1. Seawater Chemistry

2. 1.Water is a powerful solvent. The total quantity (or concentration) of dissolved inorganic solids in water is its salinity. 2.Although salinity may vary with location, the ratio of dissolved solids in seawater is a constant. 3.Gases dissolve in seawater. Cold water can hold more gas in solution than warm water. 4.The ocean is a vast reservoir of carbon. The dynamics of carbon exchange between ocean and atmosphere affect Earth’s climate. 5.The ocean’s acid-base (pH) balance varies with depth and dissolved components. Carbonate chemistry serves to moderate (buffer) wide swings in oceanic pH. Five Main Concepts

3. 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. I. The Dissolving Power of Water

4. Stepped Art Condensation Precipitation 111,000 Lakes Ocean Groundwater Precipitation 392,000 Glaciers Runoff 46,000 Transpiration and Evaporation 66,000 Evaporation 437,000 Figure 7-1 p208

5. A.A solution is made of two components, with uniform molecular properties throughout: 1.The solvent, which is usually a liquid, and is the more abundant component. 2.The solute, often a solid or gas, is the less abundant component. B.A mixture is different from a solution. In a mixture the components retain separate identities, so it is NOT uniform throughout. I. The Dissolving Power of Water

6. C.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. D.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. I. The Dissolving Power of Water

7. 1.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. D.The ions of sodium and chloride … (cont’d) I. The Dissolving Power of Water

8. 2.The ions are surrounded by water molecules that are attracted to them and become solute ions in the solvent. 3.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. C.The ions of sodium and chloride … (cont’d) I. The Dissolving Power of Water

9. A.Seawater Consists of Water and Dissolved Solids 1.About 97.2% of the 1,370 million cubic kilometers of Earth’s surface water is marine. 2.By weight, seawater is about 96.5% water and 3.5% dissolved substances, most of which are salts. II. Seawater

10. A.Seawater Consists of Water and Dissolved Solids (cont’d.) 3.Salinity is the total quantity of dissolved inorganic solids in water. 4.Water’s colligative properties are: a.The heat capacity of water decreases with increasing salinity b.As salinity increases, freezing point decreases c.As salinity increases, evaporation slows d.Osmotic pressure increases as salinity increases II. Seawater

11. B.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 (‰). II. Seawater

12. C.Components of Ocean Salinity Came From Earth’s Crust. Processes that regulate the major constituents in seawater: 1.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. II. Seawater

13. 2.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. II. Seawater C.(cont’d.)

14. D.The Ratio of Dissolved Solids in the Ocean is Constant 1.Although the total amount of dissolved solids (salinity) might vary among samples, the ratio of major salts is constant. II. Seawater

15. D.The Ratio of Dissolved Solids in the Ocean is Constant (cont’d.) 2.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. 3.This is known as Forchhammer’s principle, or the principle of constant proportions. II. Seawater

16. E.The Ocean Is in Chemical Equilibrium 1.The ocean appears to be in chemical equilibrium: the proportion and amounts of dissolved salts per unit volume of ocean are nearly constant. 2.Ions are being added to and removed from the ocean at the same rate. II. Seawater

17. E.The Ocean Is in Chemical Equilibrium (cont’d.) 3.Residence time is the average length of time an element spends in the ocean. 4.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. II. Seawater

18. F.Seawater’s Constituents May Be Conservative or Non-conservative 1.Conservative constituents a.those constituents that occur in constant proportions, b.have long residence times, and c.are the most abundant dissolved material in the ocean. II. Seawater

19. F.Seawater’s Constituents May Be Conservative or Non-conservative (cont’d.) 2.Non-conservative constituents a.have short residence times, and b.are usually associated with seasonal, biological or short geological cycles. II. Seawater

20. A.Gases Dissolve in Seawater 1.Most gases in the air dissolve readily in seawater at the ocean’s surface. 2.Plants and animals living in the ocean require these dissolved gases to survive. 3.Unlike solids, gases dissolve most readily in cold water. III. Dissolved Gases

21. A.Gases Dissolve in Seawater III. Dissolved Gases

22. B.Gas Concentrations Vary with Depth 1.Oxygen is abundant near the surface because of the photosynthetic activity of marine plants. 2.Oxygen concentration decreases below the sunlit layer because of the respiration of marine animals and bacteria. III. Dissolved Gases

23. B.Gas Concentrations Vary with Depth (cont’d.) 3.In contrast, because plants use carbon dioxide during photosynthesis, surface levels of CO 2 are low. 4.Because photosynthesis cannot take place in the dark, CO 2 given off by animals and bacteria tends to build up at depths below the sunlit layer. III. Dissolved Gases

24. 5.CO 2 also increases with depth because its solubility increases as pressure increases and temperature decreases. III. Dissolved Gases B.Gas Concentrations Vary with Depth (cont’d.)

25. The Ocean’s Acid-Base Balance Varies with Dissolved Components and Depth A.An acid is a substance that releases a hydrogen ion in solution. B.A base is a substance that combines with a hydrogen ion in solution. C.A solution containing a base is called an alkaline solution. IV. Acid-Base Balance

26. The Ocean’s Acid-Base Balance Varies with Dissolved Components and Depth (cont’d.) D.An excess of hydrogen ions (H+) in a solution makes that solution acidic. An excess of hydroxide ions (OH−) makes a solution alkaline. E.Acidity or alkalinity is measured on the pH scale. IV. Acid-Base Balance

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

28. The pH scale. IV. Acid-Base Balance

29. H.Carbon dioxide (CO 2 ) combines readily with seawater to form carbonic acid (H 2 CO 3 ). I.Carbonic acid can then lose a H+ ion to become a bicarbonate ion (HCO 3- ), or two H+ ions to become a carbonate ion (CO 3 2- ). IV. Acid-Base Balance The Ocean’s Acid-Base Balance Varies with Dissolved Components and Depth

30. J.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. K.As the double arrows indicate, all these reactions may move in either direction. IV. Acid-Base Balance The Ocean’s Acid-Base Balance Varies with Dissolved Components and Depth (cont’d.)

31. The Ocean’s Acid-Base Balance Varies with Dissolved Components and Depth IV. Acid-Base Balance

32. L.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. The Ocean’s Acid-Base Balance Varies with Dissolved Components and Depth IV. Acid-Base Balance

33. M.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 decrease the pH of water, making it less alkaline and more acidic with depth. The Ocean’s Acid-Base Balance Varies with Dissolved Components and Depth (cont’d.) IV. Acid-Base Balance

34. The Ocean’s Acid-Base Balance Varies with Dissolved Components and Depth IV. Acid-Base Balance The variation in pH with depth. The average calcium carbonate compensation depth (CCD) is represented by a red line.

35. N.The ocean has a natural buffering capacity preventing broad swings of pH when acids or bases are introduced. O.However, the ocean is becoming more acidic as it absorbs additional carbon dioxide from the atmosphere. The Ocean’s Acid-Base Balance Varies with Dissolved Components and Depth (cont’d.) IV. Acid-Base Balance

36. P.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. The Ocean’s Acid-Base Balance Varies with Dissolved Components and Depth (cont’d.) IV. Acid-Base Balance

37. The charts show changes in sea surface pH from the late 1800s to the year 2100. The Ocean’s Acid-Base Balance Varies with Dissolved Components and Depth IV. Acid-Base Balance

38. 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. Chapter 7 in Perspective