What is a GSSP or golden spike?

A GSSP is Global Boundary Stratotype Section and Point, which defines the lower boundary of a stage on the geologic time scale. It is sometimes referred to as a “golden spike”, after the marker which may be driven into the boundary. A GSSP must fulfill certain criteria:

• The lower boundary must be defined by a primary marker. This is often the first appearance of a fossil species, but can also be other markers such as radionuclides.
• There should also be secondary markers such as other fossils, chemical traces, or evidence of geomagnetic reversal.
• The horizon in which the marker appears must have minerals that can be radiometrically dated.
• The marker must have regional and global correlation in outcrops of the same age. For example, Searsville Reservoir and San Francisco Estuary provide local comparison, the East Gotland Basin and Śnieżka Peatland provide regional comparison, and all sites provide global comparison.
• The marker should be independent of the specific kind of geological site; for example, the onset of plutonium can be observed in both coral reefs and estuarine mud.  
• The outcrop must have adequate thickness to allow for potential future re-measurement.
• Sedimentation must be continuous without any changes in the basic composition of recording material. For example, estuarine mud covered by a lava flow would not be suitable for a GSSP (this is why the GSSP for the Cretaceous–Tertiary boundary is where the asteroid hit). 
• The outcrop must be accessible to research and free to access. This means that the outcrop has to be: located where it can be visited quickly (i.e. it should be close to an international airport, served by good roads, and in accessible terrain), kept in good condition (ideally a national reserve), extensive enough to allow repeated sampling, and open to researchers of all nationalities.

On which criteria have the candidate Anthropocene GSSP sites been chosen? 

They all provide a sedimentary archive of materials that should contain markers of the Anthropocene. The succession of layers must be able to be clearly dated annually, so that the mid-twentieth century period can be identified against pre-Anthropocene sediments. Although the various sites are different in depositional nature, they should all contain the globally-synchronous presence of radionuclides (from atomic weapons tests) that has been identified as a primary stratigraphic marker of the mid-twentieth century. A formalized geological boundary—a global GSSP—is often accompanied by designated auxiliary stratotypes depicting the lower boundary across a spectrum of depositional settings.

The current sites have been selected by research groups with an interest in recent sedimentation and the records of recent human and natural phenomena held within it, as well as a shared enthusiasm for investigating the presence of a geological Anthropocene.

Why are the majority of Anthropocene GSSP candidates from the Global North?

In theory, a GSSP site can be anywhere in the world that fulfills the necessary environmental and accessibility criteria. However, the site selection for the Anthropocene Working Group relied upon research groups self-nominating potential sites. Therefore, the sites do not represent a spatially balanced global selection but reflect both individual and institutional interests as well as the financial, cultural, and practical constraints which shape environmental science research. Due to the ingrained biases of the social, cultural, and economic systems in which we live, the groups with the necessary long-term interest, funding, and capabilities required are often from Europe and North America. 

What is new about the Anthropocene GSSP research?

There are no new techniques of investigation being used. The conservative nature of stratigraphic ratification procedures demands well-tested, reliable, and trusted techniques. However, some of the proxies being considered will not have been used in any previous GSSP study. Anthropogenic markers such as plutonium, microplastics, and anthropogenic pollutants are geologically novel, and therefore were not analyzed during research to ratify previous successions. It is also novel to be referring to the historical record (i.e. knowledge of nuclear tests and industrialization) to correlate with stratal evidence; evidence for earlier epochs is based on the rock record alone.

How will GSSP research be conducted in the future?

GSSP research is increasingly moving away from using fossil markers and towards geochemical ones, particularly for the last 66 million years. 

How and when will the Anthropocene be officially recognized?

Once a particular site has been recommended by the AWG to serve as the GSSP, it must pass three additional levels of international scrutiny, by a 60 percent supermajority vote at each stage. The AWG will propose their GSSP selection to the ICS Subcommission on Quaternary Stratigraphy (SQS); the next stage would be for the SQS to propose it to the International Commission on Stratigraphy (ICS); finally the proposal would go to the International Union of Geological Sciences (IUGS) for formal ratification. After this, it will be published in Episodes: Journal of the International Union of Geological Sciences, and can be added to the International Chronostratigraphic Chart. After passing the three voting stages mentioned above, official ratification is notified by letter from the Secretary General of the International Union of Geological Sciences (IUGS) to the Secretary  General of the International Commission on Stratigraphy (ICS).

