Tracers of Ocean-Water Masses

The oceans, atmosphere, continents and cryosphere are part of Earth's tightly connected climate system. The ocean's role in the climate system involves the transport, sequestration , and exchange of heat, fresh water, and carbon dioxide (CO₂) between the other components of the system.

When waters descend below the ocean surface, they carry with them dissolved atmospheric gases. The time-dependent tracers in the oceans provide information on which waters have been in contact with the atmosphere on various timescales. They also give information on the ocean circulation and its variability.

The timescale information is needed to understand and to assess the ocean's role in climate change, and its capacity to take up human-derived constituents, such as CO₂ from the atmosphere. Thus, the advantage to using tracers for ocean circulation studies is the added dimension of time: their time history is fairly well known; they are an integrating quantity; and they provide an independent test for time integration of models and biogeochemical processes.

Tracers serve as a "dye" with which to follow the circulation of ocean waters. Conventional ocean tracers include temperature, salinity, oxygen, and nutrients. Stable isotope tracers, such as oxygen-18 and carbon-13, do not decay. In contrast, other radioactive tracers do decay. The radioactive tracers are naturally occurring, such as the uranium/thorium series and radium, and those produced both naturally and by nuclear bomb tests, such as tritium and carbon-14. The bomb contributions from the latter two are called transient tracers, as are the chlorofluorocarbons (CFCs), because they have been in the atmosphere for only a short time. The designation as transient tracer implies a human-derived source and a non-steady input function.

Tracer Timescales Matched To Ocean Processes

The decision of which tracer to use in an oceanographic application depends on the process involved and its timescale. The conventional ocean tracers and stable isotope tracers have no timescale associated with them; their use would depend solely on the natural process involved. For example, to decipher the fresh-water sources in the waters exiting the Arctic Ocean, salinity and the oxygen-18 isotope would be useful. The oxygen isotope is studied because precipitation and the melting and freezing of ice each has different fractions of the ratio of oxygen-18 to oxygen-16.

The radioactive tracers decay with a known half-life. A half-life is the time it takes half of the concentration to disappear. The half-life is matched to what is known about the timescale of the ocean process. For example, to study upper ocean circulation, which occurs on timescales of the order decades, tritium (half-life of 12.4 years) is useful. To study deep ocean circulation, which occurs on timescales of the order hundreds of years, carbon14 (half-life of 5,500 years) is useful. The transient tracers are used to study ocean processes with timescales of less than decades, because that is how long they have existed. This includes upper ocean process, and circulation near the deep-water source regions.

Deep waters that fill ocean basins form primarily in the high latitudes of the North Atlantic and Southern Ocean (the ocean around Antarctica). There is a close coupling of the surface waters in high latitudes to the deep ocean through the density-driven thermohaline circulation. During the process of deep-water formation, atmospheric constituents such as CFCs and CO₂ are introduced into the newly formed water. After these waters sink, they spread out through the deep oceans. As an example of the spreading of deep water from its source in the high latitude North Atlantic, CFCs are used as a tracer.

The Chlorofluorocarbons

The chlorofluorocarbons, CFCs, are synthetic halogenated methanes. Their chemical structures are as follows: CFC-11 is CCl₃F; CFC-12 is CCl₂F₂; and CFC-113 is CCl₂FCClF₂. The CFCs have received considerable attention because they are a double-edged environmental sword. They are a greenhouse gas and a threat to the ozone layer. The CFCs are used as coolants in refrigerators and air conditioners, as propellants in aerosol spray cans, and as foaming agents. These chemicals were developed in the mid-twentieth century when no one realized they might cause environmental problems.

When released, CFCs are gases that have two sinks : the atmosphere, and to a lesser extent, the oceans. Most of the CFCs go up into the troposphere , where they remain for decades. In the ocean and in the troposphere, the CFCs pose no problem. However, some escape into the stratosphere , where they are a threat to the ozone layer. Destruction of the ozone layer by CFCs removes the atmosphere's ability to block ultraviolet radiation, and has been correlated with the increased incidence of skin cancers.

Since the recognition of the CFCs as an environmental problem in the 1970s and the signing of the Montreal Protocol in 1987, the use of CFCs has been phased out. The atmospheric concentrations have started to decrease, as shown in the figure to the right. (The curves represent the amount of CFCs released into the atmosphere [Northern Hemisphere] between 1930 and 2000.) This phase-out is an important international step toward correcting the dangerous trend of stratospheric ozone depletion.

CFCs in the Ocean.

The CFCs are gases, and like other gases they get into the ocean via air-sea exchange. There is a direct correlation between gas exchange rate and wind speed, and the direction of the gas flux between the air and ocean is from high to low concentration. For CFCs, the atmospheric concentrations greatly exceed those in the ocean. The concentrations of CFCs dissolved in the surface layer of the ocean depends on the solubility, atmospheric concentration, and other physical factors. Therefore, the colder the water the higher the CFC concentration. The solubility is only slightly dependent on the salinity.

The compounds CFC-11 and CFC-12 were first measured in the oceans in the late 1970s. Concentrations generally decrease as the ocean depth increases. An exception is the western North Atlantic, where CFCs reach to the ocean bottom, because of the proximity to the deep-water source and short circulation time.

One of the main advantages to 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. An estimate of age can be calculated from the CFC-11:CFC-12 ratio of the two compounds measured in the oceans and corrected for their solubility. The atmospheric value of the ratio is compared to the atmospheric source function to determine a corresponding date. The date is subtracted from the sample collection date to get an age. Because the atmospheric changes in the ratio of CFC-11:CFC-12 have remained unchanged since the mid1970s, application of the ratio age for CFC-11 and CFC-12 is restricted to older waters.

In regions where surface waters are converted to deep and bottom waters which then spread into a background of low-tracer water, a tracer ratio age represents that of the youngest component of the mixture. Thus, the tracers, such as CFCs, can be used to define the pathways, timescale, and transport for the spreading of deep water from its source regions.

see also Carbon Dioxide in the Ocean and Atmosphere; Mid-Ocean Ridges; Ocean Basins; Ocean Currents; Radionuclides in the Ocean; Sea Water, Gases in; Tracers in Fresh Water.

Rana A. Fine

Bibliography

Smethie, W.S. et al. "Tracing the Flow of North Atlantic Deep Water Using Chlorofluorocarbons." Journal of Geophysical Research 105 (2000):14,297–14,323.

Walker, S.J., Weiss, R.F., and Salameth, P.K. "Reconstructed Histories of the Annual Mean Atmospheric Mole Fractions for the Halocarbons CFC-11, CFC-12, CFC-113 and Carbon Tetrachloride." Journal of Geophysical Research 105 (2000): 14,285–14,296.