sea water At first glance, water seems an ordinary substance; this is not, however, the case. In fact it is only because of the very unusual physical and chemical characteristics of water that such diverse life forms have been able to evolve on land and in the sea. As we all know, water is composed of two atoms of hydrogen bonded to a single oxygen atom. The geometry of these bonds results in a water molecule in which both hydrogen atoms are aligned on one side of the oxygen atom. Because of the way in which the electrons are shared by the atoms, oxygen carries a small net negative charge and hydrogen a net positive charge. Consequently, when water molecules meet one another they tend to join together, held by the attraction of the positively charged hydrogen atoms to the negatively charged oxygen atom of the neighbouring water molecule (in much the same way that the opposite poles of bar magnets are attracted to each other). This type of bond between molecules is known as a hydrogen bond. It results in water having a highly stable molecular arrangement, which has important implications. Under ordinary conditions, water is the only substance on the planet that naturally occurs simultaneously in all three phases: solid, liquid, and gas. Were it not for the great stability of the hydrogen bonds, the boiling point and freezing point of water would be considerably lower and water would occur on Earth exclusively in the gaseous phase.
Approximately 71 per cent or 361 × 10
6 square kilometres (km
2) of the Earth's total surface is covered with water. Each year about 3.6 × 10
14 cubic metres of water evaporate from the surface of the oceans. About 90 per cent is returned to the oceans as rainfall. The remaining 10 per cent is transported over the continents, where it falls mainly as rain before eventually returning to the sea as river and groundwater run-off. This transport of water forms the basis of the hydrological cycle. Because the rates at which water is removed from and supplied to the ocean is equal, the volume of water in the ocean remains constant over time. Water is known as the universal solvent because of its unique ability to dissolve, to varying extents, the majority of the known elements. During its passage over land, water erodes vast amounts of continental rock. Each year approximately 50 × 10
6 tons of material are removed from the continents and transported to the oceans as dissolved salts and suspended particles. The bulk of this material falls to the sea floor and accumulates as thick layers of sediment close to the continental margins.
Table 1 Concentrations of the eleven most abundant constituents in sea water
Constituent | Ion | Parts per thousand | Percentage of |
|---|
| symbol | by weight (g/kg) | dissolved material |
|---|
(From: A. Allaby and M. Allaby (1990) The concise Oxford dictionary of Earth sciences.) |
Chloride | Cl− | 18.980 | 55.05 |
Sodium | Na+ | 10.556 | 30.61 |
Sulphate | SO42− | 2.649 | 7.68 |
Magnesium | Mg2+ | 1.272 | 3.69 |
Calcium | Ca2+ | 0.400 | 1.16 |
Potassium | K+ | 0.380 | 1.10 |
Bicarbonate | HCO3− | 0.140 | 0.41 |
Bromide | Br− | 0.065 | 0.19 |
Borate | H3BO3− | 0.026 | 0.07 |
Strontium | Sr2+ | 0.008 | 0.03 |
Fluoride | F− | 0.001 | 0.00 |
Total | | 34.447 | 99.99 |
The average concentrations of the eleven most common constituents found in sea water are shown in Table 1; together they account for about 99.9 per cent of the total dissolved material in sea water. A significant feature of sea water is that while the total concentration of the more abundant components varies with location, the proportions of these dissolved salts remain remarkably constant throughout the oceans and appear to have done so over the past 600 million years. The total concentration of these dissolved salts determines the saltiness or salinity of sea water (see
oceanic salinity).
Together with the major elements found in sea water, nearly all the remaining naturally occurring elements are also present in smaller quantities. In fact, several of these minor constituents, in particular nitrogen, phosphorus, silicon, zinc, copper, and iron, are important for the growth of many marine organisms. The variety of different element reactivities is highlighted by the span of the estimated average time each element spends in the ocean before being permanently incorporated in the sediment of the sea floor. These estimates, known as residence times, range over six orders of magnitude from sodium at about 2.8 × 10
8 years down to aluminium at about 100–200 years. The longest residence times are found for the major ions in sea water. These times are far longer than the average time it takes to complete a single mix of the oceans (about 1600 years); this emphasizes the lack of reactivity in sea water shown by these elements. The shortest residence times are found for elements such as aluminium, chromium, titanium, and iron, which are highly particle reactive and so are quickly removed by sinking particles. A large number of the minor constituents of sea water that are active in biological processes have intermediate residence times (about 5–50 × 10
3 years); these times are still long with respect to the oceanic mixing time. As a result, these elements undergo numerous internal cycles before being permanently removed to sediments. Such elements are removed in the surface waters and transported down through the water column by biological carriers. In deeper waters these elements are regenerated into solution from decaying organisms and are subsequently transported back to the surface in upwelling zones.
