Dating In Archaeology Lecture 3 Radiocarbon Dating Radiocarbon

Dating In Archaeology Lecture 3 Radiocarbon Dating

Radiocarbon Dating • Radiocarbon is an unstable isotope. An isotope is a version of an atom that varies in mass. 14 C 6 Mass number (total number of nucleons) Atomic number (just the number of protons)

• A radiocarbon atom has the same number of nucleons as Nitrogen, but the ratio of neutrons to protons is 8/6 = 14. There would be only 6 electrons.

14 CO 2

Decay Curve of Radiocarbon 100 % of original 75 radioisotope 50 remaining 12. 5% 25 0 Half Lives 1 Years B. P. 5730 2 11, 460 3 4 17, 190 22, 920 5 28, 650 6 34, 360 7 8 40, 110 45, 840 Decay: the reversion of a radioisotope to a stable form. Half-life: the amount of time it takes for one half of a quantity of a radioisotope to decay.

Dr. Rainer Berger in the UCLA Radiocarbon Lab. 1986. Conventional radiocarbon dating required sample of 5 g minumum

• In the early 1960’s, older radiocarbon dates (before 1200 BC) were shown to be inaccurate. The inaccuracy was due to the fact that the production of radiocarbon in the atmosphere varied year in, year out. • Hans Suess (the real Dr. Suess) found a way to correct the problems by measuring the amount of yearly radiocarbon variation in the tree rings of the bristlecone pine growing in California. He then constructed a calibration curve.

The calibration of dates these days is done with a computer program. Raw radiocarbon dates are indicated by a small bc, and calbrated dates by BC in capital letters.

Tandem Accelerator with a mass spectrometer AMS Dating

Sample Size Material Quantity Ideal Minimum Bone, antler or 600 mg ivory 100 mg Charcoal or 100 mg charred seeds 5 mg Insect remains 50 mg 5 mg Plant remains, cotton, linen or 100 mg paper Shells or other 50 mg carbonates Sediments Silk, wool or hair Wood Comments Depending on collagen preservation and method used Single seeds are often possible Depending on the methods used 20 mg 15 mg 50 mg 15 mg 120 mg 30 mg 100 mg 20 mg Depends strongly on carbon content and fraction being dated

Dating at O’Dea’s Castle

Initial chronological hypotheses • The tower house was constructed between 1470 -1490 AD. • There may have been an earlier residence on the site. • The tower house was occupied and wrecked by a Cromwellian garrison in 1651 AD. • C. 1700 O’Dea’s Castle abandoned?

Raw dates from Arizona AMS lab

• Early 14 th century date from a unit in front of the existing tower house. Calibration curve Probability density curve • Indicates earlier activity.

Late 15 th century date from same unit.

Mid 16 th century date from trench in front of bake house

th 17 Mid century date from unit in front of the bake oven unit 4 a