chemical oceanography

chemical oceanography is the study of the chemistry of the complex mixture of dilute chemicals in sea water. This is quite unlike classical chemistry in which pure unadulterated solutions are investigated.

Sea water contains every known naturally occurring element. Some like sodium and chlorine are in high concentrations but the majority occur only in trace concentrations. At the beginning of the 20th century the Germans believed they could extract enough gold from sea water to finance their war debt. They were led astray because the analyst's water samples were contaminated by his gold wedding ring, and he grossly overestimated the samples' gold content. Similar problems of contamination, caused by the wire on which the sample bottles were mounted, or even by cigarette smoke in the laboratory, have invalidated many subsequent analyses of the concentrations of more important trace metals in sea water, like iron, copper, and zinc. Sea water also contains a complex cocktail of organic compounds. Some of these are natural, but others are pollution from industrial and agricultural activities. Man has even added novel elements such as minute traces of americium, an isotope synthesized by the nuclear industry and used in smoke detectors.

Water itself is a remarkable compound. As an oxide of hydrogen (H₂O) it might be expected to be a gas as is nitrous oxide (N₂O or laughing gas), which has a much heavier molecule. However, water molecules tend to stick together in fives, increasing its effective molecular weight, so pure water is liquid above 0 °C (32 °F). The freezing point of sea water is depressed to -1.9 °C (28.6 °F) by its salt content. Ice is also exceptional, in that it floats. It is lighter than the liquid water because the molecules are arranged into an open lattice and are more spaced out. Quite of lot of heat is needed to melt a gram of ice (80 calories) and even more to evaporate it (532 calories). These ‘latent heats’ have far-reaching implications for understanding physical processes in the oceans.

The total amount of dissolved salts in sea water is known as its salinity. Many chemical constituents of sea water are conservative, so their concentrations vary in direct proportion to the salinity. This is particularly true for those occurring at concentrations greater than one part per million, which together contribute 99.9% of the salinity. Most constituents are in ‘steady state’—the rate at which they are being added to the oceans is exactly balanced by the rate that they are being lost. The total ocean is stirred about every 1,500 years (akin to taking over a millennium to stir a cup of tea). So any substance with a turnover rate of 10,000 years or more is uniformly distributed. However, the concentrations of many substances fluctuate widely, and most of these are involved in biological processes. For example, oxygen is used in the respiration of most organisms and is also produced during photosynthesis. Carbon compounds are involved in all biological processes, and many simple inorganic compounds, such as nitrates and phosphates, are essential for marine plant growth. Recently, iron has been identified as being an important trace nutrient regulating biological production in certain areas of ocean. Many organisms regulate their internal chemical contents and thereby influence the concentrations of trace elements found in sea water. Some of the rarer elements are used in trace concentrations: iron is a constituent of haemoglobins in vertebrates including whales and man, and vanadium and copper are constituents of the blood pigments of pelagic tunicates (sea squirts) and crustaceans respectively.

Trace concentrations of some elements are essential to certain life forms, but at higher concentrations they become toxic, copper being an example. Some elements are not used directly by organisms but become attached to the outside of cell walls of marine plants and bacteria, and so their distributions in sea water resemble those of nutrients.

The vertical profiles of concentrations of these substances that are involved in biological interactions typically show much reduced concentrations near the surface, where they are stripped out by the heightened biological activity, and by the sinking of detrital material into deep water. However, in the deeper water below the thermocline, detrital material is broken down, releasing these substances back into solution, so their concentrations increase. They are then returned back into the surface layers by vertical mixing during winter at high latitudes when the thermocline is broken up by cooling. So the upper few hundred metres of water are mixed by winter storms, and by upwelling.

Different behaviours of isotopes of the same element provide useful insights into differences in the dynamics of different parts of the cycles. For example, uranium-238 in continental rocks disintegrates into the radioactive gas radon, which is emitted to the atmosphere where it radioactively decays into lead-210, which is inert and insoluble. The lead isotope is washed into the ocean by rain where slowly it sinks to the bottom. However, lead-210 is also radioactive and decays into polonium-210, which readily gets absorbed onto biological surfaces and behaves like a nutrient. So measuring the ratio between the lead-210 and polonium-210 gives a measure of the differences in rates of sedimentation under gravity and the biological removal.

The exchange of gases across the surface of sea water is another important process, which is determined by the temperature of the water and the partial pressures of the gas. More gas can dissolve in cold water, because its partial pressure is lower, and if its partial pressure is lower than in the atmosphere, more gas will dissolve at the surface of the water. When the sea is rough, the breaking waves carry bubbles several metres below the surface, so the equilibrium in the gas balance between the atmosphere and the water is achieved faster. In cold polar seas up to 9 millilitres of oxygen can dissolve in each litre of sea water, whereas in the tropics, where surface water temperatures are higher than 20 °C (68 °F), only 6 millilitres of oxygen can dissolve. The same effect is seen with carbon dioxide, but here the picture is complicated because when the marine plant phytoplankton is growing rapidly and using up the carbon dioxide in photosynthesis, the oceans draw carbon dioxide out of the atmosphere. Where cold water is upwelling in the tropics, carbon dioxide is vented back into the atmosphere. So understanding the importance of the oceans in ameliorating the rise of carbon dioxide concentrations in the atmosphere is highly complicated, and their effectiveness may be reduced by climate change.

Other important processes that influence the chemistry of sea water and the sediments that accumulate on the seabed are associated first with hydrothermal vents, which discharge fluids that are rich in the metal sulphides that precipitate to form manganese modules. Secondly, the organic matter that accumulates in profusion on the seabed underlying upwelling regions is, over geological time, converted into the offshore oil and gas reserves that are now fuelling our industries.Open University, Ocean Chemistry and Deep-Sea Sediments (1989). M. V. Angel