For a long time, the kilogram was a  metal cylinder in a safe in France.

Nobody really used it to weigh  out day-to-day stuff, of course. But for scientists, it was  still a useful thing to have.

That’s because, in science, we like to  have reference points; a particular thing, like a metal cylinder, that can represent  something more abstract like “mass”. And this applies to geology, in which  case, you’re probably not dealing with questions of mass as much as time. But how do you find a physical  reference point for time?

For geologists studying the modern era,  the physical reference point for time may be a quaint little lake in Canada… one that you can probably  walk around in 15 minutes. Here’s why one little lake  might define the human age. [♪ INTRO] This is Crawford Lake. It’s a small  lake, about 250 meters at its longest.

A good swimmer could probably  cross it in about 5 minutes. It’s in a conservation area and  UNESCO biosphere reserve about 50 km southwest of Toronto, surrounded by cedar trees, reconstructed indigenous  longhouses, and a boardwalk. Also, their website says they have  maple syrup tastings there?

Canada, man. Crawford sits in a natural sinkhole formed  from the area’s soft limestone bedrock. And while the lake is small,  it’s fairly deep, reaching down about 24 meters at the deepest part.

In July, 2023, scientists from  a working group within the International Union of Geological  Sciences, which helps coordinate geologists around the world, proposed  that the bottom of this lake should serve as the marker for the  start of the Anthropocene epoch. So geologists like to divide the history  of the Earth into different chapters. You’ve probably heard of the Jurassic  or Cretaceous periods, for instance.

An epoch is one of those types of  chapters, and the Anthropocene is a proposed new epoch characterized by  humanity’s effects on the planet. The idea is that we have changed  fundamental properties of the Earth and its systems so much, with our plastics  and nuclear tests and fossil fuels, that it’s enough to say this is a  new phase of history for the planet. Not everyone agrees that we  need a new epoch for that, but the proposal has been gaining steam.

And one of the things we’d like to find in  order to really use this new slice of time in a proper, scientific sense is one  of those physical reference points. There are others out there already. For the end of the Cretaceous, for  instance, we use a layer of iridium in Tunisia deposited during the impact  event that killed the dinosaurs.

These reference points are useful  because, well, it’s a reference point. If two different paleontologists get  to arguing about whether a T. rex is 70 million years old or 75 million years  old, they can always go back to that place in Tunisia as a starting point for  where to start counting backwards from. But there’s been disagreement about  where to put this reference point for the Anthropocene.

Ice sheets? Coral reefs? The fact that those are disappearing  is… part of the whole problem.

So what’s so great about Crawford Lake?  Like an ogre or an onion, it’s got layers. In the bottom of the lake, there’s mud that  builds up, little by little, over the years. This by itself isn’t unique, that’s what mud does, but this lake’s got a combination  of interesting things about it.

Firstly, Crawford’s water doesn’t mix. Because of how deep and narrow the  lake is, the bottom layer of lake water doesn’t mix with the water above. There’s no wind powerful  enough to churn things up.

There also aren’t a lot of critters  living at the bottom of the lake. This means there’s not a lot to  mix around the water or the mud. Each layer of sediment stays  pretty much exactly where it falls.

Secondly, the chemistry of this lake  does something funny each summer. Because it’s not just dead leaves and goose  poop that fall to the bottom of the lake. Each summer, something else does too: white, chalky crystals of calcium carbonate.

Remember how I said that the  lake sits in a bed of limestone? That’s the stuff limestone is made of,  and it’s slightly soluble in water. Which means the lake always has a little  bit of that mineral dissolved in it.

But under certain conditions triggered  by warm summer weather and algae growth, the chemistry of the lake changes just  enough that the dissolved calcium carbonate falls out of solution and drifts  to the bottom as little crystals. What you end up with is clear  lines of brown mud from the fall, winter, and spring separated by  a chalky white line each summer. These layers build up like tree rings,  creating a continuous and regular rhythm, making it super easy to tell one  year’s sediments from another.

Finally, oxygen is present at the lake bed. Even though the bottom layer of water  doesn’t mix with the one on top of it, new water does trickle in through the  limestone rock bed, bringing oxygen with it. Oxygen can often be destructive, but  in this case, it’s incredibly helpful.

We’ll see why in a second. And this, the scientists say, makes for kind of the perfect marker for the  beginning of the Anthropocene. It’s a nearly pristine record of recent centuries.

Scientists have been able to use plant  pollen trapped in those layers to see things like indigenous groups coming to the area  and growing corn there for the first time. Other analyses have shown that  we can also see evidence of more modern human activity via increased coal  and oil burning in the area in the 1950’s. This is thanks to subtle changes  in nitrogen isotopes in each layer, as well as the presence of  something called fly ash, which is produced when fossil fuels are burned.

But what’s especially key is plutonium. Plutonium is an almost entirely human-made element and had been incredibly rare in nature  before we started messing with it. It’s sometimes found in  incredibly tiny quantities, like one part in a hundred  billion, in uranium ore.

But humans started to purposefully  synthesize it in labs in the 1940’s, and nuclear weapons detonations  starting in the 40’s and 50’s spread small amounts of it far and wide. This spike can now be found all around the world, and is often what people point  to as a start to the modern age. In Crawford, that signal is especially crisp.

Microbes in oxygen-free environments  seem to be able to dissolve and kind of spread around plutonium,  making that spike a lot muddier. But that doesn’t happen in  Crawford’s oxygen-rich waters. That’s why the bottom of the lake  having oxygen is so interesting.

So in the end, you have a lake that  collects layers of mud each year, effectively sampling the world around it, only to cap off each year with a chalky white cap. And whether it’s due to the lack of  critters or oxygen at the bottom, the signals from each layer  remain crisp and clear. All this makes it as good of a  reference point as you could ask for.

As for why the Anthropocene is worth delineating, we humans are changing the  world incredibly quickly, to the point where we may need a  scientific way to differentiate the modern age from everything that came before. Doing this allows us to think about  and ask about things in a better way. As of this video being written, Crawford  isn’t yet officially the reference point.

One group has picked it out, but there needs to be a bit more discussion among scientists at large. But either way, by diving into this unique lake, we can see how a time and a  place are sometimes interlinked, whether in our own memories or  in the physical world itself. And thank you for diving  into this episode with us.

It was made possible by our  generous, and may I say, extremely cool supporters on Patreon. We’ve got some neat perks to say thank you, like monthly bloopers, behind-the-scenes  peeks, and even livestreams.   If you’d like to get involved, you can  get started at patreon.com/scishow. [♪ OUTRO]