Chapter 6: Water and Seawater

1. Chapter 6: Water and Seawater Fig. 6-19

2. Atomic structure • Nucleus • Protons and neutrons • Electrons • Ions are charged atoms

3. Water molecule • H2O • Two hydrogen, one oxygen • Bonded by sharing electrons • Bend in geometry creates polarity • Dipolar molecule

5. Dipolar molecule • Weak negative charge at O end • Weak positive charge at H end • Hydrogen bonds • Weak bonds between water molecules and ions • Explains unusual properties of water

6. Fig. 6-3

7. Two unusual properties • High surface tension • Hydrogen bonding creates “skin” • Important for living organisms • Capillarity • Universal solvent • Electrostatic bond between dipolar water and ions • Ocean is salty

8. Fig. 6.4

9. Fig. 6-5b

10. Thermal properties of water • Solid, liquid, gas on Earth’s surface • Water has high freezing point • Water has high boiling point • Water has high heat capacity • Water has high latent heats

11. Fig. 6-7

12. Heat capacity • Heat absorbed or released with changes in state • Latent heats of • Melting; freezing • Vaporization, evaporation • Condensation

13. Global thermostatic effects • Moderate global temperature • Evaporation removes heat from oceans • Condensation adds heat to atmosphere • Heat re-distributed globally

14. Differences in day and night temperatures

15. Water density • Maximum density at 4oC • Ice less dense than liquid water • Atomic structure of ice • Ice floats • Increased salinity decreases temperature of maximum density

16. Fig. 6-10

17. Fig. 6-8

18. Seawater • Salinity=total amount of solid material dissolved in water (g/1000g) • Typical salinity is 35 o/oo or ppt • Brackish (hyposaline) < 33 ppt • Hypersaline > 38 ppt

20. Measuring salinity • Evaporation • Chemical analysis • Principle of Constant Proportions • Chlorinity • Electrical conductivity (salinometer)

21. Dissolved substances • Added to oceans • River input (primarily) • Circulation through mid-ocean ridges • Removed from oceans • Salt spray • Recycling through mid-ocean ridges • Biogenic sediments (hard parts and fecal pellets) • Evaporites

22. Residence time • Average length of time a substance remains dissolved in seawater • Long residence time = unreactive • Higher concentration in seawater • Short residence time = reactive • Smaller concentration in seawater • Steady state • Ocean salinity nearly constant through time

23. Dissolved gases • Solubility depends on temperature, pressure, and ability of gas to escape • Gases diffuse from atmosphere to ocean • Wave agitation increases amount of gas • Cooler seawater holds more gas • Deeper seawater holds more gas

24. Conservative vs. nonconservative constituents • Conservative constituents change slowly through time • Major ions in seawater • Nonconservative constituents change quickly due to biological and chemical processes • Gases in seawater

25. Oxygen and carbon dioxide in seawater • Nonconservative • O2 high in surface ocean due to photosynthesis • O2 low below photic zone because of decomposition • O2 high in deep ocean because source is polar (very cold) ocean

26. CO2 low in surface ocean due to photosynthesis • CO2 higher below photic zone because of decomposition • Deeper seawater high CO2 due to source region and decomposition

27. Acidity and alkalinity • Acid releases H+ when dissolved in water • Alkaline (or base) releases OH- • pH scale measures acidity/alkalinity • Low pH value, acid • High pH value, alkaline (basic) • pH 7 = neutral

28. Carbonate buffering • Keeps ocean pH about same (8.1) • pH too high, carbonic acid releases H+ • pH too low, bicarbonate combines with H+ • Precipitation/dissolution of calcium carbonate CaCO3 buffers ocean pH • Oceans can absorb CO2 from atmosphere without much change in pH

29. Fig. 6-17

30. How salinity changes • Salinity changes by adding or removing water • Salinity decreases by • Precipitation (rain/snow) • River runoff • Melting snow

31. Salinity increases by • Evaporation • Formation of sea ice • Hydrologic cycle describes recycling of water

32. Hydrologic cycle Fig. 6-19

33. Horizontal variations of salinity • Polar regions: salinity is lower, lots of rain/snow and runoff • Mid-latitudes: salinity is high, high rate of evaporation • Equator: salinity is lower, lots of rain • Thus, salinity at surface varies primarily with latitude

34. Fig. 6-20

35. Vertical variations of salinity • Surface ocean salinity is variable • Deeper ocean salinity is nearly the same (polar source regions for deeper ocean water) • Halocline, rapid change of salinity with depth

37. Density of seawater • 1.022 to 1.030 g/cm3 • Ocean layered according to density • Density of seawater controlled by temperature, salinity, and pressure • Most important influence is temperature • Density increases with decreasing temperature

38. Salinity greatest influence on density in polar oceans • Pycnocline, rapid change of density with depth • Thermocline, rapid change of temperature with depth • Polar ocean is isothermal

40. Layers of ocean • Mixed surface layer • Pycnocline • Deep ocean

41. End of Chapter 6: Water and Seawater