An Invitation to Marine Science, 7th Oceanography An Invitation to Marine Science, 7th Tom Garrison Chapter 7 Ocean Chemistry

Chapter 7 Study Plan Water Is a Powerful Solvent Seawater Consists of Water and Dissolved Solids Gases Dissolve in Seawater The Ocean’s Acid-Base Balance Varies with Dissolved Components and Depth

Chapter 7 Main Concepts Water is a powerful solvent. The total quantity (or concentration) of dissolved inorganic solids in water is its salinity. Though salinity may vary with location, the ratio of dissolved solids in seawater is a constant. Gases dissolve in seawater. Cold water can hold more gas in solution than warm water. The ocean is a vast reservoir of carbon. The dynamics of carbon exchange between ocean and atmosphere affect Earth’s climate. The ocean’s acid-base (pH) balance varies with depth and dissolved components. Carbonate chemistry serves to moderate (buffer) wide swings in oceanic pH.

Water Is a Powerful Solvent A simplified hydrologic cycle. Water moves from ocean to air, onto land, to lakes and streams and groundwater, back to the sky and ocean, in a continuous cycle. The numbers indicate the approximate volumes of water in cubic kilometers per year (km3/yr). Water is also stored in the ocean, ice, groundwater, lakes, and the atmosphere.

Water Is a Powerful Solvent What are solutions and mixtures? A solution is made of two components, with uniform molecular properties throughout: The solvent, which is usually a liquid, and is the more abundant component. The solute, often a solid or gas, is the less abundant component. A mixture is different from a solution. In a mixture the components retain separate identities, so it is NOT uniform throughout.

Water Is a Powerful Solvent Salt in solution. When a salt such as NaCl is put in water, the positively charged hydrogen end of the polar water molecule is attracted to the negatively charged Cl- ion, and the negatively charged oxygen end is attracted to the positively charged Na+ ion. The ions are surrounded by water molecules that are attracted to them and become solute ions in the solvent.

Salinity Salinity is the total quantity of dissolved inorganic solids in water. Water’s colligative properties are: The heat capacity of water decreases with increasing salinity As salinity increases, freezing point decreases As salinity increases, evaporation slows Osmotic pressure increases as salinity increases

A Few Ions Account for Most of the Ocean’s Salinity A representation of the most abundant components of a kilogram of seawater at 35‰ salinity. Note that the specific ions are represented in grams per kilogram, equivalent to parts per thousand (‰).

Components of Ocean Salinity Came From Earth’s Crust Processes that regulate the major constituents in seawater. Ions are added to seawater by rivers running off crustal rocks, volcanic activity, groundwater, hydrothermal vents and cold springs, and the decay of once-living organisms. Ions are removed from the ocean by chemical entrapment as water percolates through the mid-ocean ridge systems and seamounts, sea spray, uptake by living organisms, incorporation into sediments, and ultimately by subduction.

The Ratio of Dissolved Solids in the Ocean is Constant Forchhammer’s principle, also known as the principle of constant proportions states that although the salinity of various samples of seawater may vary, the ratio of major salts is constant. How do scientists determine the salinity of seawater? Salinity can be determined by measuring the chlorinity of the sample. Since the chlorinity is easy to measure, and the principle of constant proportions applies to all seawater, scientists can use the following formula to determine salinity: Salinity in ‰ = 1.80655  Chlorinity in ‰

The Ocean Is in Chemical Equilibrium Is the ocean becoming progressively saltier with age? No, the ocean is in chemical equilibrium. The proportion and amounts of dissolved solids remain constant and is known as the “steady state ocean.“ Ions are being added to and removed from the ocean at the same rate. Residence time is the average length of time an element spends in the ocean and can be calculated as: Residence Time = ___Amount of element in the ocean____ The rate at which the element is added to (or removed from) the ocean

Seawater’s Constituents May Be Conservative or Non-conservative Conservative constituents of seawater are those constituents that occur in constant proportions. Conservative elements have long residence times and are the most abundant dissolved material in the ocean. Non-conservative constituents have short residence times, and are usually associated with seasonal, biological or short geological cycles.

Gas Concentrations Vary with Depth How concentrations of oxygen and carbon dioxide vary with depth. Oxygen is abundant near the surface because of the photosynthetic activity of marine plants. Oxygen concentration decreases below the sunlit layer because of the respiration of marine animals and bacteria, and because of the oxygen consumed by the decay of tiny dead organisms slowly sinking through the area. In contrast, because plants use carbon dioxide during photosynthesis, surface levels of CO2 are low. Because photosynthesis cannot take place in the dark, CO2 given off by animals and bacteria tends to build up at depths below the sunlit layer. CO2 also increases with depth because its solubility increases as pressure increases and temperature decreases.

The Ocean’s Acid-Base Balance Varies with Dissolved Components and Depth What are acids and bases? An acid is a substance that releases a hydrogen ion in solution. A base is a substance that combines with a hydrogen ion in solution. A solution containing a base is called an alkaline solution. Acidity or alkalinity is measured on the pH scale.

The Ocean’s Acid-Base Balance Varies with Dissolved Components and Depth (right) The pH scale. A solution at pH 7 is neutral; higher numbers represent bases, and lower numbers represent acids.

The Ocean’s Acid-Base Balance Varies with Dissolved Components and Depth Carbon dioxide (CO2) combines readily with seawater to form carbonic acid (H2CO3). Carbonic acid can then lose a H+ ion to become a bicarbonate ion (HCO3-), or two H+ ions to become a carbonate ion (CO32-). Some bicarbonate ions dissociate to form carbonate ions, which combine with calcium ions in seawater to form calcium carbonate (CaCO3), used by some organisms to form hard shells and skeletons. When their builders die, these structures may fall to the seabed as carbonate sediments, eventually to be re-dissolved. As the double arrows indicate, all these reactions may move in either direction.

Chapter 7 in Perspective In this chapter you learned that water has the remarkable ability to dissolve more substances than any other natural solvent. Though most solids and gases are soluble in water, the ocean is in chemical equilibrium and neither the proportion nor amount of most dissolved substances changes significantly through time. Most of the properties of seawater are different from those of pure water because of the substances dissolved in the seawater. About 3.5% (35‰) of seawater consists of dissolved substances. These almost always exist as ions – “salts” do not exist in the ocean. The most abundant ions dissolved in seawater are chloride, sodium, and sulfate. Seawater is not concentrated river water or rain water – its chemical composition has been altered by circulation through the crust at oceanic spreading centers and by other chemical and biological processes. Most gases in the air dissolve readily in seawater at the ocean’s surface. Plants and animals living in the ocean require these dissolved gases to survive. In order of their relative abundance, the major gases found in seawater are nitrogen, oxygen, and carbon dioxide. The proportions of dissolved gases in the ocean are very different from the proportions of the same gases in the atmosphere because of differences in their solubility in water and air. In the next chapter you will learn how atmosphere and ocean work together to shape Earth’s weather, climate, and natural history. Our past discussions of heat, temperature, convection, dissolved gases, and geological cycles will be put to use. You will find surprising oceanic connections among storms, tropical organisms, deserts, and balloon races!