Photo caption: Ultrahigh resolution image of a cross section of a freeze core from Crawford Lake across the proposed boundary between the Anthropocene (to the left of the gap between panels) and the Holocene Epoch in panels displayed at HKW (Haus der Kulturen der Welt) as part of the Earth Indices installation, May 2022. Couplets of organic detritus capped each summer by crystals of calcite precipitated in the alkaline waters in this sinkhole in the Niagara Escarpment allow annual resolution, like tree rings. Many geologically preservable markers of the Great Acceleration and the Nuclear Age, including the products of fossil fuel combustion and fallout from nuclear weapons, increase sharply in abundance in the mid-20th century, and the proposed GSSP is at the boundary between the light-coloured calcite layer deposited in the summer of 1952 CE.
Photo credit: Mike MacKinnon.
Contributors: Francine McCarthy, Brock University, and Julia Adeney Thomas, University of Notre Dame
I suspect that few people reading this blog refute the fact that humans have substantially impacted our planet. With our superior intellect and opposable thumbs, we have altered the courses of rivers, mined entire mountains for their resources, and turned forests into pasture, with many unforeseen consequences.
Nobel Laureate Paul Crutzen, an atmospheric chemist studying the changes in atmosphere, together with paleolimnologist Eugene Stoermer coined the term “Anthropocene” for the epoch in which we live. The term emphasizes the role of humans in pushing Earth systems beyond their environmental limits. With respect to the span of our modern epoch, in Newsletter 41 of the International Geosphere-Biosphere Programme (IGBP, 2000) they wrote:
To assign a more specific date to the onset of the “Anthropocene” seems somewhat arbitrary, but we propose the latter part of the 18th century, although we are aware that alternative proposals can be made (some may even want to include the entire Holocene). However, we choose this date because, during the past two centuries, the global effects of human activities have become clearly noticeable. This is the period when data retrieved from glacial ice cores show the beginning of a growth in the atmospheric concentrations of several “greenhouse gases”, in particular CO2 and CH4. Such a starting date also coincides with James Watt’s invention of the steam engine in 1784.
The International Commission on Stratigraphy (ICS) has the final say on the geologic time scale of the world since its creation. So, its task is to identify a single point in time to define the beginning of this new Epoch called Anthropocene. This blog – whose title borrowed from the subtitle of Altered Earth, edited by historian Julia Adeney Thomas of the University of Notre Dame – summarizes the efforts made by this organization to define the term Anthropocene in a manner consistent with the previous 4.6 billion years of Earth History. This requires identifying a specific location that exemplifies the characteristics of that interval (typically the materials are sedimentary rock strata, but ice core records and cave deposits have been used to define and subdivide time through the Quaternary Period). A ‘golden spike’ marks the base of that section, which defines the beginning of that interval of time. Should the ICS accept the proposal of the Anthropocene Working Group (AWG) that a new epoch be formally erected, a GSSP (Global boundary Stratotype Series and Point) must be identified to define the beginning of the Anthropocene / end of the Holocene Epoch.
To Earth scientists, this is what “getting the Anthropocene right” means. Precisely defining the beginning of an interval of time based on geologically preservable, globally correlatable markers does not necessarily mean “getting it right” for all members of the “Anthropocene’” community. The term has been used enthusiastically by historians, human/social geographers, environmental scientists, philosophers, anthropologists, and a host of others outside of the geological community. Sometimes these communities use the term without reference to Earth science, defining it in hundreds, if not thousands, of alternative ways. Recently, however, a growing group of social scientists and humanists have been working with the scientific understanding, asking their own distinctive questions about how and why this phenomenon arose and what might be done to mitigate it. To the stratigraphic community, loose usage of a term that mimics formally defined units of geologic time (e.g., Pleistocene, Holocene) is untenable.
Even among the geological community, there is not unanimous agreement with the decision ratified in May 2019 that:
1) “the Anthropocene be treated as a formal chrono-stratigraphic unit defined by a GSSP”, and 2) “the primary guide for the base of the Anthropocene be one of the stratigraphic signals around the mid-twentieth century of the Common Era” (Working Group on the ‘Anthropocene’ | Subcommission on Quaternary Stratigraphy).
Nonetheless, with ratification of decisions made at the International Geological Congress in Cape Town in 2016, the formal search began for a GSSP that records the Great Acceleration of Steffen et al. (2015) with great chronological precision. In this case, it would be unprecedented in the geological time scale, to a single calendar year.
Together with a team of roughly 50 researchers, including social scientists and an artist, I have explored the annually laminated (varved) sediments of Crawford Lake, in the Township of Milton, Ontario, as a potential GSSP. Calcite crystals form each summer in surface waters of this small, but very deep (nearly 24 m!) lake. They sink to the lakebed, capping the organic-rich sediments that are mainly the remains of phytoplankton and their consumers that accumulate the rest of the year.
The varve couplets can be counted (similar to how tree rings can be counted to give the age of a tree) and “Team Crawford” proposed that the base of the Anthropocene Series to mark the beginning of the new epoch be the light-coloured calcite lamina deposited in the summer of 1950 CE (common era), when plutonium-239 (the main fissionable isotope used in nuclear weapons) and spheroidal carbonaceous particles/ SCPs (produced by incomplete high-temperature combustion of fossil fuels) increased sharply in abundance. If the varved succession of Crawford Lake is chosen by the Anthropocene Working Group as the proposed GSSP— and if the proposal is eventually approved by the International Commission on Stratigraphy —this date would mark the end of the Holocene Epoch.
As with other chronological boundaries on the Geologic Time Scale, the base of the Anthropocene is based on geologically preservable markers of large-scale change on the planet, exceeding the range of environmental change through the Holocene. The mid-20th century base proposed by the AWG after years of study and debate was not intended to reflect the earliest evidence of human impact on the planet, nor the greatest anthropogenic impact at any given location. Rather it was to record the profound impact on Earth systems (atmosphere, hydrosphere, cryosphere, and biosphere, and geosphere) that can be identified synchronously worldwide using markers of that change.
The plutonium-239 and SCPs that mark the proposed GSSP in a sediment core from Crawford Lake that is archived at the National Cryobank of Canada (Canadian Museum of Nature, Ottawa) are anthropogenic markers that were deposited synchronously worldwide as atmospheric fallout far from their points of origin – as a result of thermonuclear weapons tests and industrial emissions, for example. Greater anthropogenic impacts on Crawford Lake are recorded around ten centimeters below the proposed GSSP, when a lumber mill operated on the south end of the lake around the turn of the last century, and around 30 centimeters below that, recording the impact of ‘three sisters agriculture’ by several hundred people who inhabited longhouses in the northwestern part of its catchment for several centuries before the middle of the last millennium.
If adopted, a GSSP would formalize the definition of the term Anthropocene, but it would by no means imply that human impact began in the mid-20th century. That point is well illustrated by the longhouses at the Crawford Lake Conservation Area that were reconstructed by Conservation Halton based on archaeological excavations. This history is communicated to visitors by its staff and through displays. The Interpretive Centre and boardwalk around the lake is also a useful means of communicating the difference between the geological definition of an epoch and evidence of anthropogenic impact, irrespective of the decisions of the AWG and the ICS. In any case, the research done at the Crawford Lake site has allowed us to better understand the kind of impact human activity has had and continues to have on ecosystems and the Earth’s system as a whole. This understanding can potentially lead us to adopt more sustainable ways of existing.