Presentation Transcript

1. Chapter 7 Ocean Chemistry About solutions and mixtures A solution is made of two components, with uniform (meaning ‘the same everywhere’) molecular properties: 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.

2. Water is a powerful solvent and we have it everywhere – the hydrological cycle

3. Ocean Salinity • Salinity is the total quantity of dissolved inorganic solids in water. • 3.5% salt on average • measured in g/kg (ppt = parts per thousand) Ocean salinities vary in space Processes that affect salinity: evaporation, precipitation, runoff, freezing, and thawing And recall that: The heat capacity of water decreases with increasing salinity As salinity increases, freezing point decreases As salinity increases, evaporation slows (boiling point increases)

4. Temperature North Tropic of Cancer Tropic of Cancer Latitude Equator Equator Tropic of Capricorn Tropic of Capricorn South Salinity Temperature Salinity Fig. 6-14, p. 167 Ocean-Surface Conditions Depend on Latitude, Temperature, and Salinity

5. Mid Ocean Average Surface Salinity

6. Surface Salinity Northern Hemisphere Summer High = high evaporation Low = coastal regions and high precipitation regions

7. One kilogram of seawater Water 965.6 g Most abundant ions producing salinity Sodium (Na+) 10.556 g Other components (salinity) 34.4 g Sulfate (SO42−) 2.649 g Chloride (Cl–) 18.980 g Magnesium (Mg2+) 1.272 g Bicarbonate (HCO3−) 0.140 g Other Calcium (Ca2+) 0.400 g Potassium (K+) 0.380 g Fig. 7-3, p. 189

8. Dissolved salts • Major constituents and trace elements • Conservative/nonconservative constituents • Major Constituents = [] > 1 part per million • Na+ Sodium • Cl- Chloride • SO4- Sulfate • Mg2+ Magnesium • Ca2+ Calcium • K+ Potassium 86 % 99 % • Trace Elements = [] < 1 part per million

9. A few ions (charged particles) account for most of the salinity of the oceans. See Table 7.2 for minor and trace elements in seawater

10. Regulating the major constituents in seawater • Sources of salt: • Positive ions: weathering and erosion • Negative ions: gases from volcanic eruptions • Hydrothermal activity supply and remove salt from the deep ocean Balance of salt: Input: rivers, volcanic activity, groundwater, hydrothermal vents and cold springs, and the decay of once-living organisms. Output: sea spray, uptake by living organisms, incorporation into sediments, and ultimately by subduction.

11. Residence time: the average time a substance remains in solution in the ocean • Total amount divided by rate of input (or output)

12. The ratio of dissolved solids in the ocean is constant: • Well-mixed solution • Principle of Constant Proportions : the ratios between the concentrations of major conservative ions in open-ocean water are constant

13. The ocean is in chemical equilibrium • Salt composition constant for last 1.5 billion years • 5*1022g= total dissolved material • 2.5*1015g=total run off per year • salinity is not increasing! therefore: rate of addition ~ rate of removal for a 360/00 salinity water

14. Determining salinity: • Electrical conductivity is a function of salinity • More Ions = greater transmission of electricity salinometer

15. Seawater’s constituents may be conservative or nonconservative • Conservative = concentration changes only as a result of mixing, diffusion, and advection • Non-conservative = concentration changes as a result of biological or chemical processes as well as mixing, diffusion, and advection

16. Gases • Distribution with depth • Photosynthesis removes CO2 and produces O2 at the surface • Respiration produces CO2 and removes O2 at all depths • Compensation depth (Photosynthesis = Respiration) CO2 O2 photosynthesis respiration

17. Oxygen and CO2 profiles O2 Concentrations Photosynthesis Bottom water enrichment CO2 Concentrations Direct solution of gas from the atmosphere Respiration of marine organisms Oxidation (decomposition) of organic matter oxygen minimum

18. metric tons C (106) The Carbon/Carbon Dioxide Cycle - • Ocean uptake from atmosphere Depends on: pH, temperature, salinity, chemistry • Biological pump numbers in black = rates of exchange numbers in green = total amounts stored in reservoirs numbers in parenthesis = net annual changes

19. Some words to keep in mind: Ion – charged atom cat-ion (+) – positively charged ion • an-ion (-) – negatively charged ion • Dissociation = to break apart into ions • OH- • H2O (all 3 will be in water solutions) • H+ Pure Water; [] = concentration [H+]=10-7 [OH-]=10-7 Neutral solution: [H+] = [OH-]

20. Non-pure water solutions • [OH-] and [H+] are inversely proportional • Imbalance between the relative concentration of H+ and OH- produces an acidic or basic solution • pH scale-measures acidity/alkalinity • 0-14 • Logarithmic scale: pH=-log10[10-5]=5 • pH of rainwater ~ 5-6 (on the acidic side)

22. Seawater • Alkaline, pH from 7.5-8.5 • Average pH=7.8 • pH relatively constant due to buffering action of CO2 • Buffer = substance that prevents sudden or large changes in the acidity or alkalinity of a solution • Important for biological processes • pH inversely proportional to the concentration of CO2

23. CO2 Buffer 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 redissolved. As the double arrows indicate, all these reactions may move in either direction.

24. Chapter 7 - Summary • pH: acidity of seawater 7.5 - 8.5 • Carbon dioxide acts as a buffer that prevents large variations in pH • Major salt ions are in constant proportions except in coastal areas • Ocean is a net source of oxygen to atmosphere • Biological processes pump CO2 into the deep ocean

25. Chapter 7 - Summary The polar nature of the water molecule is responsible for water's remarkable ability to dissolve more substances than any other natural solvent. The most abundant ions dissolved in seawater are chloride, sodium, sulfate. The quantity of dissolved inorganic solids in water is its salinity. The proportion of ions in seawater is not the same as the proportion in concentrated river water because ongoing geological and chemical processes affect the ocean's salinity. Though most solids and gases are soluble in water, the ocean is in chemical equilibrium, and neither the proportion nor the amount of most dissolved substances changes significantly through time. Nitrogen is the most abundant dissolved gas in seawater; oxygen is the second most abundant. Carbon dioxide is the most soluble gas, and one of many substances that affect the ocean's pH balance. Seawater acts as a buffer to prevent broad swings of pH when acids or bases are introduced.

26. Good Stuff 1. Calculate the residence time of water in the oceans. 2. How many times could a water molecule have been recycled since the formation of the oceans? Ocean volume = 1,370 million km3 km3