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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.

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slide1

AMS Radiocarbon Dating:

Do You Know What You’re Dating?

Kathryn Puseman

Linda Scott Cummings

R.A. Varney

PaleoResearch Institute

slide4

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
slide5

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.
slide7

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)

slide12

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.

slide14

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)

slide15

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.

slide16

Beta Date

Unidentified/Bulk Sample

Alder Charcoal

PRI Date

Alder Charcoal

PRI Date

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

AMS Radiocarbon Dating of

Identified Charcoal at La Revive, France

slide19

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

slide20

Microcharcoal

Unidentified Hardwood twig

MicroCharcoal

Chrysothamnus Charcoal

Comparison of Microcharcoal and

“Chunk” Charcoal AMS Dates.

slide22

Residue from a single ceramic sherd

can yield information on …

Pollen

Starch

Phytoliths

FTIR

AMS Date

slide23

Ceramic residue,

Ohio

AMS Date:

1815 +/- 20 BP

Pollen and Starch

Grass seeds and maize

slide24

Ohio

Illinois

Florida

Florida

Multiplot of AMS Radiocarbon Dates for Ceramic Residue

refinement of dates
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
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 curves
Calibration: 4 Calibration Curves

CAL 86

INTCAL 98

CAL 93

INTCAL 04

what to report
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…)
slide34

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

slide35

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.