Oceans and Oceanography

I

Introduction

Oceans and Oceanography, the ocean is a vast body of salt water that covers about three-quarters of the Earth's surface; oceanography is the scientific study of the physical, chemical, and biological processes that maintain its structure and motion. Marine science is also concerned with the boundaries of the ocean, with the atmosphere above, and with the seafloor below as well as at coastlines, and with ice.

II

Ocean Basins

In the Southern hemisphere there is a circumpolar zone (the Southern Ocean) that connects the southern extremities of the S-shaped Atlantic, the vast triangular Pacific and the smaller Indian Ocean. There are several smaller semi-enclosed seas, of which the Arctic, the Baltic, and the Mediterranean are typical, that connect with larger oceans and modify their properties.

The average depth of the ocean ranges from around 4,000 m (13,100 ft) to 5,000 m (16,400 ft). Close to the land the seafloor is commonly found at shallow depths of less than 200m (655ft) which slope gently, sometimes rising to form offshore banks or islands. These shallow regions typically extend 100 to 200 km (60-120 mi) out from the land masses to form the continental shelves, regions of economic importance for fisheries, for the recovery of oil and gas, and for the disposal of waste. Seawards of the continental shelf, at the so-called shelf-break, the seafloor deepens rapidly, by about 3,500m (460 ft) to the continental rise, a gradually sloping zone of sediment extending about 600 km (360 mi) to the flat abyssal plains of the deep ocean floor.

The central axes of the main ocean basins are connected by the mid-ocean ridge system, extensive mountain chains with inner troughs that are intersected by fracture zones. The mid-ocean ridges (see Oceanic Ridge), fundamental to understanding the evolution of the ocean basins, are explained by plate tectonics. They are associated with earthquakes, volcanoes, and with hydrothermal vents that transfer hot chemically-rich fluid from the Earth's interior and are associated with unusual sulphide-dependent biological communities. From the mid-ocean ridges, molten rock wells up and spreads internally, adding new material to the Earth's rigid crustal plates. The plates are moving apart, at a few centimetres per year. In areas where the plates overlap, such as the rim of the Pacific Ocean, crust is subducted and returned to the mantle, forming trenches which reach depths exceeding 7 km (4.4mi). The deepest known depth, in the Mariana Trench east of the Philippines, is 11,033 m (36,198 ft).

It is useful to distinguish between the shallow continental shelves and the deep ocean, but it must be kept in mind that even the deepest trenches are shallow relative to the diameter of the Earth: the depth-to-width ratio is about 1 to 1,000. The ocean, like the atmosphere, consists of a shallow layer of fluid held on the rotating Earth by the force of gravity.

III

Ocean Water

The ocean contains 97 per cent of the water on Earth, the atmosphere 0.001 per cent. The processes that interchange and transform water into vapour, liquid or solid are fundamental to weather and climate, and to life itself.

Water is one of the most common substances but it has unusual physical and chemical properties. It is one of very few naturally occurring liquids and it occurs in all three phases: water vapour, liquid water, and solid ice. It has a large specific heat and latent heat, so that large amounts of energy are needed to raise its temperature, to melt ice and to vaporize water. These properties to a large extent control the distribution of temperature on Earth, oceanic climates being more equable than those of continental regions. There are many other properties of water—high solvent power, high dielectric constant, high surface tension, among others—that ensure reactions essential to life proceed rapidly. Most of these properties are not much affected by the presence of the dissolved salts that distinguish the saline seawater from the much smaller amount of freshwater.

Seawater is a complicated solution that probably contains all the stable elements; present analytical techniques have identified about half of them but most are present in only small amounts—less than 1 part per million. The major constituents of a typical kilogram of seawater are 965 grams of water together with 19.353 grams of chloride, 10,760 of sodium, 2,712 of sulphate, and 1,294 grams of magnesium together with smaller amounts of calcium, potassium, bicarbonate, bromide, strontium, boron, and fluoride. Samples of seawater from almost everywhere in the open ocean have been found to contain these major constituents in very nearly constant proportions, so that seawater can be treated as a uniform mixture of them (the salinity) diluted with a varying amount of freshwater. Because of this near-constancy of composition, the salinity can be accurately estimated by measuring the electrical conductivity of a sample at a known temperature.

The properties of freshwater depend on pressure and temperature; those of seawater are affected by salinity also. The density of seawater, for example, depends on temperature, pressure, and salinity in a complicated way: it decreases as the temperature increases but increases with the salinity and the pressure. The density is important because the ocean tends to move so that the densest water is at the bottom and the least dense at the top. Another important physical property of seawater is its high capacity for absorbing electromagnetic radiation, especially sunshine. Even in the clearest water almost all (99 per cent) of the incoming sunlight is absorbed in the top 100 m (330 ft), where it can be utilized in photosynthesis to transform inorganic carbon and nutrient elements into biological organisms such as plankton. At deeper depths the ocean is essentially dark, and its properties can be changed only by mixing processes.

Sound waves, however, can be transmitted through the ocean with relatively little loss: a depth charge detonated off Perth, Western Australia, can be detected off Bermuda in the North Atlantic. Man and marine animals therefore use sound to communicate underwater. Ocean depths are measured by echo sounding, in which the depth is calculated from the time taken by a pulse of sound to go to and into the seafloor and back. Sonar works similarly but the beam is transmitted at an angle to the vertical to detect or to image submarines, fish shoals or the shape and texture of the seafloor. GLORIA (the Geological Long Range Inclined Asdic), for example, uses a towed subsurface sonar to image the floor of the deep ocean for several kilometres each side of a surface research vessel.

