Sea Water Chemistry

1. Sea Water Chemistry Chapter 7

2. Sea Water Chemistry • determine many important oceanographic phenomena including : • Global patterns of oceanic and atmospheric circulation, and the growth and distribution of marine organisms.

3. Life on earth probably evolved in water • Most animals are 50-65% water • water exists in all three physical states of matter: solid, liquid, and gas

4. 71% of the Earth Surface - Sea Water • regulates the climate, dilute waste • major habitat for living creatures

5. polarity of water molecules results in hydrogen bonding

7. Characteristics of H20 • 1. has cohesive behavior • 2. resists changes in temperature • 3. has a high heat of vaporization and cools surfaces as it evaporates • 4. expands when it freezes • 5. is a versatile solvent

9. 1. Surface tension • measure of how difficult it is to stretch or break the surface of a liquid • water has a greater surface tension than most liquids

10. 2-3. Water's high heat of vaporization: • moderates the earth's climate. • solar heat absorbed by tropical seas dissipates when surface water evaporates

12. 4. Oceans and lakes don't freeze • because of hydrogen bonding, water is less dense as a solid than it is as a liquid. • consequently, ice floats.

13. 5. water is a versatile solvent owing it to the polarity of the water molecule • ionic compounds dissolve in water Prepared by: Prof. Rodriguez

14. Major Components of Seawater • dissolved salts - hydrated anions and cations (Table 7.1;, f. 7.3) • dissolved gases - nitrogen, oxygen, carbon dioxide • organic and inorganic - dissolved organic materials suspended particulate matter

15. Major Ions in Typical Seawater • Ion Parts per thousand by weight ‰ (ppt)(Table 7.1) • Cl- 18.98 • SO4-2 2.649 • HCO3- 0.140 • Na+ 10.556 • Mg2+ 1.272 • Ca+ 0.400 • K+ 0.380

18. On average, concentration of dissolved salts, i.e., the salinity, in seawater is 3.5% or 35‰. • The relative abundances of the ions listed above does not change, even though salinity does; are said to be conservative. • Relative abundances of minor and trace constituents do vary

20. Determining Salinity • Evaporation of water to weight the salt is an imprecise method • Because of the constancy of composition if we measure one component we can get a more precise measurement • Salinity ppt = 1.80655 x Chlorinity in ppt • If chlorinity is 19.2 ppt, what is the salinity of sea water? • 34.7 ppt = 35 ppt

21. Sources of Salt • Rivers (winds and glaciers are a less important, indirect source) • Weathering of oceanic crust • Hydrothermal Vents associated with Mid-ocean ridges and other submarine volcanoes

22. Sinks • Biologic activity • Interaction with Particulate matter: clays and organic matter absorb dissolved metals

23. Direct Precipitation • Hydrothermal Activity: (fig. 7.4) • Reaction between seawater and new oceanic crust • Minerals like magnesium is incorporated into deposits • Calcium is added to sea water

24. Physical and Chemical Properties of Water • Heat Capacity - energy added to raise temperature of 1 gram of substance by 1°C • adding energy breaks H-bonds, increases fraction of free water • important in thermal buffering and heat transport to higher latitudes

25. Latent Heats and Evaporation • heat input or release associated with phase changes (ice - liquid, liquid - vapor) • changes in water structure, H-bonding with phase changes • important in thermal buffering, heat transport and heat exchange with atmosphere

26. Density - mass per unit volume (grams/cm3) • density of water phases (ice, liquid, vapor) due to structural changes at molecular level • density maximum at 4°C in pure water • Major role in deep ocean circulation and water column structure and stability

28. Light Transmission • transparent in visible part of spectrum • Absorbed as is goes deeper in the water column • strongly absorbs infrared (heat) and ultraviolet (prevents damage to DNA)

30. Dissolving Power • hydration of solutes - interactions between solutes and free water • decreases H-bonding, increases order of free water, increases density • exclusion of solutes on freezing and evaporation • other effects of solutes: freezing point depression, boiling point elevation

31. pH (acidity or alkalinity) • measure of the dissociation of water into ions (H+, OH), (fig. 7.9) • pH = - log [H+] • pH effects on biological and geochemical reactions

34. Conservative vs. Non Conservative Properties • Conservative Properties of Seawater • those properties that can only be altered at the sea surface: temperature, salinity, inert gases • properties not altered by biological or geochemical reactions • importance in water mass identification, tracing and mixing

35. Nonconservative Properties of Seawater • those properties that can be altered anywhere in the water column • properties altered by biological and geochemical reactions

36. Dissolved Gases • The proportions of gases in the atmosphere is not the same as their proportions in seawater, (Table 7.4) • There is less N2 (nitrogen gas) in the ocean than in the atmosphere, • much more oxygen, • and even more CO2. • All this CO2 in the oceans keeps CO2 from being in the atmosphere and causing global warming.

38. The colder the water, the more gas can dissolve in it. • When you leave your can of coke in the car in the sun, then open it, what happens? • Coke sprays all over you. • That's because the gas has exsolved (come out of solution); a lot has accumulated in the little space at the top of the can.

39. Very active fish, such as trout and salmon, require very cold water to live in because they have high oxygen requirements. • They literally suffocate when the water gets too warm, and the oxygen levels drop. • This explain why these fish don't live down south. • 'Thermal pollution' occurs when electric plants put warm water into streams, lowering the oxygen level.

40. CO2 is important because it is needed by plants so they can photosynthesize. • O2 is important because animals need it for respiration. Photosynthesis: • CO2 + H2O + energy [from the sun]  O2 + sugar (organic matter) Respiration (the reverse of photosynthesis): • O2 + sugar CO2 + H2O + energy

41. Dissolved Oxygen • seawater\atmosphere exchange at air water interface only (fig.7.8) • biological processes that affect O2 concentration: photosynthesis and respiration • typical distribution of O2 in water column and processes that control this distribution

42. Phytoplankton Nutrients • inorganic sources of N, P, S and other atoms required for phytoplankton growth • photosynthesis and respiration contributes in nutrient distribution

43. Especially important, because so much is needed, are N (nitrogen) and P (phosphorus). • Si (silica) is also important for all the siliceous organisms we‘ll discuss: diatoms, radiolarians, and siliceous sponges. • N is necessary to make proteins. • P is necessary to make new cells (it's part of the cell wall), and also genetic material, DNA and RNA.

44. N is useful for plants only in these forms: • NO3-nitrate • NO2-nitrite • NH4+ammonium • N2, the gas, is not usable by most plants. • Only a few bacteria can break this very strong molecule apart and turn it into nitrate. • These are 'nitrogen-fixing bacteria'. • P is useful in the form of phosphate, PO43-

45. Thus weathering, sedimentation, and ocean chemistry are all closely linked. • Other ions in seawater, such as Cl – and SO4 , are not derived from weathering, but from volcanic degassing.

46. The Carbonate System in Seawater • CO2 in seawater is controlled by: ( f.7-10-11) • Exchange with the atmosphere • Photosynthesis/Respiration: • 6CO2 + 6H2 O C6 H12 O6 + 6O2

47. The Carbonate Buffer • Carbon Dioxide: CO2 , • Carbonic Acid: H2CO3 , • Bicarbonate: HCO3- , • Carbonate: CO32- • CO2 + H2O H2CO3 . • H2CO3HCO3- + H+2. • HCO3CO32-+ H +3.

49. Another important reaction is the dissolution and precipitation of calcium carbonate: • CO3+ Ca+2 CaCO34.. • Importance of these reactions: • Maintain constant pH (seawater is said to be buffered). • Few marine organisms can tolerate a pH very different from 8.

50. Biological Productivity • In general, shells of organisms are likely to be preserved where their production rate is high, • Siliceous shells are preserved only where the production rate is high. • Siliceous oozes occur where productivity rate is high and terrigenous sedimentation rate low.