5.8: Effects of Pressure, Temperature, and Dissolved Salts on Seawater

Many of the movements of water masses in the oceans are driven by differences in density. Solid objects that have higher density than water sink, and those that have lower density rise and float. Liquid water can also rise or sink if its density is different from that of the surrounding water (CC1, CC3). Water density is controlled by changes in pressure, temperature (CC6), and concentration of dissolved constituents (salinity).

Pressure

Because water molecules in a liquid form can be forced together only slightly as pressure increases, seawater is virtually incompressible, and its volume decreases and density increases only very slightly with pressure. Therefore, pressure changes are not as important to controlling water density in the oceans as changes in temperature and salinity are. However, the pressure at the greatest depths in the oceans is more than 1000 times as great as atmospheric pressure. This pressure change causes seawater density to be approximately 2% higher than it is in shallow water at the same temperature and salinity. Some research studies must take this small difference into account. Like all other gases, water vapor is compressible, and its density varies substantially with pressure.

Temperature

Increasing temperature adds energy that enables the molecules of solids and liquids to vibrate, rotate, and/or translate more vigorously. Thus, the average distance between the molecules generally increases. As the same number of molecules occupies a larger volume (the material expands), increasing the temperature causes the density to decrease. Ice is no exception to this rule. However, pure water (but not seawater, as discussed later) behaves anomalously, because liquid water has a density maximum at 4°C (rounded from precisely 3.98°C). Between 0°C and 4°C, water density actually increases with increasing temperature (Fig. 5-11). In the rest of liquid water’s temperature range—4°C to 100°C— pure water behaves normally and density decreases with increasing temperature.

Figure 5-11. Density of pure water plotted against temperature. Water with no dissolved salts has a maximum density at 3.98°C. At lower temperatures, water density decreases until the freezing point is reached. At the freezing  point, there is a discontinuity in the density scale. The density of ice is much less than that of liquid water because of the open structure of the ice crystal lattice. When solid ice and liquid water occur together at 0°C, the ice floats on the water because its density is less than that of the water.

The reason for the anomalous effect of temperature on water density is the hydrogen bond. Water molecules form clusters in which the molecules are arranged in a lattice-like structure. The atoms in the cluster are held in place by hydrogen bonds (Fig. 5-12). The structure is similar to that of ice, and the molecules of water in a cluster occupy a larger volume than molecules that are not clustered. The ordered clusters remain together for only a few ten-millionths of a second, but they are continuously forming, breaking, and re-forming. Both the number of clusters present at any time and the number of molecules in each cluster increase as the temperature decreases (more unbroken hydrogen bonds are present). Because clustered molecules occupy a greater volume than unclustered molecules, an increase in the number of clusters and in the number of molecules per cluster decreases the density of the water. Above 4°C, there are too few clusters to counteract the normal temperature effect on density. Below 4°C (actually 3.98°C), however, clustering increases fast enough that the decrease in density caused by the clustering is faster than the increase caused by the normal temperature effect.

Figure 5-12. The hydrogen bond plays a major role in the properties of ice and water. (a) Molecules in the ice crystal lattice are bonded to each other by hydrogen bonds and arranged in a hexagonal lattice. The resulting structure is very open, which explains the low density of ice. (b) Water molecules constantly form clusters that are temporarily held together by hydrogen bonds. The clusters consist of chains or networks of water molecules arranged in hexagons. The representation here shows this hexagonal form but, for simplicity, does not show the hydrogen bonds. (c) When water molecules are unclustered, the molecules are closer to each other than they are in clusters. Clusters are more likely to form and may persist longer at low temperatures, which is why water’s density decreases below 3.98°C.

Dissolved Salts and Density

Salts dissolved in water increase water density for several reasons. First, the ions or molecules of most substances dissolved in seawater have a higher density than water molecules. Dissolved substances also reduce the clustering of the water molecules, further increasing the density, particularly at temperatures near the freezing point.

Combined Effects of Salinity and Temperature

The density of ocean waters is determined primarily by salinity and temperature. Figure 5-13a shows the relationships among the salinity, temperature, and density of seawater. Raising the temperature of freshwater from 4°C to 30°C (the range between the temperature of maximum density and the highest temperature generally found in ocean surface waters) decreases its density by about 0.0043 (from 1.0000 to 0.9957), or about 0.4%. At a constant temperature, changing the salinity from 0 to 40 (approximately the range of salinity in surface waters) changes the density by about 0.035, or about 3.5%. These observations suggest that salinity is more important than temperature as a determinant of density. This is often true in rivers and estuaries where the water has a wide range of salinity, but the range of salinity in open-ocean waters is much smaller. In fact, 99% of all ocean water has salinity between 33 and 37, and 75% has salinity between 34 and 35 (Fig. 5-14). Similarly, 75% of ocean water has a temperature between 0°C and 5°C, and the rest has a much wider temperature range, between about –3°C and 30°C.

Figure 5-13. The effects of temperature and salinity on the density of water. (a) The relationships among salinity, temperature, and density are complex. The effects of temperature and salinity individually are more obvious if we examine (b) the relationship of temperature (–3°C to 30°C) to density at a fixed salinity, and (c) the relationship of salinity (30 to 40 PSU) to density at fixed temperature. The shaded sections of the plots in (b) and (c) represent the narrow ranges of salinity and temperature present in most ocean water.

Figure 5-14. Temperature and salinity characteristics of ocean waters. Ninety-nine percent of all ocean water has salinity and temperature within the range shown by the larger shaded area. Seventy-five percent of all ocean water has the much narrower range of salinity and temperature within the inner darker-shaded area.

With the exception of water discharged by hydrothermal vents, the highest temperatures in the oceans are in surface waters in tropical regions. Figure 5-13b,c relates salinity, temperature, and seawater density, and shows that, in most of the ocean, temperature and salinity are of approximately equal importance in determining ocean water density. However, their relative importance varies with location and depth. For example, temperature changes are more important to density variations in the tropical water column, where salinity variation is relatively small but temperature variation with depth is relatively large. In contrast, salinity is more important in some high-latitude regions, where salinity variations are relatively large as a result of high volumes of freshwater runoff and the formation and melting of ice, but temperatures are generally uniform and near the freezing point.