IV

Ocean Structure

The superficial appearance of the ocean is now well known from space observation. We live on what has been called “the blue planet” and from space see mostly blue sea, white clouds, and the relatively small amounts of land. We can distinguish ocean waves and swell and, by careful study of coastlines, the daily and half-daily movement of the whole ocean basins that constitute the tides. Such visual observations are confined to the surface; other important properties require observations from ships.

The distribution of sea surface temperature is the property best known, because it can be measured from space as well as by using simple methods and it is reported by merchant ships. In the open ocean it decreases from values of 30°C or more, near the equator to about -2°C near the ice in high latitudes. The salinity is more difficult to measure so is less well known; it is relatively low in high latitudes and has subtropical maxima at about 25°N and 25°S, with an equatorial minimum between. The distribution is linked to the difference between evaporation and precipitation, the low equatorial salinity being linked to high tropical rainfall (hence there are jungles and rainforests on land) and the twin maxima to the low rainfall of the subtropical anticyclones (with deserts on land). Both surface temperature and salinity are distributed approximately zonally, with contours running east to west. There are anomalies near coasts, associated with ocean currents and with a phenomenon known as upwelling. Upwelling regions are found near the eastern boundaries of the ocean where winds blowing along the coast can produce a mean offshore surface current. Deeper water (from perhaps 500 m/1,640 ft)) upwells to replace it, bringing lower temperatures. The upwelled water is often rich in nutrient salts, and as a result the upwelling areas are biologically highly productive and rich in fish and other forms of marine life.

Observations at depths below the surface are less numerous but scientists have a good knowledge of the mean distribution of temperature, salinity, and oxygen, with less complete information about other constituents. By far the best known is the temperature structure. The range is the same as that at the surface (-2°C to 30°C, just about the range of temperature in which people can live) but there is much more cold water than warm; the average temperature is 3.5°C. All the water warmer than 5°C is confined to a relatively shallow layer between latitudes 50°N and 50°S.

Apart from near-surface seasonal and diurnal changes, the typical structure is of a layer of nearly isothermal water near the surface, separated by a layer of relatively rapid temperature change (the main thermocline) from a thick cold layer extending to the seafloor. North and south of latitude 50° the temperature varies little with depth. In middle latitudes the surface temperature increases and the depth of the main thermocline is a maximum, about 1 km (3,300 ft). At low latitudes the surface temperature is high and the thermocline shallow (about 100 m/330 ft) with a rapid change of temperature with depth. This structure is to some extent explicable in terms of the physical properties of seawater: in general the colder the water the heavier, so it is to be expected that the densest (cold) water will sink to fill the deep ocean basins. The coldest water is found at the surface in polar regions in winter, after its heat has radiated away into the long polar night: it sinks and makes the deep ocean cold, even under the tropics and the equator. Exactly how and where the sinking takes place is still being investigated. Salinity, as well as temperature, affects the density, especially at low polar temperatures. The main sinking regions appear to be of limited extent, confined to the Weddell Sea, in the Atlantic sector of the Southern Ocean, and to the Greenland-Iceland-Norwegian Sea in the North Atlantic. The salinity structure of the ocean is more complicated than its temperature structure. In general the densest water, with the lowest temperature, is found at the bottom. Salinity usually affects the density less so can be more variable with depth. The processes that affect the salinity (rainfall to dilute the water and evaporation to concentrate it) occur at the surface and form water masses with a particular combination of salinity and temperature. Once a water mass leaves the surface its temperature and salinity can be altered only by mixing other water masses. Most of the mixing processes treat heat and salt in the same way, so a water mass tends to retain its own particular temperature/salinity (T/S) characteristic.

Temperature and salinity are the most important tracers for indicating the source regions of water masses. They are called conservative tracers because there is no process, away from the surface, that puts in or takes out heat or salt, so in the deeper layers the temperature and salinity are conserved. A T/S diagram, showing how the salinity varies with temperature in a particular water column, provides a sort of fingerprint that allows water masses to be traced over thousands of kilometres of travel, being only gradually modified by slow mixing with other water masses. The detailed processes that bring about the mixing present a central problem in modern physical oceanography.

There are other tracers which, though not conservative, are valuable because they can provide an indication of time. Water at the sea surface is usually saturated (or even supersaturated) with atmospheric gases, including oxygen. When the water leaves the surface its oxygen content is gradually reduced because it is the life support of marine creatures and is used in the decomposition of detritus. The decreasing oxygen content thus provides some indication of the time since the water left the surface. In some regions, where the water is stagnant, all the oxygen is used up and hydrogen sulphide is present instead. The Black Sea is a classic example—it is said to be so called because sulphides blacken metal objects lowered into it.

Other tracers, called transient tracers, have distributions that change with time, often due to the influence of humankind. Tritium, for instance, is the heaviest isotope of hydrogen; its concentration in the ocean is almost entirely due to radioactive fallout from nuclear weapons testing after World War II. Its spread in the ocean has clarified some ocean-circulation rates and the magnitude of mixing. Tritium is radioactive, decaying with a half-life of 1,245 years to form a stable isotope, Helium³. Measurements of the tritium and Helium³ in the same sample provide an estimate of the time since the sampled water left the surface. Both the measurements and the interpretation are complicated but are producing valuable insight into deep ocean circulation. Other man-made tracers, such as freons, are also producing valuable results, and studies are being made of the possibility of deliberately injecting tracers, such as sulphur hexafluoride, to study transport and mixing.