1. Radiocarbon Dating:
theoretical concepts & practical
applications
(all you ever wanted to know about
14C but were afraid to ask)
Robert M Chapple

2. Rule No. 1

3. • Many things about 14C dating are complex, but
the basics are simple!

4. • Analogy: your car
– you don’t need to know everything about how it
works to drive it around

5. • Analogy: your car
– BUT you should have a mastery of the basics

6. • Analogy: your car
– Adherence to basic principles prevent …

7. • Analogy: your car
– Adherence to basic principles prevent …
(metaphorically)

8. • Basics:
– Here comes the science bit …

9. • Method discovered by Willard Libby & his team in
1949 (won Nobel Prize in Chemistry in 1960)

10. • Uses naturally occurring radioisotope carbon-14 (14C)
to determine the age of carbon-bearing materials.

11. • Plants fix atmospheric carbon dioxide (CO2) into
organic material as part of photosynthesis.
• This process incorporates a quantity of 14C close to
the level of the isotope present in the
atmosphere.
• After the plants die/are eaten the 14C fraction
declines at a fixed, exponential rate due to
radioactive decay (half life: 5730±40 years).
• Comparing the remaining amount of 14C vs. the
amount expected in a ‘fresh’ sample allows a
determination of the age of the sample to be
made.

12. • ‘Raw’ results are presented as years BP (Before
Present - 1950) eg 3476±45BP
• Determinations are supplied with a ‘±’ (standard
deviation). This describes a level of uncertainty
with the date. Traditionally: statistical counting
uncertainty, but some labs also include an ‘error
multiplier’ to account for other forms of
uncertainty.
• Limits: 58,000 to 62,000 years

13. • Calibration: change determinations into calendar
dates.
• Necessary: 14C varies in the atmosphere over time
& locality.
• Standard: curve based on comparison of 14C
determination against samples of known date
(dendrochronology)

14. Reporting dates in publications/reports: some fundamentals
The radiocarbon determination: record the actual determination: 3727±49 BP
eg “this feature dated to 2286-1987 cal BC” fine in the text, but of limited use in
assessing/reusing that date
Lab Code: unique identifier for that date
Conventional vs. Measured dates: Beta Analytic Inc. return two dates
Measured: surviving 14C in sample, calculated using the Libby Half Life
Conventional: date with corrections for isotopic fractionation

15. Reporting dates in publications/reports: some fundamentals
Conventional vs. Measured dates:
May be several decades between the two. Recalibrating wrong one will lead to future
errors!
Feel free to quote both dates, but it is the Conventional Age that should be quoted first!

16. Reporting dates in publications/reports: some fundamentals
Type of analysis: conventional radiometric/AMS
eg AMS uses less carbon & may be used on small, single-entity, samples (individual pieces
of hazelnut shell). In the past use of AMS may indicate insufficient carbon for regular
date.
Pretreatment: Usually acid, alkali, acid (AAA) washes to sterilise sample & remove
modern matter.
eg techniques used to extract bone collagen may have small influence on dates: best
practice to report all available information.
Calibration: different computer programs & curves available (eg Calib, OxCal etc.)
Each program use slightly different algorithms & may return slightly different results. Best
practice: state program + version and calibration curve + version
eg: 3727±49 BP Calib 2σ: 2286-1987 cal BC
OxCal 2σ: 2287-1978 cal BC

17. Reporting dates in publications/reports: some fundamentals
The sample
Context information: clearly state which feature, which deposit, which box section?
eg lack of accuracy limits further interpretation
Material: Wood (+ identification), bone/antler?
eg discrimination between long & short-lived
species + twigs/heart wood id. Allows future researchers to reassess your work & confidently
incorporate it into future research
Entity type: is sample from one part of one plant (single entity)?
eg single twig, grain, or hazelnut shell
or
sample from unsorted charcoal from feature? (multiple entity)?
eg ‘bag of charcoal’ recovered from feature or small number of grains from one feature (no
guarantee that all are from the same plant/harvest)
Single entity is preferable, but not always possible: stating entity type helps others assess the
quality & reliability of your dating

18. Reporting dates in publications/reports: some fundamentals
Fundamental: include the ‘raw’ date with its standard deviation!
General rules : Include as much information as possible in publication
: If in doubt, include it!

19. Reporting dates in publications/reports: some fundamentals
Exception: Beta Analytic Inc. include detail ‘standard delivery’

20. Reporting dates in publications/reports: some fundamentals
Exception: Beta Analytic Inc. include detail ‘standard delivery’
Means: you paid for the standard return time, not the express service!

21. Irish Radiocarbon & Dendrochronological Dates project
Website: http://tinyurl.com/64e6qgf
Facebook: https://www.facebook.com/IrishRadiocarbonDates
Google: Irish radiocarbon dates + Chapple

22. Irish Radiocarbon & Dendrochronological Dates project
2006: Started as a response to my personal research needs: I had to write up two burnt
mound excavations and thought that comparing dates may be more
interesting/rewarding than just comparing morphology

23. Irish Radiocarbon & Dendrochronological Dates project
Problem: dates are scattered across a vast array of books, journals & reports

24. Irish Radiocarbon & Dendrochronological Dates project
Problem: dates are scattered across a vast array of books, journals & reports
Solution: start with my own library & go from there!

