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Chapter 11 Orbital-Scale Changes in Carbon Dioxide and Methane

Chapter 11 Orbital-Scale Changes in Carbon Dioxide and Methane. Reporter : Yu-Ching Chen Date : May 22, 2003 (Thursday). Outline. Introduction Ice Cores Drilling and Dating Ice Cores Trapping Gases in the Ice Orbital-Scale Change in Methane Methane and the monsoon

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Chapter 11 Orbital-Scale Changes in Carbon Dioxide and Methane

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  1. Chapter 11Orbital-Scale Changes in Carbon Dioxide and Methane Reporter : Yu-Ching Chen Date : May 22, 2003 (Thursday)

  2. Outline • Introduction • Ice Cores • Drilling and Dating Ice Cores • Trapping Gases in the Ice • Orbital-Scale Change in Methane • Methane and the monsoon • Orbital-Scale Change in CO2 • Physical Oceanographic Explanations of CO2 Changes • Orbital-Scale Carbon Reservoirs • Tracking Carbon through the Climate System • Can δ13C Evidence Detect Glacial Changes in Carbon Reservoirs? • Pumping of Carbon into the Deep Ocean during Glaciations • Changes in the Circulation of Deep Water during Glaciations • Conclusion

  3. Introduction • Methane (CH4) and carbon dioxide (CO2) have varied over orbital time scales. • Methane levels have fluctuated mainly at the 23,000-year orbital rhythm of precession, and we will evaluate the hypothesis that these changes are linked to fluctuations in the strength of monsoons in Southeast Asia. • During glaciations, atmosphere CO2 value have repeatedly dropped by 30 .

  4. Ice Cores • Drilling and Dating Ice Cores

  5. Ice Cores • Trapping Gases in the Ice • Air moves freely through snow and ice in • the upper 15m of an ice sheet, but flow is • increasingly restricted below this level. • Bubbles of old air are eventually sealed off • completely in ice 50 to 100m below the • surface. Figure 11-3. Sintering: Sealing air bubbles in ice

  6. Ice Cores • Measurements of CO2 (top) and • methane (bottom) taken on bubbles in • ice cores merge perfectly with • measurements of the atmosphere in • recent decades. Figure 11-4. Ice core and instrumental CO2 and CH4 .

  7. Orbital-Scale Change in Methane • 550~770 maxima • 350~450 minima • 12500-10000/5=23000 years/cycle • Methane record from Vostok ice • in Antarctica shows regular • cycles at Intervals near 23,000 • years (left). • This signal closely resembles the monsoon- • response signal driven by low-latitude • insolation (right). Figure 11-5. Methane and the monsoon

  8. How would changes in the strength of low-latitude monsoons produce changes in atmospheric methane concentrations?

  9. Heavy rainfall in such regions saturates the ground, reduces its ability to absorb water, and thereby increases the amount of standing water in bogs. • Decaying vegetation uses up any oxygen in the water and creates the oxygen-free conditions needed to generate methane. • The extent of these boggy areas must have expanded during wet monsoon maxima and shrunk during dry monsoon minima.

  10. Orbital-Scale Change in CO2 • A 400,000-year record of CO2 from • Vostok ice in Antarctica shows four • large-scale cycles at a period of • 100,000 years similar to those in the • marine δ18O record. • 280-300ppm maxima 180-190 minima • Abrupt increases in CO2 occur during • time of rapid ice melting. Figure 11-6. Long-term CO2 changes

  11. Orbital-Scale Change in CO2 • A record of the last 160,000 • years of CO2 variations from • Vostok ice in Antarctica • (left)resembles the marine δ18O • record (right). • CO2 concentrations in the • atmosphere changed by 30 • just a few thousand years. Figure 11-7. The most recent CO2 cycle

  12. What factors could explain the observed 90-ppm drop in CO2 levels during glacial Intervals from the levels observed Interglacial intervals?

  13. Physical Oceanographic Explanations of CO2 Changes • One possibility is that changes in the physical oceanographic characteristics of the surface ocean-its temperature and salinity. • CO2 dissolves more readily in colder seawater, atmospheric CO2 levels will drop by 9 ppm for each 1℃ of ocean cooling. • CO2 dissolves more easily in seawater with a lower salinity. • During glaciations, the average salinity of entire ocean increased by about 1.2o/oo, atmospheric CO2 levels increase 11 ppm .

