Introduction to Ocean Sciences

Chapter 6: Water and Seawater

Guide to Reading and Learning

Chapter 6 explains how and why water is a unique molecule among all the millions of molecules that are known to chemists. Most of its unique properties are the result of the hydrogen bond, which exists in only a handful of chemical substances. The hydrogen bond is a result of the polar nature of the water molecule.

Seawater is different from pure water as a result of its dissolving power. This chapter reviews the spectrum of dissolved components in seawater and their involvement in geochemical cycles. Chapters 6 and 7 are perhaps the most important chapters in this text. Without an understanding of the properties of water, many of the concepts used in later chapters would be difficult, if not impossible, to fully understand.

A basic understanding of the composition of seawater is necessary to see how seawater composition affects biological productivity and how dissolved gas exchange between the ocean and atmosphere is important to studies of the anticipated global climate effects of greenhouse gases.

This chapter is very rich in data that describes the composition of seawater in terms of its constituent dissolved components. There is no need for you to memorize or even study in detail all the numbers, especially those provided in Table 6.5. The information is very detailed, but it is provided so you can develop a general “feel” for the concentrations of various dissolved components, especially the incredibly small concentrations of some dissolved components that are critical to supporting life in the ocean.

Did you know that if water was not unique in almost all of its many properties described in Chapters 6 and 7, but especially in its ability to dissolve and transport chemical elements and compounds essential to life, life as we know it could not exist? Water’s unique properties are intimately connected with every part of our lives—including our biochemistry, our weather and climate, and the distribution of life on the planet—so the knowledge you gain here will find numerous applications in your future life.

Chapter 6 Essential to Know 

Critical Concepts used in this chapter

CC.7, CC.8, CC.9, CC.14, CC.18

6.1 Origins and Distribution of the Earth’s Water

  • Most of the Earth’s water condensed out of the atmosphere as it cooled.
  • The temperature of the Earth’s atmosphere has apparently remained fairly close to today’s temperature for billions of years, and water has probably occurred in all three forms—water, ice, and water vapor—at the Earth’s surface for all of that period.
  • Since water first condensed to form oceans some small amounts of water have been added to the Earth’s surface by volcanism and possibly from meteorites, but the total volume of water has probably remained almost unchanged for billions of years.

Uniqueness of the Earth

  • The Earth is the only planet in the solar system that has abundant water and is the only planet where water can exist in solid, liquid, and vapor phases within the range of atmospheric temperatures.
  • Water is the only known substance that is present in all three physical forms—solid, liquid, and gas—within the range of temperatures found on the Earth’s surface.
  • The presence of water in all three physical states is important for maintaining the Earth’s climate within a relatively stable state and for the stability of the Earth’s ecosystems.

Distribution of the Earth’s Water

  • More than 97% of the world’s water is in the oceans.
  • Most the world’s water that is not in the oceans is found in the ice caps and glaciers.
  • Less than one half of one percent of the Earth’s water is in the form of fresh water in lakes, rivers, streams, and groundwater, the source of almost all drinking water for the human and terrestrial animal population of the planet.

6.2 The Water Molecule

  • Water has many unusual properties that are related to the nature of its molecule.

Atoms and Electrons

  • All atoms consist of electrons orbiting a nucleus. The number of negatively charged electrons is equal to the number of positively charged protons in the nucleus. The nucleus also contains neutrons, which have no charge.
  • Atoms of different elements have different numbers of electrons.
  • Electrons are arranged in shells that orbit at various distances from the nucleus.
  • Each shell has a maximum number of electrons. The first shell contains no more than two electrons, the second contains no more than eight electrons, and so on.
  • Atoms are energetically more stable if their outer shell of electrons is full. As a result, elements such as chlorine and oxygen, which have an almost-full outer shell, have a tendency to gain electrons; atoms such as sodium, which have a less-than-half-full outer shell, tend to lose electrons.

Chemical Bonds between Atoms

  • Atoms of different elements combine to form chemical compounds.
  • There are two types of bonds between atoms in compounds: ionic and covalent.
  • In an ionic bond one atom loses one or more electrons (to empty a less-than-half-full shell) while the other atom or atoms gain one or more electron (to complete an almost-full shell).
  • In covalent bonds the two atoms share one or more electrons so that each of their outer shells is filled part-time.
  • Covalent bonds are much stronger than ionic bonds.

Van der Waals Force and the Hydrogen Bond

  • All molecules are electrically neutral but are attracted to each other by a weak force called van der Waals force.
  • In water molecules, oxygen and hydrogen form a covalent bond. The two hydrogen atoms are located on the same side of the oxygen atom with an angle of 105o between them. This makes the water molecule polar. On the hydrogen side of the molecule there is small net positive charge as the single proton in each hydrogen atom’s nucleus is partially “exposed” because the hydrogen electron is shared with the oxygen atom. In contrast, on the other side of the oxygen molecule there are two pairs of electrons, which gives this side of the molecule a small net negative charge.
  • Because the water molecule is polar, the negative side of one water molecule is attracted to the positive side of another, and vice versa. This attraction is called a hydrogen bond. The hydrogen bond is weaker than either ionic or covalent bonds but is stronger than the van der Waals forces between other molecules. The hydrogen bond is the reason that water has many unique properties.

