the next. Never discouraged, geologists at the International Commission on Stratigraphy (ICS) have managed to come together and agree on distinguishable boundaries to differentiate geologic periods, epochs, and ages. The reference boundary is referred to as the Global Boundary Stratotype Section and Point or GSSP. This point and associated stratigraphy is what becomes globally recognized as the lower marker of a given geologic time division.

In the dynamic times that we live in, the planet has been rapidly transformed by human activity. These changes are significant enough that the people tasked with studying the layers of the earth—the ICS and the Subcommission on Quaternary Stratigraphy (SQS)—created a working group to debate whether these changes signify the boundary of a new geologic time period: the Anthropocene. The folks tasked with debating the Anthropocene are known as the Anthropocene Working Group (AWG). This aggregate of geologists, stratigraphers, geochemists, and many others from a range of sub disciplines assist with deciding the future of geologic time, and have been doing so for a decade now.

Enter the Anthropocene

The 2019 meeting of the AWG took place in early November in the US at Tulane University in New Orleans, Louisiana. The timing of this meeting coincided with the Anthropocene River Campus: The Human Delta, involving students, artists, and scientists from around the world. I first journeyed to New Orleans to attend the Campus as a guide to a group of paddlers that had traversed the entirety of the Mississippi River Valley as part of the Anthropocene River Journey. Along the paddle journey, I was invited to attend the AWG meeting that would kick off Campus proceedings. Never wanting to miss out on a good geologic discussion, I bought a new outfit to wear in lieu of my standard paddling attire and prepared for two days of rock talk in the city of Mardi Gras.

When did it begin?

At a previous gathering of the AWG in 2016, the majority of attendees (88 percent) voted that the Anthropocene is stratigraphically real at the magnitude of an epoch and best represented by the atomic radiation spikes found globally in stratigraphic records from the early 1950s and 1960s. In particular, the presence of plutonium-239 following atmospheric bomb testing around 1952 seems to be the most globally recognizable primary marker and is favored by the group. Bomb testing during this time propelled radioactive dust high into the upper atmosphere, allowing it to settle back down virtually everywhere on the planet.

The 1952 date for the Anthropocene is actively met with contention from at least one landscape ecologist in attendance of the 2019 AWG meeting, outspoken in his views on such a “late” date for the start of the Anthropocene. Indeed, there are many other stratigraphic markers dated long before the plutonium-239—such as evidence of human agricultural activity in the soil record—that could also be used as indicators of the Anthropocene. However, the geochronological basis for the GSSP was decided by majority vote in 2016 and so the debate is secondary to the current work of the group deciding the most representative candidate site for the Anthropocene. It should be noted that the landscape ecologist was one of four that voted against this basis.

Candidate Sites

During the course of the two-day 2019 AWG meeting, the primary objective was to hear presentations from researchers of candidate GSSP sites and consider which sites hold the most promise for representing the boundary of the Anthropocene on a global scale. The AWG works in partner with two German institutions, the Max Planck Institute for the History of Science, and Haus der Kulturen der Welt, that will be able to offer some support to assist with research at the chosen candidate site and establish the formal stratigraphic boundary of the Anthropocene.

There are a variety of localities being considered as potential GSSP candidate sites for the Anthropocene, and the AWG is actively soliciting nominations. The primary requirement is that the location clearly captures plutonium-239 in a reliable stratigraphic sequence, though an additional requirement is the existence of prior research data. Candidate sites can be soil sections from bays, reservoirs, lakes, seas, and peat bogs, as well as other stratigraphic sequences like cores from ice, trees, speleothems, and coral.

Secondary Signals

The AWG meeting inspired several heated discussions about the concept of primary and secondary signals of the Anthropocene. Many consider the presence of microplastics, concrete, and other novel materials in the sediment as an indicator of the Anthropocene that predates the primary plutonium-239 marker. Likewise, concepts such as the spread of invasive species and the presence of soot from the burning of coal also make good secondary markers of the Anthropocene. The main issues with using these as primary markers are that they are not globally present in the sediment record and do not offer as precise of a geochronological timer as plutonium-239.

Each potential record source has drawbacks and redeeming qualities. For example, cores from ice, coral, trees, and speleothems are unlikely to contain any novel materials but they are all relatively stable. A living organism has never before been used as a GSSP, so coral and trees would be unprecedented. Sediment records from bogs, bays, lakes, reservoirs, and seas hold great potential but they are not yet lithified (hardened into rock) and thus are difficult to sample while prone to loss, compaction, and distortion. The inherent stability associated with GSSPs in the rock record is not as certain when considering unconsolidated strata.

Uncertainty

The uncertainty involved with selecting a GSSP for the Anthropocene extends beyond the drawbacks of candidate sites. While plutonium-239 offers an ideal layer to make as the base of stratigraphy for the Anthropocene, there is plenty of evidence that humans were impacting the sediment record far earlier than 1952. In addition, we cannot know the extent to which we will change the planet and the sediment record in the near future. In 500 years, geologists might be tempted to look back and decide that another major event serves as a truer marker of the Anthropocene.

Fortunately, there is a mechanism that is built into the concept of geologic time that could allow for a more honest representation of the beginning of the Anthropocene—that of uncertainty. If we are using plutonium-239 as the marker of Anthropocene’s onset, the temptation might be to make the date November 2, 1952 and the set uncertainty of this date to 1 week or so (the amount of time it took for the radioactive dust from Ivy Mike to have global reach).

If we instead acknowledge that there are indicators in the sediment record far older than this date, we can honor the gradual transition into the atomic Anthropocene with the 1952 date but an uncertainty of 200 years. This would then cover the mass adoption of the Watt steam engine in the late 1700s, the Industrial Revolution, the introduction of most invasive species, and the vast majority of fossil fuel emissions. It also offers a buffer for the uncertainty of the future and acknowledges the complexity of the issue under debate.

Regardless of the questions that remain to be answered involving the Anthropocene, and no matter how long it takes to have the boundary formally recognized, it is clear that we are currently in a new geologic time epoch that is distinguished by evidence of human activity. Something like this should not happen again within any of our lifetimes, and the geological definition will not be formally settled for at least a few more years. It’s a great time to be a Quaternary geologist, and an even better time to be an anthropomorphic organism; this is our time!

The formal recognition and boundary for the Anthropocene will not fundamentally change what the Anthropocene is socially. The term has been adopted by all reaches of culture and has taken on a meaning of its own. However, a formal geologic recognition would influence geologists, archeologists, anthropologists, and other soil scientists. Scientific literature would become consistent in the temporal definition of the boundary. It would be ideal if this geologic recognition of the Anthropocene could eventually lead to a societal recognition of our undeniable impact on the planet and ultimately a modification of our most destructive habits.