Week Four: Ocean Waters of the World

4.1 Understanding Water Density

Have you ever wondered why ice floats? Have you ever thought how different the world would be if ice didn't float? Well, start thinking!

Ice floats because it is less dense than water. This is true about any two substances: if one is less dense than the other, then it will float. Good examples here are oil on water, floating logs, and ducks and witches (a reference to Monty Python 's Holy Grail; for those not fortunate enough to have seen their marvelous treatise on the density of objects, rent this movie and watch it!).

The density  of a substance (liquid, solid, or gas) is defined as the mass of that substance per unit volume. It is an expression of the amount of molecules packed into a particular volume of space. Think about a cereal box that is packed by weight not volume. When the cereal is loaded into the box, the box is full and has a certain density, for example, one pound of cereal per box. As the box is shipped and the cereal settles, the box is no longer full. The one pound of cereal now occupies a smaller volume (i.e. less than a full box) and thus, the density of the cereal has increased. The cereal grains are packed closer to each other.

The density of liquids and gases can change depending on the temperature. Increases in temperature usually decrease the density of substances, i.e. the space between the molecules in the substance expand. Decreases in temperature typically cause the density to increase, that is, the molecules in the substance get closer together, i.e. they contract.

Variations in density also occur as a function of pressure. As pressure on a substance increases, its density increases. Where decreases in pressure occur, substances expand and become less dense. Obviously, the effects of pressure are greater on gases than liquids or solids; nonetheless, pressure affects all substances.

We should all be comfortable with the concept of density and try to understand how changes in temperature and pressure cause changes in the density of substances, particularly water. Density differences between different masses of seawater are one of the major driving forces of deep-sea circulation and may have a major influence on the climate .

Let's take a moment to look at the effects of temperature on the density  of seawater. As the temperature of water decreases, water becomes more dense, as expected. However, at temperatures below 4 degrees C, a very unusual thing happens to water—it begins to expand. In other words, the density of water reaches a maximum at 4 degrees C; below and above this temperature, the density of water decreases.

This unusual property of water is what allows ice to float . Because water freezes below 4 degrees C, i.e. at 0 degrees C, ice is less dense than water. The reason for this apparent anomaly is that at 4 degrees C, water molecules are packed as tight as they will go. Any attempt to push them closer together, such as by lowering the temperature, only makes the water molecules push back harder, i.e. they repel each other. Water molecules at the freezing point form a crystal lattice structure, like ice and snow, that is significantly less dense that liquid water. Like Ivory soap, ice floats.

Imagine a world where ice didn't float. If ice sank, it is very likely that ice skating would never have been invented; lakes would take forever to freeze from the bottom up. The polar ice caps would not be as large as they are; all the sea ice would be at the bottom of the ocean. I would venture to say that polar bears would not exist and penguins would be packed onto a much smaller Antarctic continent. On the other hand, if there were no icebergs , the Titanic  never would have sunk.

Question: Can you think of any examples where organisms would be severely affected if ice didn't float?

4.2 The Saltiness of the Ocean

Just about anything and everything dissolves in water, and this is no less true for the oceans. It is said that the oceans contain every element on the planet dissolved in one form or another. There is even some truth to the statement that if the price of gold  went high enough, the oceans would be the site of the next gold rush.

Because water is a polar molecule , it is particularly good at dissolving molecules that separate into ions. Ions are nothing more than the charged elements of atoms; an ionic solution is a liquid containing the positive and negative ions of a particular salt. The most common salt is sodium  chloride , ordinary table salt. The chemical symbol for sodium is Na and the chemical symbol for chloride is Cl, so the chemical symbol for salt is NaCl. Now it just so happens that when salt is dissolved in water, it separates into these two parts, namely, Na and Cl. However, when Na and Cl separate, they take their electrical charge with them. In the case of Na, it retains a positive charge; thus, it becomes Na+, a positive ion. In the case of Cl, it retains a negative charge; thus, it becomes Cl-, a negative ion.

