Topics Covered in this Lecture:
- What is Salinity?
- The "Salt Mill" in the Sea
- Distribution of Salinity in the World Ocean
We now come to a discussion of one of the most well-known aspects of the sea, its saltiness. For people who grew up far from the sea where the only waves were amber waves of grain, there is usually a shock of sorts when they jump in the ocean and get their first taste of seawater. It's a much different taste than the fishing hole back home! The reason for this is because the sea contains various salts and other elements. In this lecture, we examine the constituents of seawater and gain an understanding for processes that contribute and remove salt from the sea. Finally, we will see how the saltiness of the sea changes with latitude across the global oceans, a phenomenon that will be of some importance to our understanding of the circulation of the seas and the distribution of organisms within the sea.
What is Salinity?
In our last lecture, we learned that water is an amazing solvent. 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.
As shown in your book, 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").
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.
A listed in your book, four cations and three anions make up 99% of the ions dissolved in 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 important 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.
Changes in the amounts of the major constituents account for virtually all of the changes we observe in the salinity of seawater in the world's oceans. Perhaps most remarkably, it was discovered in 1819 that the proportions of the major constituents doesn't change. As first suggested by the chemist Alexander Marcet in 1819, verified by another chemist, Georg Forchhammer, in 1865, and finally composed as a "law" by William Ditmar in 1884, the principle of constant proportions states that regardless of variations in salinity, the ratios between the amount of major ions in seawater is constant. Chemists have found that in any sample of seawater, whether it has a salinity of 20 ppt or 40 ppt, chlorine accounts for 55% of the ions, sodium comprises 30.6%, sulfate makes up 7.7% and magnesium measures out at 4%.
This finding was very important because it gave scientists a much easier way to determine the concentrations of ions in seawater. Once the concentration of one ion was known, such as the chloride ion, the concentrations of the other ions could be calculated using the principle of constant proportions. In fact, chemical measurements of chlorinity, the concentration of chlorine in ppt, were the standard way of determining salinity for some time. Nowadays, salinity sensors have been developed measure the conductivity of seawater, the ability of seawater to transmit an electrical charge (which is made possible by the fact that seawater is composed of ions, much like the battery in your car). By knowing the relationship between conductivity and salinity, much easier determinations of salinity can be performed using a conductivity probe. Conductivity probes also have the advantage of giving measurements all through the water column, rather than on individual water samples collected at discrete depths.
While we're on this topic of constituents in seawater, I would like to mention two other very important group of constituents that will concern us in a few weeks. These are 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 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?
The Salt Mill in the Sea
According to Norse mythology, there is a salt mill grinding away somewhere at the bottom of the sea, and that is why the sea is salty. For many years, this myth received little serious attention. However, the discovery of hydrothermal vents in the 1970s and the realization that many minerals make their way into the sea through these vents has given some credence to this old myth. The Vikings (like Hagar the Horrible) may have been better scientists than we give them credit for.
The regulation of the salt content of the seas remains, in my opinion, an unsolved puzzle. Calculations of the amount of minerals and salts added to the oceans from rivers and the weathering of rocks far exceeds the amount currently present in the sea. Many mechanisms for removing salt and minerals have been postulated, and contribute some percentage of removal, but to date no one has convincingly presented evidence for a salt removal mechanism that balances the input to the sea. Nonetheless, the salinity of the seas remains exceptionally constant, as it has for 1.5 billion years. How is it that the salinity of the world oceans has not changed in all this time?
The total amount of salts and minerals in the world ocean on average is about 5 x 1022 grams, or 55 million billion tons. Using the average runoff of salt from land due to weathering and dissolution of rocks, and assuming that rivers have been adding salt to the sea at the same rate for the past 3.5 billion years, it becomes clear that more salt has been added to the sea than is presently accounted for in the sea. For salinity to remain constant, the input of salts and minerals must be balanced with the removal of these constituents. Thus, there has to be some way by which salt has been removed from the sea.
Since the sources of salts and minerals in the sea are fairly well defined, we'll begin our discussion here. The major mechanisms by which salt enters the sea are thought to be:
- weathering and dissolution of rocks by rainfall
- weathering and dissolution of rocks by rivers
- injection of salts and minerals from hydrothermal vents
- outgassing from volcanoes (both aboveground and under the ocean)
- recycling of crustal material and ejection by volcanoes (i.e. island arcs)
Weathering and dissolution of rocks by rainfall and rivers add about 2.75 billion tons of salt each year to the oceans, or about 0.000005% of the total salt currently present. The quantities released through hydrothermal vents and volcanic eruptions are less certain, but given that the entire volume of the oceans is thought to circulate through hydrothermal vents once every ten million years, the amounts cannot be insignificant. Whatever the contribution, any additions to the salt content of the sea can only exacerbate the problem of where all the salt is going.
To remove nearly 3 billion tons of salt each year, oceanographers have invoked several plausible, but less than satisfactory mechanisms. The major means by which salt and minerals are removed from seawater are:
- evaporite deposition (salt deposits on land)
- allochthonous salt traps (salt deposits beneath continental shelfs)
- adsoprtion and sedimentation by clays and inorganic compounds
- removal through hydrothermal vent chemical reactions
- biological processes, such as phytoplankton growth and sedimentation
- biological formation of reefs, which get buried by tectonic activity
No information is available in your text on the quantities of material removed by any of these processes. As I research this subject further, I will try to get some numbers for you.
