Sea Water, Physics and Chemistry of

Most people come in contact with the ocean only near its surface, and usually near its edges. In the huge part of the ocean that remains hidden, sea water is salty, cold, dark, and deep. Average salt content in the ocean is 35 grams per kilogram of sea water, composed mostly of six constituents: sodium (Na⁺), chloride (Cl⁻), sulfate (SO₄²⁻), magnesium (Mg²⁺), calcium (Ca²⁺), and potassium (K⁺).^* These are often referred to as conservative elements, because their ratios to each other remain constant throughout the ocean.

It is important to measure salinity of sea water accurately. The salinity and the temperature determine water density (which drives water movement), and concentrations of many elements can be indirectly determined from salinity.

Salinity

In the early twentieth century, salinity was measured by chemically titrating the sample to measure the chloride (Cl⁻) ion , then making use of the constant proportions of the major ions to Cl⁻ to calculate total salinity. In the 1950s and 1960s, it became clear that this procedure and calculation were not satisfactory for the precise measurements needed to distinguish and track different water masses in the oceans.

Drying a sample of sea water and weighing the salt residue is not practical because some of the salts tend to decompose and lose weight before all the water has been removed. The index of refraction of light changes with the salinity of a water sample; a handheld optical instrument called a refractometer can be used to find an inexact measure of salinity.

By 1960, the salinity of sea water was most often measured by a salinometer, which measures the electrical conductivity of the sample compared to that of a standard. Electrical conductivity, the ability of a substance to conduct electricity, increases for water as the amount of dissolved ions increase. Using the measurement of conductivity it is possible to measure salinity with high precision, so salinity can be determined to ±0.0001.

The conductivity can be measured in situ (without bringing the water to the surface) at the same time temperature and pressure are measured. These measurements give a profile of salinity and temperature versus depth (pressure) and are commonly collected by an instrument package referred to as a CTD (conductivity, temperature, depth).

The units by which salinity is expressed have changed as the methods of measurement changed. Salinity can be given as grams (g) of salt per kilogram (kg) of sea water (g/kg), or as parts per thousand (ppt). Titration for Cl⁻ led to the use of chlorinity units, also expressed as ppt. Sea water of 35 ppt salinity has a 19.4 ppt chlorinity.

Salinity is now defined as the ratio of the electrical conductivity of the sample at 15°C and 1 atmosphere of pressure to that of a potassium chloride (KCl) solution containing 32.4356 g KCl in 1 kg of solution. (Atmospheric pressure at sea level is about 14.7 pounds per square inch, termed "1 atmosphere.") The KCl solution is measured at the same temperature and pressure as the sample and gives a conductivity defined to correspond to a practical salinity of 35. Because this is derived from a ratio, it has no units, and is written "salinity is 35."

The above definition was formally established in 1980 by the United Nations Educational, Scientific and Cultural Organization (UNESCO) Joint Panel on Oceanographic Tables and Standards, putting an end to decades of debate about the meaning of salinity. Some authors, uncomfortable with a unitless value, use psu (practical salinity units). In practice, a sample of standard sea water with a salinity of 35.000 is used as a shipboard standard to compare conductivities and calibrate the salinometer.

Temperature and Density

The temperature of the world's ocean is highly variable over the surface of the ocean, ranging from less than 0°C (32°F) near the poles to more than 29°C (84°F) in the tropics. It is heated from the surface downward by sunlight, but at depth most of the ocean is very cold. Seventy-five percent of the water in the ocean falls within the temperature range of −1 to +6°C (30 to 43°F) and the salinity range of 34 to 35.

Variations in total salinity and in temperature cause variations in the density of sea water. Several factors can cause the salinity to deviate from35. Addition of river water or rainwater decreases salinity; excess evaporation or formation of pack ice causes salinity to increase (because ice crystals themselves do not contain salt—the salt is expelled to cracks and pores between the crystals).

Cold sea water is denser than warm sea water. There are several areas at the ocean surface where surface water becomes very cold. In these locations, surface sea water becomes denser than the surrounding water and sinks to begin the formation of slow thermohaline currents, which move deep-ocean water.

Density differences among different water masses allow physical oceanographers to calculate movements of water in the ocean. The density of a water sample is a measure of the total mass in a given unit volume. Salinity increases the density because the dissolved salts are contained in the same volume as the water. Water molecules cluster more closely around positive and negative ions in solution in a process called electrostriction, which also serves to increase sea-water density.

Density of water in the ocean, reported as sigma t (σ_t), is calculated from temperature, salinity and pressure by using the equation of state for sea water:σ_t = (σ − 1) × 1,000. At 4°C and salinity of 35, the density σ of sea water is 1.02781 gram per cubic centimeter, and σ_t = 27.81.

At depth, pressure from the overlying ocean water becomes very high (pressure at 4,000 meters is about 400 atmospheres), but water is only slightly compressible, so that there is only a minor pressure effect on density. At a depth of 4,000 meters, water decreases in volume only by 1.8 percent. Although the high pressure at depth has only a slight effect on the water, it has a much greater effect on easily compressible materials (see box on this page).

Density Structure of the Ocean.

