Lesson 6.04 – The Composition of Seawater

ES5.d Students know properties of ocean water, such as temperature and salinity, can be used to explain the layered structure of the oceans, the generation of horizontal and vertical ocean currents, and the geographic distribution of marine organisms.

 

INTRODUCTION

What is the difference between pure water and seawater? One of the most obvious differences is that seawater contains dissolved substances that give it a salty taste. These dissolved substances include sodium chloride, other salts, metals, and even dissolved gases. In fact, every known naturally occurring element is found dissolved in at least trace amounts in seawater. The salt content of seawater makes it unsuitable for drinking or for irrigating most crops and causes it to be highly corrosive to many materials. However, many parts of the ocean are full of life adapted to this environment.

 

Water is the major component of nearly every life form on Earth. Our own body fluid chemistry is similar to the chemistry of seawater. Seawater consists of about 3.5 percent dissolved mineral substances that are collectively termed “salts.” Although the percentage of dissolved components may seem small, the actual quantity is huge because the ocean is so vast.

 

INSTRUCTION

Salinity

Salinity (salinus = salt) is the total amount of solid material dissolved in water. It is the ratio of the mass of dissolved substances to the mass of the water sample. Many common quantities are expressed in percent (%), which is parts per hundred. Because the proportion of dissolved substances in seawater is such a small number, oceanographers typically express salinity in parts per thousand (‰). The average salinity of seawater is 3.5% or 35‰. Figure 1 shows the principal elements that contribute to the ocean’s salinity. Most of the salt in seawater is sodium chloride, common table salt.

 

 

Figure 1 Salts in Seawater This circle graph shows that 1000 grams of seawater with a salinity of 35‰ consists of 965 grams of water and 35 grams of various salts and other solids dissolved in the water.

 

 

 

Sources of Sea Salts

What are the primary sources of dissolved substances in the ocean? Chemical weathering of rocks on the continents is one source of elements found in seawater. These dissolved materials reach oceans through runoff from rivers and streams at an estimated rate of more than 2.3 billion metric tons per year. The second major source of elements found in seawater is from Earth’s interior. Through volcanic eruptions, large quantities of water vapor and other gases have been emitted into the atmosphere during much of geologic time. Scientists believe that this is the principal source of water in the oceans. About 4 billion years ago, as Earth’s temperature cooled, the water vapor condensed and torrential rains filled the ocean basins with water. Certain elements—particularly chlorine, bromine, sulfur, and boron—were emitted along with the water. These elements exist in the ocean in much greater quantities than could be explained by weathering of rocks alone.

 

Processes Affecting Salinity

Because the ocean is well mixed, the relative concentrations of the major components in seawater are essentially constant, no matter where the ocean is sampled. Surface salinity variation in the open ocean normally ranges from 33‰ to 38‰. Variations in salinity result from changes in the water content of the solution.

 

Figure 2 shows some of the different processes that affect the amount of water in seawater, thereby affecting salinity. Some processes add large amounts of fresh water to seawater, decreasing salinity. These processes include precipitation, runoff from land, icebergs melting, and sea ice melting.

Other processes remove large amounts of fresh water from seawater, increasing salinity. These processes include evaporation and the formation of sea ice. High salinities, for example, are found where evaporation rates are high, as is the case in the dry subtropical regions. In areas where large amounts of precipitation dilute ocean waters, as in the mid-latitudes and near the equator, salinity is lower. Both of these examples are shown on the graph in Figure 3.

 

Surface salinity in polar regions varies seasonally due to the formation and melting of sea ice. When seawater freezes in winter, salts do not become part of the ice. Therefore, the salinity of the remaining seawater increases. In summer when sea ice melts, the addition of relatively fresh water dilutes the solution and salinity decreases.

 

Ocean Temperature Variation

The ocean’s surface water temperature varies with the amount of solar radiation received, which is primarily a function of latitude. The graph in Figure 3 shows this relationship. The intensity of solar radiation in high latitudes is much less than the intensity of solar radiation received in tropical latitudes. Therefore, lower sea surface temperatures are found in high-latitude regions. Higher sea surface temperatures are found in low-latitude regions.

Figure 3 This graph shows the variations in ocean surface temperature (top curve) and surface salinity (lower curve). Interpreting Diagrams At which latitudes is sea surface temperature highest? Why?

 

Temperature Variation with Depth

If you lowered a thermometer from the surface of the ocean into deeper water, what temperature pattern do you think you would find? Surface waters are warmed by the sun, so they generally have higher temperatures than deeper waters. However, the observed temperature pattern depends on the latitude.

 

Figure 4 on page 425 shows two graphs of temperature versus depth: one for low-latitude regions and one for high-latitude regions. The low-latitude curve begins with high temperature at the surface. However, the temperature decreases rapidly with depth because of the inability of the sun’s rays to penetrate very far into the ocean. At a depth of about 1000 meters, the temperature remains just a few degrees above freezing and is relatively constant from this level down to the ocean floor. The thermocline (thermo = heat, cline = slope) is the layer of ocean water between about 300 meters and 1000 meters, where there is a rapid change of temperature with depth. The thermocline is a very important structure in the ocean because it creates a vertical barrier to many types of marine life.

