Seawater Chemistry

 

I. Introduction

A. The composition of seawater can be divided into suspended (also called particulate) and dissolved materials.     

B. Salinity is the term used to describe the concentration of inorganic, dissolved salts in seawater.

1. Unit = ppt or o/oo (per mil) by weight

2. Average for world's open ocean = 35 o/oo

3. Range    "    "      "     "   = 34-37 o/oo

4. Seawater is therefore 96.5% H2O and 3.5% salts

5. The boundary between dissolved and particulate is an arbitrary one set at a size of 0.45 microns.

a. This is what's called an "operational definition" in science, which is one that’s determined by practical considerations. In this case the practical consideration is the size of the holes in the filter paper used to filter the water sample. This is a procedure agreed upon by scientists to carry out a particular measurement or observation.

C. These dissolved ions make seawater a good conductor of electricity, unlike pure water.  The higher the salinity of the water the higher is its conductivity, so salinity is often determined by measuring the electricial conductivity.

D. The dissolved constituents of seawater can be divided into five broad categories:

1. Major constituents

2. Minor and trace components

3. Nutrients

4. Gases

5. Organics - not part of the salinity

 

II. Major Constituents (Comprise about 99.7% of all the dissolved materials)

A. Elements:

1. Primary are Cl- (55.04%) and Na+ (30.61%, which make up 85.65% of all the dissolved constituents.

2. Adding the next four most abundant (SO42-, Mg2+, Ca2+, and K+) brings the total to > 99% of the salts.

3. Finally the last 5 (HCO3-, Br-, H3BO3, Sr2+, F-) brings the total to > 99.99%.

4. Almost all of the rest of the 92 naturally occurring elements are found in the ocean but they only total to 0.01% of the TDS.

5. Although the other components are a very small % of the dissolved materials, they are essential for life on Earth.

B. Learn the first seven dissolved constituents, in order of decreasing abundance.

           Constituent       o/oo

                Cl-               19.2

                Na+          10.7

                SO42-              2.5

                Mg2+          1.3

                Ca2+                0.4

                K+            0.4

                HCO3-              0.1

 

C. By careful analysis of water samples collected during the Challenger expedition of the late 1800's it was discovered that all of the major constituents occur everywhere in the same relative proportions even though the amount of water in the mixture varies. These major solutes are called conservative because their concentrations are stable over time in the oceans.  This means that the ratios of these ions to one another are constant throughout most of the ocean because the oceans are very well stirred.

1. For example, from this list of constituents you can see that the ratio of the weight of sodium to the weight of chloride is:

              Na        10.556

              --   =   --------  =   0.556

              Cl        18.980

a. No matter what the salinity, this ratio remains unchanged if only H2O is added or taken away from the solution.

2. The conservative behavior of the major constituents doesn't hold true at the mouths of large rivers and in estuaries because the local conditions can concentrate or dilute the salt content.

a. In a humid climate such as we have here the influx of freshwater with a composition different from seawater dilutes the salts & decreases o/oo.

b. Estuaries in arid regions such as Texas, often have very saline waters due to excessive evaporation.

3. The conservative behavior also doesn't hold around mid-ocean ridges where some exchange of elements takes place.

4. Doesn't hold for Ca2+ and HCO3- in places because of the dissolution and precipitation of CaCO3 in tests of various organisms.

5. In polar regions freezing and melting of sea ice causes minor variations because sea ice preferentially excludes salts.

6. And finally it doesn’t hold in virtually land-locked seas like the Black and Baltic.

7. But in most places and at most times the ratios are constant so if you determine the concentration of only one of these constituents you can calculate the rest by multiplying by these ratios.

a. Cl- is easy, quick and cheap to determine so it's most often used.

b. For example, if the chloride content (o/oo) is 18.980, we can multiply this number by the ratio we calculated above to determine the o/oo of sodium.

c. It is also quite easy to determine the salinity of a seawater sample by measuring Cl- because the overwhelming proportion of the salinity consists of the major conservative elements. The salinity can be calculated from that using this formula:

                   Chlorinity X 1.80655 = Salinity

d. Chlorinity is determined by a procedure called a Mohr titration involving AgNO3 as the titrant:

                                      gms Ag*

          Chlorinity =  0.3285233   -------------

                                    kg seawater

* grams of Ag required to precipitate all the halides in a 0.3285233 kg sample of seawater

1) The chemical method for doing this was derived for the Challenger expedition and is still used today.