The light and heat from the Sun can only directly penetrate a short distance into the ocean. The surface water is warmer and thus less dense than deep water, which gives most of the ocean a stable density arrangement (see the figure on page 82). The temperature and density often are relatively constant in the surface zone or mixed layer (upper 100 to 200 meters), and begin to change more abruptly in deeper water. The mixed layer is only about 2 percent of the total ocean volume, but covers most of its surface.

Temperature decreases and density increases more abruptly in deeper water; the same structure exists in lakes. Many swimmers will recall the experience of finding cooler water at greater depths in lakes. The zone separating surface ocean water from deep water is the pycnocline, containing 18 percent of ocean volume. The deep zone lies below the pycnocline, and contains 80 percent of the ocean volume; its temperature and density are much less variable than those found in the pycnocline.

Light and Sound in the Ocean

Most of the ocean lies in complete blackness. Sunlight reaches only depths of about 100 meters (330 feet) in clear open water. This lighted layer is referred to as the photic zone.

The depth of light penetration is decreased by particles in the water, including any algal cells that are growing there. Coastal waters with a high sediment content, or water in which an algal bloom is occurring, have much shallower light penetration than clear open-ocean water.^*

Water absorbs different wavelengths of light differently. By a depth of 10 meters (33 feet), mostly blue-green light remains, explaining the bluish color of underwater photos taken in natural light.

Sound.

Sound travels at 1,450 meters (4,750 feet) per second in sea water compared to 334 meters (1,100 feet) per second in air. Sound in water is reflected back when it strikes a solid object. Because the speed of sound in water is well known, this behavior is used to measure distances under the ocean; a signal is sent out and the time required for the return of the reflected sound can give an accurate measure of the distance to the object that reflected it. This technique is used to measure the depth of water from the surface to the seafloor under a ship; the PDR (precision depth recorder) uses a narrow sound beam to give a continuous record of the water depth through which the ship is moving.

Some wavelengths of sound can penetrate the seafloor to some degree, and hence show the layering in sediment. Depth recorders can detect the presence of fish below the surface and record the movements of the deep scattering layer, swarms of small organisms that move toward and away from the sea surface as the time of day changes.

Sonar (sound navigation and ranging) uses sound to locate and identify targets such as submarines.^* Navies have conducted years of research on sound propagation in water. The speed of sound in water increases as temperature, salinity, and depth increase. Differences in these properties in layers of ocean water cause the sound to refract, or bend, as it travels through the ocean. Refraction can easily make the sound appear to come from a different direction than its real source location, so an accurate understanding of sound physics has been vital for naval operations.

A zone of minimum sound velocity exists at a depth of roughly 1,000 meters (about 0.6 mile) called the sofar (sound fixing and ranging) channel. Sound signals that originate in the sofar channel tend to stay in the channel rather than escaping. The sound may travel enormous distances in this channel; explosions set off in the channel in Australia have been heard in Bermuda.

A project called ATOC (Acoustic Thermometry of Ocean Climate) was being tested as of 2002. Its goal is to measure global climate change by observing changes in the speed of sound in the sofar channel that would indicate changes in ocean temperatures. Research to determine the effects of this project on marine mammals also is underway.

see also Brines, Natural; Ice at Sea; Light Transmission in the Ocean; Ocean Chemical Processes; Ocean Currents; Oceanography, Physical; Sea Water, Freezing of; Sound Transmission in the Ocean.

Martha R. Scott

Bibliography

Duxbury, Alison B., and Alyn C. Duxbury. Fundamentals of Oceanography, 4th ed. NewYork: McGraw-Hill, 2001.

Duxbury, Alyn C., Alison B. Duxbury, and Kieth A. Sverdrup. An Introduction to the World's Oceans, 6th ed. New York: McGraw-Hill, 2000.

Garrison, T. Essentials of Oceanography. Pacific Grove, CA: Brooks Cole, 2001.

Pilson, Michael E. Q. An Introduction to the Chemistry of the Sea. Upper Saddle RiverNJ: Prentice Hall, 1998.

Internet Resources

Density of Ocean Water. Windows to the Universe: University Corporation for Atmospheric Research. http://www.windows.ucar.edu/tour/link/earth/Water/density.html&eduhigh

An Ocean of Layers. San José State University, Department of Geology. <http://geosun1.sjsu.edu/~dreed/130/lab10/8.html>

Temperature of Ocean Water. Windows to the Universe: University Corporation forAtmospheric Research. <http://www.windows.ucar.edu/tour/link/earth/Water/temp.html>

COMPRESSING STYROFOAM^®

The effect of sea-water pressure on easily compressible materials has been substantiated hundreds of times in "unofficial" experiments by shipboard researchers. These enthusiasts rush at the last minute to wire a sock full of Styrofoam^® coffee cups to an instrument or sampling device as it heads over the side for deep water. On return of the instrument to the deck, the cups will have been compressed to the size of thimbles.

^* See "Algal Blooms, Harmful" for a photograph of a nearshore algal bloom.

^* See "Ocean Chemical Processes" for a table summarizing selected constituents in sea water.

^* See "Sound Transmission in the Ocean" for a photograph of a sonar device.