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7 stable isotopes-1

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7 stable isotopes-1

  1. 1. Stable Isotopesδ18O- oxygen isotopes temperature ice volumeδ13C- carbon isotopes Carbon cycle
  2. 2. Oxygen Isotopes• Oxygen has 3 isotopes #protons #neutrons isotope 8 8 16 O 8 9 17 O 8 10 18 O
  3. 3. δ O18 • