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Current time:0:00Total duration:4:15

In the last video, we
talked about the idea that if I dug up a bone
someplace, if I dug up a bone, and if I were to
measure its carbon-14, and I found that it had half
of the carbon-14 that I would expect to find in a living
animal or plant, that I said, hey, maybe one half life
has gone by, or roughly for carbon-14, one half
life is 5,730 years. So I said maybe it's 5,730
years since this bone was part of a living animal,
or it's roughly that old. Now, when I did that, I made
a pretty big assumption, and some you all have touched on
this in the comments on YouTube on the last video,
is how do I know that this estimate I made
is based on the assumption that the amount of
carbon-14 in the atmosphere would have been roughly constant
from when this bone was living to now? And so the question is,
is the amount of carbon-14 in the atmosphere
and in the water, and in living plants and
animals, is it constant? And if it isn't constant, how do
you calibrate your measurement so you can actually figure
out how much carbon-14 there is relative to living plants
and animals at that time? And the way that you can
make that calibration, because it turns out it
isn't perfectly constant, the way that you can make that
calibration, there's two ways, and I have pictures
here of both of them, one is to look at tree things. Tree rings. And I'm told this will work
up to about 10,000 years. Up to 10,000 years old. I don't know of any
10,000 year old trees, I don't think anyone
does, but maybe there's some remains of old trees. And you can look at their tree
rings, and I think most of us are familiar with this idea that
every year that a tree grows, it forms another layer of bark. And so you can look back
to that layer of bark just for the half
life of carbon-14, and then figure out how
much carbon-14 was there in the atmosphere at
that period in time. And so it's kind of a
record of the atmosphere up to 10,000 years. If you want to go
even further back, you can look at cave
deposits, and the fancy word for these cave deposits
are speleothems. Speleothems. Speleothems. You might be familiar
with stalagmites. Those are those speleothems
that are kind of coming out of the bottom of the
cave, or stalactites. Those are the
speleothems that are coming from the top of the cave. But the reason why
these are useful is these are formed
by calcium carbonate, so they have carbon in them,
and slowly over, really, tens of thousands of years,
the water in the cave deposits that calcium carbonate. So it's a record of the fraction
of carbon-14 in some of those years. And you can go down to
resolutions of as small as 10 years. And so this will give
us pretty good estimates over tens of thousands of
years, up to 50,000 years. And frankly, carbon-14
isn't even useful beyond, really, 50,000
or 60,000 years. So this gives us a good
record of carbon-14 in the atmosphere,
assuming that it's fairly uniform throughout
the atmosphere, and all evidence suggests
that, and that that uniformity through the atmosphere also
goes into the water supply, and into living
plants and animals. Now, the other thing, and I
looked into this a little bit, it actually turns out because we
are spewing so much fossil fuel right now, we are
changing the amount, or the proportion of carbon-14
much, much faster than has happened in other time periods. So just to answer
the question, it's actually probably in
really, the last 50 years where the fossil
fuel use has really exploded that we've
really been changing the proportion of
carbon-14 relative to the other isotopes of carbon. But anyway, hopefully
that rests some of your worries
about the assumption that I made in the last
video about carbon-14 being relatively constant. There are ways to look
back at specific years and figure out the relative
amounts of carbon-14, so it is a pretty
good way of estimating how old living things
are, especially things that are less
than 50,000 years old.

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