CHAPTER 20 • Studies of Water Masses Mixing in the Ross Sea (Antarctica)
447
More recently, isotopes and other chemicals such as tritium, 3Helium and chlorofluorocarbons (CFC) have been used as transient tracers of water mass formation processes and circulation.
They do not have any biogeochemical activities, and they enter the marine environment through inputs from atmosphere.
Their time-dependent input into oceanic surface waters can be used in model studies to derive renewal rates of deep water from near-surface waters as well as exchange
rates between individual deep-water masses (Mensch et al. 1998).
However, their sampling and measurements are very complicated due to their low
concentrations in the water column, atmosphere contamination possibility, and instrumentation not readily available in oceanographic laboratories.
Oxygen and nutrients (nitrate, nitrite, ammonia, phosphate, silicate), which are the
most determined oceanographic parameters, are involved in the production and degradation of organic matter, and therefore they are in themselves limited as water mass
tracers.
Surface oxygen concentration varies from about 350 flM in surface waters at 0 °C
to 200 flM in warm tropical waters due to the change of solubility of oxygen with temperature. However, because a number of effects such as phytoplanktonic activity, the
surface concentration is generally supersaturated.
Below the depth of the photic zone, oxygen is consumed by the oxidation of organic
matter. The average rate of consumption varies with depth and geographical location
(Minster 1989).
For nutrients, the picture is similar. The values at the surface waters are mostly determined by a competition between residence time of the waters, subtraction due to biological activities and regeneration processes, which contribute to their concentrations
in the deep layers of the water column.
20.2.1
"NO" and "PO" as Chemical Tracers
Broeker introduced a linear combination of the concentrations of oxygen and nitrate
and of oxygen and phosphate, such as the observed changes in concentrations as a
result of biological production or degradation, which in principle cancel each other
out (Broeker 1974).
Biological studies have shown that for each mole of O2 consumed, roughly 1/9th of
a mole of bound N is released as nitrate ion.
Thus, the combination of 9 N03 + O2 (termed NO) and of 135 P04 + O2 (termed PO)
should be nearly conservative.
The qualification "nearly" is introduced because the coefficient 9 and 135, based on
the estimation of Redfield ratio, undoubtedly varies from place to place in the sea.
Evidence that NO is appropriate as a conservative water mass tracer is shown by a
plot of NO, N03 and O2 vs. salinity, reported in Fig. 20.2. It can be seen that the concentration of O 2 and N0 3 shows a non linear variation as a function of salinity, and
that the curvature in the oxygen and nitrate vs. salinity profile are largely removed in
the NO vs. salinity plot (Broecker 1974).
NO and PO have been used in global ocean circulation studies for identifying waters of southern and northern origin in the deep Atlantic (Broecker 1974). More re-
447
More recently, isotopes and other chemicals such as tritium, 3Helium and chlorofluorocarbons (CFC) have been used as transient tracers of water mass formation processes and circulation.
They do not have any biogeochemical activities, and they enter the marine environment through inputs from atmosphere.
Their time-dependent input into oceanic surface waters can be used in model studies to derive renewal rates of deep water from near-surface waters as well as exchange
rates between individual deep-water masses (Mensch et al. 1998).
However, their sampling and measurements are very complicated due to their low
concentrations in the water column, atmosphere contamination possibility, and instrumentation not readily available in oceanographic laboratories.
Oxygen and nutrients (nitrate, nitrite, ammonia, phosphate, silicate), which are the
most determined oceanographic parameters, are involved in the production and degradation of organic matter, and therefore they are in themselves limited as water mass
tracers.
Surface oxygen concentration varies from about 350 flM in surface waters at 0 °C
to 200 flM in warm tropical waters due to the change of solubility of oxygen with temperature. However, because a number of effects such as phytoplanktonic activity, the
surface concentration is generally supersaturated.
Below the depth of the photic zone, oxygen is consumed by the oxidation of organic
matter. The average rate of consumption varies with depth and geographical location
(Minster 1989).
For nutrients, the picture is similar. The values at the surface waters are mostly determined by a competition between residence time of the waters, subtraction due to biological activities and regeneration processes, which contribute to their concentrations
in the deep layers of the water column.
20.2.1
"NO" and "PO" as Chemical Tracers
Broeker introduced a linear combination of the concentrations of oxygen and nitrate
and of oxygen and phosphate, such as the observed changes in concentrations as a
result of biological production or degradation, which in principle cancel each other
out (Broeker 1974).
Biological studies have shown that for each mole of O2 consumed, roughly 1/9th of
a mole of bound N is released as nitrate ion.
Thus, the combination of 9 N03 + O2 (termed NO) and of 135 P04 + O2 (termed PO)
should be nearly conservative.
The qualification "nearly" is introduced because the coefficient 9 and 135, based on
the estimation of Redfield ratio, undoubtedly varies from place to place in the sea.
Evidence that NO is appropriate as a conservative water mass tracer is shown by a
plot of NO, N03 and O2 vs. salinity, reported in Fig. 20.2. It can be seen that the concentration of O 2 and N0 3 shows a non linear variation as a function of salinity, and
that the curvature in the oxygen and nitrate vs. salinity profile are largely removed in
the NO vs. salinity plot (Broecker 1974).
NO and PO have been used in global ocean circulation studies for identifying waters of southern and northern origin in the deep Atlantic (Broecker 1974). More re-