Besides the solids dissolved in sea water there are also gases, which in order of their relative abundances are nitrogen, oxygen, and carbon dioxide. By volume dissolved nitrogen accounts for about 64 per cent of the total gases dissolved in sea water. This nitrogen is biologically unimportant, as most organisms cannot directly make use of it. Sea water contains a substantially higher concentration of dissolved oxygen than the atmosphere (34 per cent as opposed to 21 per cent). The supply of oxygen to the surface ocean reflects a balance between the input of oxygen across the air–sea boundary and involvement in biological processes. During photosynthesis, marine plants consume carbon dioxide and produce oxygen gas. Since photosynthesis occurs only in the illuminated surface waters, the only source of oxygen in the deeper ocean for use by deep-living aerobic organisms is the sinking of relatively oxygen-enriched surface sea water. The concentration of carbon dioxide (about 1.6 per cent), which is some fifty times greater than in the atmosphere, is also exchanged between the atmosphere and ocean at the air–sea boundary. Once dissolved in sea water, carbon dioxide enters a series of complicated reactions which prevent sudden change in the acidity or alkalinity of sea water. This regulation is known as buffering and it maintains the pH range of ocean water between 7.5 and 8.5. Carbon dioxide is also transferred to the biosphere by the photosynthesis of marine plants and production of carbonate shells by marine plants and animals. Carbon dioxide therefore has importance in both the biological and chemical cycles of the oceans. The role of the oceans as a consumer of excess atmospheric carbon dioxide has generated considerable interest in recent years as our appreciation of the so-called ‘greenhouse effect’ has increased. The other remaining gases in sea water make up only about 0.5 per cent of the total amount of gases in water.
Temperature and salinity are of particular interest to oceanographers, because combined with pressure they determine the density of sea water. This property is important, for it can be used to identify and trace characteristic water masses. The difference between adjacent water bodies allows physical oceanographers to calculate rates of water movement in the deep sea. The temperature of surface oceanic sea water is determined primarily by the latitudinal variation in solar insolation, which is related to the varying angles at which the Sun's radiation strikes the Earth's surface. The temperature tends to decrease away from the Equator towards the poles from about 30 to −2 °C. Because of its salt content, sea water does not freeze until about −2 °C. There is also vertical temperature variation in the water column, with warm surface water separated from the main body of the colder deep ocean by the thermocline, a zone of rapidly decreasing temperatures that extends from the base of the surface mixed layer down to 1000 m at some locations. Temperatures in water deeper than 1000 m are much lower that at the surface and range between –1.8 and 5 °C. The heat energy balance of the atmosphere and ocean remains in equilibrium despite short-term and local variations. This is achieved by interaction of the circulating sea water with the overlying atmosphere. Warm surface water is carried from the low latitudes to the higher polar latitudes as a surface current. In polar regions net loss of heat occurs through radiation from the ocean to the atmosphere. The decrease in temperature increases the density of the sea water. The resulting water mass then sinks to the bottom and causes the deep water to be pushed along horizontally at a depth of equal density, creating a bottom current which moves at velocities of about 1 to 0.1 mm s
−1 towards the Equator. This cyclic flow of water initiated by the cooling of surface sea water in the polar regions is known as thermohaline circulation, and is the principal example of a process called advection. Sea water motion is the combination of two processes: advection and turbulence/diffusion. Advection is a large-scale transport mechanism that involves the net movement of water from one location to another, either horizontally or vertically, whereas turbulence is essentially random mixing of water molecules caused by diffusion, in which there is no overall net transport of water, only the exchange of properties. Advection occurs at a much faster rate than turbulent mixing, and the sinking water masses therefore maintain the salinity and temperature signatures acquired from their last period at the sea surface. These characteristic signatures have been used to trace the transport paths as water masses follow the thermohaline-driven circulation. Turbulent mixing with adjacent water masses will, however, eventually homogenize the waters and destroy these unique salinity and temperature signatures.
The speed at which sound waves travel in sea water ranges between about 1400 and 1500 metres per second (m s
−1); this is some five times faster than the speed of sound in air. The speed of sound waves in sea water is related to its density and is therefore affected by any slight changes in salinity, temperature, and pressure. For every unit increase in salinity, the speed of sound increases by 1.5 m s
−1. While a 1 °C increase in the temperature causes a rise of about 4 m s
−1 in the speed of sound, increasing pressure causes speed to rise at a rate of 18m s
−1 for every 1000 m of depth. The propagation of sound waves in sea water has many applications in oceanography. One of the most important has been the use of precision echo-sounders on ships. These employ a sound pulse sent from a transmitter which is reflected from the bottom to an underwater receiver, or hydrophone, carried by the ship. If the speed of the sound wave through the water is known, the time interval between the pulse being sent and the reflection received can be used to calculate the depth. Accurate depth measurement has enabled oceanographers to construct detailed bathymetric charts of the ocean floor.
The passage of light through sea water is unaffected by salinity, temperature, or pressure. About 60 per cent of the total light energy entering open-ocean sea water is absorbed in the first metre; only 1 per cent is left at a depth of about 150 m, and no sunlight penetrates below 1000 m. The typical blue colour of the open ocean is due to the preferential scattering of the shorter-wave blue component of sunlight by water molecules and suspended particles. The short-wave blue light is the most penetrating, and therefore the colour seen is dominantly blue. Greater colour variation is seen in coastal sea water because of the pigments of photosynthetic plankton that are present in great abundance and the dissolved organic substances in freshwater run-off, which tend to emphasize the yellowish-green colours.
Ian R. Hall
Bibliography
Open University (1995) Seawater: its composition, properties and behaviour. (Oceanography Series). Pergamon Press, Oxford.
Libes, S. M. (1992) An introduction to marine biogeochemistry. John Wiley and Sons, New York.
