Biology Reference
In-Depth Information
Radiocarbon dating
The most familiar of the absolute dating techniques is probably radiocar-
bon dating because it encompasses the time span of recent human history.
It was developed in 1949 by J. R. Arnold and W. F. Libby at the Univer-
sity of Chicago, for which they won the Nobel Prize in Chemistry in 1960,
and it was initially tested on a piece of Acacia wood from the tomb of the
pharaoh Zoser (2700-2600 BCE). The technique is based on the interac-
tion of cosmic radiation and nitrogen in the Earth's upper atmosphere to
produce radioactive carbon 14 C, the stable 13 C, and the most common 12 C
which constitutes 98.89 percent of the Earth's carbon. Production of the
isotopes takes place mostly at an altitude of 9 to 15 km at the geomagnetic
poles. The isotopes drift down, or are brought to Earth in the form of 14 CO 2
by rain or snow. They are then distributed by storms, winds, ocean cur-
rents, and rivers and become incorporated into organic biomass through
photosynthesis in plants and ingestion by animals. The isotopes are present
in a wide variety of materials, such as peat, wood, seeds, bone, shell, horn,
enamel, hides, and fl esh preserved in bogs and ice. Archeological materi-
als include leather, fabrics, parchment or vellum (animal skins used for
writing), and plant-derived paper. The decay process is through emission of
beta particles, at a rate of about fi fteen per minute per gram, to form non-
radioactive nitrogen. The half-life of 14 C is 5730 years, and the technique is
applicable to material as old as about 50,000 to 60,000 years. This limit is
because after 5730 years, half of the carbon isotope in a specimen is con-
verted to nitrogen (that is, the beta emissions are cut by half); after another
5730 years, half of the remainder disappears; and after about ten cycles
(or 57,300 years), the amount left is too small to measure. Application of
the technique to recent peat deposits younger than 300 years, preserving
records of events such as the Chernobyl accident, has been described by
Turetsky and others (2004).
In the early days of radiocarbon dating, it was not known that the amount
of 14 C in the atmosphere varies over time. This variation is due to fl uctuat-
ing intensities in cosmic radiation, and to the effectiveness of the upper
atmosphere and Van Allen Belts in defl ecting radiation. Van Allen Belts are
zones of cosmic radiation (the outer belt), some of which becomes trapped
in the Earth's magnetic fi eld (the inner belt). The auroras at the poles are
part of the inner Van Allen Belt. The widely recognized depletion of the
ozone layer allows more radioactive carbon to reach the Earth's surface now
than was possible before the industrial revolution beginning in the 1800s.
Thus, the early Libby dates must be adjusted because they are too young by
Search WWH ::




Custom Search