 

Figure -4 These graphs show the variations in ocean water temperature with depth for low-latitude and high-latitude regions. Applying Concepts Why is the thermocline absent in the high latitudes?

 

The high-latitude curve in Figure 4 shows a very different pattern from the low-latitude curve. Surface water temperatures in high latitudes are much cooler than in low latitudes, so the curve begins at the surface with a low temperature. Deeper in the ocean, the temperature of the water is similar to that at the surface, so the curve remains vertical. There is no rapid change of temperature with depth. A thermocline is not present in high latitudes. Instead, the water column is isothermal (iso = same, thermo = heat).

 

Ocean Density Variation

Density is defined as mass per unit volume. It can be thought of as a measure of how heavy something is for its size. For example, an object that has low density is lightweight for its size, such as a dry sponge, foam packing, or a surfboard. An object that has high density is heavy for its size, such as cement, most metals, or a large container full of water.

 

Density is an important property of ocean water because it determines the water’s vertical position in the ocean. Density differences cause large areas of ocean water to sink or float. When high-density seawater is added to low-density fresh water, the denser seawater sinks below the fresh water.

 

Factors Affecting Seawater Density

Seawater density is influenced by two main factors: salinity and temperature. An increase in salinity adds dissolved substances and results in an increase in seawater density. An increase in temperature results in a decrease in seawater density. Temperature has the greatest influence on surface seawater density because variations in surface seawater temperature are greater than salinity variations. In fact, only in the extreme polar areas of the ocean—where temperatures are low and remain relatively constant—does salinity significantly affect density. Cold water that also has high salinity is some of the highest-density water in the world.

 

Density Variation with Depth

By sampling ocean waters, oceanographers have learned that temperature and salinity—and the water’s resulting density—vary with depth. Figure 5 shows two graphs of density versus depth. One graph shows the density for low-latitude regions and the other for high-latitude regions. Compare the density curves in Figure 5 to the temperature curves in Figure 4. They are similar. This similarity demonstrates that temperature is the most important factor affecting seawater density. It also shows that temperature is inversely proportional to density. When two quantities are inversely proportional, they can be multiplied together to equal a constant. Therefore, if the value of one quantity increases, the value of the other quantity decreases proportionately. When water temperature increases, its density decreases.

 

Figure 5 The graphs show variations in ocean water density with depth for low-latitude and high-latitude regions. Interpreting Diagrams What is the difference between the low-latitude graph and the high-latitude graph? Why does this difference occur?

 

The pycnocline (pycno = density, cline = slope) is the layer of ocean water between about 300 meters and 1000 meters where there is a rapid change of density with depth. A pycnocline presents a significant barrier to mixing between low-density water above and high-density water below. A pycnocline is not present in high latitudes; instead, the water column is about the same density throughout.

 

Ocean Layering

The ocean, like Earth’s interior, is layered according to density. Low-density water exists near the surface, and higher-density water occurs below. Except for some shallow inland seas with a high rate of evaporation, the highest-density water is found at the greatest ocean depths. Oceanographers generally recognize a three-layered structure in most parts of the open ocean: a shallow surface mixed zone, a transition zone, and a deep zone. These zones are shown in Figure 6.

 

Surface Zone

Because solar energy is received at the ocean surface, it is here that water temperatures are warmest. The mixed zone is the area of the surface created by the mixing of water by waves, currents, and tides. The mixed zone has nearly uniform temperatures. The depth and temperature of this layer vary, depending on latitude and season. The zone usually extends to about 300 meters, but it may extend to a depth of 450 meters. The surface mixed zone accounts for only about 2 percent of ocean water.

 

Transition Zone

Below the sun-warmed zone of mixing, the temperature falls abruptly with depth as was seen in Figure 4. Here, a distinct layer called the transition zone exists between the warm surface layer above and the deep zone of cold water below. The transition zone includes a thermocline and associated pycnocline. This zone accounts for about 18 percent of ocean water.

 

Deep Zone

Below the transition zone is the deep zone. Sunlight never reaches this zone, and water temperatures are just a few degrees above freezing. As a result, water density remains constant and high. The deep zone includes about 80 percent of ocean water.

 

In high latitudes, this three-layered structure of the open ocean does not exist as seen in Figure 6. The three layers do not exist because there is no rapid change in temperature or density with depth. Therefore, good vertical mixing between surface and deep waters can occur in high-latitude regions. Here, cold high-density water forms at the surface, sinks, and initiates deep-ocean currents, which are discussed in Chapter 16.

 

Figure 6 Ocean Zones Oceanographers recognize three main zones of the ocean based on water density, which varies with temperature and salinity.

 

PRACTICE

Take notes on the above information.

 

ASSESSMENT

1. Turn in your notes.
2. Take the 6.04 Quiz.