Dating

Stratigraphy

• As early as 1668 in De solido intra solidum naturaliter…the Danish scholar Niels Stensen (renamed Nicolaus Steno in college) argued that geological strata represent past depositional events. The vertical order of strata represented a sequence of deposition. This is the “priniciple of superposition.”

Younger

Older

Seriation

• Archaeologists key in on formal attributes that are chronologically sensitive when seriating artifacts.

Frequency seriation

Molds were first used in the early 17th century. Mechanical procedures for mass producing bottles date to the mid-19th century.

Dendrochronology

Andrew E. Douglass 1867-1962

Astronomer at the Lowell Observatory, Flagstaff, AZ – Interested in sun spot activity.

In 1914 he was persuaded by Clark Whistler to apply his technique to wood from prehistoric Puebloan structures of the Southwest

Logboat at the bottom of Lough Corrib

Irish Crannógs

Its not possible to date all wood, no matter

how well preserved.

Radiocarbon Dating

• Radiocarbon is an unstable isotope. An isotope is a version of an atom that varies in mass.

C14

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.

14CO2

Decay Curve of Radiocarbon

% of original radioisotope remaining

Half LivesYears B.P. 5730 11,460 17, 190 22,920 28,650 34,360 40,110 45,840

1 2 3 4 5 6 7 8

100

75

50

25

0

12.5%

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.

• 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.