An important consideration in the use of a transient tracer is knowledge of its source strength. Anthropogenic sources are generally not well defined;
however, in the case of
14
C, an important advantage
is that its contribution to the atmospheric CO 2 reservoir is well known (i.e., the excess
14 C amount
relative to
12 C), and this precisely defines its source
function. In the case of anthropogenic
3 H, this is not
the case, but a great advantage is that one can
measure both the
3
H and tritugenic
3
He in a water
sample and defines an ‘age’ of the water mass. These
measurements are by no means easy, however, but a
large database of information has been produced
which has yielded very useful insights into large-scale
ocean circulation in the upper ocean.
There are also two important nonnuclear transient
tracers, chlorofluorocarbons CFC-11 and CFC-12,
which have also proven very useful in view of their
known (changing) relative concentrations in the atmosphere. These behave essentially as conservative
tracers; any CFC losses would not be expected to
change their ratio in the oceanic water mass. The use
of new tracers (F113, CCl 4 ) has extended the timescales of CFC in both directions.
In practice, one has to work with tracers of different properties, and each of its properties can be
taken advantage of, as its special attribute. Even
nonconservative tracers, e.g.,
14 C and
32
Si, have their
own significance and merit. In fact, in the oceans the
only conservative tracers of natural origin are
3 H
(half-life 12.3 y) and
3 He (stable).
New Techniques for Measurements of
Tracers in the Oceans in the 1980s
and 1990s
By the end of the 1970s, the field of cosmogenic
tracers had clearly recognized the usefulness of most
of the cosmogenic tracers, with sufficient measurements at hand in each case. After isolated studies of a
few tracers, e.g.,
14 C,
10 Be, in individual water
samples, it became apparent that oceans can yield
their secrets only with multiple tracer attack. In early
multiple tracer investigations, detailed information
regarding the nature and rate of processes responsible for the formation of the Antarctic Bottom water
was obtained by including the tracers
3
He and
14 C.
The field was expanded in the 1980s and 1990s with
larger-scale exploitation of several tracers for answering specific questions. This came about due to a
fruitful combination of events and discoveries, which
gave a tremendous fillip to both chemical oceanography and tracer studies including nuclides belonging to U-Th series. Foremost was the decision to
study oceans in a systematic manner, along geochemical sections (GEOSECS), using a suite of tracers. GEOSECS expeditions were successfully carried
out to the principal oceans in 1972–78 and resulted
in fairly accurate tracer data. The GEOSECS concept
was very successful; it rested on the necessity for
making more precise measurements of several tracers
and ocean properties in addition this integrated study
resulted in information about temporal changes in
the property profiles at the same stations after an
elapse of one to two decades since the site was occupied in the GEOSECS expedition. and finally, it
was an artful and timely combination of theory and
experiment, which gave a tremendous boost to the
field of learning about oceanographic processes.
The 1980s also marked an era of dramatic advance
in the techniques of measurements of long-lived cosmogenic radionuclides
14
C,
10
Be, and
26
Al in the
oceans, in sediments, and in manganese nodules using
AMS (accelerator mass spectrometry). This opened up
new windows for observing in detail a host of physical, chemical, and biological processes. The ease with
which these nuclides can be measured allowed long
series of measurements in space and time to be obtained. Examples of this development are the direct
measurements of
10
Be and
26
Al concentration profiles
in sea water in the principal oceans; and profiles of
10
Be concentrations in marine sediments and in
manganese nodules which opened up a new field of
investigation in marine beryllium geochemistry.
A new field, the study of P-biodynamics in surface
waters using cosmogenic
32
P and
33
P surfaced in the
late 1980s. This tracer application was not held up for
want of a technique. In this case, it was not realized
that these short-lived nuclides (half-lives, 14.3 and
25.3 days) in fact had about the appropriate half-lives
for studying timescales of exchange of phosphorus
between dissolved inorganic P, organic P and plankton. Concurrently, technical advances were also being
made to measure short-lived radionuclides in ocean
waters, where the AMS technique does not offer any
gain in detection sensitivity, e.g.
32
P (half-life,
14.3 days),
33
P (half-life, 25.3 days) and
32
Si (half-life,
150 years). By using a standard liquid scintillation
counting system to simultaneously measure both
32
P
and
33
P activities, much higher sensitivity is attainable
than by using low-level counters, especially for samples of low specific radioactivity.
