Introduction to Archaeology F 2002 / Owen: Absolute dating: Tree rings and radiocarbon p. 1
Introduction to Archaeology: Class 5
Absolute dating: Tree rings and radiocarbon
Copyright Bruce Owen 2002
Two kinds of dating: relative and absolute
Relative dating puts things in order, older to younger, without specifying dates in years; we'll look at these later
Absolute dating gives ages in years.
In theory, this is better than relative dating, because we know both how old things are, and can put them in order
Unfortunately, most “absolute” dating methods give slightly fuzzy dates (radiocarbon dates are usually plus or minus 40 years or more), so sometimes we can get the order more precisely by lower-tech, relative methods.
also, most absolute dating methods are expensive and involve sending samples to a lab and waiting for the results
Some kinds of absolute dates:
Historical dates (coins, dated inscriptions, etc.)
not as simple as you might think
for reasons that apply to all kinds of absolute dates, not just historical ones
the key is to have a clear understanding of archaeological associations of artifacts, features, activities, and dates
Thomas throws in issues about associations while covering other things, but I think it is important to deal with them head on, right away
and historical dates give us a clear, simple case in which to do it.
say we excavate a historical burial and find a coin in it dated 1827.
was the person buried in 1827?
not necessarily.
First, we have to feel sure that the coin (or any absolute date) is truly associated with the rest of the burial activity that we want to date
that is, that the coin got into the burial as part of the burial event, not some other time
we should try to rule out the possibility that the association is false due to disturbance that introduced artifacts after the event of interest
maybe the person was buried back in 1500
and in 1830, someone looking for treasure dug into the burial and accidentally dropped the 1827 coin into it.
or in 1830 a gopher tunneled through the burial, and someone accidentally dropped the 1827 coin down the hole
we can try to rule out disturbance by carefully observing the soil and the positions of the artifacts while excavating
that is, we can say "we looked carefully for evidence of rodent burrows but did not see any"
disturbance can introduce artifacts of any age
the example above has an artifact that is much younger than the burial falling down the gopher hole into to
but it could just as well be a paleolithic spearpoint that falls into the gopher hole
that is, something much older than the burial
the point is that an unrelated artifact gets added to the context after the fact
such artifacts are not really associated with the event of interest; they have nothing to do with it at all, and tell us nothing about when the event happened
we should try to rule out the possibility that the association is false due to redeposition of earlier material
maybe the person was buried in 1900
but when they dug the grave, they dug through a layer of garbage from the 1830s that included the coin
and when they filled the grave, they redeposited the old coin right next to the casket
again, we can try to rule out redeposition by observing carefully during excavation, and by lab analysis
we can say "the surrounding soil was free of artifacts, charcoal, etc., so contamination of the burial by redeposited artifacts is unlikely."
that is, we should not just assume that things found close to each other necessarily result from the same event
we have to look carefully to rule out the possibility of disturbance or redeposition
we usually cannot be absolutely positive
some kinds of closed contexts (also called sealed contexts) give us confidence that the objects really got in there at the same time
because there is no way for anything else to have gotten in
for example, if we found the tomb or casket intact, we can be pretty sure that everything inside it really is contemporary
Assuming that the coin was deposited at the time of the burial, it still does not tell us the date of the burial
because the event that it dates is not the event we are interested in
the date on the coin tells us when the coin was made
not when it got into the burial
so even though the coin is well associated with the burial, the date is not
this is exactly the same problem that Thomas deals with extensively when discussing the dating of the Oseberg ship
they couldn't just date the wood in the ship, because that date is not closely associated with the event of the burial
they chose to date timbers that were apparently cut specifically for the burial chamber
these dates are well associated with the burial event itself
