Oceans CH 5 - The Chemistry of Seawater

Keeling Curve

A continuous measurement by David Keeling of Scripps Institution of Oceanography, from the top of Mauna Loa on the Big Island of Hawaii, of atmospheric CO2 concentration began in 1958. The Keeling curve is the most widely recognized measurement of human impact on the environment in existence

Organics

A wide variety of organic substances are present in seawater. Proteins, carbohydrates, lipids (or fats), vitamins, hormones, and their breakdown products are all present. Some are eventually oxidized or broken down into smaller molecules; others are used directly by organisms and are incorporated into their systems. Another portion of the organic matter accumulates in the sediments, where over geologic time it may slowly provide hydrocarbon molecules to form deposits of oil and gas.

Chemical Resources

About 30% of the world's table salt is extracted from seawater. The industrially produced energy required to remove the water is kept to a minimum to keep extraction costs low. In warm, dry climates, seawater is allowed to flow into shallow ponds and evaporate down to a concentrated brine solution. More seawater is added, and the process is repeated several times, until a dense brine is produced. Evaporation continues until a thick, white salt deposit is left on the bottom of the pond.

Gases

Abundant gases in the atmosphere and in the oceans are nitrogen (N2), oxygen (O2), and carbon dioxide (CO2). The percentages of each of these gases in the atmosphere and in seawater are given in table 5.6. Oxygen and carbon dioxide play important roles in the ocean because they are necessary to all life, and biological activities modify their concentrations at various depths. Although nitrogen gas is used directly only by bacteria, its use also plays an important role in ocean processes. Gases such as argon, helium, and neon are present in small amounts, but they are chemically inert and do not interact with the ocean water or its inhabitants.

Cation

An ion with a positive charge is a cation

Advantages of Reverse Osmosis

As older evaporative plants wear out, reverse osmosis plants are replacing them. The advantages of reverse osmosis include no energy requirement for heating the water, no thermal pollution from the discharge, and removal of unwanted contaminants—including pesticides, bacteria, and some chemical compounds. High-salinity wastewater returning to the coastal environment can be a disadvantage. The cost of the energy required to pump the water under pressure makes the cost of this type of desalinated water very high. In southern California, desalinated water is about fifteen times more expensive than local groundwater and five times more expensive than water imported from the northern part of the state

Bacterial Respiration

Bacteria also respire (although they, of course, do not breathe like humans). Bacterial respiration of organic matter becomes the most important factor in the removal of oxygen from seawater at depth because bacteria are the most abundant organisms deep in the water column.

Constant Proportions

Because of this thorough mixing, the ionic composition of open-ocean seawater is the same from place to place and from depth to depth. That is, the ratio of one major ion or seawater constituent to another remains the same.

anthropogenic carbon dioxide

Burning of fossil fuels and deforestation has led to a dramatic increase in the concentration of CO2 in the atmosphere. Carbon dioxide produced by human activities is known as anthropogenic carbon dioxide Definition: carbon dioxide produced by human activities such as the burning of carbonaceous fuels, including wood, coal, oil, and natural gas.

Regulating Salt Balance

Chemical and geologic evidence from rocks and salt deposits leads researchers to believe that the salt composition of the oceans has been the same for about the last 1.5 billion years. The total amount of dissolved material in the world's oceans is calculated to be 5 × 1022 g for ocean water of 36‰ salinity. Each year, the runoff from the land adds another 2.5 × 1015 g, or 0.000005% of total ocean salt. If we assume that the rivers have been flowing to the sea at the same rate over the last 3.5 billion years, more dissolved material has been added by this process than is presently found in the sea. For the oceans to remain at the same salinity, the rate of addition of salt by rivers must be balanced by the removal of salt; input must balance output.

Desalination

Desalination is the process of obtaining fresh water from salt water.

Photosynthesis

During photosynthesis, plants, seaweeds, and phytoplankton use carbon dioxide, sunlight, and inorganic nutrients (see section 5.4) to produce organic matter. In the process of photosynthesis, oxygen is generated. Phytoplankton and seaweeds grow in surface waters where there is sufficient sunlight to carry out photosynthesis.

Dissolved Salts

For millions of years, volcanism and rain washing over the land have supplied the oceans with dissolved salts. Once these reach the ocean basins, most of the dissolved salts stay in the ocean.

