Geoscience Reference
In-Depth Information
half-life (1,250 Ma), this method is not especially useful for rocks younger than
about 50 ka. As a result of the very large error terms involved in dating young
rocks using
40
K/
40
Ar dating, efforts were made to develop a more precise method
based on the ratio between two argon isotopes (McDougall and Harrison,
1999
).
The isotopes in question are
40
Ar and
39
Ar.
39
Ar is produced in the laboratory by
irradiating the sample to be dated with fast neutrons in order to convert
39
Kto
39
Ar. The
40
Ar/
39
Ar method is more precise for younger rocks because of the shorter half-lives
involved, allowing samples as young as 10 ka to be dated (
Tabl e 6 . 1
). This method has
recently been used to obtain a very precise age of 73.88
) for the super-
eruption of the youngest tephra from Toba volcano, known as the YTT (Storey et al.,
2012
).
±
0.32 ka (1
σ
6.5.2 Radiocarbon dating of organic and inorganic carbon
Radiocarbon dating is the method most widely used to date late Quaternary marine
and terrestrial sediments. Willard F. Libby invented the method (Arnold and Libby,
1949
; Libby,
1955
), for which he received the Nobel Prize for chemistry in 1960.
Like many outstanding scientific discoveries, this one arose quite by accident. In
1947, Libby and his colleagues had collected samples of methane gas produced
by Baltimore's Patapsco Sewage Plant and found that it contained trace amounts of
radioactive carbon (
14
C), showing that living organisms harboured this isotope (Balter,
2006
). Libby (
1973
, p. 7) described radiocarbon dating succinctly as 'a measurement
of the age of dead matter by comparing the radiocarbon content with that in living
matter'.
Radiocarbon is produced in the outer atmosphere by cosmic rays that generate
neutrons that then react with the nucleus of stable
14
N, detaching a proton, to form
the radiocarbon isotope of mass 14 and half-life of 5,568
±
30 years (Libby,
1955
).
In fact, the half-life is more accurately given as 5,730
40 (Godwin,
1962
), but
for convenience, the original Libby half-life estimate is still used by all radiocarbon
laboratories. Because the mean life of any one radiocarbon atom is approximately
8,300 years, there is ample time for its mixing and assimilation in atmosphere, bio-
sphere and ocean. Plants will take in some radiocarbon from the atmosphere during
photosynthesis. Marine or aquatic organisms will absorb radiocarbon dissolved in
the oceans or in freshwater, and that radiocarbon will become incorporated into their
calcareous shells. Soil and lake carbonates, speleothems and tufas (see
Chapter 14
)
likewise absorb radiocarbon dissolved in rain, run-off or groundwater during the time
(which may be of quite long duration) in which they are being precipitated. Animals
will absorb
14
C from the atmosphere as they breathe, and this becomes incorporated
into their bones and soft tissues. Once the organisms die, the
14
C within the dead
organism starts to decay, with half of the
14
C converted back to the stable isotope
14
N within about 5,730 years and half of what then remains converted to
14
N after a
±
