The Anthropocene Unconformity
It has been suggested that our current time interval is different from all the times that preceded it, that human activities are dramatically affecting the Earth’s environment, atmosphere, and oceans. Geologists have long known the time since the end of the last ice age, about 12,000 years ago, as the Holocene Epoch, but about fifteen years ago it was proposed that we have passed from the Holocene into a new interval, the Anthropocene Epoch. The Anthropocene has not been accepted as a formal geological term, and its concept is still somewhat fuzzy. For instance, it does not yet have an agreed start date; it is most often considered to have begun around the start of the Industrial Revolution in the late 1700s, but other suggestions are that it began at the dawn of agriculture about 12,000 BP (in which case it would be virtually synonymous with the Holocene), or even with the first use of atomic weapons in 1945.
It is not easy to find much information about what the geological signature of the Anthropocene might be.* A lot is written about the magnitude of human effects on the modern environment, and geochemists have made many suggestions about characteristic chemical anomalies that can be seen in sediments and glacial ice cores, but how are we actually affecting geology that might be observed with the naked eye? A geological age should be readily recognizable in the sediment and/or rock record. The following are a few thoughts on things that geologists might be considering in some distant future . . .
A few weeks ago I was working in my parents’ garden in the Maritimes. It was a rainy spring day, too wet to do much digging, so I drove to the garden centre and bought a few bags of crushed rock.** Returning to the house, I spent much of the afternoon alternating between happily trundling an empty wheelbarrow down the path through the trees, and somewhat less happily pushing a barrow full of gravel upslope through the drizzle. This sort of activity is wonderful for opening the mind, and as I spread and tramped the gravel, a thought entered that empty space between my ears: I am making an unconformity.
Which, surely, I was. The hill at Fredericton is underlain by grey and brown Upper Carboniferous sandstones between 330 and 300 million years old, which are thinly clad by soil and trees many places, and which yield the abundant fieldstone that rises to the surface with every spring melt. The sandstone is an excellent material for drystone walls, but apparently it doesn’t work well as an aggregate, and the most common crushed rock in that area is quite different: fine-grained, mid-grey material that breaks into sharp-edged pieces. When we were children who invariably scraped our knees by falling on gravel paths, we referred to this rock as “slate.” I am pretty sure that much of it comes from the quarry at Springhill, just up river from Fredericton, in which case it is geologically a wacke belonging to the Burtts Corner Formation, of mid Silurian age (about 435-420 million years old).
The basic principle of superposition tells us that sedimentary strata are younger than those they overlie, since sediment is generally deposited on top of the Earth’s surface. If sedimentation was continuous (or appears to have been virtually continuous), then the contact between a stratum and that below it can be said to be conformable. If there was a break in sedimentation that can be identified as a gap in the time/rock record, then we call that an unconformity. A rock unit will be older than that beneath it only if the area was subsequently subjected to immense geological forces that resulted in folding or thrust faulting.
A Silurian rock that has been quarried, crushed, transported, bagged, and spread on a path has to be considered as a “new” geological material. All of the value-added work of the quarrying industry has acted as a reset button on the geological age of this material. It may contain microfossils of Silurian age, but the human redeposition has made it an Anthropocene (or Holocene) deposit, here lying unconformably above the Carboniferous sandstone.
If you have read this far, you are probably thinking, “So what? Why is it at all significant that you spread a few bags of gravel on a path in the rain?” Considered by itself, this isn’t very important at all, but it is a very small example of what we humans are doing all the time. We are nothing if not industrious. Much of our industry produces results that seem fabulous to us, but that will leave virtually no trace when we are gone; I challenge you to imagine how Facebook or Twitter or The Beatles will produce any geological record. But some of our works will leave evidence long after our species has departed.
Think of how many tonnes of concrete, asphalt, and glass are required to produce one city of modest size. Consider the volume of aggregate that has been quarried for harbours and airports, the amount of iron that has been incorporated into vehicles and bridge spans, the quantity of clay that has been baked into brick for city houses. As buildings and roads are demolished to make way for newer ones, the materials are buried and incorporated into the Earth. The hundreds of years through which we have behaved in this way this may seem long to us, but they are really the blink of an eye in geological terms.
