Typically, the noble gases are observed over time at a
single location in the ocean along with other meteorological and hydrodynamic parameters to characterize and quantify the behavior of each of the
gases in response to the driving forces of gas exchange such as water temperature, wind speed, wave
characteristics, and bubbles injected from breaking
waves. Because both the amount and rate of a gas
transferred between the atmosphere and ocean depend on the solubility and diffusivities of the gas, the
noble gases have long been recognized as ideal tracers for these processes. In addition, argon and
oxygen have very similar molecular diffusivities and
solubilities, making argon an excellent tracer of the
physical behavior of oxygen. By comparing the
relative concentration changes of argon and oxygen
over time, it is possible to account for the relative
contributions of physical and biological processes
(such as photosynthesis by phytoplankton in the
surface ocean) to the overall concentrations, thus
constraining the biological signal and allowing for
estimates of the biological productivity of the surface
ocean.
The observations of anomalous helium and neon
concentrations in ice formation areas and the suggestion that these anomalies could be the result of
solubility of these gases in the ice were made in 1983,
and since that time, a number of laboratory and field
studies have been conducted to characterize and
quantify these interactions. The partitioning of the
noble gases among the three phases of gas, water,
and ice creates a very distinctive ‘signature’ of the
noble gas concentrations left behind in the residual
water. Noble gas concentrations are typically expressed in terms of ‘saturation’, which is the concentration of a gas dissolved in the water relative to
its equilibrium with the atmosphere at a given temperature. For example, a parcel of water at standard
temperature and pressure containing the concentrations of noble gases shown in column 2 of Table 1
would be said to have a saturation of 100%. Saturations that deviate from this 100% can arise when
equilibration with the atmosphere is incomplete, either because the equilibration process is slow relative
to some other dynamic process acting on the system
(for example, rapid heating or cooling, or injection
of bubbles from breaking waves), or because full
equilibration between the water and atmosphere is
prevented, as in the case of ice formation.
Typical saturations for the noble gases in the surface ocean range from 100 to 110% of atmospheric
equilibrium, due mostly to the influx of gas from
bubbles. Ice formation, however, can lead to quite
striking saturations of À 70 to À 60% for helium
and neon and þ 230% for argon in the relatively
undiluted residual water. Ice melting can also lead to
large anomalous saturations of the noble gases,
showing the reverse of the freezing pattern for the
gas saturations, where helium and neon are supersaturated while argon is undersaturated with respect
to the atmosphere.
The interactions of noble gases and ice have been
well-documented and quantified in relatively simple
freshwater systems. Observations of large noble gas
anomalies in a permanently ice-covered antarctic
lake were quantitatively explained using the current
understanding of the solubility of helium and neon in
ice and the partitioning of the gases in a three-phase
system. Characteristics of ice formed from salt water
are more complex than ice formed from fresh water,
and the modeling of the system more complex. Using
a set of equations developed in 1983 and measurements of the ice temperature, salinity, and density, it
is possible to calculate the volume of the brine
pockets in the ice and the volume of bubbles in the
ice. With this type of information, a model of the ice
and the dissolved gas balance in the various phases in
the ice and residual water can be constructed. Such
an ice model was developed during a field study of
gas–ice interactions in a seasonally ice-covered lagoon, and the model predicted the amount of argon,
nitrogen, and oxygen measured in bubbles in similar
types of sea ice. No measurements are available for
the amount of helium and neon in the bubbles of sea
ice to verify the results for these gases. It is also
possible to predict the relative saturations of the
noble gases in the undiluted residual water at the ice–
water interface, and this unique fingerprint of the
noble gases can then serve as a tracer of the mixing
and circulation of this water parcel as it leaves the
surface and enters the interior and deep ocean. In this
manner, the supersaturations of helium from meltwater have been successfully used as a tracer of water
mass mixing and circulation in the Antarctic, and the
estimated sensitivity of helium as a tracer for these
processes is similar to the use of the conventional
tracer, salinity, for these processes.
As an illustration of the ways in which the noble
gases can be used to distinguish between the effects
of ice formation, melting, injection of air bubbles
from breaking waves, or temperature changes on
dissolved gases, Figure 1 shows a vector diagram of
the characteristic changes of helium compared to
argon resulting from each of these processes.
From a starting point of equilibrium with the atmosphere (100% saturation), both helium and argon
saturations increase as a result of bubbles injected
from breaking waves. Ice formation increases the
saturation of argon and decreases the saturation of
helium, whereas ice melting has the opposite effect.
