AMS Radiocarbon Dating: Do You Know What You’re Dating? Kathryn Puseman Linda Scott Cummings R.A. Varney PaleoResearch Institute

AMS Radiocarbon Dating:

Do You Know What You’re Dating?

Kathryn Puseman

Linda Scott Cummings

R.A. Varney

PaleoResearch Institute

Charcoal is most often the preferred material for radiocarbon dating.

AMS radiocarbon dating can be used to date very small samples.

Carbon has three naturally occurring isotopes: 12C, 13C, and 14C

• 12C and 13C are stable

• 12C has 6 protons and 6 neutrons

• 13C has six protons and 7 neutrons

• 14C (radiocarbon) has 6 protons and 8 neutrons

The extra two neutrons make 14C unstable and radioactive

• Radiocarbon (14C) is constantly being produced by cosmic radiation hitting nitrogen in the upper atmosphere.

• After the radiocarbon is produced, it combines with oxygen to form 14CO2.

• While a plant is metabolically active, it takes CO2 from the atmosphere and converts it into sugar during photosynthesis.

• Metabolic processes maintain the 14C content of the living organism in an equilibrium with the atmospheric 14C.

• Once a plant is no longer metabolically active, no new carbon atoms are acquired, and the 14C present in the organism slowly decays.

Woody plants grow from the center and add rings as they grow.

The inner wood is dead and already aging from a radiocarbon perspective.

Living Cambium layer (green ring between dead

wood and bark)

Thuja plicata – Western red cedar

Radiocarbon Dates from Western Red Cedar Wood Section, Washington

Sample

No.

Radiocarbon Date

1-Sigma Calibrated Date (68.2%)

2-Sigma Calibrated Date (95.4%)

SKO1-1I(Inner rings)

400 ± 20 RCYBP

505-465 (68.2%)

CAL yr. BP

510-530 (87.2%)

350-330 (8.2%)

CAL yr. BP

SKO1-1O(Outer rings)

145 ± 15 RCYBP

270-250 (11.5%)

230-170 (28.4%)

150-130 (10%)

40-10 (18.3%)

CAL yr. BP

280-250 (15.3%)

230-170 (33.5%)

160-130 (11.8%)

120-70 (14.7%)

40-(-1) (20.22%)

CAL yr. BP

Average Life Span for Great Basin Shrubs & Trees

Scientific Name Common Name Average Life Span

Alnus incana Mountain alder 60-100 years

Artemisia tridentata Big sagebrush 25-50 years

Atriplex canescens Four-wing saltbush 20-100 years

Cercocarpus sp. Mountain mahogany 150+ years

Ephedra sp. Mormon tea, Jointfir, Ephedra

120 years

Gutierrezia sarothrae Broom snakeweed 20 years

Juniperus scopulorum Rocky Mountain juniper 200-300 years,

Up to 1500-2000 years

Pinus longaeva Great Basin bristlecone pine

4000+ years

Purshia tridentata Antelope bitterbrush 100+ years

A Tale of Two Dates

Charcoal was identified from four prehistoric hearths.

20 years ago, unidentified charcoal from hearths in the same cultural layer yielded dates of 950 ± 80 RCYBP and 870 ± 80 RCYBP.

The four hearths contained varying amounts of sagebrush, juniper, and pine charcoal, with juniper and pine dominating the assemblage.

Comparison of Life Spans for Charcoal Types Present in Four Hearth Samples

Scientific Name Common Name Average Life Span

Pinus contorta Lodgepole pine 150 years

Pinus ponderosa Ponderosa pine 300 years

Pinus monticola Western white pine 350 years

Pinus albicaulis Whitebark pine 450 years

Pinus flexilis Limber pine Up to 300 years

Juniperus scopulorum Rocky Mountain juniper 200-300 years average, up to 1500-2000 years

Artemisia sp. Sagebrush 25-50 years average,

up to 150 years

Radiocarbon dates:

Top 2 dates are “original dates” run about 20 years ago

Next 4 dates are on sagebrush charcoal from four prehistoric hearths

Bottom date is on pine charcoal from one of the four prehistoric hearths (#1)

Atmospheric data from Reimer et al (2004);OxCal v3.10 Bronk Ramsey (2005); cub r:5 sd:12 prob usp[chron]

2000BP 1500BP 1000BP 500BP 0BP

Calendar date

Previous date 1 950±80BP

Previous date 2 870±80BP

PRI-08-10-1 340±25BP

PRI-08-10-2 315±20BP

PRI-08-10-3 345±25BP

PRI-08-10-4 360±20BP

PRI-08-10-1PI 840±20BP

The original two radiocarbon dates and the recent date from the pine charcoal suggest site occupation was around 800 years ago.

Radiocarbon dates from the sagebrush charcoal suggest that site occupation was around 400 years ago.

Comparison of AMS Radiocarbon Dating of Unidentified Charcoal/Bulk Sample Vs.

