which can be made within a reasonable period, and
within available financial resources. Today, the
principal constraint is the inadequacy of spatial and
temporal coverage of the tracer data. With more
synoptic data, tracer modeling could also be refined.
Any shortcomings in these foil the goal of these
studies. We are now in the mode of understanding
details of ocean circulation and chemistry; not just
the integrated overall features. In the earliest applications of cosmogenic tracers just a few measurements of radiocarbon in the oceans were sufficient
to show that the upper ocean had a turnover time of
about 600 years. In the case of the atmosphere, the
mean time for removal of aerosols was similarly
learned very quickly from the observed fallout of
nuclear weapons tests-injected radioactivities (
90 Sr,
3 H, etc.) and from the fallout of cosmogenic
22 Na,
7
Be and
32 P. With continued applications of
these tracers, it has been shown that oceanic processes are indeed complex, especially at the interfaces (air–sea, mixed layer–intermediate waters–
deep waters, and water–sediment), and in the polar
oceans, which determine formation of bottom–deep
waters. Thus, our hasty perception of these tracers
being a panacea quickly changed, even back in the
1970s.
What is the basic nature of large-scale oceanic
circulation? The radiochemists used the Kw model in
its simple one-dimensional form; K denotes the eddy
diffusivity, and w the advection velocity. At this time
a stimulating discussion of essential mathematical
approaches for treating the tracer data was presented. Simple material balance calculations by Lal
showed that an appreciable amount of carbon was
added to the dissolved carbon inventory (J-flux) at
depths by sinking biogenic particles. This important
aspect of resetting the
14 C clock in the deep sea by Jflux is now being examined on a global basis under
WOCE experiments, and constitutes a critical parameter in climatic feedback processes. Subsequently,
tracer data showed that an important transfer of
oceanic properties occurred across pycnoclines.
Modeling of tracer data in fact reveals model inadequacies and fosters development of more appropriate physical models. There are several very basic
issues that are recognized, but not well understand.
For example, what are the roles of tides and internal
waves in large-scale ocean circulation? These questions have been asked several times, but not yet attacked properly due to our present limitations.
Today, we are far from a synthesis of comprehensive
models which are capable of providing an interactive
atmosphere–ocean coupled model which can respond to changes in climate, or predict climatic
changes as the model is run.
There are several academic and technical issues
which we are confronting today:
1. complexity of the ocean system; variable response
at different time and space scales;
2. lack of three-dimensional tracer data;
3. lack of information on temporal and spatial
changes in tracer distribution;
4. lack of understanding of physical, chemical, and
biological processes.
However, there are proven methods based on
14
C,
10
Be,
26
Al,
39
Ar,
32
Si,
7
Be,
33
P and
32
P. Improved
tracer modeling will emerge only with further advances in techniques for their measurement, and with
a better understanding of the atmospheric and oceanic
circulation, mixing, and biogeochemical processes.
On the question of ease of measurement, the radionuclide
39
Ar is an important case in point here. It is a
conservative tracer, ideally suited for studying vertical
mixing in the oceans, but to date very few measurements have been made, since they are very time consuming. Another example is that of
32
Si. Although it
has been measured at several stations in the Atlantic,
Pacific and Indian oceans, these measurements are not
currently precise enough to make detailed mixing and
transport models to define the silica cycle in the
oceans. They are, however, useful to determine vertical J-fluxes, one-diemnsional K/o ratios, and the
latitudinal inventories of
32
Si in the oceans.
Physical oceanography provides the theoretical
basis for oceanic mixing and circulation, but the
experimental data necessary to understand the nature of this circulation must be based on present day
and proxy observations of chemical composition of
sea water in space and time. Directed global scale
coordination between scientists to study important
oceanic processes, such as physical and biological
controls on biological production, and export of
carbon, are rapidly providing new insights and accelerated developments of realistic models. An example is the coordinated US Joint Global Ocean Flux
Studies (JGOFS) in the Equatorial Pacific Ocean in
1992, during a four-month period which coincided
with the maximum intensity of the warm El Nin ˜ o
event, and another three-month period during
well-developed cool surface-water conditions. The
combined physical/chemical and biological data
produced, which included
234 Th concentrations of
sinking particulates, led to new insights about the
roles of dissolved organic carbon, microzooplankton
grazing, nutrient and CO 2 fluxes, and highlighted the
importance of physical, in contrast to biological,
processes in this region, where net carbon fluxes out
of the system are very small as a result of highly
efficient biological cycling.
