238
Chemical Oceanography, 4th Edition
effects of physical and biological processes on the distribution of reactive gases such as O 2
and CO 2 . With the advent of better analytical techniques, recent advances have been and
are being made in this area of research.
The variations of nonreactive gases are studied by examining the solubility at 1 atm total
pressure and 100% relative humidity. The degree of saturation σ i is given by
σ i = [i]/[i]* × 100
(6.34)
where [i]* is the solubility concentration at a given potential temperature and salinity, and
[i] is the measured gas concentration. The saturation anomaly Δ i is also used:
Δ i = ([i] – [i]*)/[i]*) × 100
(6.35)
The examination of σ i and Δ i permits one to follow the distribution without the bias of temperature and salinity. The physical processes that can cause nonreactive gases to depart
from the expected concentration are
1. Departures from the standard pressure
2. Partial dissolution of air bubbles
3. Air injection
4. Differential heat and gas exchange
5. Mixing of waters of different temperatures
6. Radiogenic or primordial addition (He)
The radiogenic production of 40 Ar from 40 K in seawater is not significant. It would take
10 11 years of 40 K decay to produce the amount of 40 Ar in the oceans.
Changes in barometric pressure and humidity affect all the gases by an equal percentage. At 30°, the Δ i is increased by 0.9% when the humidity is decreased to 80%. To account
for departures from the standard pressure and humidity, one must study at least two
gases. When a bubble of air is submerged below the surface, it will dissolve as a result of
hydrostatic pressure. If the bubble partially dissolves before it goes back to the surface, all
the gases will partially dissolve and increase Δ i . For partial dissolution, the composition
of the gases in the bubble will remain unchanged. At a depth of 1 m, the Δ i is increased
by 10% for all the gases. If the air bubble completely dissolves, it is called air injection.
Each gas is affected differently for air injection because of the differences in solubility.
The differences in the variation of the solubility of gases with temperature are shown in
Figure 6.5. This difference in composition produces large variation in Δ i for different gases
when air is injected or totally dissolved in seawater.
Since the solubility of gases varies differently with temperature, changes in temperature
without an exchange with the atmosphere can cause Δ i to vary. The mole fraction of each
gas at equilibrium in solution is different from that of air (see Table 6.6). For example, at
15°C a 1°C decrease in temperature will cause Δ i to vary from –0.24% for He to –2.5% for
Xe (Kester, l975). When the heat exchange is more rapid than gas exchange (for example, in
upwelling areas), saturation anomalies will occur. The mixing of waters of different temperatures will also produce saturation anomalies resulting from the nonlinear behavior of
solubility with temperature. For example, the mixing of waters at 0 and 30°C will produce
saturation anomalies of 1.3% for He and 18% for Xe.
Chemical Oceanography, 4th Edition
effects of physical and biological processes on the distribution of reactive gases such as O 2
and CO 2 . With the advent of better analytical techniques, recent advances have been and
are being made in this area of research.
The variations of nonreactive gases are studied by examining the solubility at 1 atm total
pressure and 100% relative humidity. The degree of saturation σ i is given by
σ i = [i]/[i]* × 100
(6.34)
where [i]* is the solubility concentration at a given potential temperature and salinity, and
[i] is the measured gas concentration. The saturation anomaly Δ i is also used:
Δ i = ([i] – [i]*)/[i]*) × 100
(6.35)
The examination of σ i and Δ i permits one to follow the distribution without the bias of temperature and salinity. The physical processes that can cause nonreactive gases to depart
from the expected concentration are
1. Departures from the standard pressure
2. Partial dissolution of air bubbles
3. Air injection
4. Differential heat and gas exchange
5. Mixing of waters of different temperatures
6. Radiogenic or primordial addition (He)
The radiogenic production of 40 Ar from 40 K in seawater is not significant. It would take
10 11 years of 40 K decay to produce the amount of 40 Ar in the oceans.
Changes in barometric pressure and humidity affect all the gases by an equal percentage. At 30°, the Δ i is increased by 0.9% when the humidity is decreased to 80%. To account
for departures from the standard pressure and humidity, one must study at least two
gases. When a bubble of air is submerged below the surface, it will dissolve as a result of
hydrostatic pressure. If the bubble partially dissolves before it goes back to the surface, all
the gases will partially dissolve and increase Δ i . For partial dissolution, the composition
of the gases in the bubble will remain unchanged. At a depth of 1 m, the Δ i is increased
by 10% for all the gases. If the air bubble completely dissolves, it is called air injection.
Each gas is affected differently for air injection because of the differences in solubility.
The differences in the variation of the solubility of gases with temperature are shown in
Figure 6.5. This difference in composition produces large variation in Δ i for different gases
when air is injected or totally dissolved in seawater.
Since the solubility of gases varies differently with temperature, changes in temperature
without an exchange with the atmosphere can cause Δ i to vary. The mole fraction of each
gas at equilibrium in solution is different from that of air (see Table 6.6). For example, at
15°C a 1°C decrease in temperature will cause Δ i to vary from –0.24% for He to –2.5% for
Xe (Kester, l975). When the heat exchange is more rapid than gas exchange (for example, in
upwelling areas), saturation anomalies will occur. The mixing of waters of different temperatures will also produce saturation anomalies resulting from the nonlinear behavior of
solubility with temperature. For example, the mixing of waters at 0 and 30°C will produce
saturation anomalies of 1.3% for He and 18% for Xe.