OCG123, Spring 2001
Answers to third hourly exam, Monday 2 April 2001
Chapters 7, 8

Definitions (3 points each; 30 points total)
      1. pH—The negative logarithm of the hydrogen-ion concentration in a solution.
      2. Biological pump—The transfer of CO₂ and nutrients from the surface waters to the deep waters by the sinking of dead phytoplankton, dead zooplankton, and fecal matter and their subsequent decay in the deep.
      3. Reduced carbon—Carbon that is combined mainly with hydrogen, nitrogen, and other carbon rather than with oxygen.
      4. Residence time—The average lifetime of a substance in a reservoir, defined by the ratio of mass in the reservoir to rate of input or output of that mass at steady state.
      5. Phytoplankton—Free-floating photosynthetic marine plants.
      6. Impact degassing—The process by which extraterrestrial bodies release volatile substances such as CO₂ from rocks that they slam into.
      7. Lead-lead dating—The process of determining the age of rocks by measuring the abundances of two lead isotopes produced from uranium isotopes with different half-lives.
      8. Glacial striations—Parallel grooves carved into bedrock by rocks frozen into the base of a moving glacier.
      9. Terrestrial planets—The four rocky, earthlike planets Mercury, Venus, Earth, and Mars.
      10. Heavy bombardment period—The period from 4.6 to about 3.8 billion years ago when the earth was regularly bombarded by large planetesimals.

Short answers (5 points each; 40 points total)
      1. The atmosphere contains 760 Gton carbon in carbon dioxide. 60 Gton leaves each year by photosynthesis. Calculate the residence time of carbon dioxide in the atmosphere to the proper number of significant figures. 760 Gton/60 Gton yr^-1 = 12.7 yr. Rounding to one significant figure gives 10 yr. You can also say 10–20 yr.
      2. Explain the sequence of steps by which atmospheric CO₂ dissolves in ocean water. Write the reaction for each step. First the CO₂ from the air dissolves in the water. Then it reacts with a water molecule to form carbonic acid. The weak carbonic acid then ionizes in two steps to form bicarbonate and carbonate.

CO₂ (g) → CO₂ (aq)
CO₂ (aq) + H₂O → H₂CO₃ (aq)
H₂CO₃ (aq) → H⁺ (aq) + HCO₃^- (aq)
HCO₃^- (aq) → H⁺ (aq) + CO₃^2- (aq)

      3. Explain the difference between the short-term and the long-term carbon cycles. The short-term carbon cycles involve processes that cycle carbon through reservoirs with periods of up to thousands of years or so. Examples are cycling in and out of the atmosphere and soils. The long-term cycle involves sedimentary rocks, mostly marine, whose cycling times are millions of years.
      4. What is the oxygen-minimum zone, and how is it created? Use a diagram if you wish. The oxygen-minimum zone is a broad layer of the ocean centered on a depth of 1 km or so where oxygen is depleted relative to waters above and below it. The layer is created by the decay of sinking organic matter, which uses up some of the dissolved oxygen to “combust” the organic matter. the layer does not extend all the way to the bottom because most of the material is oxidized at intermediate depths. See Box Figure 7-1 on page 137 of the text.
      5. List three possible mechanisms for forming the moon and cite the pieces of evidence that point to the one currently regarded as correct. The potential mechanisms for forming the moon include gravitational capture as it was passing by, coaccretion with the earth, fission of a rapidly rotating earth, and tearing off of material as the result of a side-swipe collision with a Mars-sized planetesimal. The last one is currently favored because the moon’s oxygen isotopic ratios resemble those of the earth’s mantle (from which it would have been removed), its density is less than that of the total earth (and like the mantle), and it is depleted in volatile elements and had a molten surface (consistent with a big collision that melted the surface of the earth as it was tearing off molten matter).
      6. Explain the “faint young sun” paradox and its most likely resolution. The paradox is that the sun was faint enough in its early days (70% of the present luminosity) to make a very cold earth, but the earth wasn’t correspondingly cold then. The earth was apparently kept warm by a very large greenhouse effect created by the very high concentrations of CO₂ in the early atmosphere. The concentrations were so high because the early continents were too small to remove much of the CO₂ by weathering.
      7. How do we know that the Mesozoic was so warm at high latitudes? Because ferns grew in Siberia and dinosaurs roamed to north of the Arctic Circle. What might have made it that way? Lack of a polar ice pack and strongly developed Hadley circulation, among other things.
      8. How can a supercontinent at the equator become glaciated? By being weathered actively enough to draw down global CO₂ enough to reduce the greenhouse effect to the point that ice sheets could form at the points of land farthest from the equator and spread all over the supercontinent.

Problems and longer answers (15 points each; 30 points total)
      1. Recall that the intrahemispheric and interhemispheric mixing times of tropospheric air are several months and a year or two, respectively, and that the carbon in fallen leaves spends about 50 years in soil before it is oxidized to CO₂. Starting from your answer to short-answer question 1, describe the short-term terrestrial organic carbon cycle that is followed by the carbon atom of a molecule of CO₂ after it enters the atmosphere from the soil. Be as quantitative as possible.
      (See page 129 ff. for details of the journey of Molly the Molecule.) The molecule’s atmospheric residence time of 10–20 years allows it to fully mix within the NH and move back and forth several times between NH and SH. It then enters the pores of a leaf, where it stays for a growing season if the leaf is not eaten first. The leaf falls to the forest floor, where it gradually decays for 50 years before the newly formed CO₂ diffuses back into the atmosphere to begin the cycle over again.
2. Describe the general sequence of events by which our solar system was formed, including the sun and the planets. Explain the kind of early environment for earth this created and how it differed so strikingly from today. Show how this environment created much of the earth’s atmosphere, and note how the new explanation differs from earlier ideas. Which constituents dominated the early atmosphere?
      No tricks here—see pages 155 ff. in the book. Briefly, an irregular cloud of interstellar dust collapsed into a rotating blob that collapsed further into a central star and a plane of material that became a series of planets. The nearby planets lost their volatiles; the outer planets didn’t. For the first billion years there were all sorts of planetesimals flying around, which gradually coalesced into planets that were regularly bombarded by other planetesimals. These impacts formed much of the atmosphere and ocean by impact degassing and from their own materials, but made the surface uninhabitable for a billion years. The early atmosphere was mostly CO₂ and nitrogen; the oxygen came later. This violent past of the earth differs greatly from earlier ideas of a cold, quiet place the changed only gradually.