Who votes on the proposed Anthropocene GSSP?

The twenty-two voting members of the Anthropocene Working Group can be viewed in the 2020 AWG newsletter. All have chronostratigraphic experience and are skilled in techniques relevant to the work.

Can a GSSP be revoked or changed after it has been ratified?

The International Commission on Stratigraphy’s guidelines for the establishment of global chronostratigraphic standards notes that a GSSP can be changed if subsequent research shows such a change to be necessary. But this can only occur ten years after the original ratification (“the practical value of the boundary definition” should be tested for this duration). The only exceptions to this are if the established GSSP is permanently destroyed or becomes inaccessible, or if “a violation of accepted stratigraphic principles” is discovered after the ratification.

Where did the term “Anthropocene” originate?

Paul Crutzen and Eugene Stoermer published an article in May 2000 in the IGBP Global Change Newsletter that introduced the term, although they acknowledge that the idea of human activity being geologically significant has a much longer history, going back at least to nineteenth-century Italian geologist Antonio Stoppani who wrote of an “anthropozoic era” in 1873. Crutzen’s later article, published in Nature in 2002, is also considered significant in popularizing the term.

How are geologic time units usually named?

According to the International Commission on Stratigraphy, “the names of most formal stratigraphic units consist of an appropriate geographic name combined with an appropriate term indicating the kind and rank of the unit”. For example, the Jurassic Period was named for the Jura mountains between France and Switzerland, which are chiefly formed of oolitic limestone characteristic of the Jurassic period, while the Cambrian Period is named after Wales (“Cambria” comes from the Old Celtic word for Wales), where rocks of that period were first officially identified. But there are exceptions to this geographic rule. The Hadean Eon is named after Hades, the Greek god of the underworld, the Archaean Eon is named after the Greek word for “ancient”, and the Proterozoic means “first-life”. 

Other start dates for the Anthropocene have been suggested, such as the anthropogenic megafaunal extinctions in the Pleistocene, or the Industrial Revolution at the beginning of the nineteenth century. Why does the GSSP research focus on the mid-twentieth century?

The AWG voted in 2016 to decide that the mid-twentieth century was the most appropriate time for the Anthropocene lower boundary. This “Great Acceleration” is visible in sediments through a variety of signals, including radionuclides from the “bomb pulse”, nitrogen isotope changes (caused by fertilizers), technofossils, urban deposits, persistent organic pollutants and increased concentrations of heavy metals from industry, human-driven biotic change (invasive species and extinctions), traces of fossil-fuel extraction and burning, oil spills, and landscapes significantly changed by dams. Of these, the “bomb pulse” is probably the clearest and most-globally synchronous signal, making it the prime candidate marker for the Anthropocene. This does not mean, however, that it is the first or even the most significant marker of human impact on the planet: it just best fulfills the criteria needed for a GSSP. A detailed justification for the mid-twentieth century choice can be found in “When did the Anthropocene begin? A mid-twentieth century boundary level is stratigraphically optimal”, Quaternary International 383 (2015).

The AWG proposes the Anthropocene as a new epoch on the geological time scale, with a lower boundary coinciding with the mid-twentieth century Great Acceleration. However, some scientists have criticized this choice, suggesting instead that the Anthropocene should be seen as a long-running “event” that describes the range of human impact on the planet since the Pleistocene. What is the difference between an epoch and an event, and why does the AWG think an epoch is best suited to describing the planetary change in question?

A geological epoch is a chronostratigraphic unit used to divide time on the geological time scale, as represented by the International Chronostratigraphic Chart. Epochs are defined by their lower boundary, and signal the ending of the previous epoch (so the start of the Anthropocene would mean the Holocene has ended). A geological event refers to something that has happened—either instantaneously or over a considerable duration of time—and left traces in the geological record. Examples of geological processes termed “events” include volcanic eruptions and meteorite impacts, but also planetary changes that have occured over millions of years, such as changes in ocean chemistry and mountain building. However, events do not appear in the geologic time scale (though they may coincide with the lower boundary of an epoch).

The two concepts are not mutually exclusive. The AWG recognizes that some kind of longer-running anthropogenic modification event may be useful alongside the notion of the Anthropocene epoch, but they emphasize the importance of formalizing the epoch as such in order to recognize the magnitude of the global transformation visible in the mid-twentieth century. Without it, they say, “the Geological Time Scale, which typically reflects major phases of Earth’s evolution, would depart from observed geological reality.”