Examples of Oceanic Data Derived
Using Cosmogenic Tracers
With this foreground what has been learnt about the
oceanic processes using cosmogenic tracers is now
COSMOGENIC ISOTOPES 231
however, in the case of
14
C, an important advantage
is that its contribution to the atmospheric CO 2 reservoir is well known (i.e., the excess
14 C amount
relative to
12 C), and this precisely defines its source
function. In the case of anthropogenic
3 H, this is not
the case, but a great advantage is that one can
measure both the
3
H and tritugenic
3
He in a water
sample and defines an ‘age’ of the water mass. These
measurements are by no means easy, however, but a
large database of information has been produced
which has yielded very useful insights into large-scale
ocean circulation in the upper ocean.
There are also two important nonnuclear transient
tracers, chlorofluorocarbons CFC-11 and CFC-12,
which have also proven very useful in view of their
known (changing) relative concentrations in the atmosphere. These behave essentially as conservative
tracers; any CFC losses would not be expected to
change their ratio in the oceanic water mass. The use
of new tracers (F113, CCl 4 ) has extended the timescales of CFC in both directions.
In practice, one has to work with tracers of different properties, and each of its properties can be
taken advantage of, as its special attribute. Even
nonconservative tracers, e.g.,
14 C and
32
Si, have their
own significance and merit. In fact, in the oceans the
only conservative tracers of natural origin are
3 H
(half-life 12.3 y) and
3 He (stable).
New Techniques for Measurements of
Tracers in the Oceans in the 1980s
and 1990s
By the end of the 1970s, the field of cosmogenic
tracers had clearly recognized the usefulness of most
of the cosmogenic tracers, with sufficient measurements at hand in each case. After isolated studies of a
few tracers, e.g.,
14 C,
10 Be, in individual water
samples, it became apparent that oceans can yield
their secrets only with multiple tracer attack. In early
multiple tracer investigations, detailed information
regarding the nature and rate of processes responsible for the formation of the Antarctic Bottom water
was obtained by including the tracers
3
He and
14 C.
The field was expanded in the 1980s and 1990s with
larger-scale exploitation of several tracers for answering specific questions. This came about due to a
fruitful combination of events and discoveries, which
gave a tremendous fillip to both chemical oceanography and tracer studies including nuclides belonging to U-Th series. Foremost was the decision to
study oceans in a systematic manner, along geochemical sections (GEOSECS), using a suite of tracers. GEOSECS expeditions were successfully carried
out to the principal oceans in 1972–78 and resulted
in fairly accurate tracer data. The GEOSECS concept
was very successful; it rested on the necessity for
making more precise measurements of several tracers
and ocean properties in addition this integrated study
resulted in information about temporal changes in
the property profiles at the same stations after an
elapse of one to two decades since the site was occupied in the GEOSECS expedition. and finally, it
was an artful and timely combination of theory and
experiment, which gave a tremendous boost to the
field of learning about oceanographic processes.
The 1980s also marked an era of dramatic advance
in the techniques of measurements of long-lived cosmogenic radionuclides
14
C,
10
Be, and
26
Al in the
oceans, in sediments, and in manganese nodules using
AMS (accelerator mass spectrometry). This opened up
new windows for observing in detail a host of physical, chemical, and biological processes. The ease with
which these nuclides can be measured allowed long
series of measurements in space and time to be obtained. Examples of this development are the direct
measurements of
10
Be and
26
Al concentration profiles
in sea water in the principal oceans; and profiles of
10
Be concentrations in marine sediments and in
manganese nodules which opened up a new field of
investigation in marine beryllium geochemistry.
A new field, the study of P-biodynamics in surface
waters using cosmogenic
32
P and
33
P surfaced in the
late 1980s. This tracer application was not held up for
want of a technique. In this case, it was not realized
that these short-lived nuclides (half-lives, 14.3 and
25.3 days) in fact had about the appropriate half-lives
for studying timescales of exchange of phosphorus
between dissolved inorganic P, organic P and plankton. Concurrently, technical advances were also being
made to measure short-lived radionuclides in ocean
waters, where the AMS technique does not offer any
gain in detection sensitivity, e.g.
32
P (half-life,
14.3 days),
33
P (half-life, 25.3 days) and
32
Si (half-life,
150 years). By using a standard liquid scintillation
counting system to simultaneously measure both
32
P
and
33
P activities, much higher sensitivity is attainable
than by using low-level counters, especially for samples of low specific radioactivity.
Examples of Oceanic Data Derived
Using Cosmogenic Tracers
With this foreground what has been learnt about the
oceanic processes using cosmogenic tracers is now
COSMOGENIC ISOTOPES 231