Extract from ABC News
A small lake near the Canada-US border has been chosen as the site that may define the start of a new geological epoch called the Anthropocene.
Key points:
- Some scientists believe Earth has entered a new epoch called the Anthropocene
- Defining the beginning of a new epoch requires identifying rapid change in the global geological record
- An international panel voted Crawford Lake in Canada as the primary location to define the start of the Anthropocene
The spot — named Crawford Lake — was picked from a list of a dozen contenders by a panel called the Anthropocene Working Group, who announced their decision at the International Congress on Stratigraphy in Lille overnight.
The vote is the culmination of a three-year project to find the most suitable site, Colin Waters, chair of the working group, said.
It's also one step closer to confirming that Earth has entered a new epoch — one triggered by burning fossil fuels, nuclear bomb tests and other human activity.
The current epoch, the Holocene, started roughly 11,700 years ago. As seen in climate signatures from cores drilled from a Greenland icesheet, it's when the planet started warming after the relatively chilly Pleistocene.
The idea that humans have forced changes to the planet so substantive that we've shifted into a new epoch was first popularised by the late Dutch chemist and Nobel Laureate Paul Crutzen in 2000.
He proposed the term "Anthropocene" — derived from the Greek "anthropo" for "man" and "cene" for "new" — at a conference.
The reaction, Jürgen Renn from the Max Planck Institute of Geoanthroplogy in Germany said, was "very intense".
"It pointed to these global impacts of humanity ... it's not just about climate change. It's not just biodiversity loss. It's not just ... that humans are moving. It's all of this together."
How a golden spike nails down a new epoch
Professor Crutzen suggested the Anthropocene began with the Industrial Revolution.
But, Dr Waters said, while natural climate archives such as sediments deposited in some lakes do show hints of the industrialisation of the time, those signs are not consistently seen around the globe.
"If you're based in Northern Europe, if you're based in the UK, yes, you do see the first evidence of the Industrial Revolution happening early in the 1800s.
"But if you're based in large parts of Asia, Australasia, the southern hemisphere … the sediments just do not show any significant effect from the Industrial Revolution."
This is why the Anthropocene Working Group, which formed in 2009, decided the new epoch would be defined as starting in the mid-20th century.
Post-World War II, mining, fossil fuel burning, trade and pollution ramped up — a time US historian John McNeill called the "Great Acceleration" — and this left its mark in the geological record around the world.
The working group needed a site on Earth that clearly showed traces of the Great Acceleration and could provide a primary marker for the new geological epoch.
Such sites are also known as "golden spikes", after the physical objects sometimes driven into the rock at those locations.
The group came up with a dozen potential golden spike sites, including one on Flinders Reef in Australia. And after three rounds of voting, the Anthropocene Working Group finally decided on Canada's Crawford Lake as its preferred next golden spike.
A lake small in size, rich in records
From above, Crawford Lake, just 60 kilometres west of Toronto, looks entirely unremarkable.
It formed in a collapsed limestone cave, and is quite small, only 250 metres long and 125m at its widest point. It's ringed by reeds, cedar forest and a walking track.
But below the lake's placid surface lies a rare archive. Cores of sediment drilled from the bottom of the lake provide a clear record of human activity down to individual years.
This high-resolution record-keeping happens for a few reasons, according to Francine McCarthy of Brock University and lead scientist for Crawford Lake.
Stuff in the air, such as fly ash from coal combustion and pollen from the surrounding trees, constantly falls into the lake and gently sinks and settles on the bottom.
Also in the water are ions of calcium and carbonate, weathered from the surrounding limestone.
"When the water gets warm enough, the calcium and carbonate ions … crystallise into little crystals of calcite that … fall slowly through the water column and they form, each summer, a white layer [on the lake bed]," Professor McCarthy said.
The lake is also fairly deep — around 24m — and water at the bottom of the lake doesn't mix with the water near the top, so what falls to the lake bed stays on the lake bed.
As time wears on, white summer calcite layers alternate with the brownish matter that's blown in, building up a geological record with the most recent year on top.
So when scientists pull out a core of sediment, they can pinpoint precise years in which layers were laid down simply by counting the brown and white stripes.
When Professor McCarthy's team at Brock University and independent teams at other laboratories analysed the sediments, they saw distinct spikes in plutonium and carbon isotopes in layers that formed in the early 1950s — telltale signs of nuclear testing fallout.
They also saw increases in the amount of fly ash, as well as a sharp decline in elm pollen, coinciding with the arrival of the deadly Dutch elm disease in the region in the late 1940s.
Even the calcite layers were affected. Acid rain that fell in the area during the Great Acceleration prevented as much calcite from forming, so the white layers around 1950 are relatively thin.
These signatures of human activity aren't the only ones to be found in Crawford Lake sediments, Professor McCarthy said.
Corn, bean and squash pollen grains hint at the presence of Iroquoian-language peoples, who lived by the lake from the early 13th to 15th centuries.
But, unlike the human activity evident in the layers from 1950, those changes are not nearly enough to signal the end of the Holocene.
Why lakes are such excellent record-keepers
Of the 12 potential golden spike sites, three were lakes, while others included an ice shelf in Antarctica, reefs in the Coral Sea and Gulf of Mexico and a Polish peatland.
All sites showed the distinct fingerprints of human activity around 1950, revealing changes in the environment in different ways.
For instance, anchovy scales are found all through sediment cores extracted from Beppu Bay in Japan.
Scientists discovered a shift in carbon and nitrogen isotopes in anchovy scales from sediment layers that formed after 1952, the result of increased atmospheric carbon dioxide and nitrogen fertilisers that ran into the ocean.
Lakes, in particular, are good at recording changes in the land environment, said James Cook University's Emma Rehn, who analyses charcoal in lake cores extracted from northern Australia.
For the kind of high-resolution records needed to analyse recent years, it helps if a lake deposits sediment quickly, and doesn't have any rivers running into or out of them.
"Plus you also want them to be wet all the time, because if they seasonally dry out, the stuff at the bottom is more likely to break down, and it's not going to be as well preserved," Dr Rehn said.
"So if it stays wet all the time, and nothing's pushing it around, a lake can just happily sit there and build up in layers over time, ready for us to come back and have a look down through the layers later."
What happens now?
The Anthropocene Working Group's vote for the golden spike site doesn't mean that the Anthropocene has been officially recognised as a new epoch.
Evidence from Crawford Lake and other locations will be assessed and voted on in three more rounds by various scientific groups.
The International Commission on Stratigraphy will give the final thumbs up or down. Dr Waters expects the process will be completed in time for the International Geological Congress in Busan, Korea in August 2024.
But not everyone agrees that we have the evidence needed to definitively say that we've shifted into a new epoch.
Some people don't believe 70-odd years is long enough to be sure that human activity has changed the planet for the very long-term, and it's not just a geological "blip".
In that case, it may be decided next year that our current epoch remains the Holocene, but we enter a new age, one named after the newest golden spike — the Crawfordian Age.
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