AMS Radiocarbon Dating of Identified Charcoal at La Revive, France


4000CalBP 3500CalBP 3000CalBP 2500CalBP 2000CalBP 1500CalBP

Calibrated Date

2360 ± 120 BP

2515 ± 15 BP

2535 ± 15 BP

Beta Date

PRI Date

PRI Date

Unidentified/Bulk Sample

Alder Charcoal

Alder Charcoal

Ash Stains sometimes don’t contain big charcoal.

What’s even smaller? Microscopic Charcoal.

Microscopic Charcoal

10X

50X

If properly sampled, all of the laminations that you see here can be individually dated with microcharcoal


3000CalBP2700CalBP2400CalBP2100CalBP1800CalBP1500CalBP1200CalBP 900CalBP

Calibrated date

PRI-09-19-4.4MC 1995±25BP

PRI-09-19-4.3 1985±25BP

PRI-09-19-6.2MC 1250±20BP

PRI-09-19-6.2 1275±30BP

Comparison of Microcharcoal and

“Chunk” Charcoal AMS Dates.

Chrysothamnus Charcoal

MicroCharcoal

Microcharcoal

Unidentified Hardwood twig

Highlighted pairs of charcoal and microscopic charcoal from the same features


4000BP 3500BP 3000BP 2500BP 2000BP 1500BP 1000BP 500BP 0BP

Calendar date

PRI-07-84-1 220±15BP

PRI-09-19-6.2MC test 1250±20BP

PRI-09-19-6.2 1275±30BP

PRI-08-79-53 1295±15BP

PRI-09-107-RC10 1340±15BP

PRI-08-127-9339-F9 1465±20BP

PRI-09-107-RC06 1490±15BP

PRI-07-64-6773-15 1510±20BP

PRI-09-107-RC12 1850±15BP

PRI-09-19-4.3 1985±25BP

PRI-09-19-4.4MC test 1995±25BP

0.0018Microcharcoal

0.0025Microcharcoal

0.0018Chryothamnus charcoal

0.00267Microcharcoal

0.0004Microcharcoal

0.0031Microcharcoal

0.0006Microcharcoal

0.0021Microcharcoal

0.0022Microcharcoal

0.0025Unidentified Hardwood twig-vitirified

0.0025Microcharcoal

Residue from a single ceramic sherdcan yield information on …

PollenStarchPhytolithsFTIRAMS Date

Atmospheric data from Reimer et al (2004);OxCal v3.10 Bronk Ramsey (2005); lin r:5 sd:12 prob usp[chron]

CalBC/CalAD 200CalAD 400CalAD

Calibrated Date

1600BP

1700BP

1800BP

1900BP

2000BPR

ad

ioca

rbo

n D

ete

rmin

atio

n09-67-128 : 1815 ± 20 BP

68.2% Probability (13.9%) AD 135-155 (25.2%) AD 165-195 (29.1%) AD 205-240 95.4% Probability (95.4%) AD 130-250

Ceramic residue,Ohio

AMS Date: 1815 +/- 20 BP

Pollen and StarchGrass seeds and maize

Multiplot of AMS Radiocarbon Dates for Ceramic Residue


3000CalBP 2500CalBP 2000CalBP 1500CalBP 1000CalBP

Calibrated Date

1815 ± 20 BP

1275 ± 20 BP

1670 ± 15 BP

2180 ± 50 BP

Ohio

Illinois

Florida

Florida

Refinement of Dates

• Small sigmas (+/- 15 to 25 years) are possible.• Nutshell is not the only annual or short-lived

charcoal to date. • Dating ceramic residue offers the most direct

evidence of human activity.• Occupations are year by year, not century by

century or millenium by millenium.• Relevant questions for multiple dates:

– Are people coming back or not?– Permanent or semi-permanent residences?

Calibrations

• Reporting dates in radiocarbon years is extremely valuable for preserving our future ability to calibrate accurately.

• Four calibration curves between 1986 and present, all yielding different calibrated dates from the same radiocarbon age.

• Decalibrating is dependent on knowing the calibration curve used originally.

Calibration: 4 Calibration CurvesM. Stuiver and R.S. Kra eds. 1986 Radicarbon 28(2B): 805-1030;OxCal v3.10 Bronk Ramsey (2005); cub r:5 sd:12 prob usp[chron]

6000CalBC 4000CalBC 2000CalBC CalBC/CalAD 2000CalAD

Calibrated date

CAL 86

0BP

2000BP

4000BP

6000BP

8000BP

10000BP

Rad

ioca

rbo

n d

eter

min

atio

n

CAL 86

M. Stuiver, A. Long and R.S. Kra eds. 1993 Radiocarbon 35(1); OxCal v3.10 Bronk Ramsey (2005); cub r:5 sd:12 prob usp[chron]

20000CalBC15000CalBC10000CalBC 5000CalBCCalBC/CalAD

Calibrated date

0BP

5000BP

10000BP

15000BP

20000BP

25000BP

30000BP

Rad

ioca

rbo

n d

eter

min

atio

n

Atmospheric data from Stuiver et al. (1998);OxCal v3.10 Bronk Ramsey (2005); cub r:5 sd:12 prob usp[chron]

