Seawater major elements present

Because water is a universal solvent, at least some of virtually every element is present as a solute in seawater. As shown in Table 3.1, the most abundant substances in seawater are the major ions (Cl , Na", SO4 , Mg ", Ca ", and K" ). They are present in nearly constant proportions in the open ocean because their concentrations are largely controlled by physical processes associated with water movement, such as transport by currents, mixing via turbulence, evaporation, and rainfall. These solutes are also referred to as conservative ions. Most of the rest of the solutes in seawater are not present in constant proportions because their concentrations are altered by chemical reactions that occur faster than the physical processes responsible for water movement. These chemicals are said to be nonconservative. Though most substances in seawater are nonconservative, they collectively comprise only a small fraction of the total mass of solutes and solids in the ocean. 

The average elemental composition of seawater is shown in Table 1 [6]. The elements which exist at more than l/igml 1 are the major elements, and those less than l/rgml-1 are the trace elements. Of these, only twelve elements are present at a concentration greater than 1/igml-1, and most other elements range from 0.5 pg ml-1 to much less than 1 ng ml-1 in content. 

The major constituents in seawater are conventionally taken to be those elements present in typical oceanic water of salinity 35 that have a concentration greater than 1 mg kg excluding Si, which is an important nutrient in the marine environment. The concentrations and main species of these elements are presented in Table 1. One of the most significant observations from the Challenger expedition of 1872-1876 was that these major components existed in constant relative amounts. As already explained, this feature was exploited for salinity determinations. Inter-element ratios are generally constant, and often expressed as a ratio to Cl%o as shown in Table 1. This implies conservative behaviour, with concentrations depending solely upon mixing processes, and indeed, salinity itself is a conservative index. 

Chlorine and sulfixr are two major elements of seawater that can be foimd, very imequally distributed, in marine organisms. In the algae, sulfur is mainly in the form of sulfate in the primary metabolites that are polyholosides and, much more rarely, in the secondary metabolites. For the invertebrates, on the other hand, sulfixr is very largely represented in the secondary metabolites and is found in all its oxidation steps. Halogens are found in abundance in only one class of algae, the red algae (Rhodophyceae or rhodobionts), but they are present in most phyla of invertebrates. 

Though great progress has been made in the past four decades, many gaps remain in our understanding of the chemical processes that occur in the sea. There are several reasons for this. First, except for water and the six major ions, all the other substances in seawater are present at very low concentrations. The combination of trying to detect low concentrations in the presence of large amounts of salts makes measurement of the trace constituents in seawater very difficult. To make matters even more complicated, most elements are present in several different forms, or species, in seawater. The speci-ation of an element determines its reactivity. Thus, the concentration of each species of an element must be known to fully understand the chemical behavior of that element. 

Radioactive substances are widely distributed on the earth. Some are found in the atmosphere, but the major part is present in the lithosphere. The most important ones are the ores of uranium and thorium, and potassium salts, including the radioactive decay products of uranium and thorium. Uranium and thorium are common elements in nature. Their concentrations in granite are about 4 and 13mg/kg, respectively, and the concentration of uranium in seawater is about 3 pg/l. Some uranium and thorium minerals are listed in Table 1.1. The most unportant uranium mineral is pitchblende (UsOs). Uranium is also found in mica. The most important thorium mineral is monazite, which contains between about 0.1 and 15% Th. 

An element that is relatively conservative through water-rock reaction is chlorine in the form of the anion chloride. Chloride is key in hydrothermal fluids, because with the precipitation and/or reduction of SO4 and the titration of HC03"/C03, chloride becomes the overwhelming and almost only anion (Br is usually present in the seawater proportion to chloride). Chloride becomes a key component, therefore, because almost all of the cations in hydrothermal fluids are present as chloro-complexes thus, the levels of chloride in a fluid efiectively determine the total concentration of cationic species that can be present. A fundamental aspect of seawater is that the major ions are present in relatively constant ratios—this forms the basis of the definition of salinity (see Volume Editor s Introduction). Because these constant proportions are not maintained in vent fluids and because chloride is the predominant anion, discussions of vent fluids are best discussed in terms of their chlorinity, not their salinity. 

The shiny purple-black crystals of elemental iodine were first prepared in 1811 from the ashes of seaweed. Several species of seaweed concentrate the iodine that is present in low proportions in seawater, and for many years, seaweed was the major practical source of this element. Most iodine is now produced from natural brines via oxidation of iodide ion with chlorine. 

Eight major ions account for almost 99% of the mass of salts present in seawater, as shown in Table 3.1. The dominant ions are chloride (i.e. the anion CF) and sodium (the cation Na+). Virtually all known elements have been detected in seawater, but most are at extremely low concentrations (below the parts per million, or ppm, level). 

Magnesium is the third most abundant element in seawater, behind sodium and chorine, and has an average concentration of approximately 1300 ppm. Table 3.2 displays the major and some minor elemental constituents of seawater. Eleven major constituent ions account for 99.5% of the total solutes present in seawater. These 11 are chloride, sulfate, bicarbonate, bromide, fluoride, sodium, magnesium, calcium, potassium, strontium, and boron, and they largely determine the chemistry of seawater. 

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