200
evaluate the age of organic material by measuring
its content of radioactive carbon (
14 C).
Radiocarbon is formed in the upper atmosphere by the reaction of nitrogen (
14 N) with neutrons (n):
14 N + n →
14 C +
1 H. The production of
14 C has been nearly constant over time, but had
been varying because of changes in the supply of
neutrons produced by cosmic radiation. Neutron
quantity is, thus, dependent upon the intensity of
cosmic radiation. The cosmic-ray fl ux in the
upper atmosphere is infl uenced by two factors,
namely,
(a) The intensity of earth’s magnetic fi eld
(b) Short-term changes in solar wind magnetic
properties
Fewer cosmic rays reach the upper atmosphere when the earth’s geomagnetic fi eld is
strong. Conversely, when it is weak, the inverse
occurs. Radioactive carbon decays back to
14 N
with emission of a β
− particle and has a half-life
of 5,568 years.
The
14 C of the atmosphere is a very small part
of the earth’s CO 2 reservoir. For purposes of
14 C
dating, it is assumed that
14 C produced in the
atmosphere is in rapid equilibrium with the CO 2
reservoirs.
During photosynthesis,
14 CO 2 is photosynthetically incorporated into organic matter in proportion to its availability in the atmosphere. When
organisms die, they enter into the sediments. The
14
C contained in the organic matter continues to
decay back to
14
N with the emission of a β
−
particle. However, the half-life of
14 C disintegration is constant. An estimate of the age of the
organic matter is obtained when the residual specifi c activity of
14
C in the carbon of old organic
matter is accurately radioassayed. The decay rate
of
14 C generally permits age determination back
to a limit of about 75,000 years. Newer methods
are being evolved (Grootes 1978 ).
Considerable efforts have been made in evaluating the accuracy of
14 C dating of organic deposits (Krishnaswami and Lal 1978 ). However,
recent changes in the past atmospheric
14 C levels
could be determined by measuring the presentday
14
C activity of wood laid down in trees.
However, dating of very recent sediments could
be problematic with
14 C methodology. More
recent dates of sediment deposits have been
obtained by analyses of lead-210 (
210 Pb). Also,
radium-226 in soils decays to radon-222, which
escapes to the atmosphere, where it decays to
210
Pb. This
210
Pb enters lake via precipitation and
is eventually incorporated into the sediments.
Concomitant to above, properties of magnetic
intensity and geomagnetic declination (direction)
are retained in the sediments as a remnant
magnetism after sediments are deposited in a lake.
The magnetic characteristics may refl ect the past,
but relatively short (approx. 1,000 years), variations
in the direction of the earth’s magnetic fi eld.
It may be noted here that hematite (Fe 2 O 3 )
possibly carries much of the magnetic remnants
(Creer et al. 1972 ). Preliminary studies done in
Lake Windermere in England revealed that the
direction of the horizontal magnetisation oscillates about a mean direction with an amplitude of
approx. ± 20° and a frequency of 2,700 years.
Remnant magnetism was similarly demonstrated
in sediments of a number of other lakes, but with
somewhat different periods of oscillation
(Tolonen et al. 1975 ; Thompson et al. 1980 ). This
method helps to date sediments rapidly by nondestructive methods once the detail chronology
has been established within a given lake region.
Concomitant to above, relatively coarse sediments are brought to recipient lakes in glacial
environments, with the infl ow of meltwater in the
spring and summer. These are deposited during
the period of thermal stratifi cation. Subsequently,
fi ner sediments are deposited over these during
winter when the lake is covered with ice. Similar
bimodal deposition could often be found in temperate lakes, particularly in the meromictic ones,
in which the sediments are not disturbed by water
circulation (Simola 1977 ).