6.3 The Dissolving Power of Water

  • Water can dissolve greater numbers substances and higher concentrations of most of these substances than any other known liquid.
  • Compounds with ionic bonds are particularly soluble because the negative ion (cation) is surrounded by water molecules oriented with their positive hydrogen atom sides toward the ion and the positive ion (anion) is similarly surrounded by water molecules with their negative oxygen atom sides oriented toward it. This is called hydration and it breaks the ionic bond and allows the ions to move about individually,  and free from each other..
  • Most elements dissolved in seawater are in the form of hydrated cations or anions.
  • Water’s great ability to dissolve chemicals is important to all life processes because these processes depend on the transport of elements in the dissolved form.

6.4 Sources and Sinks of Chemicals Dissolved in Seawater

  • Concentration of elements in seawater are determined by their behavior in global biogeochemical cycles.

Biogeochemical Cycles

  • Chemical compounds are cycled through complicated pathways among the oceans, ocean sediments and rocks, and terrestrial rocks. The cycle, extremely simplified, takes elements from the ocean to ocean sediments and rocks, then through subduction zones to be added to the continental crust or subducted and then erupted through volcanoes back into the continents and islands, and then returned to the oceans through erosion and rivers.
  • Global biogeochemical cycles have been ongoing for billions of years and have had time to reach a steady state in which the total amounts of each element in the oceans, ocean sediments, and terrestrial rocks respectively do not change with time.

Steady-State Concentrations

  • Because global biogeochemical cycles are in an approximate steady state, the concentrations of dissolved salts in seawater (averaged over the ocean basins) are also at an approximate steady state and do not change with time.

Residence Time

  • The residence time of an element in the oceans is the average length of time an atom of the element that enters the oceans stays in the oceans before being removed to the sediment or otherwise removed.
  • Elements in the oceans have a wide range of residence times. The residence time is a major factor in determining seawater concentration, although the total crustal abundance of the element is also a factor.
  • Elements with long residence times tend to have higher ocean water concentrations, whereas elements with short residence times have very small concentrations in seawater.

Constancy of Ocean Water Composition

  • Concentrations of elements in seawater are believed to have remained virtually unchanged for more than 500 million years. This constancy of composition has undoubtedly aided the development and evolution of life in the oceans.

6.5 Salinity

  • Salinity is a measure of total salt concentration in seawater and is usually measured, by the water’s electrical conductivity.
  • Salinity, originally defined as the total weight of dissolved salts per unit volume of seawater, is now defined by the ration of its electrical conductivity compared to the electrical conductivity of a known concentration of potassium chloride.  This ratio is called practical salinity units (PSU).  Salinity cannot be expressed by SI units so it normally appears as a number with no units.
  • Salinity is very important in determining the density of seawater.

6.6 Dissolved Chemicals in Seawater

  • All of the stable elements and thousands (or millions) of organic compounds are found dissolved as anions or cations in seawater, although some are at very low concentrations.
  • Concentrations of some elements in seawater are as low as, or lower than, one part per trillion parts of water—the equivalent of 1 second in 31,700 years or less.

Major Constituents

  • Seawater has six major constituent dissolved elements (concentrations above 100 ppm) in seawater: chlorine, sodium, magnesium, sulfur (as sulfate), calcium, and potassium.
  • The six major constituents together comprise 99.28% of all the dissolved salts.
  • Inputs of the major constituents to the oceans from rivers and other sources are very small compared to their total amounts in the oceans, and their residence times in the oceans are very long. Their relative concentration are the same in seawater throughout the oceans, except in some enclosed marginal seas where ocean water is mixed with substantial amounts of river water, which has a much more variable composition than seawater.

Minor Constituents

  • Seawater has six minor constituents (concentrations between 1 and 100 ppm): bromine, carbon, strontium, boron, silicon, and fluorine. Nitrogen and oxygen are not considered to be minor constituents because they are present primarily as dissolved gases.
  • Some of the minor constituents, including carbon and silicon, are extensively used by marine organisms and, as a result, their concentrations in seawater are variable.

Trace Elements

  • Aside from the 12 elements that are major or minor constituents and some elements that are dissolved gases, all other dissolved elements in seawater are present at concentrations less than 1 ppm and are called trace elements.
  • Many trace elements are essential to life.
  • All elements are toxic at some concentration, but some are toxic at relatively low concentrations and a few are toxic at concentrations that are not far above their seawater concentration.

Radionuclides

  • Seawater contains a variety of naturally occurring and human-made radionuclides, all at extremely low concentrations.
  • The concentrations of radionuclides are relatively easy to determine in seawater. Because radionuclides behave in a virtually identical manner to nonradioactive isotopes of the same elements, they can be used as tracers to follow the movements of the element through the biogeochemical cycle.
  • Some radionuclides, especially human-made radionuclides whose time and location of entry into the oceans is known, are useful for tracing the movements of ocean water.

