Radiocarbon dating theoretical concepts & practical applicationsPresentation Transcript
Radiocarbon Dating:theoretical concepts & practical applications (all you ever wanted to know about 14C but were afraid to ask) Robert M Chapple
Rule No. 1
• Many things about 14C dating are complex, but the basics are simple!
• Analogy: your car – you don’t need to know everything about how it works to drive it around
• Analogy: your car – BUT you should have a mastery of the basics
• Analogy: your car – Adherence to basic principles prevent …
• Analogy: your car – Adherence to basic principles prevent … (metaphorically)
• Basics: – Here comes the science bit …
• Method discovered by Willard Libby & his team in 1949 (won Nobel Prize in Chemistry in 1960)
• Uses naturally occurring radioisotope carbon-14 (14C) to determine the age of carbon-bearing materials.
• 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.
• ‘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
• 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)
Reporting dates in publications/reports: some fundamentalsThe radiocarbon determination: record the actual determination: 3727±49 BPeg “this feature dated to 2286-1987 cal BC” fine in the text, but of limited use in assessing/reusing that dateLab Code: unique identifier for that dateConventional vs. Measured dates: Beta Analytic Inc. return two datesMeasured: surviving 14C in sample, calculated using the Libby Half LifeConventional: date with corrections for isotopic fractionation
Reporting dates in publications/reports: some fundamentalsConventional 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!
Reporting dates in publications/reports: some fundamentalsType of analysis: conventional radiometric/AMSeg 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 + versioneg: 3727±49 BP Calib 2σ: 2286-1987 cal BC OxCal 2σ: 2287-1978 cal BC
Reporting dates in publications/reports: some fundamentalsThe sampleContext information: clearly state which feature, which deposit, which box section?eg lack of accuracy limits further interpretationMaterial: Wood (+ identification), bone/antler?eg discrimination between long & short-livedspecies + twigs/heart wood id. Allows future researchers to reassess your work & confidently incorporate it into future researchEntity type: is sample from one part of one plant (single entity)?eg single twig, grain, or hazelnut shellorsample 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
Reporting dates in publications/reports: some fundamentalsFundamental: include the ‘raw’ date with its standard deviation!General rules : Include as much information as possible in publication : If in doubt, include it!
Reporting dates in publications/reports: some fundamentalsException: Beta Analytic Inc. include detail ‘standard delivery’
Reporting dates in publications/reports: some fundamentalsException: Beta Analytic Inc. include detail ‘standard delivery’Means: you paid for the standard return time, not the express service!
Irish Radiocarbon & Dendrochronological Dates project2006: 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
Irish Radiocarbon & Dendrochronological Dates projectProblem: dates are scattered across a vast array of books, journals & reports
Irish Radiocarbon & Dendrochronological Dates projectProblem: dates are scattered across a vast array of books, journals & reportsSolution: start with my own library & go from there!
Irish Radiocarbon & Dendrochronological Dates projectOriginally: just for my own use & interestSummer 2010: published ‘Just and expensive number?’ in Archaeology Ireland (24.2). Noted that I had this resource & was willing to shareNovember 2010: made publically downloadable versionCurrent version (March 2012): 6093 radiocarbon determinations & 240 dendro dates
Irish Radiocarbon & Dendrochronological Dates projectDates 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
Irish Radiocarbon & Dendrochronological Dates projectContext: M J O’Kelly’s 1989 textbook Early Ireland: An Introduction to Irish Prehistory lists 109 radiocarbon datesJ Waddell’s 2000 textbook The Prehistoric Archaeology of Ireland (2nd edn) lists 307 radiocarbon datesNRA Database:482 dates not availablefrom any other source!
