Index Trace Fossils of the Anthropocene

When I visit a city, I suspect that I sometimes look quite eccentric. Although I take in the usual tourist sites, I can also often be seen with my nose close to the wall of a stone building, or crawling around as I examine some small ground-level feature. This “insane” behaviour is, of course, driven by my interest in things geological. The average modern city is chock-a-block with diverse geological features, many of them originating from other parts of the world.

So there I was in Halifax last week, spending a few minutes walking around the town’s charming old centre. Imagine my delight when I saw this section of sidewalk ahead of me, illuminated by the low-angle light of the afternoon sun:

The non-dinosaur avian footprints had presumably been made by a pigeon (or two pigeons) while the concrete was still wet. The lovely preservation of these tracks started me thinking about what might happen to them in the future. Concrete is a tremendously durable material, effectively a human-made stone. If by some chance this sidewalk happened to be preserved for millennia, perhaps buried in sediment and later exhumed, would the pigeon footprints then be trace fossils? Can a trace fossil be preserved in a man-made material, as opposed to a natural one? (FYI, a trace fossil is the preserved record of the activity of an organism)

In recent years there has been a lot of interest in the idea of an Anthropocene Epoch, the suggestion that our modern human activities are having so much impact on the planet that we have entered a new interval of geological time. The Anthropocene would be distinct from the Holocene Epoch, which encompasses the 12,000 or so years since the end of the last Ice Age.

When scientists talk about the Anthropocene concept they consider aspects such as the preservation of chemical signatures in sediment (e.g., evidence of atomic bomb testing), and the influence of human activities on sedimentation or even on earthquakes. We paleontologists tend to joke about the preservation of a sedimentary garbage layer, and about the idea of fossilized garbage.

Joking aside, how would all these human-made materials and substrates affect the definitions of “fossils” and “rocks”? Are bird tracks in concrete really trace fossils-in-progress, or would they be something else? If a Studebaker gets preserved in sandstone, would it be a body fossil or a trace fossil?

Would a fossilized bug be equivalent to a fossilized insect?

Since many organisms evolved and became extinct quickly in geological terms, their fossils can be used to date the rocks in which they occur (for example, the occurrences of various kinds of trilobites can be used to determine the relative ages of different parts of the Cambrian Period). Such fossils are often called index fossils or zone fossils. The human-related “trace fossils” of the Anthropocene might allow an unprecedented degree of stratigraphic resolution  for future scientists, given that they document a rapid diversification and evolution of human-made materials and technological products.

I just hope that they will not also document that other feature of useful index fossils: a geologically brief interval of occurrence.

© Graham Young, 2013

Just to prove that I did also look up from the sidewalk in my time in Halifax, here are a couple of “golden hour” images of the famous Halifax Town Clock.