Chemical Oceanography - Chapter 5

I. Chemical Bonding (Appendix IV)

A. Matter is made of atoms (Figure 5.1), which are the smallest particles into which an element can be subdivided that still retains the chemical properties of that element.

1. Nucleus -positively charged,contains protons & neutrons.

2. Electrons - very light-weight, negatively charged & surround nucleus like a cloud.

3. Atoms are neutral & have same number of electrons & protons

4. Elements are arranged in the periodic table in order of increasing number of protons in nucleus (called atomic number) (Fig. A.4.1 in Appendix IV)

5. Electron configurations-scientists have discovered that certain arrangements of electrons around the nucleus are much stabler than others. The most stable configuration is a filled outer shell of electrons (i.e., The Inert Gases; Fig. A.4.2 in Appendix IV)

B. Types of bonds

1.Atoms combine with each other to achieve filled outer shells forming two types of bonds:

a. ionic-transfer of electrons (NaCl; Figure A.4.4 in Appendix IV)

b. covalent-sharing of electrons (H & O in water; Figure A.4.5 in Appendix IV)

1)When neutral atoms take, give up, or share electrons they become ions

2. In the case of covalent bonds that link hydrogen and oxygen ions together in water, the electrons are not shared equally causing water to be very strongly dipolar (Figure 5.2-5.3).

a. Polar nature of H₂O causes adjacent molecules to stick together to form hydrogen bonds (Figure 5.3).

II. Water is a very unique substance whose unusual properties make life on Earth, as we know it, possible (Study Table 5.2, Fig. 5.5-5.7 and the CD=Phase Changes of Water carefully).

1. Extremely high melting and boiling points, for a compound made up of such light elements,makes water stable as a liquid over most of the surface of the Earth.

2. Very high heat capacity allows water to absorb a lot of heat with only a small increase in temperature. This thermal buffering protects life on Earth from otherwise potentially lethal temperature fluctuations (Figure 5.9).

a. Heat capacity = amount of heat (calories) needed to raise temperature of 1 gram of a substance by 1^oC.

b. Understand the difference between heat and temperature

c. Uunderstand changes of state described in Figure 5.6.

3. High latent heats of fusion and vaporization-significant impact on Earth's climate (Fig 5.8).

4. Solid H₂O (ice) is less dense than water preventing mid-latitude lakes from freezing solid in winter. (Figure 5.10)

5. Dipolar liquids like water are great solvents (Fig.5.4)

6. Water's ability to transmit light and sound are also important to life in the oceans.

III. Introduction to seawater chemistry

A. Salinity-total amount of dissolved inorganic salts in seawater

1.Unit = parts per thousand (ppt,o/oo). Average= 34-36 o/oo.

2. Includes 3 of the 5 categories of dissolved constituents found in seawater: major & minor/trace constituents, and nutrients --organics and gases are not part of salinity

IV. MAJOR CONSTITUENTS -Learn the names and symbols for the 7 components that make up greater than 99% of the dissolved salts in seawater (Figure 5.12; Table 5.3). Learn in order of decreasing abundance: chloride (Cl^-), sodium (Na⁺), sulfate (SO₄^2-), magnesium (Mg²⁺), calcium (Ca²⁺), potassium (K⁺), bicarbonate (HCO₃^-).

A. These solutes, along with bromide, strontium, boron, and fluoride, are called conservative because their concentrations are stable over time in the oceans, so the ratios of these ions to one another are constant (THIS IS THE LAW OF CONSTANT PROPORTIONS).

1. LAW OF CONSTANT PROPORTIONS - does not hold at the mouths of large rivers (Table 5.6), in estuaries, near mid-ocean ridges or where extensive dissolution and precipitation of CaCO₃ occurs.

B. Salinity averages 34-37 o/oo in the open ocean (Fig.5.20-5.22) because of evaporation and freezing of water, and rain.

1. When evaporation of seawater occurs, ONLY THE H₂O LEAVES THE OCEAN - SO SALINITY AND DENSITY BOTH INCREASE

2. On the other hand, precipitation (i.e., rainfall) adds only water (no salt) so the SALINITY AND DENSITY ARE DECREASED

3. Know how seawater salinity varies geographically in the surface ocean and know how climatic factors affect it.

V. Minor and trace constituents are the very rare components of seawater whose concentrations are measured in parts per million or parts per billion. These include such elements as Fe, Mn, Zn, Co, Cu, V, which are nonconservative but still very important.

