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Ch7 Ocean Chemistry

Ch7 Ocean Chemistry. Mr. Sternschein Marine Science. Water, as a Solvent. Water is a polar molecule (chapter 6) Water will attract other polar substances or ions. What is an ion? Solutions are composed of: Solvent- the more abundant substance, usually a liquid

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Ch7 Ocean Chemistry

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  1. Ch7 Ocean Chemistry Mr. Sternschein Marine Science

  2. Water, as a Solvent • Water is a polar molecule (chapter 6) • Water will attract other polar substances or ions. • What is an ion? • Solutions are composed of: • Solvent- the more abundant substance, usually a liquid • Solute- the less abundant substance, often a solid • Salt, NaCl is held together by ionic bonds (electrostatic forces) • Salt dissociates in water to Na+ and Cl- • Solutes can diffuse through water until homogenous. • A saturated solution can dissolve no more solute.

  3. Components of Seawater • Seawater is 96.5% water, 3.5% dissolved substances. • Sodium and chlorine ions, from salt, are the major constituents. • Colligative properties: • Freezing point depression • Slower evaporation • Higher osmotic pressure • Decreased heat capacity

  4. Measuring Salinity • Salinity is measured in parts per 1000 (g./kg.) • Dissolved substances = 3.5% or 35 ppt. • The ratio of dissolved substances in the ocean is constant. (Forchhammer’s principle) • Chlorinity- a method of measuring salinity, uses this principle. • Salinometers measure salinity using conductivity with great accuracy.

  5. Sources of minerals and elements • Dissolved solids originated from the erosion of crustal rocks • Rivers running off continental crust • Wave action on shorelines • Excess volatiles: components of ocean water that are not accounted for by the weathering of surface rocks. • Outgassing of volcanoes • Hydrothermal vents • Decay of once-living organisms

  6. The ocean is in a steady state The ocean is in chemical equilibrium. The proportion and amounts of dissolved solids remain 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 and can be calculated as:

  7. Residence Time

  8. Constituent time • Conservative constituents of seawater are those constituents that occur in constant proportions. • Conservative elements 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.

  9. Dissolved gases • Gases from the atmosphere dissolve at the ocean’s surface. • Nitrogen (conservative) • Oxygen (non-conservative) • Carbon Dioxide (non-conservative) • Different proportions than in the atmosphere due to rate of solubility in the ocean

  10. Dissolved gases continued • Unlike solids, gases dissolve best in cold water. • Cold polar water will have more dissolved gases than the warm tropics • Dissolved Oxygen (DO) in the tropics may be so low that animals cannot survive • Tropical waters may be stressed more due to pollutants that consume oxygen (examples: sewage and agricultural runoff)

  11. Temperature and Dissolution

  12. Nitrogen & Oxygen Gas • Nitrogen in the most abundant gas in seawater (48%) • Upper layers are usually saturated • Nitrogen is essential for: • living things to make amino acids • amino acids then help to build proteins • plant life • Cannot be taken directly from the atmosphere • Must first be bound, or fixed, into usable forms (by bacteria) • Dissolved oxygen is critical to marine life. (36%) • Vital for organisms that extract oxygen with their gills • Sources of oxygen: • photosynthetic activity • diffusion from the atmosphere

  13. The ocean is a vast carbon reservoir • Carbon dioxide (CO2) is low because of demand by photosynthetic organisms (15%) • Concentration is 60x more in ocean than the atmosphere • Can combine with calcium to make calcium carbonate (CaCO3) • CaCO3 is used to build shells and skeletons. • Organisms die, sink and become limestone rock

  14. Gas Concentrations Vary with Depth • Oxygen levels are more abundant near the surface. why? • Photosynthesis occurs there • Decreases with depth as marine organisms consume it. • CO2 levels are low near the surface (again, photosynthesis) • Increase with depth as organisms produce it.

  15. Acid and base refresher • When water dissociates, it breaks into OH- and H3O+ • Solutions that have excess H3O+ (hydronium ions) are acids. • Solutions that have excess OH- (hydroxide ions) are bases. • Solutions that have equal amounts are neutral (water) • pH indicates whether a solution is acidic or basic • Acids have pH less than 7 • Bases have pH less greater than 7 • Basic solutions are also called alkaline

  16. The ocean is slowly becoming more acidic • CO2 and water form Carbonic acid • The more CO2 in the atmosphere, the more there is in the ocean. • This is causing the oceans pH to drop and become more acidic. • Some marine life cannot adapt and will perish.

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