4 Ways to Date an Archaeological Site
Summary
TLDRThis script explores various scientific dating methods, starting with carbon dating's limitations and introducing alternative techniques like potassium-argon and argon-argon dating. It also covers less destructive methods such as electron spin resonance (ESR) and the stratigraphic approach to relative dating. The video explains how each method works and their applications, from dating Egyptian artifacts to determining the age of moon rocks and prehistoric cave paintings.
Takeaways
- 🕰️ Carbon dating is a method to estimate the age of once-living things by measuring the remaining radioactive carbon-14, but it's only reliable for objects up to about 50,000 years old.
- 🔬 Radiometric dating includes techniques that measure the ratio of isotopes of different elements, like carbon-14 to carbon-12 for carbon dating, to determine the age of various materials.
- 🗻 Potassium-argon dating is a common method for dating rocks, based on the decay of potassium-40 into argon-40, but it has assumptions that can be affected by geological processes.
- 🔄 Argon-argon dating is an updated method that reduces uncertainties by comparing argon-40 to argon-39, which is produced by irradiating the rock sample in a nuclear reactor.
- 🌕 Argon-argon dating has been used to date moon rocks and is more accurate than potassium-argon dating, as it allows simultaneous measurement of both argon isotopes and checks for initial argon or loss over time.
- 🦷 Electron spin resonance (ESR) is a less invasive method that uses magnetic fields and microwaves to measure the number of trapped electrons in materials like teeth, seashells, and stalactites, providing dates from 50,000 to millions of years ago.
- 📚 Stratigraphy is a relative dating method that uses the order of layers (strata) to infer the age of fossils or artifacts, assuming that lower layers are older than those above.
- 🎨 Seriation is a dating method that involves comparing the style of artifacts within the same culture and geographical region to determine a chronological order, though it doesn't provide exact dates.
- 🏺 The script mentions that different dating methods have their own strengths and weaknesses, and scientists must choose the appropriate technique based on the material and context of the sample.
- 🔍 Scientists have to be creative with the evidence available to them, as dating methods are limited by the nature of the sample and the technology at their disposal.
- 🌳 The script concludes by emphasizing the importance of these dating techniques in bringing us closer to understanding the past, despite the limitations of current technology.
Q & A
What is the scientific method proposed by Willard Libby in 1946 for dating objects?
-Willard Libby proposed a method known as carbon dating in 1946, which involves measuring the amount of radioactive carbon (carbon-14) left in an object to estimate its age.
Why is carbon dating not reliable for objects older than about 50,000 years?
-Carbon dating is not reliable for objects older than 50,000 years because the half-life of carbon-14 is approximately 5,730 years, and after several half-lives, there is too little carbon-14 left to measure accurately.
What are radiometric dating techniques and how do they work?
-Radiometric dating techniques are methods that rely on measuring the ratio of abundance of two different isotopes of an element to determine the age of an object. They work by tracking the decay of a radioactive isotope into a stable one over time.
How does potassium-argon dating work and what are its limitations?
-Potassium-argon dating involves measuring the ratio of radioactive potassium-40 to stable argon-40. Its limitations include the assumption that no extra argon-40 was incorporated into the rock when it formed and that none of the argon formed had a chance to escape the rock.
What is the argon-argon dating method and how does it improve upon potassium-argon dating?
-Argon-argon dating is a method that compares argon-40 produced from potassium decay against a lighter isotope of argon, argon-39. It improves upon potassium-argon dating by reducing uncertainties, allowing for simultaneous measurement of both argon isotopes, and providing a way to check for initial argon or loss over time.
How does electron spin resonance (ESR) dating work and what materials is it suitable for?
-ESR dating works by measuring the trapped electrons in the crystalline structure of minerals, which align in a magnetic field and absorb microwave energy at a specific frequency. It is suitable for dating materials like teeth, seashells, and stalactites, and is best for dating minerals from 50,000 to 800,000 years ago.
What is stratigraphy and how is it used for relative dating?
-Stratigraphy is a method of relative dating that relies on the principle of superposition, which states that in undisturbed layers of sediment, the lower layers are older than the layers above. It is used to determine the relative ages of fossils and artifacts found within these layers.
How can the style of an artifact be used to date it, and what are the limitations of this method?
-The style of an artifact can be used to date it by comparing it to known styles from different time periods, a method known as seriation. However, it has limitations such as the inability to determine the exact age, the need to compare similar types of objects, and the potential influence of external cultural styles.
