Mobility and Migration: Strontium Isotopes

Welcome to the second case study of the mobility and migration module.

In this video we will learn about strontium isotopes and how they have been

increasingly used to infer patterns of migration in past populations.

Recall that video 3.2 reviews the basics about stable isotopes.

What they are, how they work and

how osteoarchaeologists can detect different isotopes.

That video was focused on the isotopes that are commonly used for

the reconstruction of diet.

But as you will learn, there are also isotopes that are used to tell if

a person moved from one geographic area to another in their lifetime.

Perhaps the strontium isotope values of your bones and

teeth would reveal that you had moved from one part of the world to another.

You can tell us on the online discussion forum.

Strontium is a trace element found in rocks which through weathering gets into

water and soil.

Which there by becomes incorporated into the tissues of plants and animals.

There are four naturally occurring strontium isotopes

present in different proportions as shown here.

Osteoarchaeologists use the ratio of two of these,

strontium 87 to strontium 86 for migration research.

That's because the amount of strontium 87 varies in rocks according

to the age of the rock.

For example, very old rocks, such as granites and

shales, have higher strontium 87 to 86 ratios than younger rocks.

So the main type or types of rocks underlying where you live,

referred to as bedrock, will give you a strontium isotope signature that

varies from geographic areas composed of different bedrock.

This figure shows the strontium-87 to 86 values of bedrock from the United States.

Such maps are called isoscapes.

Importantly, the world's oceans have a near constant strontium-87

to 86 ratio of 0.7092.

Areas along the coast will have the same or at least a very similar value.

Moving inland and away from coastal environments,

bedrock strontium-87 to 86 ratios have been found

to vary between 0.703 and 0.760.

In 1985, Ericson was the first archeologist to demonstrate

that the strontium-87 to 86 ratio in human bones and

teeth reflects the strontium ratio of the geographic area that

a person lived in while that tissue was being formed.

One may ask how does strontium even get into our bones and teeth.

Well, strontium has a molecular structure similar to calcium.

Calcium, as you know, is a major component of the mineral of both bones and teeth.

And quite simply a very small amount of strontium

isotopes will replace calcium in hydroxyapatite.

Now recall that tooth enamel forms during infancy and childhood.

The last bit of your enamel in your third molars forms at 12 to 13 years of age.

And after this, the chemical composition of your enamel does not change.

So for your entire life, this enamel holds a record of your childhood home.

In contrast, your bones as you know are always turning over, remodeling.

And so, always changing and reflecting the most recent period of life.

So the strontium-87 to 86 value of an adult's bone mineral

will reflect their adult home.

Different strontium-87 to 86 values in

a person's enamel compared to their bone can therefore indicate a geographic

change of residence from childhood to adulthood that is a migration event.

Another common approach is to characterize the local bio-available range

of these strontium isotope ratios and

consider anyone who falls within two standard deviations of the average as local,

and anyone who falls beyond this 95% range is non-local.

This is shown in a figure here by Boric and Price,

where a relatively high number of non-locals entered the Danube Gorge's

region in the North Central Balkans around 8,200 years ago,

which they interpret as evidence for early farmers entering the region.

The non-local could come from an adjacent, close-by region that has different

bedrock, or from hundreds or even thousands of kilometres away.

Matching the non-local strontium isotope value to that of geographic regions with

the same value can allow us to see what possible homelands they came from.

And sometimes in combination with other isotopes,

such as oxygen and sulphur, as well as clues from the skeleton itself and

material culture items, we can deduce which homeland was most likely.

Now the principle underlying the use of strontium isotopes

to reconstruct migration in past populations is quite straight forward,

but the reality is seldom as simple.

And we have to keep a few things in mind that impact the feasibility

of this kind of research.

First, in order to use this method there has to be sufficient isotopic

variation in the geographic area under study.

There are large areas of the world underlain by the same bedrock or

with bedrocks are very similar strontium isotope values,

such that migration within and between this areas simply cannot be detected.

Second, one's diet can influence if their strontium isotope

values are useful to track migration.

One example of this is the consumption of a lot of seafood.

As regardless of where one lives, if they eat a lot of seafood they will

have a strontium-87 to 86 value close to or of sea water.

Another example concerns modern day and some select past groups and

individuals who ate a lot of imported foods.

If you eat a lot of non-locally grown food as is the case for

many of us today in western developed nations, your strontium-87 to 86 value

will not be reflective of where you live, but rather where your food came from.

Third, the accuracy of the method of determining what the local strontium

isotope value of a region is is important to consider.

Archaeologists have characterized the strontium isotope value of an area in

different ways including by direct measurements of exposed bedrock or soil;

or by determining the value in plants; or from the bones of small mammals; or

the shells of land snails,

animals that can only roam over a small area.

