Mark Williams : “Cities will leave a clear and globally widespread geological record of their impact that will persist for millions of years”.

By Lucas Tiphine

Mark Williams is Professor of Palaeobiology at the University of Leceister and a member of the Anthropocene Working Group of the International Commission on Stratigraphy (ICS). In this short interview he explains how cities can be viewed from the perspective of an Anthropocene geological approach.

How did cities enter into your research on the Anthropocene?

In the early 21st century Homo sapiens (our species of human) has become a predominantly urban species for the first time in 300,000 years. There are now hundreds of cities with more than a million people and more than 30, like São Paulo, Lagos and Jakarta, exceed 10 million. By the end of the 21st century some cities may grow to perhaps 80 or 90 million inhabitants. Though occupying a small component of the land surface, urban ecologies consume vast energy, and uniquely extract their materials from a hinterland that is global. This is a big contrast to natural terrestrial ecosystems, which are grounded in the energy of sunlight, and source materials provided by their immediate environment. Because the reach of cities is so great, their patterns of consumption threaten the stability of Earth’s co-evolving systems — the atmosphere, lithosphere, biosphere and hydrosphere, and as a result, cities may become a major disruptive phenomenon.

Chart Source: Williams, M., Edgeworth, M., Zalasiewicz, J. et al. 2019. “Underground metro systems: a sustainable geological proxy of rapid urban population growth and energy consumption during the Anthropocene. In Benjamin, C. et al (eds), The Routledge Companion to Big History. Routledge (p.435). Data compiled by Will Steffen from sources indicated in the chart reproduction and referring to the bibliography of the article by Williams, M., Edgeworth, M., Zalasiewicz, J. et al. 2019 (supra).

Cities will leave a clear and globally widespread geological record of their impact that will persist for millions of years into the future. The structures of cities above ground, everything from skyscrapers to rail stations will be eroded over time, turned into sediment to be deposited in rivers, lakes and seas, just as geological processes have eroded great mountain chains for millions of years. Fragments of these above-ground structures will survive, for example monuments and monumental buildings hewn from resistant stones like granite or marble. So, a future geologist might be lucky enough to find a fragment of Michelangelo’s David preserved in a sedimentary rock.

But cities also build vast structures below ground, such as drains, sewers and metro systems. Deep metro systems, for example, penetrate pre-existing geology, and as a result, are likely to preserve much longer into the future than the above-ground structures of cities. Metro systems have spread to all continents over the past 160 years, since their inception in London, so they provide a preservable global signal of a period of rapid urban growth (and hyper-consumption) that began in the 19th century and accelerated in the second half of the 20th century.

Cutaway of London’s five levels of traffic at Charing Cross Station (now Embankment Station), taken from the Popular Science Magazine, January 1921, pp. 44–45, drawing by S.W. Clatworthy. The District Railway (just below ground) represents an example of a cut and cover construction, whilst the Hampstead and Bakerloo railways are deep tunnels. In this way metro systems like that in London demonstrate the evolution of behaviour within a single trace fossil complex, evolving from ‘at surface’ burrows to deep burrows over time.

More specifically, how does the Anthropocene — and cities — redefine the way we can think about fossils?

The Anthropocene is characterised by the rapid spread of thousands of species beyond their native range as a result of human intervention, sometimes deliberate, but often indeliberate. Some of these species, like the Giant African snail or Zebra mussel damage the ecologies they invade. And because they have hard shells, they leave a clear fossil record that is often also associated with the extirpation of indigenous organisms. From a purely technical perspective the way we study and examine body fossils (those that preserve something of the original body of the plant or animal, like the shell) is no different in the Anthropocene from the way we would use fossil molluscs or arthropods to identify Ordovician or Jurassic strata.

Where there is a greater distinction between deep time palaeontology and the Anthropocene, is with regard to trace fossils. These are traces of organism activity (rather than the organism itself), and they include things like footprints, trails and burrows. We can find trace fossils preserved in rocks from hundreds of millions of years ago (they are amongst the first evidence we have for the evolution of animals, and they are good evidence for how Jurassic and Cretaceous dinosaurs walked on the land). But there is a scale change both in size and complexity with regard to the trace fossils left by humans. For example, a metro system beneath a city can be considered as a giant burrow system. That does not preclude an archaeologist, engineer or historian examining the metro system from their own disciplinary perspectives. But equally these are geological structures, giant trace fossils made by humans.

The relationships of metros with surface anthroturbation, the archaeosphere and deep geology. Metros are part of the set of human trace fossils (including mines and quarries) that, uniquely for the biosphere, penetrate deep into the subsurface geology. Note that the above ground city, the archaeosphere, and the metro tunnels are not drawn to scale (chart source : Williams, M., Edgeworth, M., Zalasiewicz, J. et al. 2019. “Underground metro systems: a durable geological proxy of rapid urban population growth and energy consumption during the Anthropocene”. In Benjamin, C. et al. (eds), The Routledge Companion to Big History. Routledge, p. 437.

When is the next deadline for official recognition of the Anthropocene by the International Commission on Stratigraphy? And in your opinion, what are the main scientific blocks that could still prevent this process from coming to a positive end at this time?

There isn’t a fixed deadline as such, and the Anthropocene Working Group is gathering information in a careful and considered way. But we hope to have assimilated the major datasets by 2022. We have summarized a huge body of evidence in the 2019 Cambridge University Press volume ‘The Anthropocene as a geological time unit’. We are now focused on key sites around the world where a physical record of the Anthropocene can be established. An example of the kind of places we are investigating is San Francisco Bay in California. Here the ecosystem has been heavily disrupted by human activities for over 100 years, most notably by a succession of introduced species, many of which have become highly invasive. Some, such as the Amur River Clam (which comes from East Asia), have left a fossil record in the bay, and because we know when they were introduced, we can use these clams to date the accumulation of sediments. In this way we may be able to identify an Anthropocene horizon. This work — trying to establish the geological level where we can identify the Anthropocene — is a global effort.

References:

Williams, M., Edgeworth, M., Zalasiewicz, J. et al. 2019. “Underground metro systems: a durable geological proxy of rapid urban population growth and energy consumption during the Anthropocene”. In Benjamin, C. et al. (eds), The Routledge Companion to Big History. Routledge.

Zalasiewicz, J., Waters, C., Williams, M., & Summerhayes, C. (Eds.). (2019). The Anthropocene as a Geological Time Unit: A Guide to the Scientific Evidence and Current Debate. Cambridge: Cambridge University Press.