Organic Compounds

  • There are thousands of organic compounds dissolved in seawater. However, their concentrations are so low that little is known about them.

Dissolved Gases

  • Gases are free to move between the ocean and atmosphere at the ocean surface.
  • The net direction of the exchange is determined by the saturation solubility of the gas in seawater. If seawater is undersaturated, the gas migrates into the water from the atmosphere. If seawater is oversaturated, the gas is released to the atmosphere.
  • The relative proportions of nitrogen, oxygen, and carbon dioxide in the oceans are very different from the atmosphere.
  • Carbon dioxide makes up only a faction of a percent of atmospheric gases but is approximately 15% of the dissolved gases in the ocean surface water and more than 80% in deep waters. This is, in part, because carbon dioxide reacts with water to produce highly soluble carbonate and bicarbonate ions.
  • The total quantity of carbon dioxide in the oceans is about 70 times greater than the total quantity in the atmosphere.
  • Oxygen constitutes a larger proportion of the dissolved gases in the ocean surface waters than it does in the atmosphere, but only a much smaller proportion in deep waters.

Oxygen and Carbon Dioxide

  • In surface waters, oxygen exchanges with the atmosphere and is usually at or near its saturation solubility. Oxygen is also released by photosynthesis in surface waters.  As a result, oxygen concentration can be oversaturated in surface waters in areas of high photosynthetic activity.
  • Below the surface layer, oxygen is consumed and carbon dioxide is released to solution by decomposition of organic matter. Thus, the oxygen concentration falls and the carbon dioxide concentration rises with time after a water mass has left the surface.
  • The saturation solubility of gases generally decreases as temperature increases and is generally lower at ocean salinities than in freshwater. As a result, oxygen concentrations are normally high in cold polar and subpolar waters, whereas oxygen concentrations are much lower in tropical waters.
  • The low oxygen concentration in tropical waters makes them more vulnerable to marine pollution than waters in cooler regions.

Other Gases

  • There are several other gases dissolved in seawater. With the exception of sulfur dioxide all of these other gases—including nitrous oxide, methane, methyl iodide and dimethyl sulfide—are produced primarily by marine organisms so their net transport is from the ocean to the atmosphere..
  • Several of the minor dissolved gases, including methane, are potent greenhouse gases when they are introduce to the atmosphere. The release of methane to the atmosphere must therefore be taken into account in studies of global climate change.
  • Sulfur dioxide in the atmosphere comes primarily from burning fossil fuels. When sulfur dioxide is transfered to the oceans it reacts with water to produce sulfuric acid. This process has led to a slight but steady increase in the acidity of the oceans that, if continued, may adversely affect the ability of marine organisms, including reef-building corals, to produce calcium carbonate shell or structural material

pH and Buffering

  • pH is a measure of the acidity or alkalinity of a solution. Ocean water is mildly alkaline.
  • The equilibrium between dissolved molecular carbon dioxide, bicarbonate, and carbonate ions in seawater buffers seawater (maintains the pH of sweater within a narrow range). Adding acid, which would normally lower the pH, pushes the equilibrium toward higher carbonate and carbon dioxide concentration, which removes some hydrogen ions and raises the pH. Conversely, adding alkali would produce more bicarbonate and hydrogen ions and raise the pH to offset the effect of the alkali.
  • Recent studies have found that ocean surface waters have experienced a small, but possibly significant, decrease in pH due to sulfur dioxide, despite the protective buffering ability of seawater.

Critical Concept Reminders:

CC.7 Radioactivity and Age Dating (p. 143)

  • Seawater contains naturally occurring and manmade radioisotopes of many elements at very low concentrations. These are useful for tracing movements of the elements through biogeochemical cycles. To read CC7 go to page 18CC.

CC.8 Residence Time (p. 137, 138)

  • The residence time of an element in seawater is the average length of time atoms of the element spend in the oceans and it depends, to a large extent, on the rate of processes that remove them from solution. Residence time is a major factor in determining the concentration of elements in seawater. To read CC8 go to page 19CC.

CC.9The Global Greenhouse Effect (p. 144)

  • The oceans play a major part in studies of the greenhouse effect because the oceans store large amounts of carbon dioxide and are a source of other greenhouse gases including methane. To read CC9 go to page 22CC.

CC.14 Photosynthesis, Light, and Nutrients (p. 138)

  • Chemosynthesis and photosynthesis, the processes by which simple chemical compounds are made into the organic compounds of living organisms, depend on the availability of a number of dissolved nutrient elements. To read CC14 go to page 46CC.

CC.18 Toxicity (pp. 138, 142)

  • Many dissolved constituents of seawater become toxic to marine life at concentrations above their natural levels in seawater. A number of these constituents are nutrients that can limit primary production if their concentrations are too low. Thus, life depends on the relatively invariable concentrations of dissolved chemicals in the oceans. To read CC18 go to page 54CC.

 

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