When you Google "geological epoch", the one that comes up most often is the "Anthropocene".
"That tells us the thing that is most relevant to people right now is how we are impacting the planet," said Colin Waters, a geologist at the University of Leicester.
Countless words — and a Nick Cave song — have been written about the impact of humans on planet Earth since Nobel Laureate Paul Crutzen first coined the term "Anthropocene" more than two decades ago.
Yet, despite sharp rises in carbon dioxide levels, plastic pollution and extinctions in the modern era, the Anthropocene is not yet an official epoch.
"There's still no consensus amongst the scientific community that the Anthropocene should be defined at all," Professor Waters said.
The sticking point centres around finding evidence of human impact in the geological record — the layers of sediment and rocks that record how Earth has changed over time.
But a group of scientists led by Professor Waters hope to change that.
In the same week as the world's population ticked over to 8 billion people, the Anthropocene Working Group (AWG) began voting on a short list of locations around the world they believe contain evidence showing when the Anthropocene began.
Flinders Reef, off the coast of Queensland, is one of the sites in the running to be the "golden spike", the location that marks the boundary between epochs.
The group hopes to reach a decision by December 16.
Let's dig a bit deeper into what that means, what the sites tell us, and what happens next (hint: things happen slowly on a geological scale).
So, we're not in the Anthropocene?
No. We're in the Holocene epoch, which started 11,700 years ago at the end of the last glacial maximum.
The geological timescale is divided into eons, eras, periods, epochs and ages.
The Holocene, which is the second epoch in the Quaternary Period, marks a time when temperatures warmed, many large megafauna became extinct, and humans shifted from hunter-gatherers to agrarian and then urban lifestyles.
But Professor Waters said many geologists were reluctant to accept the Anthropocene.
"To them [the record] is too young, too thin, it's not like deep-time rock material, it's all gloopy sediment," he said.
Many of the deep-time epochs are marked by changes in the fossil record, while shifts in the Quaternary are seen in chemical changes in the atmosphere or sea documented in ice cores from Greenland and a cave in India.
"The accumulation rates of sediment is so slow that the Anthropocene is probably a millimetre in thickness."
So even though human civilisation has made such a significant change to the planet's biosphere in recent times, it's very challenging to detect in the rock record.
What kind of evidence could prove we're in the Anthropocene?
Defining the beginning of a new epoch all comes down to identifying rapid global change at a consistent point in time in rocks across the planet.
"We've had significant impacts on the planet, but the problem is, how do you quantify those impacts?" Professor Waters said.
"In many cases [such as extinctions], what you're seeing is slow, gradual change spreading across the planet from continent to continent.
"That's not the sort of thing that you look for to define the start of a new geological time interval."
Originally the group of scientists were planning to look for global changes kickstarted by the Industrial Revolution at the end of the 19th century.
"When we started looking at that in detail, it was quite clear that that was a very Western-centric focus," Professor Waters said.
Instead, they are now focusing on a time around 1950 known as the Great Acceleration.
Signs of that time include increasing levels of carbon isotopes from fossil fuel combustion; nitrates and pesticides from large-scale agriculture; spikes in plutonium from nuclear testing, the appearance of plastics; and the spread of marine organisms that have hitchhiked on vessels around the globe.
How do they choose the 'golden spike'?
Even though signals of human impact may be seen in multiple places around the world, the group must choose one site.
"It's a horrendous job because it's so difficult to separate [the sites]," Professor Waters said.
Once approved, the Global boundary Stratotype Section and Point, or "golden spike", becomes the definitive location of the boundary between different stages of Earth's history.
"It is comparable to the former concept of a metre measured as a platinum bar, with the definitive metre held in Paris," Professor Waters explained.
"There are plenty of tape measures showing you what a metre is, but if you really need to know very precisely, then you need to consult the original."
After trawling through the scientific research over the past two years, the group initially settled on 12 potential sites: a mix of marine sediments from undisturbed bays and remote lakes; cores from coral reefs, Antarctica, a peat bog and a cave; and post-World War II rubble filled with human-made remnants and plastic.
- Beppu Bay, Japan (marine sediment)
- Crawford Lake, Canada (lake sediment)
- East Gotland Basin, Baltic Sea (marine sediment)
- Ernesto Cave, Italy (stalagmite)
- Flinders Reef, Australia (coral core)
- Palmer Ice Sheet, Antarctica (ice core)
- San Francisco Bay, US (marine sediment)
- Searsville Reservoir, US (lake sediment)
- Sihailongwan Lake, China (lake sediment)
- Sniezka, Poland (peat bog)
- West Flower Garden, US (coral core)
- Vienna, Austria (post-WWII site)
Already three sites – San Francisco Bay, Vienna, and Ernesto Cave – have been knocked out because their records only started in the 1950s or in the case of the cave, there was up to a 20-year lag between when the carbon signal appeared in the atmosphere and when it finally found its way into the stalagmite.
That leaves nine sites left in the running to potentially become the golden spike and have a geological age named after them (for instance if Flinders Reef is chosen, it will mark the beginning of the Flinderian age).
What are the chances of Flinders Reef being chosen?
Flinders Reef is one of the remotest sites selected, said Simon Turner of University College London, and secretary of the AWG.
Corals are incredibly good at recording environmental changes like sea surface temperatures and changing water chemistry.
"They have these really exquisite tree ring-like structures as seasonal growth patterns develop," Dr Turner said.
The Flinders Reef coral also records nuclear testing in the 1950s and 60s.
"You see a really beautiful radiocarbon pulse of an affected atmosphere," he said.
But the flip side of being remote is that the radiocarbon signal appears towards the end of the 1950s, about five years later than those seen in the Northern Hemisphere.
There can also be a delay for signals such as changes in carbon to get from the atmosphere to the ocean.
The decision rests with the 23 voting members, who have to make up their minds in a secret ballot.
For a site to be selected, it has to win 60 per cent of the vote.
"That's quite a difficult thing to do," said Professor Waters, adding that the high bar also helped rule out biases and favouritism by the mainly European and North American members of the group.
What happens once a site is chosen?
Given the odds, unless there's a stand-out winner, the group of nine will be whittled down to a smaller group and a second vote might be needed.
"We can't do this again for another 10 years, it's to our benefit to pick the best site, the one most likely to get through," Professor Waters said.
Locations that are eliminated earlier will provide backup evidence to support the main site chosen to represent the "golden spike" of the Anthropocene.
But even if the working group settles on an outright winner, there's still a long road ahead.
The leading site has to be voted on by the broader geological community, firstly by the subcommittee that decides about anything to do with the Quaternary geology, then by the International Committee of Stratigraphy, which includes geologists that oversee all the other epochs.
"We have to try and convince them that this science is exactly the same as the processes they go through; it's just got a different focus."
If the chosen site passes by 60 per cent at both of those steps, it can then be ratified by the International Union of Geological Sciences. But it could fall over at any time.
"If you come up with a strange idea and it's not supported, it doesn't get through."
Professor Waters predicts a final decision about the Anthropocene will be handed down in 2024.