The formation of evaporites occurs as shallow seas, such as the Salton Sea, become isolated and evaporate. A giant salt deposit, more than 1000 feet thick, is present in the deepest parts of the Mediterranean Basin. These salt deposits formed as the Mediterranean was cut off from the Atlantic as a result of plate tectonics. Huge salt deposits are present beneath the ocean floor in parts of the Gulf of Mexico and the Atlantic Ocean. However, these salt deposits alone cannot account for the removal of all the minerals and salts that are needed to balance the salt cycle.
Adsorption of minerals onto clays and particles that settle onto the sea floor as deposits is another possible way that salts and minerals are removed. Clay particles may enter the oceans through rivers and land runoff and probably by atmospheric processes. Volcanic eruptions such as Mt. Pinatubo likely deposit vast amounts of clay-like particles that absorb ions and carry them to the sea floor. Clays appear to bind potassium easily, and may be responsible for sedimentation of this element.
Hydrothermal vents can act both as a source and removal mechanism for salts. Magnesium and sulfate appear to react with rocks in the crust. Magnesium forms mineral deposits while sulfate is transformed to hydrogen sulfide, a source of energy for chemoautotrophic bacteria. Whether these processes actually remove these salts permanently is hard to tell; magnesium could re-dissolve unless buried and sulfur could be recycled to seawater by vent organisms. Certainly, hydrothermal vents play an important part in regulating the composition of seawater. As more is learned about their chemistry, their role in removing salts and minerals will become more clear. One point in their favor: hydrothermal vents are continuously active and widespread through the oceans, unlike the formation of evaporite deposits. Thus, they have a better potential to regulate seawater chemistry than other mechanisms.
Biological activity is, without a doubt, an important means by which ions are removed from seawater. Silica is removed in great quantities by a form of phytoplankton known as diatoms. As mentioned in an earlier lecture, these siliceous organisms are deposited on the sea floor and thrust under the continents in subduction zones. Melting of siliceous deposits creates pyroclastic lavas that are highly explosive. Calcium is another ion used by marine phytoplankton. Calcareous "shells" are constructed by a group of phytoplankton known as coccolithophorids, who bloom in great profusion in the North Atlantic Ocean and parts of the Pacific Ocean. The shells of these organisms are quite ornate and decorated with knobby extensions called coccoliths. When present in sufficient numbers, they can turn the water a milky white, owing to the calcium present in their "shells." Regardless, they represent one means by which calcium at least can be removed from seawater and deposited to the seafloor as these organisms are eaten and defecated or as they die and sink to the ocean floor.
James Lovelock, founder of the Gaia Hypothesis, has proposed a most interesting mechanism for the removal of salt from the sea. He contends that coral reefs may represent a kind of evaporation pond built by organisms for the purpose of removing salts from seawater. He asks: "Is it possible that the Great Barrier Reef, off the north-east coast of Australia, is the partly finished project for an evaporation lagoon?" He goes on to point out the presence of vast coastal limestone cliffs and other biologically derived sedimentary deposits on land that point to the ability of marine organisms to change the shape and composition of the continents.
The regulation of the mineral content of the sea by marine organisms has many aspects that are appealing. Biological processes, like hydrothermal vents, are always active. Biological processes also tend to remove elements in specific ratios, as the metabolic processes that govern growth occur in very specific chemical sequences. Moreover, the influence of phytoplankton on seafloor deposits is unquestioned; many parts of the ocean sea floor are pure silica. Around land margins, where biological activity is the greatest, deposits derived from marine organisms have caused uplifting. As mentioned above, biogenic sediments also influence the composition of lava. Given the feedback mechanisms that marine organisms and perhaps more importantly, marine communities have developed; given the origins of many kinds of phytoplankton species 1.5 to 2 billion years ago; and given the constancy of the ocean's salinity since that time; strong arguments might be made for a "Gaian" or at least biological control of salinity. Only further research can reveal the magnitude of this activity.
One last concept that is important in understanding how salts and minerals are regulated in the ocean environment is residence time. Residence time refers to the average time that a particular constituent spends in seawater before it is removed by one of the processes mentioned above. Your book lists the approximate residence times for some of the major constituents. The table provided below, taken from Gross (Oceanography: A View of the Earth, 4th Edition) also lists the major mechanism by which a particular ion is removed. By knowing how an ion is removed, it is easier to understand its particular residence time.
Reactive substances that form insoluble compounds have short residence times. The ions of aluminum, iron, and chromium fall into this category. Soluble ions such as sodium, potassium, and magnesium, have long residence times. Despite the differences in residence times, the major constituents still maintain relatively constant proportions to each other. This implies that elements with short residence times also have quick replenishment; otherwise they would disappear altogether.
|Element||Time in Years||Removal Process|
|Sodium (Na+)||68 million||Evaporite deposition|
|Chloride (Cl-)||100 million||Evaporite deposition|
|Magnesium (Mg2+)||10 million||Reactions with newly formed oceanic crust|
|Sulfate (SO42-)||10 million||Reactions with newly formed oceanic crust|
|Potassium (K+)||7 million||Reactions with clay|
|Calcium (Ca2+)||1 million||Shell formation|
|Lead (Pb)||4 hundred||Removed by particles|
|Aluminum (Al)||1 hundred||Absorbed on clay particles|
(modified after Gross, 4th Edition, 1994)
from The Remarkable Ocean World