8.15: Tracing Water Masses

Although direct measurements have been made of deep-ocean currents, most of what we know about deep-ocean circulation was learned from studies of the horizontal and vertical distribution of water masses. Water masses are most often characterized by their temperature and salinity, but the concentrations of several dissolved constituents are also used to distinguish and trace water masses (CC6).

Conservative and Nonconservative Properties

Conservative properties of seawater are properties that are not changed by biological, chemical, or physical processes within the ocean. A conservative property can be changed only by mixing with other water masses that have different values of that property, or by processes that occur at the ocean surface, or where rivers or other sources, such as hydrothermal vents, enter the oceans. For example, salinity, a conservative property, can be changed at the ocean surface by evaporation or precipitation, or at river mouths by the introduction of freshwater. Small salinity changes can also occur at hydrothermal vents, where high-salinity waters are emitted. Similarly, temperature is a conservative property until water contacts the atmosphere or seafloor, with which it can exchange heat.

Dissolved constituents of seawater, such as Na+ and Cl–, whose concentrations are not significantly affected by biological or chemical uptake or removal from ocean water, are also conservative properties. In contrast, concentrations of constituents such as oxygen, carbon dioxide, phosphate, and many trace metals are substantially altered by biological and chemical processes that remove and release them to solution within the ocean. These are nonconservative properties.

Conservative properties are particularly useful in tracing water masses because they can be used to identify the masses as they are formed, to trace their transport through the oceans, and to determine how they mix with other water masses. When two water masses mix, the value of the conservative property in the mixed water mass is determined by the proportions in which the two water masses have mixed. For example, consider the mixing of two water masses, one with a salinity of 35 and the other with a salinity of 37. If the salinity of the mixed water mass is 36, the mixture must consist of equal volumes of the two water masses (2 volumes of 36 = 1 volume of 37 + 1 volume of 35). If the salinity of the mixture is 35.5, the mixture must consist of three parts water of salinity 35 and 1 part water of salinity 37 (4 volumes of 35.5 = 3 volumes of 35 + 1 volume of 37).

TS Diagrams 

We can identify different water masses by plotting temperature against salinity in what are called “TS diagrams” (Fig. 8-29). Water samples from the same water mass have the same salinity and temperature and therefore will appear as a single point on a TS diagram. Samples from water masses that have different temperature and salinity appear as separated points. When two water masses are mixed, the temperature and salinity of the mixed water lie along a straight line drawn between the two points representing the original water masses. The location of the TS value for a mixed water sample on the line between the TS values of the two water sources indicates the relative proportions of the two original water masses in the mixture.

Figure 8-29. TS diagrams are used to trace water masses. In this figure, hypothetical vertical temperature and salinity profiles are used to construct examples of TS diagrams. (a) A simple three-layer system. All TS points plot at one of three locations, representing the three water masses. (b) If the three layers mix, but some of the unmixed mid-depth water mass is still present (at 700 m depth), the TS points plot on two straight lines that connect the three original water-mass characteristic points. (c) Even if all of the mid-depth water mass is mixed with at least some of the other two water masses, a TS plot can be used to identify the temperature and salinity characteristics of the original unmixed water mass. (d) A typical TS diagram for a station in the South Atlantic Ocean.

When three water masses mix in such a way that no unmixed sample of one of the three water masses remains, some of the mixed water samples will have TS values that are no longer on straight lines between the three original TS points (Fig. 8-29c). However, in such cases, the temperature and salinity of the original water mass, which now exists only as a part of mixtures, can often be deduced by extrapolation of the straight-line portions of the TS curve (Fig. 8-29c).

A TS diagram for water samples taken at different depths at a single station in the South Atlantic Ocean is shown in Figure 8-29d. At this station, the principal water masses are warm, saline surface water; North Atlantic Central Water; water that is a mixture of surface water and North Atlantic Deep Water; Antarctic Intermediate Water; North Atlantic Deep Water; and Antarctic Bottom Water. We need not be concerned with how these water masses acquired their different temperature and salinity characteristics. What is important is that, once the source properties are known, the presence of a particular water mass in the area for which the TS diagram is drawn can be determined. Note that we can detect the presence of Antarctic Intermediate Water in the diagram, even though no unmixed Antarctic Intermediate Water is present at the station. We can also calculate from the diagram the percentage of Antarctic Intermediate Water present in the mixed water at any depth. For example, at 800 m, where the proportion of this water mass is greatest, Antarctic Intermediate Water is about 55% of the mixture.

By drawing TS diagrams for many different stations across the oceans, oceanographers can trace movements of individual water masses and investigate mixing processes at different points within the oceans.

Tracers

Salinity and temperature are excellent tracers, but they tell us only which water masses are mixed and where the mixed water is transported. Without other information, these tracers cannot tell us how fast the water masses move and mix. Several different dissolved components of seawater are now used in conjunction with temperature and salinity to trace ocean water masses and to provide information on their rate of movement.