20000CalBC15000CalBC10000CalBC 5000CalBCCalBC/CalAD

Calibrated date

0BP

5000BP

10000BP

15000BP

20000BP

25000BP

30000BP

Rad

ioca

rbo

n d

eter

min

atio

n


25000CalBC20000CalBC15000CalBC10000CalBC 5000CalBCCalBC/CalAD

Calibrated date

0BP

5000BP

10000BP

15000BP

20000BP

25000BP

30000BPR

adio

carb

on

det

erm

inat

ion

CAL 86

CAL 93

INTCAL 98

INTCAL 04


6400CalBC 6300CalBC 6200CalBC 6100CalBC 6000CalBC 5900CalBC

Calibrated date

7000BP

7100BP

7200BP

7300BP

7400BP

7500BP

Rad

ioca

rbon

det

erm

inat

ion

INTCAL 04 : 7300±15BP 68.2% probability 8170BP (68.2%) 8050BP 95.4% probability 8180BP (95.4%) 8030BP

Atmospheric data from Stuiver et al. (1998);OxCal v3.10 Bronk Ramsey (2005); cub r:5 sd:12 prob usp[chron]

6400CalBC 6300CalBC 6200CalBC 6100CalBC 6000CalBC 5900CalBC

Calibrated date

7100BP

7200BP

7300BP

7400BP

7500BP

Rad

ioca

rbon

det

erm

inat

ion

INTCAL 98 : 7300±15BP 68.2% probability 8170BP (44.0%) 8100BP 8090BP (24.2%) 8030BP 95.4% probability 8180BP (95.4%) 8020BP

M. Stuiver, A. Long and R.S. Kra eds. 1993 Radiocarbon 35(1); OxCal v3.10 Bronk Ramsey (2005); cub r:5 sd:12 prob usp[chron]

6300CalBC 6200CalBC 6100CalBC 6000CalBC 5900CalBC

Calibrated date

7100BP

7200BP

7300BP

7400BP

7500BP

Rad

ioca

rbon

det

erm

inat

ion

CAL 93 : 7300±15BP 68.2% probability 8120BP (47.0%) 8060BP 8050BP ( 8.8%) 8030BP 8020BP (12.3%) 8000BP 95.4% probability 8130BP (95.4%) 7990BP

M. Stuiver and R.S. Kra eds. 1986 Radicarbon 28(2B): 805-1030;OxCal v3.10 Bronk Ramsey (2005); cub r:5 sd:12 prob usp[chron]

5450BC 5400BC 5350BC 5300BC 5250BC

Calendar date

CAL 86 : 7300±15BP 68.2% probability 7320BP (68.2%) 7285BP 95.4% probability 7335BP (95.4%) 7270BP

0.0

0.2

0.4

0.6

0.8

Rel

ativ

e pr

obab

ilit

y

Charcoal Assemblages can Vary

What to Report?

• Date in RCYBP• Calibrated date (including calibration curve used

in the methods)• A description of what was dated:

– Nutshell– Wood charcoal identified to genus (or family)– Seed– Bone– Shell

• A date of 1815 +/- 20 RCYBP, which calibrates to AD 130-250 at the two-sigma level, is reported on ceramic residue (or alder charcoal or nutshell…)

Know What You’re Dating! (and report it!)



Calibrated Date

600BP

700BP

800BP

900BP

1000BP

1100BPR

adio

carb

on D

eter

min

atio

nPRI-08-10-1PI : 840 ± 20 BP

68.2% Probability (68.2%) 775-725 BP 95.4% Probability (95.4%) 790-695 BP

Radiocarbon date for pine charcoal

Atmospheric data from Reimer et al (2004);OxCal v3.10 Bronk Ramsey (2005); lin r:5 sd:12 prob usp[chron]


Calibrated Date

PRI-08-10-1 340 ± 25 BP

PRI-08-10-2 315 ± 20 BP

PRI-08-10-3 345 ± 25 BP

PRI-08-10-4 360 ± 20 BP

Radiocarbon dates for sagebrush charcoal from four prehistoric hearths

14C has a half-life of about 5730 years, meaning that half (50%) of the 14C present in an organism will turn into 14N in 5730 years. In another 5730 years, only 25% of the original 14C will remain, and so on.

14C

↓

↓

Both solar activity and geomagnetic field strength affect the amount of cosmic radiation hitting the earth.

Because these two phenomena are variable, the amount of radiocarbon produced in the atmosphere has fluctuated over time.

The amount of radiocarbon present in living organisms also has fluctuated, creating the need to calibrate the radiocarbon age to determine the sample’s age in calendar years.



Calibrated Date

2500BP

2600BP

2700BP

2800BP

2900BPR

adio

carb

on D

eter

min

atio

nPRI-08-56-SBC2-2CO : 2685 ± 15 BP

68.2% Probability (68.2%) 2785-2755 BP95.4% Probability (12.6%) 2845-2805 BP (82.8%) 2800-2750 BP

Dating Residue on Ceramics

Documents

AMS Radiocarbon Dating: Do You Know What You’re Dating? Kathryn Puseman Linda Scott Cummings R.A. Varney PaleoResearch Institute