VI. A nutrient is anything other than the elements C, H, and O that is needed in the synthesis of organic matter and whose scarcity may limit biological productivity. The most important nutrients in seawater are nitrogen in the form of nitrate (NO₃^-), phosphorus in the form of phosphate (PO₄^3-) and silica (SiO₂) (Table 5.3).

A. Productivity is measured in terms of the rate of production of organic carbon from inorganic carbon (gms C/m²/day or gms C/m²/yr).

B. The concentraion of nutrients generally increases with depth in the oceans showing a significant depletion in surface waters. This distribution pattern is called a nutrient profile (Figure 12.20).

VII. The dominant dissolved gasses in seawater are N₂, O₂ (Figure 12.20), and CO₂. These gasses have very different solubilities in seawater which has very important implications for life in the oceans and on Earth in general (Figure 5.18-5.19).

VIII. PHOTOSYNTHESIS AND RESPIRATION

A. Nutrients, O₂ and CO₂ are intimately involved in photosynthesis and respiration (Figure 13.1):

106 CO₂ + 122 H₂O + 16 NO₃^- + PO₄^3- + 19 H⁺= (CH₂O)₁₀₆(NH₃)₁₆H₃PO₄ + 138 O₂

1. You need to understand thoroughly the chemical components of this equation, where they come from, how they are distributed in the oceans and what factors affect their distribution.

IX. Origin of seawater and its components

A. The H₂O (as well as the other volatile (Figure 1.17) components such as chlorine (Cl), carbonate (CO₃), nitrogen (N), sulfur (S) & fluorine (F)) originally came from outgassing of the mantle during the early stages of Earth history.

B. The nonvolatile, originally solid components dominated by Na, Mg, Ca & K were weathered out of the igneous rocks with which the original ocean came in contact.

X. 5th broad category of dissolved constituents includes the organic compounds like fats, proteins, carbohydrates, etc., which occur in very small amounts but which are very important to marine organisms.

XI. The oceans are basically well-mixed due to convection (motion of large masses of seawater). On a microscopic scale mixing occurs by diffusion (Figure 12.12), which is important for movement of materials in and out of cells. Diffusion is the movement of the individual ions, atoms, and molecules through seawater. Diffusion by itself could not create well-mixed solution because it is very slow.

XII. Geochemical Cycles describe the path that a given element or chemical component follows as it is 1) transported across the sur-face of the Earth through various envirnoments, and 2) involved in various chemical, physical and biological processes (Figure 13.16).

A. These chemical, physical and biological processes do not cause the destruction of an element, but merely cause a change in the form in which it occurs (NaCl can occur as a solid (table salt) or it can occur as dissolved ions in seawater (Na⁺ & Cl^-)).

1. Reservoir = storage compartment for a component

a. Source - reservoir from which a component comes

b. Sink - reservoir to which a component goes

B. Understand the major transport mechanisms, processes, and reservoirs involved in the simple cycles such as the hydrologic cycle (Figure 5.16; CD=Earth's H₂O and hydrologic cycle)and those of the conservative major elements of seawater.

1. Cycle for sodium as an example of a simple cycle:

a. Reservoirs: rocks on the continents, marine sediments, or seawater itself (as dissolved ions)

b. Transport mechanisms and processes = rivers, weathering flux (flow), hydrothermal activity around mid-ocean ridges, sea spray, evaporation, precipitation

C. Arguably, the most important cycle in the oceans is the carbonate-carbon dioxide cycle which affects biological activity, atmospheric concentrations of CO₂, and pH (hydrogen ion concentration = acidity; Figure 5.17) of the ocean (Fig.5.18/5.19).UNDERSTAND IT

ATMOSPHERE

CO₂ SEA SURFACE

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Life Processes Chemical Processes

CO₂ CO₂ + H₂O H₂CO₃ (carbonic acid)

(gas) (dissolved)

H⁺ + HCO₃^- (bicarbonate ion)

Photosynthesis

Respiration

H⁺ + CO₃^2- (carbonate ion)

O₂ (gas)

Precipitation Dissolution

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OCEAN BOTTOM CaCO₃ (carbonate sediments)

E. Nutrient cycles such as that for phosphorus and nitrogen are heavily influenced by biological activity (Figure 13.16)

F. Residence Time =the average time that a substance remains in soution in sewter. This time depends on how many ways, and how easily a substance is added to or removed from seawater.