if the association is secure, what the coin does tell us is that the person was buried some unknown time after the coin was minted in 1827
1827 is the date after which the burial must have taken place
this kind of "date after which" is called a terminus post quem
these are extremely useful, but be careful: we don't know how long after…
dendrochronology (tree ring dating)
Low tech, but the most precise method there is
Most trees grow by adding one layer or “ring” of wood per year: a low-density, light-colored part in the rainy season, and a high-density, dark-colored part in the dry season
The thickness of the rings varies depending on the climate each year
a complex combination of rainfall, temperature, sunlight
If you count inwards from the bark of a recently felled tree, the widths of the rings are a record of the climate of each year back to when the tree sprouted
Actually, they usually use narrow cores drilled out of the tree, so they don’t have to cut old trees down for this
Any given period of years has a unique pattern of ring widths
If you have a piece of wood with numerous rings, you can match its ring width pattern to the old tree and tell exactly which years those rings grew in
The pattern can be extended back further into time by finding older logs that have ring width patterns that overlap
this is extremely accurate – to the exact year
someone must create a separate master sequence for each region, sometimes even for different types of trees
this is a very time-consuming, expensive process
and depends on both luck and persistence to find samples that overlap enough and leave no gaps
range of dating:
varies by region; up to 8000 years ago in a few places like northern Europe for Irish oaks and the US Southwest
sometimes the best we can do is a "floating chronology"
that is a tree-ring sequence that does not extend to a known date
pieces of wood that grew during the period covered can be precisely dated relative to others, say "tree A was felled 32 years before tree B"
but the starting date of the whole sequence is unknown
the method cannot be used in all regions
some areas have no suitable trees
or the climate does not vary enough from year to year
or the microclimates vary so much from place to place in the region that no single sequence would work
or the trees are irrigated, so their ring widths reflect a combination of climate and irrigation activities
this is the problem with most trees in coastal Peru
including colonial olive trees that I collected wood from
but also prehistoric trees that typically grew alongside canals
or the work to establish a master sequence simply has not been done yet
the method cannot be used at all sites
it requires relatively large chunks of wood with quite a few rings
if you don’t have beams or posts at your site, you generally can’t use this method
the real trick, as with all methods, is ensuring a meaningful archaeological association of the date with an event that interests us
we care about human behavior, not when a certain bit of wood grew
dendrochronology is usually useful for logs that are preserved all the way out to the bark, or to the smooth surface just under it
because it can then tell us when the tree died
which is probably when someone cut it down
which is probably near when it was used
but:
what if the tree died naturally, and was collected later for use?
what if the tree was cut down and left for years to dry before it was used?
what if a log in the building we want to date was not cut down for that building, but instead was salvaged from an old, abandoned structure?
if squared beams or planks were cut from the log, or artifacts were made from small pieces of the log, an unknown number of outer rings have been removed
so the growth of the wood in the object can be dated, but the tree might have lived many years after that before being chopped down
even so, that gives us a date after which the artifact must have been made
the object cannot be older than the tree rings present in it
a terminus post quem
this can help us bracket events in time, but we have to be careful not to confuse a terminus post quem with the actual age of the thing, since all we know is that it was made an unknown time after the terminus post quem.
Radiocarbon dating (carbon-14 dating, or14C dating)
Thomas's presentation of this is slightly confused; read it, but focus on the version posted on the class web site and discussed here
Go through the online handout for an explanation of the method
A minor wrinkle: two different estimates of the half-life of 14C have been used
The original, early work used the "Libby" half-life of 5568 years.
Later work produced a better estimate that is now universally used: 5730 years.
So some early radiocarbon dates have to be recalculated to get a more accurate result.
How is the measurement done?
