Using this method we can analyse both minerals
and whole rocks, since we assume that the
87 Sr formed
does not escape from the rock as easily as argon (in the
K-Ar method). When dating sediments one should
choose the finest-grained clay sediments with low
permeability. It can then be assumed that after deposition the sediment was homogeneous with respect to
strontium isotopes, and will provide an isochron which
dates any diagenesis occurring a relatively short time
after deposition. Larger clastic fragments, however,
will contain a strontium isotope ratio which
corresponds to the age of the source rock, and the
obtained date will be intermediate between the age of
the source rock and the time of deposition of the
sediments.
7.6.4 Carbon 14 Method
The carbon 14 method is the one most commonly used
for dating the youngest sediments, from about 50,000
years old up to the present.
14 C is formed in the
atmosphere by cosmic rays when a
14 N atom absorbs
a neutron and gives off a proton.
14 C is unstable, and
decays to
14 N. The production of
14 C thus occurs only
in the atmosphere, at altitudes over 10,000 m. The
14 C
then mixes with the lower air layers and the seawater.
The
14 C method is based on the assumption
(discussed below) that the
14 C content of the atmosphere has been constant for a long time, due to an
equilibrium between the
14 C added from the atmosphere and the
14 C which decays to
14 N. The half-life
of
14 C is 5,730 years. This means that after 5,730 years
half of the
14 C atoms will have changed to
14 N.
14 C
enters the carbon dioxide (CO 2 ) in the air and is taken
up by plants through photosynthesis. If we assume that
the carbon dioxide in the air in the past had as much
14 C as now, the
14 C content of older plants or plant
remains is an expression of their age. This age can be
determined analytically with relatively great accuracy,
for example 10,200 Æ 100 years, depending on the
nature of the sample. When 50–60,000 years has
elapsed since plant material formed (i.e. since it
ceased to take up CO 2 ), the
14 C content will be so
small that we will be approaching the limit of detection. This is then the upper limit to the age of material
we can analyse. We now know that the concentration
of
14 C has varied in the last 10,000 years and more due
to variation in cosmic radiation.
14 C from the atmosphere also becomes mixed with
sea water and freshwater. Carbonate-secreting
organisms which live in the sea and in lakes will take
up CO 2 from the water, and the amount of
14 C in their
CaCO 3 skeletons can be measured. Molluscan and
foraminiferal shells are particularly suitable for age
dating. Chemically precipitated carbonates can also
be dated in this way. Living organisms have a tendency to fractionate the lightest isotopes from the
heaviest. The ratio between two stable carbon
isotopes,
12
C and
13 C, can therefore be used to correct
14 C determinations.
7.6.5 Other Radiometric Methods
There are numerous other radiometric dating methods,
of which the uranium-lead method and the lead-lead
method (relationship between different lead isotopes)
are the most important.
The methods based on fission processes mentioned
above, involve long half-lives, 10
9 –10
10 years, so for
young sediments the quantity of decay products available to be analysed will be small and the accuracy
poor. An exception is the protactinium method
231 Pa=
230 Th
, which has given age determinations
on younger sediments showing good agreement with
the
14 C method.
The fission-track method is especially well suited
for younger rocks. It is applied to glass or minerals
which contain a sufficient amount of uranium 238. The
material will show tracks from fission products which
are observed as deformations. By counting the number
of such tracks, an expression of the age is obtained. In
apatite the tracks disappear (aneal) at about 100
C,
which is called the blocking temperature. The frequency of tracks is therefore an expression of the
time elapsed since rocks have been uplifted and cooled
to below the blocking temperature.
7.7
Chemostratigraphy
7.7.1 The
87 Sr/
86 Sr Method
The
87 Sr=
86 Sr ratio is particularly useful for
carbonates and other minerals precipitated in the
ocean. In the ocean water the
87 Sr=
86 Sr ratio has varied
7 Stratigraphy
245
and whole rocks, since we assume that the
87 Sr formed
does not escape from the rock as easily as argon (in the
K-Ar method). When dating sediments one should
choose the finest-grained clay sediments with low
permeability. It can then be assumed that after deposition the sediment was homogeneous with respect to
strontium isotopes, and will provide an isochron which
dates any diagenesis occurring a relatively short time
after deposition. Larger clastic fragments, however,
will contain a strontium isotope ratio which
corresponds to the age of the source rock, and the
obtained date will be intermediate between the age of
the source rock and the time of deposition of the
sediments.
7.6.4 Carbon 14 Method
The carbon 14 method is the one most commonly used
for dating the youngest sediments, from about 50,000
years old up to the present.
14 C is formed in the
atmosphere by cosmic rays when a
14 N atom absorbs
a neutron and gives off a proton.
14 C is unstable, and
decays to
14 N. The production of
14 C thus occurs only
in the atmosphere, at altitudes over 10,000 m. The
14 C
then mixes with the lower air layers and the seawater.
The
14 C method is based on the assumption
(discussed below) that the
14 C content of the atmosphere has been constant for a long time, due to an
equilibrium between the
14 C added from the atmosphere and the
14 C which decays to
14 N. The half-life
of
14 C is 5,730 years. This means that after 5,730 years
half of the
14 C atoms will have changed to
14 N.
14 C
enters the carbon dioxide (CO 2 ) in the air and is taken
up by plants through photosynthesis. If we assume that
the carbon dioxide in the air in the past had as much
14 C as now, the
14 C content of older plants or plant
remains is an expression of their age. This age can be
determined analytically with relatively great accuracy,
for example 10,200 Æ 100 years, depending on the
nature of the sample. When 50–60,000 years has
elapsed since plant material formed (i.e. since it
ceased to take up CO 2 ), the
14 C content will be so
small that we will be approaching the limit of detection. This is then the upper limit to the age of material
we can analyse. We now know that the concentration
of
14 C has varied in the last 10,000 years and more due
to variation in cosmic radiation.
14 C from the atmosphere also becomes mixed with
sea water and freshwater. Carbonate-secreting
organisms which live in the sea and in lakes will take
up CO 2 from the water, and the amount of
14 C in their
CaCO 3 skeletons can be measured. Molluscan and
foraminiferal shells are particularly suitable for age
dating. Chemically precipitated carbonates can also
be dated in this way. Living organisms have a tendency to fractionate the lightest isotopes from the
heaviest. The ratio between two stable carbon
isotopes,
12
C and
13 C, can therefore be used to correct
14 C determinations.
7.6.5 Other Radiometric Methods
There are numerous other radiometric dating methods,
of which the uranium-lead method and the lead-lead
method (relationship between different lead isotopes)
are the most important.
The methods based on fission processes mentioned
above, involve long half-lives, 10
9 –10
10 years, so for
young sediments the quantity of decay products available to be analysed will be small and the accuracy
poor. An exception is the protactinium method
231 Pa=
230 Th
, which has given age determinations
on younger sediments showing good agreement with
the
14 C method.
The fission-track method is especially well suited
for younger rocks. It is applied to glass or minerals
which contain a sufficient amount of uranium 238. The
material will show tracks from fission products which
are observed as deformations. By counting the number
of such tracks, an expression of the age is obtained. In
apatite the tracks disappear (aneal) at about 100
C,
which is called the blocking temperature. The frequency of tracks is therefore an expression of the
time elapsed since rocks have been uplifted and cooled
to below the blocking temperature.
7.7
Chemostratigraphy
7.7.1 The
87 Sr/
86 Sr Method
The
87 Sr=
86 Sr ratio is particularly useful for
carbonates and other minerals precipitated in the
ocean. In the ocean water the
87 Sr=
86 Sr ratio has varied
7 Stratigraphy
245