12.2 Inorganic Chemistry
External source materials in dissolved and particulate forms usually leave the drainage basin
and enter the recipient lake basin. They are infl uenced markedly by vegetative cover. However,
long-term effects on the productivity of lakes are
infl uenced by climatic variations as well as by
12 Historical Records (Palaeolimnology)
evaluate the age of organic material by measuring
its content of radioactive carbon (
14 C).
Radiocarbon is formed in the upper atmosphere by the reaction of nitrogen (
14 N) with neutrons (n):
14 N + n →
14 C +
1 H. The production of
14 C has been nearly constant over time, but had
been varying because of changes in the supply of
neutrons produced by cosmic radiation. Neutron
quantity is, thus, dependent upon the intensity of
cosmic radiation. The cosmic-ray fl ux in the
upper atmosphere is infl uenced by two factors,
namely,
(a) The intensity of earth’s magnetic fi eld
(b) Short-term changes in solar wind magnetic
properties
Fewer cosmic rays reach the upper atmosphere when the earth’s geomagnetic fi eld is
strong. Conversely, when it is weak, the inverse
occurs. Radioactive carbon decays back to
14 N
with emission of a β
− particle and has a half-life
of 5,568 years.
The
14 C of the atmosphere is a very small part
of the earth’s CO 2 reservoir. For purposes of
14 C
dating, it is assumed that
14 C produced in the
atmosphere is in rapid equilibrium with the CO 2
reservoirs.
During photosynthesis,
14 CO 2 is photosynthetically incorporated into organic matter in proportion to its availability in the atmosphere. When
organisms die, they enter into the sediments. The
14
C contained in the organic matter continues to
decay back to
14
N with the emission of a β
−
particle. However, the half-life of
14 C disintegration is constant. An estimate of the age of the
organic matter is obtained when the residual specifi c activity of
14
C in the carbon of old organic
matter is accurately radioassayed. The decay rate
of
14 C generally permits age determination back
to a limit of about 75,000 years. Newer methods
are being evolved (Grootes 1978 ).
Considerable efforts have been made in evaluating the accuracy of
14 C dating of organic deposits (Krishnaswami and Lal 1978 ). However,
recent changes in the past atmospheric
14 C levels
could be determined by measuring the presentday
14
C activity of wood laid down in trees.
However, dating of very recent sediments could
be problematic with
14 C methodology. More
recent dates of sediment deposits have been
obtained by analyses of lead-210 (
210 Pb). Also,
radium-226 in soils decays to radon-222, which
escapes to the atmosphere, where it decays to
210
Pb. This
210
Pb enters lake via precipitation and
is eventually incorporated into the sediments.
Concomitant to above, properties of magnetic
intensity and geomagnetic declination (direction)
are retained in the sediments as a remnant
magnetism after sediments are deposited in a lake.
The magnetic characteristics may refl ect the past,
but relatively short (approx. 1,000 years), variations
in the direction of the earth’s magnetic fi eld.
It may be noted here that hematite (Fe 2 O 3 )
possibly carries much of the magnetic remnants
(Creer et al. 1972 ). Preliminary studies done in
Lake Windermere in England revealed that the
direction of the horizontal magnetisation oscillates about a mean direction with an amplitude of
approx. ± 20° and a frequency of 2,700 years.
Remnant magnetism was similarly demonstrated
in sediments of a number of other lakes, but with
somewhat different periods of oscillation
(Tolonen et al. 1975 ; Thompson et al. 1980 ). This
method helps to date sediments rapidly by nondestructive methods once the detail chronology
has been established within a given lake region.
Concomitant to above, relatively coarse sediments are brought to recipient lakes in glacial
environments, with the infl ow of meltwater in the
spring and summer. These are deposited during
the period of thermal stratifi cation. Subsequently,
fi ner sediments are deposited over these during
winter when the lake is covered with ice. Similar
bimodal deposition could often be found in temperate lakes, particularly in the meromictic ones,
in which the sediments are not disturbed by water
circulation (Simola 1977 ).
12.2 Inorganic Chemistry
External source materials in dissolved and particulate forms usually leave the drainage basin
and enter the recipient lake basin. They are infl uenced markedly by vegetative cover. However,
long-term effects on the productivity of lakes are
infl uenced by climatic variations as well as by
12 Historical Records (Palaeolimnology)