Plastic rocks, plutonium, and chicken bones: the markers we're laying down in deep time

• Not on our human-scale time, but deep time: the almost unimaginable span of billions of years which have already come and gone.
• Let’s say you’re in the far future and you’re looking for evidence of previous civilisations.
• Here are five of the markers we’re leaving for the future.

What markers are we laying down in rock?

• Everyone is familiar with periods such as the Jurassic.
• Usually, a change in the global environment so large it leaves permanent evidence visible in the rock layers.
• So to declare that we’re in a new geological epoch – and that we’ve left the balmy post-ice age Holocene behind – requires finding evidence of unmistakably clear markers.

1. Plastics and plastic rocks

• Finding plastics in a sediment or rock layer is a clear sign that the layer dates from modern times.
• There are also plastiglomerates, the mutant offspring of plastics and rock.

2. Concrete

Concrete is now the most abundant human-made “rock” on the planet’s surface. Future archaeologists could dig down through mud and detritus to identify when widescale use of concrete first became obvious. This would tell them they’d struck the 20th century. Concrete, of course, has been used for millennia – ancient Roman concrete is still standing in some places. But it didn’t become ubiquitous until recently.

3. Chicken bones

• Humans like chicken.
• As of 2018, we were eating about 65 billion of these birds a year.
• But why would chicken bones be a telltale sign we were here?

4. Plutonium and nuclear residue

• Nuclear testing began in the 1940s and accelerated through the 1950s and 60s before being phased out.
• The amount of plutonium spread by testing has left a clear spike, like a fingerprint, in the environment.
• Even now, we can identify samples from the 1950s and 1960s by the presence of plutonium and other radionuclides.

5. Fossil fuels and climate change

• We’ve been digging up and burning fossil fuels for a long time.
• The carbon (CO2) pollution from burning the fuels will also eventually be recorded in rock.

Markers upon markers

• There are many more markers, from sudden shifts in distribution of animal species, soil erosion and pollution, to refined metals, to looming mass extinctions of species.
• Will these markers be recognisable long term?
• And – as some geologists argue – can we even say this is a distinct epoch, given it’s only just begun in geological terms?