This discussion would not be complete without
mention of the ‘purposeful tracer experiment’ carried
out as a component of WOCE, using SF 6 released in
the open ocean. The results confirmed the earlier
estimates of very low diapycnal diffusivity, of the
order of 0.1 cm
2 s
À1 , implying that heat, salt, and
tracers must penetrate the thermocline primary by
transport along, rather than across, density surfaces.
Clearly, understanding of the large-scale ocean
circulation will come from multiple approaches, with
directed research to simultaneously understand
transfer and mixing of ‘properties’ and ‘substances’
and their relationship to climatic changes. It has taken
five decades to develop techniques to make relevant
oceanographic measurements, and one should expect
rapid advances in our knowledge of oceanic processes
as a result of innovative research, and international
observational programs such as Tropical Oceans and
the Global Atmosphere (TOGA) and WOCE.
It must be realized that a state must be reached
where experiments and theory go hand in hand,
leading to the development of better (more realistic)
models, and acquisition of critical tracer data. In the
absence of a knowledge of the processes involved,
models employed often yield very erroneous results.
Thus, whereas even a few tracer data are quite informative (since a few data points can be treated only
with zero order models), any attempts to understand
oceanic processes in detail pose a serious challenge.
A few examples are considered here, where tracer
data have contributed to the development of realistic
models. As mentioned earlier, simple one-dimensional models were developed earlier on using two
parameters K and w, to consider vertical transfer of
tracers through an oceanic column. Even today these
are used, in the absence of better alternatives, and in
reality, because of a lack of tracer data in the threedimensional space. The result is that as yet the general validity of the K-w models in space is not known
or their dependence on climate. The latter arises
because there are experimental tracer data for ocean
waters only during the Holocene.
The recent significant developments in oceanic
general circulation are a result of transient tracer
experiments, and order of magnitude improvements
in a number of fields, including orbit dynamics,
gravity field estimation and atmospheric variability.
High accuracy data on ocean surface elevation by
satellite altimetry is leading to hopes of complete
theee-dimensional time-evolving estimates of ocean
circulation, which would also improve estimates of
oceanic heat, and several property fluxes.
There has been prolific growth in the field of tracer
oceanography within the last two decades, but there
are acute limitations in providing a consistent picture
of the interrelationships between physical, chemical,
and biological processes, which are needed to develop a coupled atmosphere–ocean model that responds to climate in an interactive manner. It is not
clear how this will be achieved in the near future.
Finally, it is gratifying to see that the cosmogenic
radiotracer field has evolved highly from an academic curiosity in the 1950s and 1960s to its presentday form, wherein it aims to become an integral part
of realistic atmosphere–ocean global atmosphere and
ocean circulation models.
See also
Carbon Cycle. Carbon Dioxide (CO 2 ) Cycle.
Phosphorus Cycle. Radiocarbon. Stable Carbon
Isotope Variations in the Ocean.
Further Reading
Broecker WS (1981) Geochemical tracers and ocean
circulation. In: Warren BA and Wunsch C (eds.)
Evolution of Physical Oceanography, pp. 434--460.
Cambridge, MA: MIT Press.
Broecker WS and Peng TH (1982) Tracers in the Sea. New
York: Lamont-Doherty Geological Observatory.
Jenkins WJ (1992) Tracer based inferences of new primary
production in the sea. In: Falkowski PG and Woodhead
AD (eds.) Primary Productivity and Biogeochemical
Cycles in the Sea. New York: Plenum Press.
Lal D (1962) Cosmic ray produced radionuclides in the
sea. J. Ocean. Soc. Japan: 20th Anniv. Vol. 600–614.
Lal D and Peters B (1967) Cosmic ray produced radioactivity on the earth. Handbuch der Physik 46(2):
551--612.
Lal D (1999) An overview of five decades of studies of
cosmic ray produced nuclides in the oceans. Science of
the Total Environment 237/238: 3--13.
Ledwell JR, Watson AJ, and Law CS (1993) Evidence for
slow mixing across the pycnocline from an open ocean
tracer-release experiment. Nature 364: 701--703.
Libby WF, Anderson EC, and Arnold JR (1949) Age
determination by radiocarbon content: world-wide
assay of natural radiocarbon. Science 109: 227--228.
Measures CI and Edmond JM (1982) Beryllium in the water
column of the Central Pacific. Nature 297: 51--53.
Murray JW, Barber RT, Roman MR, Bacon MP, and Feely
RA (1994) Physical and biological controls on carbon
cycling in the equatorial Pacific. Science 266: 58--65.
Raisbeck GM and Yiou F (1999)
129 I in the oceans: origins
and applications. Science of the Total Environment 237/
238: 31--41.
Schlosser P and Smethie WM Jr (1995) Transient tracers
as a tool to study variability of ocean circulation.
In: Natural Climate Variability on Decade-to-century
Time Scales, pp. 274--289. Washington: National
Research Council.