8. These days salinity is measured by measuring the electrical conductivity of a seawater sample.  The conductivity is directly proportional to the content of dissolved salts and can be, therefore, parlayed into a salinity determination.

9. When seawater evaporates or is diluted by precipitation, only the proportion of H2O changes so the proportions of these salts are unchanged.

a. In climates with an excess of evaporation over precipitation salinities get higher; that is the total content of dissolved salts per kilogram of seawater increases but the ratios of the major components to one another remain the same.  At the same time, in areas of excessive precipitation over evaporation salinities decrease, while the ratios remain the same.

b. What do you get if you evaporate seawater?

1) If you evaporate it to 1/2 of its original volume you get CaCO3 to precipitate out.

2) Evaporate to 1/5 and CaSO4.2H2O

3) Evaporate to 1/100 and all the NaCl comes out

4) Evaporate to dryness and get K and Mg chlorides and sulfates.

5) In ancient times people extracted NaCl from seawater by a partial evaporation to remove the CaCO3.  Then they drained off the remaining brine and evaporated it down to 1/100 of the original volume to get the NaCl out. They had to be careful not to evaporate it all the way to dryness or they would get the very bitter K and Mg salts.

 

III. Salinity variations in the open oceans

A. 34 - 37 o/oo is the range and 35 o/oo is the average

1. Red Sea = 40-42 o/oo because there is little incoming fresh water and high evaporation.

2. Gulf of Bothnia (off Finland) = 5 o/oo because of just the opposite situation

3. Completely isolated seas and lakes can reach as high 250 o/oo if there is little influx of fresh water and lots of evaporation. For example, the salinity of the Dead Sea is about 240 o/oo, and the salt is mainly MgCl2.

B. Processes =  EVAPORATION, PRECIPITATION, FREEZING

1. Evaporation and freezing cause an increase in salinity because only the H2O is removed from the seawater, the salts do not go along with the water.

a. Evaporation = liquid to gas transition

b. Freezing = liquid to solid transition

c. As a result the proportion of salts in seawater increases and therefore the salinity increases

2. Precipitation causes a decrease in salinity because rainwater contains very little in the way of dissolved constituents and, therefore, merely dilutes the concentrations of salts by increasing the proportion of H2O.

Example I

 

                                35 grams salt

Starting salinity = 35 o/oo = -------------------

                             1000 grams seawater

 

This means the 1 kg sample of seawater contains 35 grams of salt and 965 grams of H2O. After 100 grams of H2O have been evaporated, the remaining sample still contains 35 grams of salt but now it only contains 865 grams of H2O for a total seawater sample weight of 900 grams. Therefore, the new salinity is:

                                35 grams salt

      New salinity = 35 o/oo = -------------------

                              900 grams seawater

 

Clearly, this will yield a larger number than the starting salinity because the denominator is now a smaller number and the numerator has not changed. The new salinity is 38.9 o/oo.

 

Example II

As a second example, imagine starting with the same seawater sample,

                                35 grams salt

Starting salinity = 35 o/oo = -------------------

                             1000 grams seawater

 

but now assume it rains and 100 grams of virtually pure rainwater is added to the sample. Our new sample contains 35 grams of salt and 1065 grams of H2O for a total seawater sample weight of 1100 grams. Therefore, the new salinity is:

                                35 grams salt

      New salinity = 35 o/oo = -------------------

                              1100 grams seawater

 

Clearly, this will yield a smaller number than the starting salinity because the denominator is now a larger number and the numerator has not changed. The new salinity is 31.8 o/oo.

C. Evaporation and precipitation are by the far the major processes and their variations are climatically controlled.

1. Where  EVAP > PRECIP  seawater salinities are higher

a. Around  20-30oN and 20-30oS

b. Coincides with the world's great deserts

2. Where PRECIP > EVAP seawater salinities are lower

a. Around the equator and at higher latitudes

3. When we discuss the relationship between oceans and climates you'll find out why these conditions occur where they do.