What is the significance of the argon-argon dating method in studying moon rocks and asteroids?
-The argon-argon dating method is significant for studying moon rocks and asteroids because it requires a smaller sample, making it ideal for extraterrestrial materials where obtaining additional samples is not possible.
How did scientists determine the age of Australian Aboriginal rock art using stratigraphy?
-Scientists determined the age of Australian Aboriginal rock art by dating wasp nests, specifically bits of charcoal incorporated into the nest. By comparing nests buried beneath painted rock walls to those not covered, they could establish an age range for the art.
What are the benefits of using a combination of dating methods to study historical objects?
-Using a combination of dating methods allows researchers to cross-verify the age of objects, account for potential inaccuracies or limitations in individual methods, and gain a more comprehensive understanding of the history and chronology of the objects being studied.
Outlines
🔬 Radiometric Dating Techniques
This paragraph introduces the concept of radiometric dating, a scientific method for determining the age of objects by measuring the ratio of radioactive isotopes to their stable counterparts. The focus is on carbon-14 dating, which is effective for objects up to 50,000 years old but has limitations. It also mentions the broader application of radiometric dating using different elements like potassium in argon-argon dating. The paragraph discusses the assumptions and potential inaccuracies of these methods, and introduces an improved technique that involves measuring both argon isotopes to account for uncertainties. This method has been instrumental in dating the age of moon rocks and determining the age of rocks from the Apollo 11 mission.
🧲 Electron Spin Resonance and Relative Dating
The second paragraph delves into the less invasive dating method known as Electron Spin Resonance (ESR), which is suitable for smaller objects like human teeth. It explains how ESR uses magnetic fields and microwaves to measure the number of trapped electrons, which correlates with the age of the object. The method is effective for dating objects between 50,000 to 800,000 years old and can extend to millions of years under ideal conditions. The paragraph also introduces relative dating through stratigraphy, which involves understanding the order of geological or archaeological layers to deduce the age of objects within them. Stratigraphy is applied in various fields, including dating rock art and determining the age of artifacts based on their style and cultural context.
🎨 Seriation and Dating Prehistoric Art
The final paragraph discusses the method of seriation, particularly its use in dating prehistoric art such as cave paintings. Seriation relies on the gradual evolution of cultural styles and the comparison of similar objects to establish a chronological order. This method is used to date artworks by analyzing the motifs, materials, and representational styles that change over time. The paragraph highlights the use of seriation in dating cave paintings, where the presence of extinct animals like mammoths can indicate the artwork's age relative to the last ice age. The paragraph concludes by emphasizing the creativity and importance of these dating methods in bringing us closer to understanding the past.
Mindmap
Keywords
💡Carbon Dating
💡Radiometric Dating
💡Isotopes
💡Potassium-Argon Dating
💡Argon-Argon Dating
💡Electron Spin Resonance (ESR)
💡Stratigraphy
💡Relative Dating
💡Style Seriation
💡Radioactive Decay
💡Nuclear Reactor
Highlights
In 1946, chemist Willard Libby introduced carbon dating, a method to estimate the age of dead organic material by measuring remaining radioactive carbon.
Carbon dating is limited to objects younger than approximately 50,000 years and requires the presence of carbon in the object.
Radiometric dating techniques measure the ratio of abundance of two different isotopes to determine the age of an object.
Argon-argon dating is an updated method that compares argon-40, produced from potassium decay, against argon-39, providing a more accurate age estimation.
Argon-argon dating involves converting potassium-39 into argon-39 in a nuclear reactor and then measuring the argon isotopes released from the rock.
This method allows for simultaneous measurement of argon isotopes and can check for initial argon presence or loss over time.
Argon-argon dating is less invasive and requires a smaller sample size, making it suitable for unique or rare samples like moon rocks.
Electron Spin Resonance (ESR) uses magnetic fields and microwaves to measure the number of trapped electrons in a sample, indicating its age.
ESR is applicable for dating small objects like teeth, seashells, and stalactites, and can date materials up to millions of years old.
Stratigraphy is a relative dating method that uses the order of sediment layers to determine the age of fossils or artifacts.
Stratigraphy can provide an age range for fossils found within specific layers and can infer the age of unknown species based on known ones.
Style-based dating, or seriation, compares the stylistic elements of artifacts to place them in chronological order within a culture.
Seriation is useful for dating prehistoric art, such as cave paintings, by analyzing the evolution of depicted subjects and artistic techniques.
