A Beginning to the Anthropocene?

By Jan Zalasiewicz (2014)

The Earth’s history stretches back a little over four and a half billion years, beginning with the mysterious Hadean eon, from which little remains. From the beginning of the subsequent Archaean eon, some 3.8 billion years ago, there are preserved rock strata, from which past states of the Earth can be gleaned—the result of much painstaking and ingenious detective work carried out by many geologists over the last two centuries or so.This history is enormous, and encompasses a seemingly countless succession of changes in geography, landscape, sea level, climate, biology. For almost all of it, there have been no human observers to watch and record it (hence the interrogation of those rock strata). But, without documented observations, how does one build a coherent time framework for that long history?

Over the first century of serious geological study, the measure of choice of the human historian—the year—was simply impossible to apply. In the nineteenth century, the enormous scale and complexity of Earth’s history was becoming apparent. Yet the length of that history remained frustratingly opaque, despite ingenious attempts to measure it (how long might it take the Earth to cool down, for instance, or the oceans to become salty). Estimates ranged from a few million years to many billions of years—but no one really knew. Therefore, that history was resolved into a succession of dynasties of Earth time, characterized by different physical, chemical and (especially) biological states, all deduced from those information packed strata. These were given names such as Cambrian, Jurassic and Pleistocene, with large units (e.g. eons) being successively divided into smaller units—eras, periods, epochs and ages.

The history grew complex, and still no one knew how long a time it represented. Then, early in the 20th century, Henri Bequerel’s discovery of radioactivity was exploited to establish the duration of those time divisions in millions of years.

Geologists, did not, though, discard their elaborately compiled dynastic framework to simply use numerical time (although suggestions to that end were made). Those stratal dynasties proved too useful—and their reflection of real genuine changes in Earth state made them into a kind of convenient aide memoire of the major phases of Earth history. Besides, the means to trace those historical events around the Earth to construct a four-dimensional history, notably through the use of fossils as relative time markers, was usually easier and more effective than direct numerical dating of rocks, which remains time-consuming and expensive. Hence, those labels—Jurassic, Pleistocene and so on—persist as the primary units of geological time. For the last half-billion years of Earth time, since fossils became plentiful, these units are defined by physical reference points, carefully selected (after decades of intense debate) within one stratal level at one place in the world, to represent the time instant when one epoch, or era, or period, gave way to the next.

These are the “golden spikes” that define geological time units, more formally known as Global Stratotype Sections and Points (GSSPs). For older rocks, that represent the time before fossils became plentiful, more or less arbitrary numerical boundaries are chosen, that geologists try to work with as best they can. The boundary between the Archaean and Proterozoic eons, for instance, is set at 2.5 billion years ago. These are Global Standard Stratigraphic Ages (GSSAs). Enter the Anthropocene, very recently—proposed in 2000 by Paul Crutzen and Eugene Stoermer as the newest epoch of geological time. The Anthropocene is still informal, but is being analyzed to see whether it might become a formal part of the Geological Time Scale. It is remarkable in many ways, reflecting the dramatic, and geologically rapid, human-driven reshaping of the Earth’s physical, chemical and biological characteristics. It is by far the shortest of the Earth’s epochs. There are still debates about where its boundary should be placed, but there is a growing consensus that it should be associated with the mid-twentieth century “Great Acceleration” of global economic activity, and the beginning of the nuclear age.

If so, the Anthropocene is currently less than 70 years old, compared Human Impacts and Their Consequences with the average length of a geological epoch at something over ten million years. It is at its very beginning: human action has irrevocably changed the course of Earth history, even if we do not know exactly how that future history will unfold. How, then, should the Anthropocene be defined? One possibility is to establish, somewhere, a GSSP or golden spike for it. Perhaps this might be in the layers of sediment within a lake or an undisturbed deep sea, or even within the annual layers of ice and snow that have accumulated on a major icecap (the “golden spike” marking the beginning of the Holocene epoch has been placed within Greenland ice layers, for instance). As with more ancient geological time boundaries, considerable research would be needed to identify the best location for such a ‘spike’. The Anthropocene, though, coincides with a very precisely dated observational and historical record—constructed, of course, by humans.

Therefore, it may well be simpler and more pragmatic to simply select a numerical date of the human calendar and use that as the time boundary—that is, a GSSA. Again, in detail, there are discussions as to what might be the most appropriate date. It might be 1945, perhaps, to mark the first scattering of atom bomb-derived particles across the world, or 1950 (neatly bisecting the century, and being the ‘base date’ for radiocarbon dating). As with much else with the Anthropocene, this is work in progress. The Anthropocene represents an extraordinary, unprecedented and important phase of Earth history. It is also ferociously complicated in detail. Defining it precisely will not be easy.