Geology Reference
In-Depth Information
them are experimental or empirical and need calibra-
tion to produce numerical ages.
originally present to be reduced by half is called the
half-life
. Fortunately, the half-lives of suitable radioac-
tive isotopes vary enormously. The more important
isotopic transformations have the following half-lives:
5,730 years for carbon-14, 75,000 years for thorium-
230, 250,000 years for uranium-234, 1.3 billion years
for potassium-40, 4.5 billion years for uranium-238,
and 47 billion years for rubidium-87. These isotopes
are found in environmental materials.
Sidereal methods
Sidereal methods, also called calendar or annual meth-
ods, determine calendar dates or count annual events.
Apart from historical records, the three sidereal meth-
ods are as follows:
4
Radiocarbon
.
Carbon-14
occurs in wood,
charcoal, peat, bone, animal tissue, shells,
speleothems, groundwater, seawater, and ice. It is
a boon to archaeologists and Quaternary palaeo-
ecologists, providing relatively reliable dates in Late
Pleistocene and Holocene times.
5
Cosmogenic nuclides
. Radioactive
beryllium-
10
is produced in quartz grains by cosmic
radiation. The concentration of beryllium-10 in
surface materials containing quartz, in boulders for
instance, is proportional to the length of exposure.
This technique gives a very precise age deter-
mination. Aluminium-26, chlorine-36, helium-3,
and carbon-14 are being used experimentally in a
similar manner.
6
Potassium-argon
. This is a method based on
the radioactive decay of
potassium-40
trapped in
potassium-bearing silicate minerals during crystal-
lization to argon-40.
7
Uranium series
. This is a method based on
the radioactive decay of uranium and daughter
nuclides in biogenic chemical and sedimentary
minerals.
8
Lead-210
. This is a method based on radioactive
decay of lead-210 to lead-206.
9
Uranium-lead
. This method uses normalized lead
isotopes to detect small enrichments of radiogenic
lead from uranium and thorium.
1
Dendrochronology
or
tree-ring dating
.Tree
rings grow each year. By taking a core from a
tree (or suitable timbers from buildings, ships, and
so on) and counting the rings, a highly accurate
dendrochronological timescale can be established
and cross-referenced with carbon-14 dating. For
example, an 8,000-year carbon-14 record has been
pieced together from tree-rings in bristlecone pine
(
Pinus aristata
).
2
Varve chronology
. The distinct layers of sedi-
ments (varves) found in many lakes, especially
glacial lakes, are produced annually. In some
lakes, the varve sequences run back thousands
of years. Varves have also been discerned in
geological rock formations, even in Precambrian
sediments.
3
Sclerochronology
. This is an experimental
method based upon counting annual growth bands
in corals and molluscs.
Isotopic methods
These measure changes in isotopic composition due to
radioactive decay or growth or both. The environmen-
tal record contains a range of 'atomic clocks'. These
tick precisely as a parent isotope decays radioactively
into a daughter isotope. The ratio between parent and
daughter isotopes allows age to be determined with a
fair degree of accuracy, although there is always some
margin of error, usually in the range
Radiogenic methods
±
5-20 per cent.
The decay rate of a radioactive isotope declines expo-
nentially. The time taken for the number of atoms
These methods measure the cumulative effects of
radioactive decay, such as crystal damage and electron
energy traps.
