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: 12 C, 13 C, and 14 C 12 C and 13 C are stable 12 C has 6 protons and 6 neutrons 13 C has six protons and 7 neutrons 14 C (radiocarbon) has 6 protons and 8 neutrons The extra two neutrons make 14 C unstable and radioactive

Radiocarbon ( 14 C) is constantly being produced by cosmic radiation hitting nitrogen in the upper atmosphere. After the radiocarbon is produced, it combines with oxygen to form 14 CO 2. While a plant is metabolically active, it takes CO 2 from the atmosphere and converts it into sugar during photosynthesis. Metabolic processes maintain the 14 C content of the living organism in an equilibrium with the atmospheric 14 C. Once a plant is no longer metabolically active, no new carbon atoms are acquired, and the 14 C 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%) SKO1-1 I (Inner rings) 400 ± 20 RCYBP (68.2%) CAL yr. BP (87.2%) (8.2%) CAL yr. BP SKO1-1O (Outer rings) 145 ± 15 RCYBP (11.5%) (28.4%) (10%) (18.3%) CAL yr. BP (15.3%) (33.5%) (11.8%) (14.7%) 40-(-1) (20.22%) CAL yr. BP

Average Life Span for Great Basin Shrubs & Trees Scientific NameCommon NameAverage Life Span Alnus incanaMountain alder years Artemisia tridentataBig sagebrush25-50 years Atriplex canescensFour-wing saltbush years Cercocarpus sp.Mountain mahogany150+ years Ephedra sp.Mormon tea, Jointfir, Ephedra 120 years Gutierrezia sarothraeBroom snakeweed20 years Juniperus scopulorumRocky Mountain juniper years, Up to years Pinus longaevaGreat Basin bristlecone pine years Purshia tridentataAntelope bitterbrush100+ 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 NameCommon NameAverage Life Span Pinus contortaLodgepole pine150 years Pinus ponderosaPonderosa pine300 years Pinus monticolaWestern white pine350 years Pinus albicaulisWhitebark pine450 years Pinus flexilisLimber pineUp to 300 years Juniperus scopulorumRocky Mountain juniper years average, up to years Artemisia sp.Sagebrush25-50 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)

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 Beta Date PRI Date Unidentified/Bulk Sample Alder Charcoal

Ash Stains sometimes don’t contain big charcoal.

What’s even smaller? Microscopic Charcoal.

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

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

Residue from a single ceramic sherd can yield information on … Pollen Starch Phytoliths FTIR AMS Date

Ceramic residue, Ohio AMS Date: /- 20 BP Pollen and Starch Grass seeds and maize

Multiplot of AMS Radiocarbon Dates for Ceramic Residue Ohio Illinois 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 Curves CAL 86 CAL 93 INTCAL 98 INTCAL 04

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 /- 20 RCYBP, which calibrates to AD at the two-sigma level, is reported on ceramic residue (or alder charcoal or nutshell…)

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

Radiocarbon date for pine charcoal

Radiocarbon dates for sagebrush charcoal from four prehistoric hearths

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

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.

Dating Residue on Ceramics