Radiocarbon

14-C is a radioactive isotope of carbon. It was discovered in 1934 by Grosse as an unknown activity in the mineral endialyte. In the same year, Kurie (Yale) exposed nitrogen to fast neutrons and observed long tracks in a bubble chamber. He had produced 14-C. Discovered in atmospheric CO2 by Libby in 1946. He determined the half life to be 5568 years. This half life has later been re-determined by Godwin (5730 years).

Radiocarbon

Libby recognized that due to its occurrence in natural materials, 14-C can be used as a dating tool for materials that contain carbon compounds derived from atmospheric CO2 either by simple mixing processes or by carbon exchange. 14-C is a widely used tool to establish climate records for the Holocene and Pleistocene.

The challenge in 14-C dating is the determination of the initial 14-C content of a substance at the time of recharge, i.e., at the time when it is isolated from exchange with the air.

There is also a stable isotope of carbon, 13-C. This isotope is important in that it allows us to correct for carbon isotope fractionation in nature and during analytical procedures.

• Atomic number is the number of protons, the number of protons determines the type of atom. All carbon atom have 6 protons, all nitrogen atoms have 7 protons. If the number of protons changes, the element changes.

• Mass number is the total mass of the atom, the number of protons and the number of neutrons are added. Elements can have different numbers of neutrons, if the number of neutrons is different then the atoms are isotopes. For example: 14C has 6 protons and 8 neutrons,13C has 6 protons and 7 neutrons,12C has 6 protons and 6 neutrons. 14C, 13C, and 14C are all isotopes of carbon.

• Radioactivity is the spontaneous emission of radiation from unstable nuclei. In general, isotopes with more neutrons than protons tend to be unstable and will emit radioactivity.

The global inventory of natural 14-C is about 75 tons.

Correction for isotopic mass fractionation

• d14-C represents the 14-C relative to the NIST standard and d13-C represents the deviation in the 13C/12C ratio relative to the Pee Dee Belemnite (Pee Dee River, South Carolina) standard. The 13C/12C ratio is measured by mass spectrometry.

Simple calculation• 14C(t) = 14C(t0) exp[-l (t-t0)] • ln 14C(t) = ln 14C(t0) -l (t-t0)• ln [14C(t)/14C(t0)] = -l (t-t0) • D t = (t - t0) = -1/l ln [14C(t)/14C(t0)]

= T1/2 / ln2 {ln [14C(t0)/14C(t)]}

• l: radioactive decay constant of 14C

• 1/l = mean life time = T1/2/ln2

Carbon 14 Dating

• 14-C is continually produced in the atomosphere and it is incorporated into all living creatures (via CO2 and food). The amount of 14-C in a living creature is constant.

• When the organism dies, the amount of 14-C starts to decrease, because no new 14-C from the atmosphere is being incorporated. By counting the remaining radioactivity, it is possible to tell how old an item is.

• Question: A piece of fossilized wood has a carbon 14 radioactivity that is 1/4 that of new wood. The halflife of carbon14 is 5730 years. How old is the fossilized wood? a) 1 x 5730 = 5730 years b) 2 x 5730 = 11,460 yearsc) 3 x 5730 = 17,190 yearsd) 0.25 x 5730 = 1432 years

This method is good for dating objects that are between 500 and 50,000 years old.

d14-C = -50 = (conc –stdt)/stdt 1000

conc/stdt= 1 – 50/1000 = 0.95

ln (conc/stdt) = ln (a_sample) = -0.051

T=8267 0.051 = 424.04 years

Anthropogenic 14C production

• The main source of anthropogenic 14C is so-called ‘bomb’ 14C, i.e., 14C produced during atmospheric testing of nuclear weapons.

• At the peak of surface testing of nuclear devices in 1963, the atmospheric 14C activity had reached about twice that of natural 14C-

• The bomb 14C has been produced by interaction of atmospheric nitrogen with the high neutron flux from the explosion of nuclear devices (mainly thermonuclear devices).

• Local increases in atmospheric 14C have been observed in the vicinity of nuclear power plants.

Development of 14C in atmospheric CO2 in the Northern Hemisphere in the last 50 years. Data before 1959 have been derived from tree rings (Stuiver and Quay, 1981). From 1959 to 1983 measurements were performed at the Alpine site Vermunt subsequent data from 1984 onwards are from the Schauinsland station in the Black Forest.