In some applications, nonconservative properties, such as oxygen and carbon dioxide concentrations, can be used as tracers. When water sinks below the pycnocline and is no longer in contact with the atmosphere, the concentrations of these gases are altered by the decomposition of organic particles, which consumes oxygen and releases additional carbon dioxide to solution. Therefore, if we know the oxygen and carbon dioxide concentrations in water masses when they sink below the mixed layer, changes in those concentrations can provide information about the relative ages (since the time when they sank) of water masses in different parts of the oceans.

Human activities have provided a number of useful tracers during the past half century. These tracers include radionuclides created by nuclear weapons testing, and certain synthetic organic compounds, particularly chlorofluorocarbons (CFCs). Some radionuclides that do not occur naturally are now found dissolved in ocean water in extremely small concentrations. They are derived primarily from nuclear bomb tests in the atmosphere, which peaked in the 1950s. These radionuclides are good tracers because the time and location of their introduction to the oceans is well known, and they decay at known, constant rates (CC7).

Tritium (3H), a radioactive isotope of hydrogen, is a particularly useful tracer. Tritium is produced naturally by cosmic rays in the atmosphere. Thus, it occurs naturally in ocean water, but only at extremely small concentrations. In water unaffected by human releases, approximately one of every 1019 atoms of hydrogen is a tritium atom. Releases of tritium during the nuclear bomb testing era of the mid-twentieth century raised the tritium concentration in surface seawater to a concentration of approximately 6 atoms of tritium in 1018 atoms of hydrogen. Tritium reacts quickly with oxygen in the atmosphere to form tritiated water, which enters the oceans in rainfall. Because the tritium is incorporated in the water molecule itself, it is a perfect tracer for water masses. Tritium has a half-life of only approximately 12 years, so bomb-produced tritium will continue to be useful as a tracer for only a few decades, until all the tritium introduced by nuclear bomb testing has decayed.

In the Northern Hemisphere, most tritium was produced by nuclear tests in the Soviet Arctic. Therefore, tritium concentrations are particularly high in water flowing into the Atlantic Ocean from the Arctic Ocean. This tritium is now distributed throughout the mixed layer of the oceans and is currently being transported into the deep oceans with sinking cold water masses created in polar regions. The progress of deep water-mass formation has been monitored using tritium as a tracer (Fig. 8-30).

Figure 8-30. (a) Tritium distribution with depth in the western North Atlantic Ocean (1972–1973). Released primarily by nuclear bomb tests in the 1950s, tritium has spread throughout the ocean surface layers and is being transported steadily into the deep oceans with North Atlantic Deep Water, which is formed by the cooling of surface waters in the North Atlantic. (b) Distribution in 1981. The values are corrected for the radioactive decay that occurred during the 9 years between the two surveys. The strong source in the surface layer at 50°N is the outflow from the Arctic Ocean where the Soviet Union did most of its nuclear bomb testing. Comparing the two diagrams shows the progressive movement of tritium into the deep layer as North Atlantic Deep Water continuously forms, sinks, and moves southward.

Chlorofluorocarbons (CFCs), which were used as refrigerants and are thought to be responsible for ozone depletion in the atmosphere (Chap. 7), are also excellent water-mass tracers (Fig. 8-31). These compounds do not occur naturally. They are dissolved at very low concentrations in seawater and are not readily decomposed by biological processes.

Figure 8-31. The concentrations of CFC-11, a specific chlorofluorocarbon compound, have been measured throughout the water column to the east of Abaco Island in the Bahamas, periodically between 1986 and 2004. This figure plots the vertical distribution of the concentrations of CFC-11 in this area as a function of time. The data show the arrival at this subtropical location of CFC-11 in recently formed deep water masses originating from the high latitude, deep water mass formation areas in the polar and subpolar regions of the North Atlantic (Figs. 8-26, 8-28). There are three distinct water layers seen arriving in this area, centered at about 1200 m, 1800 m, and 4000 m. The deepest layer is deep water formed from the outflow of the Arctic Ocean, and the two shallower layers are both deep water formed near Greenland. The 1800 m layer arrived near Abaco later than the shallower layer, and appears to reflect a small reduction in the temperature of deep water formed near Greenland that had been observed since about a decade earlier.

Unfortunately, for most water-mass tracers, in situ instrumentation is not sensitive enough, and thus, water samples must be collected for laboratory analysis. Because the cost of ship time required to collect water samples from deep within the water column is very high, sampling cannot be as intensive either spatially or temporally as would be desirable in most instances. Thus, many studies rely on mathematical modeling (CC10) using the limited data that are available.

Studying the recent and ongoing processes of deep water mass formation is extremely important to our understanding of the effects of fossil fuel burning on global climate. Estimates suggest that approximately 30% of the carbon dioxide released by burning fossil fuels since the Industrial Revolution in the mid-1800s is now dissolved in ocean water. However, there is still considerable uncertainty about the accuracy of the estimates. Ocean water that sinks below the mixed layer carries carbon dioxide from the atmosphere into the deep oceans, where it is locked away from contact with the atmosphere for hundreds or thousands of years (or longer if it is incorporated in carbonate sediments; Chap. 6). Tracer studies will help us to determine how much carbon dioxide from fossil fuels has already been transported safely, at least for now, below the pycnocline and to estimate how quickly additional carbon dioxide will follow it.