Irish Radiocarbon & Dendrochronological Dates projectExample: Site 19, Gransha, Co. Derry/LondonderryUnusual MBA enclosed cemetery/ritual site
Irish Radiocarbon & Dendrochronological Dates projectExample: Site 19, Gransha, Co. Derry/LondonderryCharred barley grains from cist C1976UBA-9321 3082±22 BP
Irish Radiocarbon & Dendrochronological Dates projectUBA-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
Irish Radiocarbon & Dendrochronological Dates projectUBA-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
Irish Radiocarbon & Dendrochronological Dates projectUBA-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
“Radiocarbon Landscapes”Direct Benefits:Innovative way of interrogating the archaeological literary mountainPresents 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.!
“Radiocarbon Landscapes”Downsides:Not a research ‘silver bullet’ – for use as one of a number of complimentary research toolsInconsistencies, 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
New Kid on the Block!
New Kid on the Block!Bayesian Statistics
New Kid on the Block!Bayesian Statistics
New Kid on the Block!Bayesian Statistics Rev. Thomas Bayes (c. 1701 – 7 April 1761) English mathematician and Presbyterian minister
New Kid on the Block!Bayesian Statistics
New Kid on the Block!Bayesian StatisticsStatistical means for modifyingbeliefs in the light of newinformation.Ideas about the likelihood of A aremodified by observing B
Two kinds of Bayesian modelsType 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 beliefType 2: No stratigraphic information, only assumptions about the mathematical distribution of dates in a single phase of activity. Known as: uninformative prior belief
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)
Example: Gransha, Site 12. possible Early Neolithic houseINSTAR Cultivating Societies: Assessing the Evidence for Agriculture in Neolithic Ireland6 single entity dates AMS dated at 14Chrono, QUBThanks to Rick Schulting, Paula Reimer & Nikki Whitehouse for permission to use this data
Seems to span 200-300 years … possibly as much as 400 yearsBased on these dates: how long was the site in use?
Prior beliefs are uninformative: based on assumptions of the mathematical distribution ofdates within a single activity phaseQ: If activity really started c3960 cal BC & ended c3400 cal BC, and we randomly took 7samples for dating ... how likely are their calibrated ranges to look like this?
A: Not very likely at all!
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 calibratedrange
OxCal program asks this question a lot of times (100K - millions) to come up with the bestsolution - ie the best estimate of the true span of activity sampled by the dates within themodel
Gransha, site 12: unmodelled vs. modelled dates
But thats not all ... our 7 samples may not adequately represent the duration of the phase.OxCal provides boundary start & boundary end statistical functions
Full report: UJA 67, 2008Together, these provide a statistical estimate of the actual life of this site:Combine all dates (2σ): 3696-3638 cal BCBoundary start (2σ): 3725-3642 cal BCBoundary end(2σ): 3689-3597 cal BCSpan 1σ: 0-48 cal yearsSpan 2σ: 0-115 cal yearsCompare: original date on charcoal: up to 303 years (2σ)
Problems with Bayesian modelling?Has recently come to prominence in archaeology & is somewhat being accepteduncriticallyProblem: 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.
Case Study: The Neolithic House HorizonCormac McSparron1) 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)
Case Study: The Neolithic House Horizon3) 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 18Positive: good geographical spread:12 houses in 7 different locations& relatively representative of the variety offorms & sizes encountered.
Case Study: The Neolithic House HorizonAssessment of ‘Gold Standard’ dates:construction/use of these houses began between 3715 & 3650 cal BCended between 3690 & 3625 cal BC (c. 100 years)assumption: these are representative of theentirety of the evidencesuggests: sudden appearance around 3700 cal BCacross the island – colonising group?Dating compatible with ‘Landnam’ (after 3850 cal BC) –decline in tree pollen & rise in grass & other pollenassociated with open landscapeKEY: close analysis (reanalysis) of 14C dating hasprovided fertile ground for new discoveries!
Thank You for Listeningfirstname.lastname@example.org@RMChapplermchapple.blogspot.comIrish Radiocarbon &Dendrochronological Dateshttps://sites.google.com/site/chapplearchaeology/Dingbat19and17