Carbon dating Carbon has 3 isotopes: 12C – stable 13C – stable 14C – radioactive Abundance: 12C = 98.89% 13C = 1.11% 14C = 10-8 % = .00000001%

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

Vocabulary Isotope Parent isotope Daughter isotope Nuclear decay Half-life

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

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

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

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