Two methods: conventional and AMS (accelerator mass spectrometer)
Conventional
pretreat the sample to remove contaminants
burn the sample and collect the carbon dioxide gas (CO2) that is produced
convert the CO2 through several steps to benzene (a carbon compound)
put a measured amount of the benzene next to an instrument (basically a geiger counter) that responds every time it is hit by a beta particle (high-speed electron)
every time a 14C nucleus decays, it emits a beta particle, and some of these hit the detector
after letting your sample sit there for hours or days, the number of hits on the detector gives you a measure of how often 14C nuclei are decaying in the sample, which is proportional to how many are in there
you do exactly the same thing, with the same setup, using a "modern" carbon sample
(actually a standard with a known 14C content relative to the pre-bomb atmosphere)
if the archaeological sample is emitting beta particles at, say, 1/4 the rate of the modern sample, it must contain 1/4 as much 14C
from there, you can calculate how long it has been since the sample died
in this case, two half-lives, or 11,460 years
AMS
Pretreat the sample to remove contaminants
Burn the sample, collect the CO2, convert the carbon in it to a solid “graphite target”
The target is mounted in a particle accelerator
The accelerator makes carbon nuclei stream off the target in a narrow beam
The beam contains a mixture of the three isotopes, reflecting the proportions of isotopes in the target
The beam passes by some strong magnets, which bend the beam due to the electric charge of the nuclei
the nuclei of the different carbon isotopes are each deflected to a different degree, due to their differing mass
so the magnetic field splits the beam into three parts, each with nuclei of just one isotope
a instrument is located in the path of each of the three beams that records the number of nuclei that strike it
so the device literally counts how many nuclei of each isotope comes off the target during a given period of operation
so it is easy to calculate the fraction that is 14C
pros and cons
AMS dating can use much smaller samples
so it can be used when no large amounts of organic were found
or on objects that you don't want to seriously damage by removing a big piece
AMS dating is often more precise (smaller error estimate)
AMS dates typically cost more, but they are getting more reasonable
sample preparation
both methods require that the sample be treated to eliminate possible contaminants: carbon that is older or younger than what we want to measure
the methods vary depending on the material to be dated
mechanical cleaning or sorting, often under a microscope
often done by the archaeologist before submitting the sample to a lab
chemical treatments that remove known kinds of contaminants that contain carbon, like humic acids from soils
this requires physical chemists with a special background in radiocarbon issues
usually done by experts at the lab
Understanding the error term
Both methods are based on a measurement of the amount of 14C present
Like all measurements, these have some degree of uncertainty
So radiocarbon dates come with an error term
Like 500 BP ± 40
The error term is the standard deviation (often called sigma, or ó) of the probability distribution (a “normal” or “bell” curve) of the estimated date
The “500 BP” is the mean, or center, of that distribution
The error term “± 40” is an indication of how wide the central portion of the probability distribution is
The error term tells us that there is a 68.26% chance (not 67%, as Thomas says) that the true date falls in the indicated range (in this example, 460 to 540 BP)
That still leaves almost a 1 in 3 chance that the date falls outside that range
In order to be more certain, people sometimes double the error estimate (they give the “two sigma” error term)
There is a 95% chance that the true date falls within this wider range
That still leaves a 5% (1 in 20) chance that the true date is outside this range
One way to reduce this uncertainty is run numerous dates
There are statistical methods for combining multiple dates of the same event in order to narrow the range of uncertainty
Comparing dates
Because you don't know the actual date, but instead just a probability distribution of where it is most likely to fall, comparing dates can be complicated
Say you date charcoal from a fire pit at 100 ± 40 AD, and a burned bone nearby at 150 ± 40 AD.
is it possible that the bone was burned in the fire pit?
or was the bone burned somewhere else at a later time?
or could the bone even have been burned before the fire pit was used?
You simply cannot know for sure
But using statistics (or a computer program that does this), you can estimate how likely each of those scenarios is
or how unlikely it is that they are not correct
Combining dates
Say you have 3 dates:
100 ± 40 AD
150 ± 40 AD
200 ± 40 AD
They all come from the same excavated house.
How long was the house occupied?
From 100 to 200 AD?
Not necessarily; there is a good chance that the earliest date is actually from before 100 AD, and/or that the latest date is actually from after 200 AD
From 60 to 240 AD?
Not necessarily; there is a good chance that the starting date is less than 40 away from the central estimate, or even on the plus side of it; same for the ending date
There is also a smaller but real chance that one or both dates might be even further from the central estimate
Worse yet, we might not have even sampled the whole period of occupation
We didn’t necessarily get samples from the very first and the very last moments of occupation
Odds are that our samples fall somewhere within the period of interest, but not all the way at either end of it
This is a complicated problem that many archaeologists are too intimidated to acknowledge
a lot of otherwise competent archaeologists misuse radiocarbon dates because they don’t understand (or want to deal with) the statistical problems involved
nothing can be done about the problem of samples that don’t cover the whole period of interest
but if we have reason to think that they do cover the whole period, there are statistical ways to estimate the duration of the period and the uncertainty of the starting and ending dates
the more dates, the less uncertainty in the estimates
OxCal is the program I prefer for this kind of work; it is available free online from the Oxford Radiocarbon lab