Radiocarbon dating: what you always wanted to know but were afraid to ask.
Paula J. Reimer 3 October 2009 Cultivating Societies Workshop
Intro Sample selection Sample processing Measurement and corrections Reporting of radiocarbon dates Use of radiocarbon for context integrity Calibration basics Limitations to the method
14C
Measurement
Sample Pretreated
Converted to CO2
Gas counter (Beta decay) or
Converted to Benzene
Scintillant added
Converted to Graphite
AMS (atoms counted)
Liquid Scintillation Counter (Beta decay)
14C
AMS dating •Small sample size (< 1 mg carbon) but > contamination potential •Rigorous pretreatment or separation into fractions •Sample specific background material (for >30,000 BP) •Short measurement time (10-15 minutes)
Rigorous pretreatment or separations Wood - Cellulose Bone -- Collagen (Ultrafiltration), amino acids Sediments - Humic & Humin fractions Organics -- ABOX, Stepped combustion Compound specific (some labs)
Vivaspin Ultrafilter 30kD
Contamination prevention
•Natural !
root penetration - physically remove
!
humic acids - remove with alkali
!
carbonate formation - remove with acid
!
fungal or bacterial growth – avoid by drying sample
•Human !
Preservatives – chemically remove if possible
!
Packing material – physically remove
!
Labeling – chemically or physically remove
Tracers (run tests on questionable provenance samples before accepting)
!
Sample pretreatment Physical separation* Basic chemical pretreatment* Biochemical extractions
*additional steps may be need if preservatives were used or other contamination is suspected
Physical separation Removal of contamination •removal of outer layers of bone or shell •removal of roots Selection of specific material •Sieving of charcoal from soil •Picking out forams or pollen to date •Picking out macrofossils to date
Radiocarbon dating wood, seeds, macrofossils
‘Old wood’ effect Tree may be several hundred years old when used Select short-lived or coppiced trees, bark, small twigs or branches Seeds – charred indicates probable human activity Macrofossils – peat stems, leaves - Avoid aquatic in lake sediments (unless reservoir correction known for lake)
Genus
Common name
Typical lifespan
Alnus
Alder
Hardwood
Up to 100 years
Betula
Birch
Hardwood
Up to 100 years
Carpinus
Hornbeam
Hardwood
Over 100 years
Corylus
Hazel
Hardwood
50-60 years
Crataegus
Hawthorn
Hardwood
Probably >100
Fraximus
Ash
Hardwood
200 years +
Fagus
Beech
Hardwood
Usually 1950 AD >Modern or pMC = 110 (percent modern carbon) F14C = 1.10 (fraction modern carbon)
•14CHRONO Centre •Queens University Belfast •42 Fitzwilliam Street •Belfast BT9 6AX •Northern Ireland
•Paula Reimer •Queen's University Belfast •42 Fitzwilliam Street •Belfast BT9 6AX •UK
Radiocarbon Date Certificate Laboratory Identification:
UBA-7853
Date of Measurement:
2007-08-07
Site:
Banks' valley, St Helena
Sample ID:
StH-Limpet2
Material Dated:
shell
Pretreatment:
Acid Etch
Submitted by:
Ron Reimer
14C
808±25
Date:
AMS δ13C:
1.5*
*not for paleoedietary or ecological analysis
Why is radiocarbon calibration needed?
Constant initial 14C
Constant initial 14C
IntCal04 Calibration curve
Constant initial 14C
calibration curve
Reporting calibrated radiocarbon age ranges Radiocarbon 2 σ calibrated age ranges * age BP
