Calibrating Radiocarbon Ages
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Transcript Calibrating Radiocarbon Ages
Calibrating Radiocarbon Ages
Matt Baillie
4/1/04
HWR696T
Outline
Calibration of 14C records
Dendrochronology
Marine coral records
Marine varve records
Available calibration programs
Calib
OxCal
BCal
Calibration of
14C
Records
“It now appears that there is some evidence for a
deviation in the period 4000-5000 B.P. (2000-3000 B.C.)
in the direction of the radiocarbon dates being too young
by perhaps 500 or 600 years for Egyptian samples at the
time of the First Dynasty (4800 to 5000 B.P.). Good
agreement generally has been found for the period back
to about 4000 B.P. The question of the source of the
disagreement in the fifth millennium remains to be
resolved. . . . If it is in the method itself, grave corrections
might prove necessary for all dates older than 4000
years.”
-Libby (1965), from Libby (1963)
Calibration of
14C
Records
Dendrochronology
Tree-ring record goes back continuously from the present
to 9905 B.C. (11,850 years before present (BP), where
present refers to the year 1950).
Patterns in trees correspond to climatic patterns; trees can be
correlated to each other based on these patterns
Tree rings of known actual age can be compared to their
radiocarbon age to create a calibration curve
Trees in equilibrium with atmospheric CO2
Yearly time scale back to 1950, decadal time scale to
11,850 BP
Calibration of
14C
Dendrochronology
Records
Calibration of
Records
Marine Corals
Radiocarbon ages of marine corals can be checked
against 234U/230Th ages
Gives systematic offset in marine ages, which are
affected by reservoir effects
14C
500-year correction for ages greater than 10,000 BP
Extends calibration curve back to 24,000 BP
Marine Varves
Chronology matched to dendrochronology
Used to help calibrate marine sample ages
Calibration of
14C
Records
Calibration Programs
Calib:
http://radiocarbon.pa.qub.ac.uk/calib
Calibration based on a probability distribution of a
sample’s true age versus the calibration dataset
Gives 2σ age (intercept range) and 95% probability
age (probability range)
Marine carbon correction can be made in data input
screen
Online and PC-based versions
Calibration Programs
1σ: 4573-4599, 4610-4770, 4780-4845 B.P.
2σ: 4440-4490, 4500-4970 B.P.
1σ: 1879-2000 B.P.
2σ: 1824-1852, 1859-2068, 2079-2110 B.P.
Calibration Programs
OxCal
http://www.rlaha.ox.ac.uk/orau/oxcal.html
Similar calibration method to Calib
Can be used in conjunction with groups of samples,
and dates can be modified to reflect known historical
dates within the sample set, or known relative age
relations between samples
PC-based only
Calibration Programs
Atmospheric data from Stuiver et al. (1998); OxCal v3.9 Bronk Ramsey (2003); cub r:4 sd:12 prob usp[chron]
Atmospheric data from Stuiver et al. (1998); OxCal v3.9 Bronk Ramsey (2003); cub r:4 sd:12 prob usp[chron]
2400BP
samp1 : 4200±100BP
Radiocarbon determination
4800BP
68.2% probability
4850BP (17.2%) 4780BP
4770BP (44.9%) 4610BP
4600BP ( 6.1%) 4570BP
95.4% probability
5000BP (95.4%) 4400BP
4600BP
4400BP
4200BP
4000BP
3800BP
samp2 : 2000±55BP
68.2% probability
2010BP (68.2%) 1870BP
95.4% probability
2120BP ( 3.7%) 2080BP
2070BP (91.7%) 1820BP
2200BP
2000BP
1800BP
1600BP
3600BP
3500CalBC
3000CalBC
2500CalBC
Calibrated date
2000CalBC
400CalBC
200CalBC CalBC/CalAD 200CalAD
Calibrated date
400CalAD
Calibration Programs
Bcal
http://bcal.shef.ac.uk/
Uses Bayesian statistics to calibrate radiocarbon
sample ages
Online only
Stores individual user data on a server, accessible from any
computer on the internet
Also allows input of already-known relationships
between your samples (relative dating)
Calibration Programs
Bcal - Bayesian Statistics:
Uses empirically derived data to temper probability estimates for
an event
Dependent upon subjective interpretation of data
“With the Bayesian approach, different individuals might specify
different prior distributions.”
“Bayes's theorem provides a mechanism for combining a prior
probability distribution for the states of nature with new sample
information, the combined data giving a revised probability
distribution about the states of nature, which can then be used as
a prior probability with a future new sample, and so on.”
Allows for constant revision of assumptions (accepted by all?)
Source: http://www.abelard.org/briefings/bayes.htm
References
Libby, Willard F. Radiocarbon Dating. University of
Chicago Press. 1965.
http://radiocarbon.pa.qub.ac.uk/calib
http://www.rlaha.ox.ac.uk/orau/oxcal.html
http://bcal.shef.ac.uk/
http://www.abelard.org/briefings/bayes.htm