COSMOGENIC ISOTOPES 233
within available financial resources. Today, the
principal constraint is the inadequacy of spatial and
temporal coverage of the tracer data. With more
synoptic data, tracer modeling could also be refined.
Any shortcomings in these foil the goal of these
studies. We are now in the mode of understanding
details of ocean circulation and chemistry; not just
the integrated overall features. In the earliest applications of cosmogenic tracers just a few measurements of radiocarbon in the oceans were sufficient
to show that the upper ocean had a turnover time of
about 600 years. In the case of the atmosphere, the
mean time for removal of aerosols was similarly
learned very quickly from the observed fallout of
nuclear weapons tests-injected radioactivities (
90 Sr,
3 H, etc.) and from the fallout of cosmogenic
22 Na,
7
Be and
32 P. With continued applications of
these tracers, it has been shown that oceanic processes are indeed complex, especially at the interfaces (air–sea, mixed layer–intermediate waters–
deep waters, and water–sediment), and in the polar
oceans, which determine formation of bottom–deep
waters. Thus, our hasty perception of these tracers
being a panacea quickly changed, even back in the
1970s.
What is the basic nature of large-scale oceanic
circulation? The radiochemists used the Kw model in
its simple one-dimensional form; K denotes the eddy
diffusivity, and w the advection velocity. At this time
a stimulating discussion of essential mathematical
approaches for treating the tracer data was presented. Simple material balance calculations by Lal
showed that an appreciable amount of carbon was
added to the dissolved carbon inventory (J-flux) at
depths by sinking biogenic particles. This important
aspect of resetting the
14 C clock in the deep sea by Jflux is now being examined on a global basis under
WOCE experiments, and constitutes a critical parameter in climatic feedback processes. Subsequently,
tracer data showed that an important transfer of
oceanic properties occurred across pycnoclines.
Modeling of tracer data in fact reveals model inadequacies and fosters development of more appropriate physical models. There are several very basic
issues that are recognized, but not well understand.
For example, what are the roles of tides and internal
waves in large-scale ocean circulation? These questions have been asked several times, but not yet attacked properly due to our present limitations.
Today, we are far from a synthesis of comprehensive
models which are capable of providing an interactive
atmosphere–ocean coupled model which can respond to changes in climate, or predict climatic
changes as the model is run.
There are several academic and technical issues
which we are confronting today:
1. complexity of the ocean system; variable response
at different time and space scales;
2. lack of three-dimensional tracer data;
3. lack of information on temporal and spatial
changes in tracer distribution;
4. lack of understanding of physical, chemical, and
biological processes.
However, there are proven methods based on
14
C,
10
Be,
26
Al,
39
Ar,
32
Si,
7
Be,
33
P and
32
P. Improved
tracer modeling will emerge only with further advances in techniques for their measurement, and with
a better understanding of the atmospheric and oceanic
circulation, mixing, and biogeochemical processes.
On the question of ease of measurement, the radionuclide
39
Ar is an important case in point here. It is a
conservative tracer, ideally suited for studying vertical
mixing in the oceans, but to date very few measurements have been made, since they are very time consuming. Another example is that of
32
Si. Although it
has been measured at several stations in the Atlantic,
Pacific and Indian oceans, these measurements are not
currently precise enough to make detailed mixing and
transport models to define the silica cycle in the
oceans. They are, however, useful to determine vertical J-fluxes, one-diemnsional K/o ratios, and the
latitudinal inventories of
32
Si in the oceans.
Physical oceanography provides the theoretical
basis for oceanic mixing and circulation, but the
experimental data necessary to understand the nature of this circulation must be based on present day
and proxy observations of chemical composition of
sea water in space and time. Directed global scale
coordination between scientists to study important
oceanic processes, such as physical and biological
controls on biological production, and export of
carbon, are rapidly providing new insights and accelerated developments of realistic models. An example is the coordinated US Joint Global Ocean Flux
Studies (JGOFS) in the Equatorial Pacific Ocean in
1992, during a four-month period which coincided
with the maximum intensity of the warm El Nin ˜ o
event, and another three-month period during
well-developed cool surface-water conditions. The
combined physical/chemical and biological data
produced, which included
234 Th concentrations of
sinking particulates, led to new insights about the
roles of dissolved organic carbon, microzooplankton
grazing, nutrient and CO 2 fluxes, and highlighted the
importance of physical, in contrast to biological,
processes in this region, where net carbon fluxes out
of the system are very small as a result of highly
efficient biological cycling.
COSMOGENIC ISOTOPES 233