W . M . W h i t e G e o c h e m i s t r y Chapter 15: Oceans Chapter 15: The Oceans as a Chemical System Introduction ntoine Lavoisier* called seawater Òthe rinsings of the Earth.Ó Given the tenuous understand- ing of geological processes existing at the time (the late 18th century), this is a remarkably Ainsightful observation. Most of the salts in the oceans are derived from weathering of the con- tinents and delivered to the oceans by rivers. But the story of seawater is more complex than this. Some components of seawater are derived from hydrothermal metamorphism of the oceanic crust. Other components in seawater, most notably the principal anions as well as water itself, are derived from neither weathering nor hydrothermal reactions. These so-called Òexcess volatilesÓ are derived from volcanic degassing. Furthermore, salts do not simply accumulate in seawater. This point was overlooked by John Jolly in his attempt to estimate the age of the Earth, described in Chapter 8, from the mass of salts in the sea and the amount added annually by rivers. His result, 90 million years, is a factor of 50 less than the actual age of the Earth. The ocean is a dynamic, open system, and it is ul- timately the balance between addition and removal of an element that dictates it concentration in the ocean. This was recognized by Georg Forschhammer in 1865 when he wrote: ÒThe quantity of dif- ferent elements in seawater is not proportional to the quantity of elements which river water pours into the sea, but is inversely proportional to the facility with which the elements are made insoluble by general chemical or organo-chemical actions in the sea.Ó One of our objectives in this chapter will be to examine the budget of dissolved matter in the oceans; that is, to determine the sources and sinks and the rates at which salts are added and removed from the oceans. Elements also cycle between different forms within the oceans; these include both organic and inor- ganic solids as well as various dissolved species. This internal cycling is intimately tied to the vari- ous physical, geological, and biological processes occurring within the ocean. The biota plays a par- ticularly crucial role both in internal chemical cycling and in controlling the overall composition of seawater. A second objective of this chapter will be to examine how elements and compounds are dis- tributed within the ocean, and how they cycle between various forms. LavoisierÕs statement also reminds us that the oceans are part of a grander geochemical system. Sediments deposited in the ocean provide a record of that system. On human time scales at least, the ocean appears to be very nearly in steady state. It is tempting to apply LyellÕs principal of uniformi- tarianism and assume that the composition of the seawater has also been constant on geologic time scales. There is, however, strong evidence that some aspects of seawater composition do change over time, as we found in Chapters 8 and 9. Precisely because these variations are related to changes in other geological processes, such as plate tectonics, climate, life, and atmospheric chemistry, they can tell us much about the EarthÕs history and the workings of the planet. Interpreting these past changes begins with an understanding of how the modern ocean works and the controls on its composi- tion. This understanding is our main goal for this chapter. Some Background Oceanographic Concepts Salinity, Chlorinity, Density, and Temperature A useful concept in oceanography is salinity. Salinity can be thought of as the total dissolved sol- ids in seawater. More precisely, salinity is defined as: the weight in grams of the dissolved inorganic matter in one kilogram of water after all the bromide and iodide have been replaced by the equiva- lent amount of chloride and all carbonate converted to oxide (CO2 driven off). This unfortunate defi- *Antoine Lavoisier, born in France in 1843, is often called the father of modern chemistry. He died a t the guillotine in 1794. 