234 COSMOGENIC ISOTOPES
mention of the ‘purposeful tracer experiment’ carried
out as a component of WOCE, using SF 6 released in
the open ocean. The results confirmed the earlier
estimates of very low diapycnal diffusivity, of the
order of 0.1 cm
2 s
À1 , implying that heat, salt, and
tracers must penetrate the thermocline primary by
transport along, rather than across, density surfaces.
Clearly, understanding of the large-scale ocean
circulation will come from multiple approaches, with
directed research to simultaneously understand
transfer and mixing of ‘properties’ and ‘substances’
and their relationship to climatic changes. It has taken
five decades to develop techniques to make relevant
oceanographic measurements, and one should expect
rapid advances in our knowledge of oceanic processes
as a result of innovative research, and international
observational programs such as Tropical Oceans and
the Global Atmosphere (TOGA) and WOCE.
It must be realized that a state must be reached
where experiments and theory go hand in hand,
leading to the development of better (more realistic)
models, and acquisition of critical tracer data. In the
absence of a knowledge of the processes involved,
models employed often yield very erroneous results.
Thus, whereas even a few tracer data are quite informative (since a few data points can be treated only
with zero order models), any attempts to understand
oceanic processes in detail pose a serious challenge.
A few examples are considered here, where tracer
data have contributed to the development of realistic
models. As mentioned earlier, simple one-dimensional models were developed earlier on using two
parameters K and w, to consider vertical transfer of
tracers through an oceanic column. Even today these
are used, in the absence of better alternatives, and in
reality, because of a lack of tracer data in the threedimensional space. The result is that as yet the general validity of the K-w models in space is not known
or their dependence on climate. The latter arises
because there are experimental tracer data for ocean
waters only during the Holocene.
The recent significant developments in oceanic
general circulation are a result of transient tracer
experiments, and order of magnitude improvements
in a number of fields, including orbit dynamics,
gravity field estimation and atmospheric variability.
High accuracy data on ocean surface elevation by
satellite altimetry is leading to hopes of complete
theee-dimensional time-evolving estimates of ocean
circulation, which would also improve estimates of
oceanic heat, and several property fluxes.
There has been prolific growth in the field of tracer
oceanography within the last two decades, but there
are acute limitations in providing a consistent picture
of the interrelationships between physical, chemical,
and biological processes, which are needed to develop a coupled atmosphere–ocean model that responds to climate in an interactive manner. It is not
clear how this will be achieved in the near future.
Finally, it is gratifying to see that the cosmogenic
radiotracer field has evolved highly from an academic curiosity in the 1950s and 1960s to its presentday form, wherein it aims to become an integral part
of realistic atmosphere–ocean global atmosphere and
ocean circulation models.
See also
Carbon Cycle. Carbon Dioxide (CO 2 ) Cycle.
Phosphorus Cycle. Radiocarbon. Stable Carbon
Isotope Variations in the Ocean.
Further Reading
Broecker WS (1981) Geochemical tracers and ocean
circulation. In: Warren BA and Wunsch C (eds.)
Evolution of Physical Oceanography, pp. 434--460.
Cambridge, MA: MIT Press.
Broecker WS and Peng TH (1982) Tracers in the Sea. New
York: Lamont-Doherty Geological Observatory.
Jenkins WJ (1992) Tracer based inferences of new primary
production in the sea. In: Falkowski PG and Woodhead
AD (eds.) Primary Productivity and Biogeochemical
Cycles in the Sea. New York: Plenum Press.
Lal D (1962) Cosmic ray produced radionuclides in the
sea. J. Ocean. Soc. Japan: 20th Anniv. Vol. 600–614.
Lal D and Peters B (1967) Cosmic ray produced radioactivity on the earth. Handbuch der Physik 46(2):
551--612.
Lal D (1999) An overview of five decades of studies of
cosmic ray produced nuclides in the oceans. Science of
the Total Environment 237/238: 3--13.
Ledwell JR, Watson AJ, and Law CS (1993) Evidence for
slow mixing across the pycnocline from an open ocean
tracer-release experiment. Nature 364: 701--703.
Libby WF, Anderson EC, and Arnold JR (1949) Age
determination by radiocarbon content: world-wide
assay of natural radiocarbon. Science 109: 227--228.
Measures CI and Edmond JM (1982) Beryllium in the water
column of the Central Pacific. Nature 297: 51--53.
Murray JW, Barber RT, Roman MR, Bacon MP, and Feely
RA (1994) Physical and biological controls on carbon
cycling in the equatorial Pacific. Science 266: 58--65.
Raisbeck GM and Yiou F (1999)
129 I in the oceans: origins
and applications. Science of the Total Environment 237/
238: 31--41.
Schlosser P and Smethie WM Jr (1995) Transient tracers
as a tool to study variability of ocean circulation.
In: Natural Climate Variability on Decade-to-century
Time Scales, pp. 274--289. Washington: National
Research Council.
234 COSMOGENIC ISOTOPES