Argon-argon dating helped determine that rocks from the Apollo 11 mission are nearly four billion years old.
Scientists use a variety of dating methods to study the past, each with its own strengths and limitations.
The creativity of scientists in developing dating methods allows us to better understand and bring us closer to the past.
Transcripts
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in 1946 chemist Willard Libby proposed a
new way to scientifically estimate how
old a dead thing is by measuring the
amount of radioactive carbon left in it
we've used this method to date a bunch
of stuff from the wood of an Egyptian
pharaohs boat to fossilized human poop
but carbon dating is only reliable for
objects younger than 50 thousand years
or so and what if the thing you want to
date never had any carbon to begin with
what then here are four clever ways
scientists figure out how old stuff is
carbon dating belongs to a larger group
of techniques known as radiometric
dating these methods rely on measuring
the ratio of abundance of two different
atoms
sometimes these atoms are two different
elements other times it's the same
element with different numbers of
neutrons called isotopes for carbon
dating it's the ratio of radioactive
carbon-14 a carbon with eight neutrons
and stable carbon-12 a carbon with six
neutrons because the rate at which the
radioactive isotope decays is consistent
over time scientists can calculate how
long ago a living thing died by
measuring the ratio of carbon 14 to
carbon 12
at least until too much of the carbon-14
has decayed for us to measure it that
works best with living things which
continually accumulate both kinds of
carbon over their lifetimes but we can
still measure the age of other things
like rocks if we use different elements
one of the most common is potassium
argon dating which is based around
potassium-40 decaying into argon-40 but
that method operates under a few
assumptions that don't always hold for
one it assumes no extra argon-40 was
incorporated into the rock when it
formed and that you're only measuring
argon that came from potassium it also
assumes that none of the argon that was
formed had a chance to escape the rock
but those things can happen and that
messes with your date so some scientists
have turned to a related updated test
and while it's a bit less direct it does
get rid of a lot of uncertainties this
one compares the same argon-40
made from potassium decay against a
lighter isotope of argon argon 39 they
actually make the argon 39 by putting a
sample of the rock they're dating inside
a nuclear reactor converting some of its
potassium 39 atoms into argon then they
determine the amount of argon 40 by
heating up the rock so it releases the
argon as a gas this might sound weird
but what they're doing is really clever
see the amount of argon 39 produced is
directly related to the amount of
potassium 39 it came from and at least
on earth the ratio of potassium 39 to
potassium 40 is constant so by measuring
argon 39 you can do some math and
ultimately work out how much potassium
40 was in your rock it's an indirect way
of providing the same age that we'd have
used in potassium argon dating but it's
a lot more accurate for one you can
measure both argon isotopes at the same
time rather than having to separately
measure the argon and the potassium and
you can also check if your rock actually
started with extra argon or lost some
over time by slowly heating up the rock
and stages and dating each stage if you
get different ages then one of your
fundamental assumptions was wrong and
neither potassium argon or argon argon
is going to give you a precise age what
this new method does have weaknesses of
its own like because the amount of argon
39 created in the reactor depends on
factors like how much potassium there
was to begin with and how long the
sample was irradiated there are a bunch
of variables that could throw off your
measurement so scientists have to
compare it against a standard mineral
they already know the age of also both
potassium argon and argon argon dating
methods require you to destroy your
sample in order to date it which isn't
always ideal but argon argon requires a
smaller sample that makes this method
better for stuff like moon rocks and
asteroids where we can't exactly hop
back for more on short notice
in fact this method is actually what
helped us figure out that the rocks
astronauts brought back from Apollo 11
are almost four billion years old now
other methods are less invasive like
electron spin resonance or ESR which
uses magnets so let's say you've got
something a bit smaller than a pot a
human tooth perhaps and your fossil site
is too
for carbon-dating but doesn't have the
right materials for other dating methods
Huth enamel is made of minerals and
minerals have a crystalline structure
meaning their atoms are locked into a
rigid pattern that structure can trap
electrons some of which are jolted loose
from their usual position by radiation
and some of which are part of that
radiation the radiation can come from
the sediment from the tooth itself and
even from space they build up over time
so the longer the tooth has been buried
the more trapped electrons it has these
trapped electrons collectively cause the
tooth to react if you expose it to a
magnetic field a moving charge creates
its own magnetic field and electrons are
charged particles constantly moving
around so each is actually also a
subatomic magnet physics is just like
that
when paramagnetic materials are put into
an external magnetic field all those
lone electrons snap into the same
position aligning their own magnetic
fields parallel to the external one it
gets weirder here's how scientists