Water molecules align themselves around Na and Cl in accordance with the positive and negative charges of hydrogen and oxygen . (Remember that opposite charges attract and negative charges repel.) This ability of water to arrange itself around positive and negative ions is what makes water such an excellent solvent and allows it to dissolve so many different substances. In formal chemical terms, the dissolution  of NaCl in seawater is expressed as:

NaCl ---- Na+ + Cl-

In this nomenclature, the positive ion (Na+) is called a cation , which refers to any ion with a positive charge (to remember this, think of "cat" and "paws"). The negative ion Cl- is called an anion , which refers to any ion with a negative charge (to remember this, think of "an" as "a negative"). You might be familiar with this terminology if you know anything about batteries, like the battery in your car. The positive pole is called the cathode  and the negative pole is called the anode .

The various ions dissolved in seawater are what make it salty. Oceanographer define salinity  as the amount of salts dissolved in seawater. More formally, salinity is defined as the total amount of dissolved solids in seawater in parts per thousand (ppt) by weight when all the carbonate has been converted to oxide, all the bromide and iodide have been converted to chloride , and all organic matter has been completely oxidized. You may choose to remember which ever definition you like.

Four cations and three anions make up 99% of the ions dissolved in seawater. These seven ions are known as the major constituents of seawater . In order of abundance, these are chloride  (Cl-), sodium  (Na+), sulfate (SO42-), magnesium (Mg2+), calcium (Ca2+) and potassium (K+), and bicarbonate (HCO3-). Other elements present in seawater make up what are known as minor elements  or trace elements . Many of these latter elements are essential to the growth of organisms in the sea.

Note that many of the trace elements  are concentrated by the actions of marine organisms, particularly iodine, which is abundantly present is many seaweeds. This ability of marine organisms to concentrate trace elements is called bioaccumulation , and it is an area of extreme interest to marine biologists today. By sequestering certain elements, marine organisms are able to produce exotic chemicals, many of which have medicinal or commercial uses.

While we're on this topic of constituents in seawater, I would like to mention two other very important group of constituents: 1) the biologically important nutrients ; and 2) dissolved organic matter .

Biologically important nutrients  are those chemicals that are necessary for the growth of phytoplankton . Nitrogen compounds, including nitrate , nitrite  and ammonium , are particularly important, as are phosphorus , silica , and iron . These compounds are called nonconservative elements , because their proportions change in seawater as a result of the growth and reproduction  of phytoplankton and the activities of organisms that feed on them. The second major group, dissolved organic matter , is highly important to the growth and reproduction of bacteria and many soft-bodied marine invertebrates. Marine research in the last decade has revealed that many organisms can absorb organic matter directly through their "skin" and obtain nutrition without eating. Absorption of dissolved organic matter is highly important to the larval stages of mussels , abalone , and many other marine invertebrates.

Question: What kinds of substances don't dissolve easily in seawater?

4.3 Water Masses

The physical processes that change the density of seawater differ in very distinct ways across the world ocean. As a result of differences in the rates of evaporation and precipitation, different regions of the world ocean give rise to distinctive water masses  that can be identified based on their temperature and salinity characteristics. In addition, the formation of these waters drive the circulation patterns of deep-sea, or abyssal, waters. Because this circulation depends on the density differences between water masses, driven by changes in the temperature and salinity of seawater, this type of deep-ocean circulation is called thermohaline circulation .

Water masses  exist as five different types:

  1. surface waters  (to a depth of about 200 m)
  2. central waters  (making up the lower half of waters above the permanent thermocline ; a thermocline is a region of rapid change in temperature)
  3. intermediate waters  (below the permanent thermocline  and above the deep and bottom waters
  4. deep waters  (water below the intermediate water but not usually in contact with the bottom
  5. bottom waters

The end result is a layered ocean about which we can form some generalities and for which we can derive individual names for the particular water masses  that form in a given region.

Let's start with the Atlantic  Ocean. A water mass formed off the coast of Iceland , known as the North Atlantic Deep Water , appears to drive the major deep ocean currents of the world ocean. This region of the world is characterized by cool temperatures and lots of precipitation , resulting in a water mass with a salinity of approximately 34.9 ppt and a temperature of 2 to 4 degrees C. North Atlantic Deep Water sinks to the bottom of the ocean and flows south along the east side of the Atlantic  Ocean. Eventually, as the story goes, this water loops its way into the Indian Ocean  and surfaces in the equatorial upwelling  zone in the Pacific  Ocean!

