briefly examined. We are obviously interested in
learning about cycles of principal nutrient elements,
and processes by which the ocean chemistry evolves
(as regulated by aeolian and fluvial fluxes from the
land, biogeochemical cycles within the oceans, and
large-scale oceanic circulation). This problem can be
approached in bits and pieces only, and then the
interconnections and feedbacks examined. A comprehensive mosaic of all the interactions and controls
may or may not be achieved. The records of present
day ocean biogeochemical processes are recorded in
the sediments. It is therefore important to study the
chronology and the makeup of ocean sediments to
get a comprehensive picture of the temporal evolution of ocean chemistry and climate through aeons.
The suite of tracers listed in Table 3 has provided
sufficient information on oceanic processes in four
broad fields.
1. Biogeochemical cycling of nutrients and trace
elements
2. Chronology of marine sediments and manganese
nodules
3. Principal features of large-scale oceanic
circulation
4. Biogeochemical and ocean circulation controls on
climate.
In each of these studies, the task is complemented
by the availability of radiotracers belonging to the UTh series. It should be stressed that tracers each have
some particular unique features for studying critical
problems in oceanography. All tracers are not created equal: some are more equal than others. This
social expression also finds a rightful place in the
realm of oceans. This can be illustrated by citing
unique features of two of the cosmogenic nuclides,
14 C (half-lifem, 5730 years) and
10 Be (half-life, 1.5
My), which have a special status among all natural
tracers.
The great ‘virtues’ or attributes of
14 C are that (1)
it is a carbon isotope, and is introduced in to the
carbon cycle reservoirs as carbon dioxide in the
earth’s atmosphere, and (2) its half-life is well suited
to study late Quaternary events and processes, including dating of sediments and timing of deep and
bottom water formation.
In the oceans,
14 C does not behave as a conservative tracer, since carbon (and its compounds) is
not distributed uniformly in the oceans. But this does
not present any problems; rather its studies allow
determination of carbon fluxes within the ocean. In
fact, if
14 C was a conservative tracer, it could not
have been used to date marine sediments. It has been
added in significant amounts to the atmosphere as a
result of nuclear weapons tests during the 1950s and
1960s. Consequently,
14
C can be used both as a
‘steady-state’ and as a ‘transient’ tracer.
The second most attractive ocean tracer is the
cosmogenic
10 Be, which serves to delineate pathways
of particle active elements through the water column,
and is useful for dating sediments and in manganese
nodules to about 10 My BP. The particle active nature
of
10 Be leads to its preferential deposition in the
coastal regions of the oceans. Recent studies have
demonstrated that using special chemical techniques,
the activity of cosmogenic
26
Al can be measured in
the oceanic environment, and it has been suggested
that it should be a useful tracer for studying changes
in the past biological productivity of the oceans. This
application arises from the higher chemical reactivity
of
26
Al, compared to
10 Be. If this suggestion is borne
out from future studies,
26
Al would constitute an
invaluable tracer for studying temporal and spatial
variations in biological productivity. Its studies
would complement the information obtained using
the cosmogenic
10 Be.
Recent measurements of cosmogenic
32 P (half-life,
14.3 days) and
33
P (half-life, 25.3 days) in surface
ocean waters have opened up new possibilities of
quantifying P-biodynamics with complementary information on eddy diffusivity in the waters, based on
the cosmogenic
7 Be (Table 3). A wealth of new
32
P,
33 P data have been added on the distribution of
cosmogenic
32 P and
33
P in the surface ocean waters,
and in plankton.
Epilogue
A large number of cosmogenic tracers are available
for oceanic studies, and the use of these tracers has
steadily increased to date. But of course, tracers are
not the complete answer to the mysteries of the
ocean. It is necessary to learn how to use tracers,
how to model them, how to combine them with
other tracers, singly and multiply, etc. The usefulness and application of a tracer cannot be discussed
on an absolute basis, because such an approach
would result in a largely academic discussion. A
tracer may have the appropriate physical and
chemical attributes, but its source strength may be
too weak, or its source function may not be known
at the present time. Tracer suitability has therefore
to be evaluated periodically as frontiers of knowledge expand. With the freedom in thinking about
what type of tracer measurements can be made, a
great deal of valuable information will probably be
derived from it; however, one has to think about
applying it within practical constraints. An important constraint is the number of measurements,
232 COSMOGENIC ISOTOPES
learning about cycles of principal nutrient elements,
and processes by which the ocean chemistry evolves
(as regulated by aeolian and fluvial fluxes from the
land, biogeochemical cycles within the oceans, and
large-scale oceanic circulation). This problem can be
approached in bits and pieces only, and then the
interconnections and feedbacks examined. A comprehensive mosaic of all the interactions and controls
may or may not be achieved. The records of present
day ocean biogeochemical processes are recorded in
the sediments. It is therefore important to study the
chronology and the makeup of ocean sediments to
get a comprehensive picture of the temporal evolution of ocean chemistry and climate through aeons.