actually date that tooth inside that
magnetic field they shoot microwaves at
it at a specific wavelength the energy
of those microwaves will get absorbed by
the trapped electrons causing them to
flip their magnetic field in the
opposite direction that change of state
is referred to as resonance hence the
name electron spin resonance by varying
either the strength of the external
magnetic field or the microwave
frequency you can measure the resonance
signal given off by those electrons and
the intensity of that signal tells you
how many trapped electrons you have and
how old your tooth is if the tooths not
too important scientists actually grind
the enamel into a powder and create a
bunch of samples but if you'd rather not
do that you can take a small solid piece
and perform the experiment over and over
again ESR also works for other minerals
associated with living things like
seashells and even nonliving things that
grow like stalactites it's best for
dating minerals from between 50,000 to
800,000 years ago but in ideal
conditions can go back two or three
million years all of the methods we've
covered so far provide absolute dates
there is some amount of error but you
an actual number for how old something
is but sometimes the best you can do is
figure out if something's older or
younger than something else that's known
as relative dating and the most popular
version is called stratigraphy
stratigraphy relies on a couple of
surprising common-sense ideas first
sediment tends to build up in horizontal
layers called strata over time
consequently anything found in lower
strata generally has to be older than
anything found in the strata above it
it's not quite that easy though
sometimes geologic events can disturb
the layers so that sections of rock
including many strata get shifted up or
down relative to another section erosion
and volcanic activity can complicate
things as well assuming you've taken any
of those events into account there are a
couple of ways you can use your strata
for dating if you have a fossil in one
layer sandwiched between two others you
have an age range that fossil has to be
younger than the fossils below it and
older than the fossils above it or if
you have known and unknown fossils from
the same layers you can infer that they
lived about the same time and get an age
for the unknown species stratigraphy
isn't just used for geology or
paleontology of course it's used at
archaeological sites as well for example
it's really hard to date australian
aboriginal rock art because the pigments
the artists use don't have the isotopes
used in radiocarbon dating so in one
instance scientists determined the ages
of rock art in western australia by
dating wasps nests specifically bits of
charcoal that had been incorporated into
the nest since some nests had been
buried beneath the painted rock walls in
some hadn't they could get an age range
they couldn't pin down an exact age but
the data suggested the art across 14
different sites was around 12,000 to
13,000 years old finally what if you
can't date your sample by digging around
a bunch of layers or grinding it up and
putting it in a machine like
stratigraphy siriusian can at least hint
at how old a piece of art is relative to
a similar piece made by the same culture
at some other point in time dating
objects by their style was proposed back
in the mid 1700s if you knew what
features for example classical Roman
statues tended to share at different
points in time
it'd be fairly easy to assign a newly
found statue to the appropriate time
frame but what if you don't have that
background knowledge like say you're in
the 1800s with artifacts from a bunch of
different prehistoric Egyptian graves
with no clear ages carbon dating hasn't
been invented and stratigraphy doesn't
apply well if you assume a culture style
changes gradually over time you can
compare how different style elements of
a certain artifact overlap across the
grave sites and place them in
chronological order there's no way to
tell which end of the order was the
oldest and which was the youngest but
it's a start by necessity Siri a shin
has to be extremely focused you can only
compare the same kind of object against
other versions because the way an
ancient artist decorates a pot isn't
necessarily going to be the same way
another decorates a sword even if
they're made in the same year you can
also only compare the different versions
of that object within not just one
culture but a small geographical region
also you have no way of knowing if there
was some outside influenced by another
culture that could have changed the
style of your artifact or if the culture
itself went through a huge period of
nostalgia for an earlier style but for
prehistoric art siriusian can be an
important tool like stratigraphy it's
been used to date cave paintings because
what Paleolithic humans chose to
represent and the materials they used to
do so changed over time dots and spirals
hand stencils people and animals drawn
with varying levels of detail and
sometimes those animals that get
represented go extinct so if you've got
an old painting with a mammoth in it
it's got a good chance of being painted
before the end of the last ice age until
humanity invents a time machine to
actually go back and see when the things
we want to study were made we'll have to
rely on clues like these so it's a good
thing that we've been able to get so
creative with the evidence we have it
brings us closer to the past thanks for
watching this scishow list show and
thanks to our patrons for helping to
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out patreon.com/scishow
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you
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