645 January 25, 1998 W . M . W h i t e G e o c h e m i s t r y Chapter 15: Oceans nition has an interesting historical basis. Robert Boyle found that he could not reproduce total dis- solved solid measurements by drying and weighing. The salinity definition is due to Thorensen, who would bubble Cl2 gas, which substitutes for Br and I, through seawater. The salt could then be heated, converting carbonate to oxide, and a constant weight achieved. Salinity is now determined by measuring electrical conductivity, which increases in direct relation to the concentrations of ions in water, and hence with salinity. Another useful definition is chlorinity, which is the halide concen- tration in grams per kilogram measured by titration with silver and calculated as if all the halide were chloride (total halides are actually 0.043% greater than chlorinity). Chlorinity can also be measured by conductivity. As we shall see, Cl is always present in seawater as a constant proportion of total salt, and therefore there is a direct relationship between chlorinity and salinity. By defini- tion: S‰ = 1.80655 Cl‰ 15.1 ÒStandard seawaterÓ, which is close to average seawater, has a salinity of 35.000 parts per thousand (ppt or ä) and a chlorinity of 19.374 ä. Open ocean water rarely has a salinity greater than 38ä or less than 33ä. Temperature, along with salinity, determines the density of seawater. Since density differences drive much of the flow of ocean water, these are key oceanographic parameters. Temperature in the oceans can be reported as potential, or in situ temperature, but the former is the most commonly used. In situ temperature is the actual temperature of a parcel of water at depth. Potential temperature, denoted θ, is the temperature the water would have if brought to the surface. The difference between the two is thus the temperature difference due to adiabatic expansion. Since water cools when it ex- pands, potential temperature is always less than in situ temperature (except for surface water, where there is not difference). The difference is small, on the order of 0.1¡C. While this difference is im- portant to oceanographers, it is generally negligible for our purposes. Temperature and salinity, and therefore also density, are conservative properties of seawater, which is to say that they can be changed only at the surface. The density of seawater is 2 to 3 percent greater than that of pure water. Average seawater, with a salinity of 35ä and a temperature of 20¡C, has a density of 1.0247 g/cc. Density is usually reported as the parameter σ, which is the per mil deviation from the density of pure water (1 g/cc). Thus i f density is 1.0247 g/cc, σ is 24.7. Again, one can distinguish between in situ and potential density, po- tential density being the density water would have if brought to the surface, and is always lower than in situ density. The difference is small, a few percent, and generally negligible for our purposes. Circulation of the Ocean and the Structure of Ocean Water The concentrations of dissolved elements vary both vertically and horizontally in the ocean. To fully understand these variations, we need to know something about the circulation of the ocean. This circulation, like that of the atmosphere, is ultimately driven by differential heating of the Earth: solar energy is gained principally at low latitudes and lost at high latitudes. Because the mecha- nisms of surface and vertical circulation in the oceans are somewhat different, it is convenient to treat them separately. Surface Circulation Surface circulation of the ocean is driven primarily by winds; hence the surface circulation is some- times also called the wind-driven circulation. Figure 15.1 is a simplified map of the wind-driven cir- culation. The important features are as follows: ¥ Both north and south of the climatic equator, known as the Inter-Tropical Convergence, or ITC, water moves from east to west, driven by the Trade Winds. These currents are known as the North Robert Boyle (1627-91) was another of the founders of modern chemistry. He defined the chemical element as the practical limit of chemical analysis, and deduced the inverse relationship between the pressure and volume of gas, a version of the ideal gas law. 646 January 25, 1998 W . M . W h i t e G e o c h e m i s t r y Chapter 15: Oceans C a n a r Ca ream y lif St C C. orni lf . io a u h G os Cu r r u r K e n t iC. al m o S B . r a C C zi a u l l r e C e u P . g n e B Figure 15.1. Surface and Deep Circulation of the oceans. Arrows show the direction of wind driven cur- rents. Gray areas are regions of upwelling. Red stippled areas are regions of deep water production. In the Indian Ocean, black arrows show current directions in northern hemisphere winter, red arrows show current direction in northern hemisphere summer. and South Equatorial Currents. Between these two currents, the Equatorial Counter Current runs from west to east. ¥ Two large gyres operate in both the Atlantic and Pacific Oceans, one in the northern and one in the southern hemisphere. Rotation is clockwise in the northern hemisphere and counter-clockwise in the southern hemisphere. The Coriolis Force, an apparent force that results from the EarthÕs rota- tion, is largely responsible for this circular current pattern.