The burial of these materials could be considered as a single depositional event, generating an immense human-made stratigraphic horizon that extends around our planet. They have all been re-worked and redeposited, and most lie incongruously above the natural surfaces that they hide. On floodplains or lakebeds the human deposits may rest on top of silts only decades to centuries old, but even there the dividing line is crisp, obvious, and mappable. In many other places the bricks, concrete, and asphalt lie directly on much older surfaces, such as that Carboniferous sandstone.
Considering how much of the world is now occupied by our activities, and what a proportion we have paved over in one way or another, it is not unreasonable to talk about an Anthropocene Unconformity. And when we take the city deposits, and add to them the mine waste, and the garbage dumps, and the smelting slag, and all the refuse that will eventually fall out of the Pacific trash vortex and be redeposited on the deep ocean floor, and the general debris left in places as difficult to reach as Mount Everest, then maybe we should even call it the Great Anthropocene Unconformity.***
Although the horizon composed of specifically human-made and human-modified materials would be huge, it would also be patchy. To really understand the scale of the Anthropocene Unconformity, we will need to consider secondary factors that also affect the geological record. In various regions, the erosion due to farming and logging may result in considerable sediment being redeposited in places where there wasn’t sediment before. In some places, fluid injection into deep wells is causing earthquakes to become common where they were previously rare or nonexistent, so we might expect to see an increase in earthquake induced deposits such as landslides and seismites.
But the really big player in the Anthropocene depositional story will probably be sea level rise. Various estimates suggest that global sea level will rise by 20 centimetres to two metres by the year 2100, and possibly by 4-6 metres in the coming centuries. It has been calculated that 26,000 square kilometres of land would be inundated by a sea level rise of just 66 centimetres, so clearly the loss of land would be immense if a rise of several metres takes place.
In the sediment and rock record, what is the signature of a sea level rise? There is considerable variation, of course, but the stratigraphic record of sea level rise (or transgression) is generally relatively abrupt, with land surfaces or shallow-water deposits being quite sharply replaced by sediment deposited in substantially deeper water. A marine flooding surface, evidence of the sea moving across a formerly eroded subaerial surface, is a classic form of unconformity. Future scientists looking at the sedimentary evidence of the third millennium sea level rise would expect to see river valleys filled with marine sediment, coastal sand dunes replaced by shallow shelf deposits, and drowned reefs and barrier islands covered with deeper water sediments.
Locations currently occupied by low-lying coastal cities such as Miami will be the best places for field studies of the Anthropocene. In Miami, a huge mass of concrete, glass, and other debris, mixed with sediment resulting from human-caused erosion, will rest almost directly on top of porous limestone dating back to the Pleistocene Epoch. The top of these anthropogenic materials will form a flooding surface, overlain by subsequent marine deposits. The completeness of this record might permit the suggestion that Miami Beach could make a very good type locality for the base of the Anthropocene.
For any geologists considering Earth in the distant future, the Anthropocene Unconformity may well be some of the best evidence of our presence here: a sharp and nearly horizontal base to the varied mass of Anthropocene deposits that overlie it, recognizable in many places around the planet. Thinking about this a bit more, I just hope that the top of the Anthropocene deposits is not also defined by a crisp surface that can be traced globally. It would be particularly bad if that upper contact is marked by an ash horizon, a boundary clay, or an anomaly rich in Caesium-135.
* The great thing about publishing a blog post from a position of some ignorance is that readers will point you toward what you should have known before you published the post. My friend Roy Plotnick suggested I should look at this interesting paper:
Zalasiewicz, J., Waters, C. N., and Williams, M., 2014, Human bioturbation, and the subterranean landscape of the Anthropocene: Anthropocene, v. 6, no. 0, p. 3-9.
This is in a volume titled “Landscapes in the Anthropocene: State of the art and future directions”, which contains quite a few worthwhile publications on related topics.
** Here in Manitoba we would have called it “quarter down”, as it consists of particles of mixed size that are smaller than about 1/4 inch across. In New Brunswick, I think it was referred to as “crusher dust”. It is fascinating how variable some of these terms are, even within Canada.
*** I have searched the Internet, and it seems that the term Anthropocene Unconformity has only been used in a local sense, and Great Anthropocene Unconformity has never been used before. It seems like a very useful concept, and I would be interested to know if there is some comparable term already out there.
© Graham Young, 2015