accuracy of the measurements is about 1.5% and
precision is about 0.7%. The solubility increases
with decreasing temperature, at a rate of about 4%
for 11C. Therefore, the colder the water the higher
the CFC concentration. At a constant salinity the
temperature effect is about two times greater for
CFCs than for oxygen. The solubility is only slightly
dependent on the salinity, and it decreases with increasing salinity.
Surface Saturation
The approach to equilibrium condition or the saturation state is dependent on the mixed layer depth
and air–sea transfer rate. It takes from days up to a
few weeks after a change in temperature or salinity
for ‘normal’ (not very deep) oceanic surface layers to
come to equilibrium with the present atmosphere.
While the surface waters of the world’s oceans are
close to equilibrium with the present day atmospheric concentration of CFCs, there are exceptions.
At times of rapid warming, such as in the spring, the
surface waters will tend to be a few percent supersaturated with the gas due to lack of time to equilibrate with the atmosphere. Likewise at times of rapid
cooling the surface waters will be a few percent
undersaturated with the gas. Typically there are
undersaturations within a few degrees of the equator
due to upwelling of deeper less saturated waters. In
high latitudes, where there are deep convective
mixed layers that do not readily equilibrate with the
atmosphere, there are likely to be undersaturations
of as much as 60%. These have been observed in the
Labrador Sea. The undersaturations in the high
latitude water mass source regions need to be taken
into account when using the CFCs to put timescales
on oceanic processes.
Oceanic Distribution
The compounds CFC-11 and CFC-12 were first
measured in the oceans in the late 1970s. The first
systematic and intensive survey was carried out in
the tropical North and South Atlantic oceans starting
in the early 1980s. Since then CFCs have been part of
the measurements made during physical oceanography field work. A global survey was conducted
as part of the World Ocean Circulation Experiment
during the 1990s. Typical vertical profiles versus
pressure for stations in the North Atlantic and North
Pacific oceans are presented in Figure 2 along with
other properties. Although CFC-12 has higher concentrations in the atmosphere, CFC-11 is more soluble in sea water, so its concentrations are about
twice that of CFC-12. Note that there are measurable concentrations of CFCs in the western North
Atlantic that reach to the ocean bottom, while they
reach to only 1000 m in the North Pacific. The difference between the CFC concentrations of the
North Atlantic as compared with the North Pacific,
reflects the formation of deep waters in the North
Atlantic and the absence in the North Pacific. Concentrations generally decrease as the ocean depth
increases. However, there may be subsurface concentration maxima due to the lateral intrusion of
water that has been in more recent contact with the
atmosphere (see applications below). The concentrations of CFCs and oxygen should behave similarly
except where the biological effects on the oxygen
distribution cause the differences, for example, the
oxygen minimum at mid-depth.
Combining a series of vertical profiles, as in Figure 2, will give a slice or section through the ocean.
Sections through the eastern Pacific and Atlantic are
shown in Figure 3. The absence of CFCs in the deep
waters of the Pacific Ocean shows the relative isolation of the deep Pacific from contact with the atmosphere on timescales of decades. In contrast, the
North Atlantic north of 351N has CFCs in deep and
bottom waters, because these waters form in the high
latitudes of the North Atlantic and easily spread
equatorward on timescales of 10–20 years. As part of
the density-driven, thermohaline circulation some of
these waters will eventually be transported into the
Pacific, but it will take hundreds of years. The upper
waters of both oceans are in contact with the atmosphere on much shorter timescales. These upper
waters are part of the wind-driven circulation.
CFC Ages in the Ocean
Age Calculations
One of the main advantages of using CFCs as tracers
of ocean circulation is that the time-dependent
source function permits the calculation of timescales
for these processes. A tracer age is the elapsed time
since a water parcel was last exposed to the atmosphere. The tracer-derived age is the elapsed time
since a subsurface water mass was last in contact
with the atmosphere. Two estimates of ‘age’ can be
calculated, one from the CFC-11/CFC-12 ratio and
one from the partial pressure of either dissolved
CFC. In both cases, the atmospheric value of either
the ratio or partial pressure with which the water
had equilibrated is compared to the atmospheric
source function to determine the corresponding date.
