Chapter 7: Ocean Chemistry Insert: Textbook cover photo

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 Five Main Concepts Water is a powerful solvent. The total quantity (or concentration) of dissolved inorganic solids in water is its salinity. Although 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. Water is also stored in the ocean, ice, groundwater, lakes, and the atmosphere.

Water Is a Powerful Solvent 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 Unlike the electron sharing found in covalently bonded molecules such as water, the sodium atoms in NaCl have lost electrons, and chlorine atoms have gained them. The ions of sodium and chloride in NaCl are said to be held together by ionic bonds, electrostatic attraction that exists between ions that have opposite charges.

Water Is a Powerful Solvent 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. Note that NaCl does not exist as “salt” in seawater; its components are separated when salt crystals dissolve in water, but they are joined when crystals reform as water evaporates.

Seawater Consists of Water and Dissolved Solids About 97.2% of the 1,370 million cubic kilometers of Earth’s surface water is marine. By weight, seawater is about 96.5% water and 3.5% dissolved substances, most of which are salts. 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 Although the total amount of dissolved solids (salinity) might vary among samples, the ratio of major salts is constant. For example, when the solids are isolated from any seawater sample, whether from the high-salinity North Atlantic or low-salinity Arctic oceans, 55.04% of those solids will be chloride ions. This is known as Forchhammer’s principle, or the principle of constant proportions.

The Ocean Is in Chemical Equilibrium The ocean appears to be in chemical equilibrium: the proportion and amounts of dissolved salts per unit volume of ocean are nearly constant. 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. Because of the vigorous activity of currents, the mixing time of the ocean is thought to be on the order of 1,600 years, so the ocean has been mixed hundreds of thousands of times during its long history.

Seawater’s Constituents May Be Conservative or Non-conservative Conservative constituents those constituents that occur in constant proportions, 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.

Gases Dissolve in Seawater 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. Unlike solids, gases dissolve most readily in cold water.

Gas Concentrations 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. 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 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. An excess of hydrogen ions (H+) in a solution makes that solution acidic. An excess of hydroxide ions (OH−) makes a solution alkaline. Acidity or alkalinity is measured on the pH scale.

The Ocean’s Acid-Base Balance Varies with Dissolved Components and Depth A solution at pH 7 is neutral; higher numbers represent bases, and lower numbers represent acids. Seawater is slightly alkaline; its average pH is about 8.0.

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. As the double arrows indicate, all these reactions may move in either direction.

The Ocean’s Acid-Base Balance Varies with Dissolved Components and Depth In areas of rapid photosynthesis, pH will increase because CO2 is used by plants and plantlike organisms. Therefore, surface pH in warm productive water is usually around 8.5. At middle depths and in deep water, cold temperatures, high pressure, bacterial respiration and no photosynthetic plants to remove the build-up of CO2 will reduce the pH of water, making it less alkaline with depth. (LEFT) The variation in pH with depth. The average calcium carbonate compensation depth (CCD) is represented by a red line.

The Ocean’s Acid-Base Balance Varies with Dissolved Components and Depth The ocean has a natural buffering capacity preventing broad swings of pH when acids or bases are introduced. However, the ocean is becoming more acidic as it absorbs additional carbon dioxide from the atmosphere. A less alkaline environment will make it more difficult for organisms to build hard structures containing calcium (shells, coral skeletons, among others) from dissolved carbonates. (RIGHT) The charts show changes in sea surface pH from the late 1800s to the year 2100.