Each of these methods has it's own advantages and

disadvantages that have to be kept in mind.

And finally bone strontium ratios can be potentially altered due to chemical,

physical or biological changes after death

particularly from the soil, if the bone was buried,

and the assessment of any such alteration must be done for each and every sample.

But if all of these factors can be taken into account,

which is often the case, strontium isotope analyses of past peoples

can be a powerful method to reconstruct migration.

Now to talk to us about one example from the Caribbean region is

Dr Jason Laffoon is a postdoctoral researcher with Leiden University, and

the Free University Amsterdam specializing in isotope bio-archaeology.

Jason, thank you very much for joining us today.

So to begin, I'd like to ask you to tell us about your research area, and

what some of the main goals of this research are.

>> Yes, thank you for having me.

The research area is the Caribbean region which is consisted of series of island

archipelagoes and the surrounding costal mainland of Central and South America.

The earliest element of the islands of the Caribbean occurred for about 5,000 BC.

But not very much known about this early inhabitants

mostly because of issues of preservation.

The lack of well preserved archaeological or osteological materials to study.

My research primarily focuses on the later occupation

of the islands of the Caribbean.

What we call the late ceramic age that dates from approximately AD 500 to

AD 1500.

And the previous research in the region had revealed the widespread

circulation of various aspects of material culture agent from ceramic

styles to shared lithic resources, to similar manufacturing techniques.

So we knew the people were probably moving from the mainland to the island,

in between the islands and my research question focused on where

exactly people were moving from and what were the patterns of migration over time.

>> So in order to investigate the patterns of migration over time,

you use strontium isotopes.

As our users have been learning, the first step in such research is to characterize

the strontium isotope variability of the region that you want to investigate.

So can you tell us a bit how you undertook that task?

>> Certainly.

The method that we found that worked best in the Caribbean region

was the bioavailable method.

And that's where you characterize the variation for

a site or population based on independent sampling of biological samples.

>> Okay. >> So we focused out primarily

on modern plants and archaeological and modern animal remains.

And we analyzed more than 300 samples from all the different islands and

island groups in the Caribbean region.

>> Fantastic, so that's a very good base with which to start.

The next step then being the analysis of the humans from these

different archaeological sites in different regions and time periods.

Can you tell us a bit about the sample size that you were able to look at,

and also what tissue you analyzed?

>> Certainly, in terms of the tissues, we focused solely on dental enamel.

And that's for two primary reasons.

Dental enamel is the most resistant to diagenic alteration.

That's changes to the tissue based on being buried in the ground for so long.

And we tried to develop a very large data set.

So what we did is we focused on as many individuals as we could, and for

my dissertation we analyzed more than 360 human individuals,

from 26 different sites on 12 different islands.

>> Amazing, very, very large sample size.

So with all of that data, you must have been able to say quite a bit.

If you could tell us, then, about some of the more remarkable or

spectacular results that came out of this research.

>> Yes, of course.

So the first thing that we found that was one of the most interesting results

actually was that, for every single population that we analyzed,

we identified non-local immigrants.

And the proportions of immigrants at each population were highly variable,

ranging up to more than 30% for some populations.

The second very interesting result that we found is that these non-local

immigrants for each population were not all originating from the same place, and

they displayed a high degree of isotopic diversity, indicating diverse origins for

different immigrants.

Other very interesting results included that there was

no sex-based differences in immigration, so that an equal number of male and

female immigrants proportionally over time.

And that suggested that was there was no migration

linked to post marital residents, such as patrilocal or matrilocal residents.

For some populations but not others there were clear differences between local and

non local immigrants in terms of both dietary practices and burial treatment.

The overall chronological pattern that we observed was a

decrease migration over time.

And we think this is most likely linked to demographic variables such as

increasing population densities and

subsequent decrease in social distances between settlements.

And lastly for the sole early colonial period of population that we analyzed,

we identified several very long distance migrants, individuals originating

from the Maya regions of Mesoamerica and from Western Africa.

>> Wow, very interesting.

Thank you very much for taking the time to share your research with us today.

>> Thank you for having me.

>> In this video,

you learned about strontium isotope analyses of the mineral in teeth and

bone as a method to reconstruct aspects of an individual's migratory history.

With this method, it is in some cases possible

to determine that an individual was born far away from where they died.

You also heard about the strontium isotope research in the Caribbean, and

how such data have improved our knowledge of the prehistoric colonization and

migration of the area.

Coming up next is the final case study for this module where you'll hear about

how the genetic information contained in each and every one of us and

in fossils going back hundreds, thousands, and even hundreds of thousands of years,

has revealed a tremendous amount about our migration.

Both at global and continental scales, as well as at smaller regional scales.

See you soon.

Osteoarchaeology: The Truth in Our Bones

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