25. Irish Radiocarbon & Dendrochronological Dates project
Originally: just for my own use & interest
Summer 2010: published ‘Just and expensive number?’ in Archaeology Ireland (24.2).
Noted that I had this resource & was willing to share
November 2010: made publically downloadable version
Current version (March 2012): 6093 radiocarbon
determinations & 240 dendro dates

26. Irish Radiocarbon & Dendrochronological Dates project
Dates from NRA publications – instrumental to the success of this resource!
Literally hundreds of dates from Monograph & Seminar series of publications + same
again from the NRA Database

27. Irish Radiocarbon & Dendrochronological Dates project
Context: M J O’Kelly’s 1989 textbook Early Ireland: An Introduction to Irish Prehistory lists
109 radiocarbon dates
J Waddell’s 2000 textbook The Prehistoric Archaeology of Ireland (2nd edn) lists 307
radiocarbon dates
NRA Database:
482 dates not available
from any other source!

28. Irish Radiocarbon & Dendrochronological Dates project
Example: Site 19, Gransha, Co. Derry/Londonderry
Unusual MBA enclosed cemetery/ritual site

29. Irish Radiocarbon & Dendrochronological Dates project
Example: Site 19, Gransha, Co. Derry/Londonderry
Charred barley grains from cist C1976
UBA-9321 3082±22 BP

30. Irish Radiocarbon & Dendrochronological Dates project
UBA-9321 3082±22 BP.
A search for 14C dates on 25 radiocarbon years on either side of this date (3107-3057 BP)
brings back 73 dates, from 57 sites, across 21 counties

31. Irish Radiocarbon & Dendrochronological Dates project
UBA-9321 3082±22 BP.
Some detail:
Burials
• Cloncowan II, Co. Meath (possible ring ditch)
• Magheramenagh, Portrush, Co. Londonderry (ring ditch)
• Derrycraw, Co. Down (token burial/possible cremation in cairn & ring ditch (2 dates))
• Island, Co. Cork (wedge tomb)
• Ballybannon, Co. Carlow (cremated Remains)
• Killoran 10, Co. Tipperary (cremation cemetery)
• Rathcannon, Co. Limerick (cremation cemetery)
• Cooradarrigan, Co. Cork (boulder burial)
• Ballybar Lower, Co. Carlow (flat Cemetery)*
• Priestsnewtown 6b, Co. Wicklow (cremation pit)*
• Templenoe, site 163.1, Co. Tipperary (Bronze Age grave)*
• Edenagarry, Co. Down (Burial Cairn) * = NRA Scheme site

32. Irish Radiocarbon & Dendrochronological Dates project
UBA-9321 3082±22 BP.
Some detail:
Houses/structures
• Corrstown, Co. Londonderry, (Structures 4 (2 dates), 17, 30, 37, 45, 47 & 68)
• Knockgraffon, site 137.1, Co. Tipperary*
• Toome (Brecart Td.), , Co. Antrim
• Grace Dieu West 8, Co. Waterford*
• Carrigillihy, Co. Cork
• Ballydrehid, site 185.5, Co. Tipperary*
• Cloghabreedy, site 125.1, Co. Tipperary*
• Ballyvergan West 1 AR 26, Co. Cork*
• Knockdomny, Co. Westmeath
• Mitchelstown 1, Cork (3 dates)*
• Cloghabreedy, site 125.4, Co. Tipperary*
• Chancellorsland, Site A, Co. Tipperary (2 dates) * = NRA Scheme site

33. Irish Radiocarbon & Dendrochronological Dates project
UBA-9321 3082±22 BP.
Some detail:
Burnt mounds
• Sranagalloon 1, Co. Clare*
• Cahircalla Beg (AR126) , Co. Clare*
• Dromnea, Co. Cork
• Doughiska, Co. Galway
• Cahiracon, Co. Clare
• Newtown 3, Co. Westmeath*
• Ballyglass West, Co. Galway*
• Sonnagh II, Co. Mayo*
• Demesne or Mearsparkfarm 6.1-6.5, Co. Westmeath*
• Monreagh 2, Co. Clare*
• Sonnagh V, Co. Mayo*
• Carrigane, site C. 3.1, Co. Cork* * = NRA Scheme site

34. Irish Radiocarbon & Dendrochronological Dates project
UBA-9321 3082±22 BP.
Some detail:
Trackways
• Timahoe, Co. Kildare
• Creggan, Track B, Co. Roscommon
• Killoran 230, Co. Tipperary
• Killoran 305, Co. Tipperary
• Lissanure, Co. Longford

35. “Radiocarbon Landscapes”
Direct Benefits:
Innovative way of interrogating the
archaeological literary mountain
Presents a more holistic approach to
archaeological research, where
connections are made between
contemporary events in different parts of
the island (vs. some reports which draw
only from sites on the same scheme, or
sites of similar morphology)
Indirect Benefits:
Data set is freely available to the entire
archaeological world & has been used by
INSTAR projects, Stephen Shennan’s
Prehistoric Demography, & various PhDs,
etc.!