This circulation of deep water in the oceans is known as the "global conveyor belt", a concept developed by Wallace Broeker  at Lamont-Doherty Earth Observatory . Although North Atlantic Deep Water  is not the most dense water formed in the world ocean, it is by far the most abundant water mass formed in the world ocean. For this reason, it has been suggested that North Atlantic Deep Water drives the circulation of the all the deep ocean. This circulation of the deep ocean driven by the formation of North Atlantic Deep Water has been called the global conveyor belt. Although it remains to be substantiated, the idea of a global conveyor belt has many important implications for the distribution of gases and elements, transfer of heat throughout the oceans, dissemination of larval stages of many organisms, global climate  and other processes. Considerable effort is underway to understand the nature and importance of this oceanic circulation.

The densest water in the world is produced in the Antarctic Ocean , primarily in the Weddell Sea  in the winter, along the edge of the Antarctic continent. The combination of high salinity and cold temperatures leads to water with the highest density of any water formed in the ocean. Antarctic Bottom Water , as it is called, has a temperature of approximately -0.5 degrees C and a salinity of around 34.8 ppt. Although Antarctic Bottom Water has the highest density, it is formed in small quantities. For the most part, it is limited to the bottom regions along the coast of South America . Here the Mid-Atlantic ridge keeps it from extending into the eastern side of the Atlantic . However, according to other sources, Antarctic Bottom Water has been found in the Pacific  Ocean at the equator (a journey of 1,000 years) and as far north as the Aleutian Islands  (another 650 years from the equator). In the Atlantic  Oceans, Antarctic Bottom Water has been found as far north as 40 degrees N, a journey of 750 years. Thus, at times, sufficient quantities may form to creep far from its origins. Its sluggish movement and confinement to the deep sills of the southern oceans means that this water mixes slowly and allows Antarctic Bottom Water to retain its characteristics for up to 1,600 years!

Intermediate waters are those water masses  that form a layer above deep and bottom waters , and generally represent the top of the permanent thermocline  in the oceans of the world. Atlantic Intermediate Water  forms as a result of the mixing of cold waters from the north and saline waters from the south. The resultant intermediate water floats as a layer on top of the North Atlantic Deep Water  and flows south. Its counterpart, the Antarctic Intermediate Water , forms in a similar way, although its density is not as great as Atlantic Intermediate Water. This water flows north towards the equator at a depth of approximately 500-1000 meters.

In the Pacific  Ocean, Intermediate Water masses  are again dominated by Antarctic Intermediate Water  below the equator and formed as intermediate water, known as North Pacific Intermediate Water, in the North Pacific . In the Indian Ocean , the intermediate waters  consist of Antarctic Intermediate Water.

Central waters are those water masses  that form directly above the permanent thermocline . Generally, they are confined to regions closer to the equator. In all three oceans, central water masses correspond to the regions where they exist. In the Atlantic  Ocean, we have the North Atlantic Central Water  and the South Atlantic Central Water . In the Pacific , the North Pacific Central Water  and South Pacific Central Waters  are present. In the Indian Ocean , central waters  consist of Southern Indian Central Water  and Equatorial Central Water.

A few "specialized" water masses  should be noted. In the Atlantic  Ocean, water coming out of the Mediterranean and flowing over the sill at Gibraltar forms water known as Mediterranean Water . Traces of Mediterranean Water have been found as far as 2,500 miles away from its source! This water is very distinct and imparts a greater complexity to deep water circulation in the Atlantic  Ocean. In the Indian Ocean , the Red Sea  serves a similar role, providing a distinct characteristic water mass to deeper waters in the northern Indian Ocean.

Identification and characterization of these water masses  is one of the favorite pastimes of physical oceanographers. As the role of the oceans in controlling the climate  of our globe has become better understood, the need to understand the factors which govern the formation and circulation of these water has become increasingly important. Understanding the properties of these water masses and defining their distribution across the globe are one means to better understand how they are formed.

The "take-home" message here is simple, yet profound. The oceans consist of distinct layers or "lenses" (or "blobs") of water that move about the globe and retain their characteristics for hundreds, if not thousands, of years. This "layer cake" model of the world ocean serves to define the "structure" of the oceans. This structure of the sea functions much in the same way that the "structure" of a forest functions; it creates distinct zones and regions on which organisms can depend and to which these organisms can adapt. It creates a type of framework on which the tapestry of ocean ecosystems can be weaved.