The suite of tracers listed in Table 3 has provided
sufficient information on oceanic processes in four
broad fields.
1. Biogeochemical cycling of nutrients and trace
elements
2. Chronology of marine sediments and manganese
nodules
3. Principal features of large-scale oceanic
circulation
4. Biogeochemical and ocean circulation controls on
climate.
In each of these studies, the task is complemented
by the availability of radiotracers belonging to the UTh series. It should be stressed that tracers each have
some particular unique features for studying critical
problems in oceanography. All tracers are not created equal: some are more equal than others. This
social expression also finds a rightful place in the
realm of oceans. This can be illustrated by citing
unique features of two of the cosmogenic nuclides,
14 C (half-lifem, 5730 years) and
10 Be (half-life, 1.5
My), which have a special status among all natural
tracers.
The great ‘virtues’ or attributes of
14 C are that (1)
it is a carbon isotope, and is introduced in to the
carbon cycle reservoirs as carbon dioxide in the
earth’s atmosphere, and (2) its half-life is well suited
to study late Quaternary events and processes, including dating of sediments and timing of deep and
bottom water formation.
In the oceans,
14 C does not behave as a conservative tracer, since carbon (and its compounds) is
not distributed uniformly in the oceans. But this does
not present any problems; rather its studies allow
determination of carbon fluxes within the ocean. In
fact, if
14 C was a conservative tracer, it could not
have been used to date marine sediments. It has been
added in significant amounts to the atmosphere as a
result of nuclear weapons tests during the 1950s and
1960s. Consequently,
14
C can be used both as a
‘steady-state’ and as a ‘transient’ tracer.
The second most attractive ocean tracer is the
cosmogenic
10 Be, which serves to delineate pathways
of particle active elements through the water column,
and is useful for dating sediments and in manganese
nodules to about 10 My BP. The particle active nature
of
10 Be leads to its preferential deposition in the
coastal regions of the oceans. Recent studies have
demonstrated that using special chemical techniques,
the activity of cosmogenic
26
Al can be measured in
the oceanic environment, and it has been suggested
that it should be a useful tracer for studying changes
in the past biological productivity of the oceans. This
application arises from the higher chemical reactivity
of
26
Al, compared to
10 Be. If this suggestion is borne
out from future studies,
26
Al would constitute an
invaluable tracer for studying temporal and spatial
variations in biological productivity. Its studies
would complement the information obtained using
the cosmogenic
10 Be.
Recent measurements of cosmogenic
32 P (half-life,
14.3 days) and
33
P (half-life, 25.3 days) in surface
ocean waters have opened up new possibilities of
quantifying P-biodynamics with complementary information on eddy diffusivity in the waters, based on
the cosmogenic
7 Be (Table 3). A wealth of new
32
P,
33 P data have been added on the distribution of
cosmogenic
32 P and
33
P in the surface ocean waters,
and in plankton.
Epilogue
A large number of cosmogenic tracers are available
for oceanic studies, and the use of these tracers has
steadily increased to date. But of course, tracers are
not the complete answer to the mysteries of the
ocean. It is necessary to learn how to use tracers,
how to model them, how to combine them with
other tracers, singly and multiply, etc. The usefulness and application of a tracer cannot be discussed
on an absolute basis, because such an approach
would result in a largely academic discussion. A
tracer may have the appropriate physical and
chemical attributes, but its source strength may be
too weak, or its source function may not be known
at the present time. Tracer suitability has therefore
to be evaluated periodically as frontiers of knowledge expand. With the freedom in thinking about
what type of tracer measurements can be made, a
great deal of valuable information will probably be
derived from it; however, one has to think about
applying it within practical constraints. An important constraint is the number of measurements,
232 COSMOGENIC ISOTOPES