4 Ways to Date an Archaeological Site

Summary

This 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

00:00

🔬 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.

05:01

🧲 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.

10:02

🎨 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.

Keywords

💡Carbon Dating

Carbon dating, also known as radiocarbon dating, is a method used to determine the age of an object containing organic material by measuring the amount of carbon-14 remaining in it. It is a central theme in the video, illustrating how scientists estimate the age of ancient objects. The script mentions that carbon dating is reliable for objects up to about 50,000 years old, as beyond this time frame, the carbon-14 has decayed to undetectable levels.

💡Radiometric Dating

Radiometric dating is a broader category of techniques that includes carbon dating. It involves measuring the ratio of abundance of two different isotopes of an element to determine the age of a sample. The video explains that this method is based on the principle of radioactive decay, which is consistent over time, allowing scientists to calculate the age of various materials.

💡Isotopes

Isotopes are variants of a particular chemical element which differ in neutron number. In the context of the video, isotopes such as carbon-14 and carbon-12 are crucial for carbon dating, while potassium-40 and argon-40 are key in potassium-argon dating. The script explains how the ratio of these isotopes can be used to estimate the age of materials.

💡Potassium-Argon Dating

Potassium-argon dating is a specific radiometric dating technique that involves measuring the ratio of potassium-40 to argon-40 in a sample to determine its age. The video points out that this method has certain assumptions, such as no initial argon-40 in the rock and no argon loss over time, which can affect the accuracy of the dating.

💡Argon-Argon Dating

Argon-argon dating is an improved version of potassium-argon dating that involves creating argon-39 from potassium-39 in a nuclear reactor and then measuring it alongside argon-40. The video describes this method as more accurate because it accounts for potential uncertainties in the dating process, such as initial argon content or argon loss.

💡Electron Spin Resonance (ESR)

Electron Spin Resonance is a dating technique that uses the magnetic properties of trapped electrons in crystalline structures to estimate the age of a sample. The video explains that ESR is useful for dating smaller objects like teeth or seashells, and it can provide age estimates for materials up to millions of years old.

💡Stratigraphy

Stratigraphy is a relative dating method that involves analyzing the order of layers, or strata, in geological formations. The video describes how objects found in lower strata are generally older than those in higher strata, and this method is used to deduce the relative ages of fossils and artifacts.

💡Relative Dating

Relative dating is a method that determines the age of a sample in relation to other samples without providing an exact date. The video mentions stratigraphy as a form of relative dating, which helps to establish the chronological sequence of events in the history of a site.

💡Style Seriation

Style seriation is a relative dating technique that involves arranging artifacts in chronological order based on their style and features. The video explains that this method is used when direct dating methods are not applicable, and it relies on the assumption that cultural styles change gradually over time.

💡Radioactive Decay

Radioactive decay is the process by which an unstable atomic nucleus loses energy by emitting radiation. In the context of the video, the rate of radioactive decay is used in radiometric dating methods to estimate the time elapsed since the death of an organism or the formation of a rock.

💡Nuclear Reactor

A nuclear reactor is a device that maintains and controls a nuclear chain reaction. In the video, it is mentioned in the context of argon-argon dating, where a sample is placed inside a nuclear reactor to convert potassium-39 into argon-39, which is then used to determine the age of the sample.

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

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