36. “Radiocarbon Landscapes”
Downsides:
Not a research ‘silver bullet’ – for use as one
of a number of complimentary research
tools
Inconsistencies, repetitions & lack of robust
error checking: stems from being a
personal research project
‘One man band’ project – reliant on my own
book-buying power/donations &
occasional grant funding

37. New Kid on the Block!

38. New Kid on the Block!
Bayesian Statistics

39. New Kid on the Block!
Bayesian Statistics

40. New Kid on the Block!
Bayesian Statistics
Rev. Thomas Bayes (c. 1701 – 7 April 1761)
English mathematician and Presbyterian minister

41. New Kid on the Block!
Bayesian Statistics

42. New Kid on the Block!
Bayesian Statistics
Statistical means for modifying
beliefs in the light of new
information.
Ideas about the likelihood of A are
modified by observing B

43. Two kinds of Bayesian models
Type 1: Strong reasons for assigning
chronological order to a series of events
(eg Stratigraphy). This information will
strongly influence the model. Known as:
'informative prior belief'
Type 2: No stratigraphic information, only
assumptions about the mathematical
distribution of dates in a single phase of
activity. Known as: 'uninformative prior
belief'

44. Example: Gransha, Site 12. possible Early Neolithic house
Dug for commercial company: was allowed 1 date
Beta-227766 4930±70 BP. 2σ: 3943-3640 cal BC (303 years)

45. Example: Gransha, Site 12. possible Early Neolithic house
INSTAR Cultivating Societies: Assessing the Evidence for Agriculture in Neolithic Ireland
6 single entity dates AMS dated at 14Chrono, QUB
Thanks to Rick Schulting, Paula Reimer & Nikki Whitehouse for permission to use this data

46. Seems to span 200-300 years … possibly as much as 400 years
Based on these dates: how long was the site in use?

47. Prior beliefs are 'uninformative': based on assumptions of the mathematical distribution of
dates within a single activity phase
Q: If activity really started c3960 cal BC & ended c3400 cal BC, and we randomly took 7
samples for dating ... how likely are their calibrated ranges to look like this?

48. A: Not very likely at all!

49. Q: What if activity started c3800 cal BC and ended c3500 cal BC?
A: More likely, but dates still do not fit particularly well. Would expect a wider calibrated
range

50. OxCal program asks this question a lot of times (100K - millions) to come up with the best
solution - ie the best estimate of the true span of activity sampled by the dates within the
model

51. Gransha, site 12: unmodelled vs. modelled dates

52. But that's not all ... our 7 samples may not adequately represent the duration of the phase.
OxCal provides 'boundary start' & 'boundary end' statistical functions

53. Full report:
UJA 67, 2008
Together, these provide a statistical estimate of the actual life of this site:
Combine all dates (2σ): 3696-3638 cal BC
Boundary start (2σ): 3725-3642 cal BC
Boundary end(2σ): 3689-3597 cal BC
Span 1σ: 0-48 cal years
Span 2σ: 0-115 cal years
Compare: original date on charcoal: up to 303 years (2σ)

54. Problems with Bayesian modelling?
Has recently come to prominence in archaeology & is somewhat being accepted
uncritically
Problem: it is a statistical model!
"All models are wrong, some models are useful“ (Box 1979 cited in Bayliss et al. 2007)
Model is only as good as our baseline assumptions & the data we put into it!
Bayliss, A., Bronk Ramsey, C., van der Plicht, J. & Whittle, A. 2007 'Bradshaw and Bayes:
towards a timetable for the Neolithic' Cambridge Archaeological Journal 17, 1-28.

55. Case Study: The Neolithic House Horizon
Cormac McSparron
1) Collect all known radiocarbon dates for Neolithic
houses (63 dates from 25 houses at 14 locations)
2) Span over 1000 years
AI 22.3
Reconstruction of Gortore, Co. Cork (NRA excavation)

56. Case Study: The Neolithic House Horizon
3) McSparron argued for the adoption of a ‘Gold Standard’ for acceptable dates
i) single entity samples
ii) from short-lived species (hazelnut shell, cereal grains etc.)
iii) from secure contexts
… no. of usable dates drops to 18
Positive: good geographical spread:
12 houses in 7 different locations
& relatively representative of the variety of
forms & sizes encountered.

57. Case Study: The Neolithic House Horizon
Assessment of ‘Gold Standard’ dates:
construction/use of these houses began between 3715 & 3650 cal BC
ended between 3690 & 3625 cal BC (c. 100 years)
assumption: these are representative of the
entirety of the evidence
suggests: sudden appearance around 3700 cal BC
across the island – colonising group?
Dating compatible with ‘Landnam’ (after 3850 cal BC) –
decline in tree pollen & rise in grass & other pollen
associated with open landscape
KEY: close analysis (reanalysis) of 14C dating has
provided fertile ground for new discoveries!

58. Thank You for Listening!
rmchapple@hotmail.com
@RMChapple
rmchapple.blogspot.com
Irish Radiocarbon &
Dendrochronological Dates
https://sites.google.com/site/chapplearchaeology/
Dingbat19and17