To normalize the concentrations for the effects on
the solubility of temperature and salinity CFCs are
expressed in terms of their partial pressures, pCFC,
where the pCFC is the concentration divided by the
CFCS IN THE OCEAN 157
precision is about 0.7%. The solubility increases
with decreasing temperature, at a rate of about 4%
for 11C. Therefore, the colder the water the higher
the CFC concentration. At a constant salinity the
temperature effect is about two times greater for
CFCs than for oxygen. The solubility is only slightly
dependent on the salinity, and it decreases with increasing salinity.
Surface Saturation
The approach to equilibrium condition or the saturation state is dependent on the mixed layer depth
and air–sea transfer rate. It takes from days up to a
few weeks after a change in temperature or salinity
for ‘normal’ (not very deep) oceanic surface layers to
come to equilibrium with the present atmosphere.
While the surface waters of the world’s oceans are
close to equilibrium with the present day atmospheric concentration of CFCs, there are exceptions.
At times of rapid warming, such as in the spring, the
surface waters will tend to be a few percent supersaturated with the gas due to lack of time to equilibrate with the atmosphere. Likewise at times of rapid
cooling the surface waters will be a few percent
undersaturated with the gas. Typically there are
undersaturations within a few degrees of the equator
due to upwelling of deeper less saturated waters. In
high latitudes, where there are deep convective
mixed layers that do not readily equilibrate with the
atmosphere, there are likely to be undersaturations
of as much as 60%. These have been observed in the
Labrador Sea. The undersaturations in the high
latitude water mass source regions need to be taken
into account when using the CFCs to put timescales
on oceanic processes.
Oceanic Distribution
The compounds CFC-11 and CFC-12 were first
measured in the oceans in the late 1970s. The first
systematic and intensive survey was carried out in
the tropical North and South Atlantic oceans starting
in the early 1980s. Since then CFCs have been part of
the measurements made during physical oceanography field work. A global survey was conducted
as part of the World Ocean Circulation Experiment
during the 1990s. Typical vertical profiles versus
pressure for stations in the North Atlantic and North
Pacific oceans are presented in Figure 2 along with
other properties. Although CFC-12 has higher concentrations in the atmosphere, CFC-11 is more soluble in sea water, so its concentrations are about
twice that of CFC-12. Note that there are measurable concentrations of CFCs in the western North
Atlantic that reach to the ocean bottom, while they
reach to only 1000 m in the North Pacific. The difference between the CFC concentrations of the
North Atlantic as compared with the North Pacific,
reflects the formation of deep waters in the North
Atlantic and the absence in the North Pacific. Concentrations generally decrease as the ocean depth
increases. However, there may be subsurface concentration maxima due to the lateral intrusion of
water that has been in more recent contact with the
atmosphere (see applications below). The concentrations of CFCs and oxygen should behave similarly
except where the biological effects on the oxygen
distribution cause the differences, for example, the
oxygen minimum at mid-depth.
Combining a series of vertical profiles, as in Figure 2, will give a slice or section through the ocean.
Sections through the eastern Pacific and Atlantic are
shown in Figure 3. The absence of CFCs in the deep
waters of the Pacific Ocean shows the relative isolation of the deep Pacific from contact with the atmosphere on timescales of decades. In contrast, the
North Atlantic north of 351N has CFCs in deep and
bottom waters, because these waters form in the high
latitudes of the North Atlantic and easily spread
equatorward on timescales of 10–20 years. As part of
the density-driven, thermohaline circulation some of
these waters will eventually be transported into the
Pacific, but it will take hundreds of years. The upper
waters of both oceans are in contact with the atmosphere on much shorter timescales. These upper
waters are part of the wind-driven circulation.
CFC Ages in the Ocean
Age Calculations
One of the main advantages of using CFCs as tracers
of ocean circulation is that the time-dependent
source function permits the calculation of timescales
for these processes. A tracer age is the elapsed time
since a water parcel was last exposed to the atmosphere. The tracer-derived age is the elapsed time
since a subsurface water mass was last in contact
with the atmosphere. Two estimates of ‘age’ can be
calculated, one from the CFC-11/CFC-12 ratio and
one from the partial pressure of either dissolved
CFC. In both cases, the atmospheric value of either
the ratio or partial pressure with which the water
had equilibrated is compared to the atmospheric
source function to determine the corresponding date.
To normalize the concentrations for the effects on
the solubility of temperature and salinity CFCs are
expressed in terms of their partial pressures, pCFC,
where the pCFC is the concentration divided by the
CFCS IN THE OCEAN 157