1. Carbon dating Carbon has 3 isotopes: 12C – stable 13C – stable
14C – radioactive
Abundance:
12C = 98.89%
13C = %
14C = % = %

2. l ½ = 5730 yr. Radiocarbon Forms in upper atmosphere:
Decays everywhere:
l ½ = 5730 yr.

3. Vocabulary
Isotope
Parent isotope
Daughter isotope
Nuclear decay
Half-life

4. Early ideas Basic physical model (1939)
production high in upper atmosphere as 14CO2
fixed in the biosphere by photosynthesis
equilibrium between atmosphere and organisms
no equilibrium after death
14C concentration drops steadily after death

5. Checking the Radiocarbon Calibration through Time
What you need:
absolute age & radiocarbon age
for the same object
Where you find it:
tree rings
coral growth
marine varves
cave deposits
What you discover:
a history of 14Catmos that shows some
fluctuations over time
radiocarbon
sampling
transect
tree cut in 1999A.D.
1821A.D. by ring-counting

6. The Radiocarbon Calibration Curve (atmospheric 14C history)
Principle: compare radiocarbon dates with independent dates
Examples of independent dating: tree-ring counting, coral U-Th dates, varve counting,
correlation of climate signals in varves with ice core
data from:
corals (bright red)
lake varves (green)
marine varves (blue)
speleothems (orange)
tree rings (black)
Observation:
radiocarbon dates
are consistently younger than calendar ages
equiline
time
Hughen et al., 2004

7. Sources of Error in 14C dating
Geomagnetic field strength partly controls 14C production in the atmosphere by deflecting cosmic rays.
Modulation of the cosmic-ray flux by increased solar activity (e.g., sunspots, solar flares)
Exchange rate between ocean reservoir and atmospheric reservoir will affect the level of 14C in the atmosphere.
Industrial revolution pollutes recent organisms:
decrease in atmospheric 14C: 12C ratio from burning fossil fuels
creation of excess 14C by nuclear bomb testing