ANTH 1500: Radiocarbon Dating and Dendrochronology
Introduction
Radiocarbon measurements from laboratories are always reported in uncalibrated years
“before present” (B.P.). The uncalibrated dates are typically represented as Year 14C B.P.
or Year uncalibrated B.P. In this case, the term “Present” refers to 1950 so that “present”
is a constant and you do not have to know the year in which the measurement was made
(1950 was chosen instead of 2000 because Libby discovered radiocarbon dating in 1949).
Radiocarbon measurements are directly based on the proportion of radiocarbon
found in the sample and are calculated assuming that the atmospheric radiocarbon
concentration has always been the same as it was in 1950. This is not the case. The
proportion of radiocarbon in the atmosphere has varied by a few percent over time due
to variations in the intensity of cosmic rays. Consequently, in order to establish a calendar
date in calibrated years BP or BCE/CE, raw radiocarbon measurements need to be
calibrated. Calibrations curves are calculated using 14C measurements of old wood that
can be accurately dated through dendrochronology or examination of samples that can
be independently dating by other methods (including ice cores, deep ocean sediments,
lake sediment varves, etc.).
There are two main methods used for calculating age ranges from the calibration curve:
1. Intercept method: This is done by drawing intercepts on a graph. First, the
uncalibrated date is located on the y-axis and a horizontal line is drawn from this
point to the curve. Where the line hits the curve, a vertical line is drawn to intersect
with the x-axis. If the horizonal line hits the curve in multiple places, vertical lines
are drawn down from the first and last points it hits. This intersection point provides
the calibrated date in years BP. These dates have a margin of error (2 standard
deviations of the date).
2. Probability method: This method requires a computer since the calculations are
more complicated. It gives the time range, from which you can be 95% sure the
sample came. We now see scholars using Bayesian statistics to further refine the
± error associated with their dates. Reducing or narrowing the ± error range results
in more refined chronologies.
Text above adapted from Radiocarbon Web-Info, Oxford Radiocarbon Accelerator Unit:
http://c14.arch.ox.ac.uk/embed.php?File=calibration.html
Please complete the calibration exercise overleaf. Insert your answers into the
handout provided.
ANTH 1500: Radiocarbon Dating and Dendrochronology Worksheet
Exercise I: Radiocarbon Dating Calibration
Using the intercept method and the calibration curve below, calculate the calibrated dates
in years B.P. (Before Present, where present = 1950) for the dates provided on the next
page. Then convert the dates you calculate in “cal. B.P.” to “B.C.E.” (Before the Common
Era) using the year 1950 as Present.
Remember: for the intercept method, the uncalibrated date is first found on the y-axis. A
horizontal line is drawn from this date to the curve. Where the horizontal line touches the
curve, a vertical line is drawn down to intersect with the x-axis. This point on the x-axis is
the calibrated date in years B.P. Note that the horizontal line can hit the curve in multiple
places in some instances! When this happens, vertical lines are drawn down from the first
and last points it hits. These dates have a margin of error (2 standard deviations of the
date). In order to convert Calibrated Dates B.P. to Calibrated dates in B.C.E. remember
that “present” is 1950 (you will need to subtract 1950 from the B.P. dates to convert to
B.C.E.). If you are confused by this, chat to your TA or re-watch the early part of the Çatal
Höyük lecture.
Using the provided calibration curve, please complete the table below:
Uncalibrated date
Calibrated
date Calibrated
(B.P.)
(B.C.E.)
2600 14C B.P.
1a. 2750
1b. 800
2350 14C B.P.
2a. 2350
2b. 400
2475 14C B.P.
3a. 2700
3b. 750
2950 14C B.P.
4a. 3150
4b. 1200
date
Radiocarbon calibration short answer:
Some parts of the curve are “flatter” (more horizontal) than others. How does the slope of
the calibration curve affect calibrated dates? Be sure to compare flatter parts of the curve
with more vertically sloped parts.
Please include your response in the space below.
I think that the slope of the calibration curve affects the calibrated dates in a way that the
more vertical the dates are form each other, the more it’s going to cluster. When it’s
horizontal, there’s a greater change.
Exercise 2: Dendrochronology
Dendrochronology can be used to date sites where a large piece of wood has been
preserved Typically at least 50 rings are needed to securely match a new wood specimen
to an existing sequence. Because pieces of wood with 50 rings are not found as frequently
as smaller fragments or other datable organic remains (like seeds and bone),
dendrochronology is not used as widely as C14 (radiocarbon) dating. When it is used,
however, it is possible to get very accurate dates of when the wood was growing (not
necessarily when it was in use!). At present, complete (fully anchored)
dendrochronological sequences extend back for roughly 12,500 years.
Instructions for exercise
1. Closely examine the growth rings for the four tree cores that you have. In this instance,
you know which end is the bark end (the outer, younger part of the tree labelled in
black) end and the pith end (the inner, older part, labeled in tan). In reality, wood is
often recovered without bark, which makes the job of a dendrochronology specialist
much harder!
2. Match up the sequences so that series of wide and narrow rings are aligned.
3. Count the rings of your completed sequence and answer the following
questions in the space provided below:
a) What dates are associated with labels A, B, and C?
A: 1962
B: 1988
C: 1942
b) When did the oldest tree start to grow?
Year: 1942
c) When was the oldest tree cut down?
Year: 1970
d) What can you infer about growing conditions in 1986? Why do you think this?
The conditions were really dry because it got thin.