Calibrating Radiocarbon Ages Matt Baillie 4/1/04 HWR696T

Outline Calibration of 14 C records  Dendrochronology  Marine coral records  Marine varve records Available calibration programs  Calib  OxCal  BCal

Calibration of 14 C Records “ It now appears that there is some evidence for a deviation in the period B.P. ( 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 14 C 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 CO 2  Yearly time scale back to 1950, decadal time scale to 11,850 BP

Calibration of 14 C Records Dendrochronology

Calibration of 14 C Records Marine Corals  Radiocarbon ages of marine corals can be checked against 234 U/ 230 Th ages  Gives systematic offset in marine ages, which are affected by reservoir effects 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 14 C Records

Calibration Programs 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: , , B.P. 2σ: , B.P. 1σ: B.P. 2σ: , , B.P.

Calibration Programs OxCal   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

Bcal   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:

References Libby, Willard F. Radiocarbon Dating. University of Chicago Press