NOBLE GASES AND THE CRYOSPHERE 137
single location in the ocean along with other meteorological and hydrodynamic parameters to characterize and quantify the behavior of each of the
gases in response to the driving forces of gas exchange such as water temperature, wind speed, wave
characteristics, and bubbles injected from breaking
waves. Because both the amount and rate of a gas
transferred between the atmosphere and ocean depend on the solubility and diffusivities of the gas, the
noble gases have long been recognized as ideal tracers for these processes. In addition, argon and
oxygen have very similar molecular diffusivities and
solubilities, making argon an excellent tracer of the
physical behavior of oxygen. By comparing the
relative concentration changes of argon and oxygen
over time, it is possible to account for the relative
contributions of physical and biological processes
(such as photosynthesis by phytoplankton in the
surface ocean) to the overall concentrations, thus
constraining the biological signal and allowing for
estimates of the biological productivity of the surface
ocean.
The observations of anomalous helium and neon
concentrations in ice formation areas and the suggestion that these anomalies could be the result of
solubility of these gases in the ice were made in 1983,
and since that time, a number of laboratory and field
studies have been conducted to characterize and
quantify these interactions. The partitioning of the
noble gases among the three phases of gas, water,
and ice creates a very distinctive ‘signature’ of the
noble gas concentrations left behind in the residual
water. Noble gas concentrations are typically expressed in terms of ‘saturation’, which is the concentration of a gas dissolved in the water relative to
its equilibrium with the atmosphere at a given temperature. For example, a parcel of water at standard
temperature and pressure containing the concentrations of noble gases shown in column 2 of Table 1
would be said to have a saturation of 100%. Saturations that deviate from this 100% can arise when
equilibration with the atmosphere is incomplete, either because the equilibration process is slow relative
to some other dynamic process acting on the system
(for example, rapid heating or cooling, or injection
of bubbles from breaking waves), or because full
equilibration between the water and atmosphere is
prevented, as in the case of ice formation.
Typical saturations for the noble gases in the surface ocean range from 100 to 110% of atmospheric
equilibrium, due mostly to the influx of gas from
bubbles. Ice formation, however, can lead to quite
striking saturations of À 70 to À 60% for helium
and neon and þ 230% for argon in the relatively
undiluted residual water. Ice melting can also lead to
large anomalous saturations of the noble gases,
showing the reverse of the freezing pattern for the
gas saturations, where helium and neon are supersaturated while argon is undersaturated with respect
to the atmosphere.
The interactions of noble gases and ice have been
well-documented and quantified in relatively simple
freshwater systems. Observations of large noble gas
anomalies in a permanently ice-covered antarctic
lake were quantitatively explained using the current
understanding of the solubility of helium and neon in
ice and the partitioning of the gases in a three-phase
system. Characteristics of ice formed from salt water
are more complex than ice formed from fresh water,
and the modeling of the system more complex. Using
a set of equations developed in 1983 and measurements of the ice temperature, salinity, and density, it
is possible to calculate the volume of the brine
pockets in the ice and the volume of bubbles in the
ice. With this type of information, a model of the ice
and the dissolved gas balance in the various phases in
the ice and residual water can be constructed. Such
an ice model was developed during a field study of
gas–ice interactions in a seasonally ice-covered lagoon, and the model predicted the amount of argon,
nitrogen, and oxygen measured in bubbles in similar
types of sea ice. No measurements are available for
the amount of helium and neon in the bubbles of sea
ice to verify the results for these gases. It is also
possible to predict the relative saturations of the
noble gases in the undiluted residual water at the ice–
water interface, and this unique fingerprint of the
noble gases can then serve as a tracer of the mixing
and circulation of this water parcel as it leaves the
surface and enters the interior and deep ocean. In this
manner, the supersaturations of helium from meltwater have been successfully used as a tracer of water
mass mixing and circulation in the Antarctic, and the
estimated sensitivity of helium as a tracer for these
processes is similar to the use of the conventional
tracer, salinity, for these processes.
As an illustration of the ways in which the noble
gases can be used to distinguish between the effects
of ice formation, melting, injection of air bubbles
from breaking waves, or temperature changes on
dissolved gases, Figure 1 shows a vector diagram of
the characteristic changes of helium compared to
argon resulting from each of these processes.
From a starting point of equilibrium with the atmosphere (100% saturation), both helium and argon
saturations increase as a result of bubbles injected
from breaking waves. Ice formation increases the
saturation of argon and decreases the saturation of
helium, whereas ice melting has the opposite effect.
NOBLE GASES AND THE CRYOSPHERE 137