D. At the poles freezing plays a part in increasing salinity as mostly H2O goes into the ice leaving a more concentrated salt solution behind.

E. These effects are extremely important in influencing the vertical movement of water in the deep oceans.  Along with temperature variations, these salinity variations generate the so-called thermohaline currents that move because of the density differences caused by temperature and salinity variations.

1. Increased o/oo == increased density = sinks

   Decreased o/oo == decreased density = rises

 

IV. Minor and trace components

A. Some lists put Sr, B, and F into this category.

B. Many of these components are non-conservative.

1. The ones which are involved in biological    processes.

 

2. Others which vary because of chemical processes such as oxidation are also non-conservative.

C. Usually called trace constituent if it occurs in amounts < 1 ppm. For example the concentration of Fe is 0.01 ppm:

                         0.01 grams Fe

                   --------------------------

                    1,000,000 grams seawater

D. Many minor and trace elements are essential for the survival  of organisms, both in the sea and on land. Some of these elements (Fe, Mn, Mo, Zn, Co, Cu, and V) are so rare that their concentrations are commonly described in terms such as parts per billion (ppb). The precise roles of many of these elements in organism metabolism have not been established.

 

V. Nutrients - elements other than C, H, and O that are required for the synthesis of organic matter and whose scarcity can limit productivity. (Productivity = production of organic carbon from inorganic carbon -- accomplished largely by plants.)

A. The common limiting nutrients in the oceans are nitrogen, phosphorus, and silicon. Most marine plants can't use these in elemental form but must use the dissolved forms indicated

1. N = nitrate (NO3-), nitrite (NO2-), ammonia (NH4+)

2. P = phosphate (PO43-)

3. Si = SiO2

B. The variations of these components in seawater are large both vertically and horizontally. The typical profile of nutrient concentrations in the ocean is to be seriously depleted at the surface where photosynthesis is occurring the most rapidly. The nutrient concentration increases with depth as creatures die and release their absorbed nutrients back into the water column. The rate of nutrient increase decreases and stops with depth as the sinking organic matter is completely dissolved. Irregularities in the subsurface pattern of nutrient distribution are caused by variations in the circulation patterns for water masses in the various oceans. Remember the important link between biological activity and nutrient concentration.

 

VII. Dissolved Gases

A. The most abundant gases dissolved in seawater are:

             % in       % in surf.   Water

  Gas      dry air      seawater     Air      Solubility

 

Nitrogen      78          47.5       0.6       Lowest

 

Oxygen        21          36.0       1.7       Interm.

 

Carbon       0.03         15.1       500       Greatest

Dioxide

 

Ar, H,        1            1.4       1.5       Interm.

Ne, He

 

                                 N2    O2   CO2

              Typical

          concentrations (ml/L)  10    5   40

 

B. This should come as no surprise because these are common and important gases in our atmosphere. But you can see that their relative concentrations in seawater differ from their relative concentrations in the atmosphere.  This is because of differing solubilities.  Just like solids have a solubility in seawater, gases also have a solubility. CO2 is extremely soluble in water.  C. The concentration of nitrogen gas in seawater is quite constant and has no biological implications.

D. On the other hand, the concentrations of carbon dioxide (CO2) and oxygen (O2) vary significantly from place to place in the ocean and their distributions are significantly influenced by biological activity near the sea surface. 

E.  Oxygen and carbon dioxide are intimately involved in      photosynthesis and respiration. (Photosynthesis is the production of organic matter from inorganic carbon and respiration is the destruction of that organic matter by reaction with oxygen gas.) Look again at the photosynthesis equation.  From this equation you would predict that as photosynthesis occurs rapidly at the surface carbon dioxide should be depleted and oxygen should be enriched.  When the photosynthesizing organisms die and sink down through the water column respiration occurs and photosynthesis ceases. Organic matter that was generated at the surface reacts with oxygen at depth and is reconverted to carbon dioxide and nutrients are released into the water again. The concentration of carbon dioxide changes like the nutrients and oxygen shows a distinct oxygen minimum layer where respiration is most vigorous - consuming oxygen. The respiration is performed either by the original plant or by organisms which have consumed the materials produced by the plant.  At greater depth, there is less animal activity, less oxygen consumed, and fewer nutrients released.