Salinity

In the major ocean basins, 3.5% of the weight of seawater is, on the average, dissolved salt and 96.5% is water, so a typical 1000 g or 1 kg sample of seawater is made up of 965 g of water and 35 g of salt. Oceanographers measure the salt content of ocean water in grams of salt per kilogram of seawater (g/kg), or parts per thousand (‰). The total quantity of dissolved salt in seawater is known as salinity, and the average ocean salinity is approximately 35‰.

Nutrients

Ions required for plant or phytoplankton growth are known as nutrients; these are the fertilizers of the oceans. As on land, phytoplankton require nitrogen and phosphorus in the form of nitrate (NO3−) and phosphate (PO43-) ions. A third nutrient required in the oceans is the silicate ion (SiO44-), which is needed to form silica (SiO2), the hard outer wall of the single-celled diatoms and the skeletal parts of some protozoans. These three nutrients are among the dissolved substances brought to the sea by the rivers and land runoff. Despite their importance, they are present in very low concentrations

Osmosis

Osmosis is the movement of water across a semipermeable membrane; the water moves from the side with the higher concentration of water molecules (or low salinity) to the side with the lower concentration of water molecules (or high salinity); this movement creates a higher pressure on the low-water concentration (or high-salinity) side of the membrane

Reverse Osmosis

Reverse osmosis produces fresh water from seawater by applying pressure to seawater and forcing the water molecules through a semipermeable membrane, leaving behind the salt ions and other impurities

pH of Ocean

Seawater is slightly alkaline with a pH between 7.5 and 8.5. The pH of the world's ocean averaged over all depths is approximately 7.8. Surface water currently has an average pH of about 8.2.

pH

The acidity or alkalinity of a solution is measured using the pH scale, which ranges from a low of 0 to high of 14 (fig. 5.7). The pH scale is a logarithmic scale that measures the concentration of the hydrogen ion (written [H+]) in a solution. The formal definition of pH is: In pure water, where the concentrations of H+ and OH− are both 10-7, the pH is equal to 7,

Residence Time

The average, or mean, time that a substance remains in solution in the ocean is called its residence time Definition: mean time that a substance remains in a given area before being replaced, calculated by dividing the amount of a substance by its rate of addition or subtraction.

Ocean Acification

The increase in CO2 in the atmosphere has resulted in a corresponding increase in the concentration of the gas in the ocean as CO2 is absorbed by seawater at the sea surface. The increasing concentration of CO2 in the water is causing a decrease in the pH of the water, an effect that is referred to as ocean acidification ocean acidification Definition: a decrease in pH and consequent increase in acidity of seawater.

Ionic Bonds

The salts in seawater are present in dissolved form as cations and anions. A good example is the salt sodium chloride (NaCl). Sodium chloride is held together by ionic bonds ionic bond the electrostatic force that holds together oppositely charged ions.

The pH of Seawater

The water molecule, H2O, can dissociate (break apart) to form a hydrogen cation, H+, and a hydroxide anion, OH−. Consequently, in any water solution, there will always be a combination of H2O molecules, H+ ions, and OH− ions. The concentration of H2O molecules always greatly exceeds the concentrations of the two ions. In a pure water solution (one in which there is only water molecules) at 25°C, a very small fraction of the water molecules, about 10-7, will spontaneously dissociate into H+ and OH− ions.

Zooplankton

The zooplankton are in turn eaten by other consumers, and eventually the nutrients are returned to the oceans by death and bacterial decomposition. Excretory products from zooplankton and larger animals are also added to the seawater, broken down, and used by a new generation of zooplankton and phytoplankton. Nutrients are nonconservative; they do not maintain constant ratios in the way most major salt ions do.

Ion

When immersed in water, compounds can break apart into individual atoms or groups of atoms that have opposite electrical charges. A charged atom or group of atoms is called an ion Definition: positively or negatively charged atom or group of atoms.

Anion

an atom with a negative charge is an anion

The ___ zone is shallow in coastal waters...

euphotic

Principle of Constant Proportion (Constant Composition)

the ratios between the abundance of major constituent ions in seawater remain constant regardless of seawater salinity.

dissolved constituent concentrations in water are expressed in terms of ___, ___, and molarity

weight, volume