  14. Physical Oceanographic Explanations of CO2 Changes

  15. Orbital-Scale Carbon Reservoirs Figure 11-8. Exchange of carbon The large changes in atmospheric CO2 in ice cores over intervals of a few thousand years must involve rapid exchanges of carbon among the near-surface reservoirs.

  16. Orbital-Scale Carbon Reservoirs Figure 11-9. Interglacial-glacial changes in carbon reservoirs During the glacial maximum 20,000 years ago, large reductions of carbon occurred in the atmosphere, in vegetation and soils on land, and in the surface ocean. The total amount of carbon removed from these reservoirs (> 1000 gigatons) was added to much larger reservoir in the deep ocean.

  17. Tracking Carbon through the Climate System Figure.11-11 Photosynyhesis and carbon isotope factionation Photosyntheis on land and in the surface ocean converts inorganic carbon to organic form and causes large negative shifts in δ13C values of the organic carbon produced.

  18. Tracking Carbon through the Climate System Figure 11-10. Carbon reservoir δ13C values The major reservoirs of carbon on Earth have varying amounts of organic and inorganic carbon, and each type of carbon has characteristic carbon isotope values.

  19. Tracking Carbon through the Climate System BOX 11-1. Carbon Isotope Ratios

  20. Can δ13C Evidence Detect Glacial Changes in Carbon Reservoirs? • We can use a mass balance calculation to estimate the effect of adding very negative carbon to the inorganic carbon already present in the deep sea: (38,000) (0%) + (530) (-25%) = (38,530) (x%) Inorganic C Mean C added Mean Glacial ocean Mean in ocean δ13C from land δ13C carbon total δ13C x=-0.34

  21. Can δ13C Evidence Detect Glacial Changes in Carbon Reservoirs? • Fig. 11-12

  22. Pumping of Carbon into the Deep Ocean during Glaciations • During glaciations(A), 12C-enriched • from the land to the ocean at the • same time that 16O-enriched water • vapor is extracted from the ocean • and stored in ice sheets. • During interglaciations (B), 12C-rich • carbon returns to the land as 16O- • rich water flows back into the ocean. Figure 11-13. Glacial transfer of 12C and 16O

  23. Pumping of Carbon into the Deep Ocean during Glaciations • Ocean carbon pump hypothesis • Carbon was exported from surface waters to the deep ocean by higher rates of photosynthesis and biologic productivity. • CO2+H2O CH2O+O2

  24. Pumping of Carbon into the Deep Ocean during Glaciations Figure 11-14. Annual carbon production in the modern surface ocean

  25. DO wind Fertilize the Glacial Ocean? BOX 11-2. Iron fertilization of ocean surface waters

  26. Pumping of Carbon into the Deep Ocean during Glaciations • Photosynthesis in ocean surface waters • sends 12C rich organic matter to the deep • sea, leaving surface waters enriched in • 13C (left). • At the same time, photosynthesis • extracts nutrients like phosphate (PO4--2) • from surface waters and sends them to • deep sea. As a result, seawater δ13C • values and phosphate concentrations are • closely correlated (right). Figure. 11-17. Link between nutrients and δ13C values

  27. Pumping of Carbon into the Deep Ocean during Glaciations Figure 11-16. Measuring changes in the ocean carbon pump

  28. Pumping of Carbon into the Deep Ocean during Glaciations • If the ocean carbon pump • affects atmospheric CO2 levels, • the net difference between • surface and deep-water δ13C • values should increase when • CO2 levels are low. • Measured δ13C differences • show some correlation with • past changes in atmospheric • CO2 Figure 11-17. Past changes in the carbon pump

  29. Changes in the Circulation of Deep Water during Glaciations Figure 11-18 Modern deepocean δ13C patterns

  30. Changes in the Circulation of Deep Water during Glaciations • Present-Day Controls on Regional δ13C Values Figure 11-19. Regional δ13C difference

  31. Changes in the Circulation of Deep Water during Glaciations • Past Changes in Regional δ13C Values Figure 11-20. Change in deep Atlantic circulation during glaciation

  32. Changes in the Circulation of Deep Water during Glaciations • The percentage of deep water • Originating in the North • Atlantic and flowing to the • equator during the last1.25 • Myr has been consistently • lower during glaciations than • during interglaciations. Figure 11-21 Changing sources of Atlantic deep water.

  33. Changes in the Circulation of Deep Water during Glaciations • Changes in Ocean Chemistry Figure 11-22. Carbon system controls on CO2 in the glacial atmosphere

  34. Conclusion

  35. Thanks For Your Attention

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