1. The variation in available sunlight is the major factor, other than nutrient availability, that determines the relative rate of photosynthesis and respiration/decay. Sunlight only penetrates a few hundred meters down into the ocean (even in the clearest, cleanest water, such as that found in the centers of the oceans, far from the continents).

a. PSYN is not fastest right at surface because many marine plants can't deal with such high levels of sunlight, instead most photosynthesize most efficiently a few inches below the surface.

b. At some point, however, the sunlight levels decrease so much that the rate of PSYN begins to decrease and is eventually overwhelmed by the rate of respiration which returns nutrients and dissolved carbon to the oceans and uses up oxygen.

c. Respiration is at a maximum in the OXYGEN MINIMUM ZONE

2. The CO2 profile is similar to the nutrient profiles because it is used up during PSYN just like the nutrients are.

3. The nutrient concentration increases with depth as creatures die and release their absorbed nutrients back into the water column.

a. The rate of nutrient increase decreases and stops with depth as the sinking organic matter is completely dissolved.

b. Later, you will learn about an important process called upwelling that returns these dissolved nutrients to the surface water where they can be utilized anew by marine plants.  Without upwelling, it is estimated that the seas would be dead within a year after it stopped.

4. The oxygen profile is a mirror image of the CO2 profile because it is produced while CO2 is consumed.

F. The large solubility of CO2 in water has extremely important implications for life in the oceans and all over the surface of the Earth. 

1. Many life forms in the sea are dependent on dissolved CO2 gas as a source of carbonate to make their CaCO3 shells and tests.

2. CO2 plays another and extremely important role in the ocean: in water it acts as a pH buffer. A buffer prevents sudden, extreme changes in the acidity or alkalinity (i.e., the pH) of water. Such rapid changes would be fatal for most marine organisms.

3. Finally, CO2's extreme solubility makes life on the surface of the Earth possible by removing the primary insulating "Greenhouse gas" from our atmosphere, thereby keeping the temperature at the surface of the Earth at a livable level.

 

VIII. Dissolved organics

A. Not part of salinity

B. Fats, proteins, carbohydrates, hormones, vitamins, etc.

C.  Occur in low concentrations (?)

 

IX. Movement of materials in seawater

A. Mostly pretty well-mixed due to wind, waves, surface and thermohaline currents which move material by convection that is by moving the water itself.

B. Diffusion which is the movement of the individual atoms and molecules is extremely slow and by itself could not possibly create a well-mixed solution.

 

X.Origin of Earth’s surface water and its components

A. The fossil and sedimentary rock records indicate that the oceans were present as far back in time as at least 3.4 billion yrs. Prior to that time the Earth was too hot for liquid water to be stable on its surface and the vaporized H2O was lost to outer space.

B. After the Earth cooled enough to permit liquid water to accumulate, volcanic emanations provided a steady supply which eventually filled up the ocean basins. The solidification of what was once a  mostly molten planet resulted in the separation of the more volatile components (water, HCl gas, CO2 gas, etc.) from the less volatile  components such as Fe, Mg,  Si,  etc. These volatile components are naturally quite light and rise towards the surface of the Earth.

1. Process often called outgassing. 

2 Measurements of the current rate of outgassing during volcanism yield rates that can account for the total amount of surface water we observe today given 3 billion years for it to accumulate.

3. Also measurements of the amounts of these volatile substances trapped in bubbles in solidified igneous rock show that they contain these volatiles in about the correct abundances to explain the surface abundances.

4. Volcanic emanations provided the following elements to the original ocean:

     S         C        Cl       Br             N

Sulfur, Carbon, chlorine, bromine, and nitrogen gases.

C. The nonvolatile elements: (e.g. Na, Mg, Ca, K) came from rain moving over and through rocks on the continent, dissolving constituents from those rocks and carrying them down to the oceans in the rivers.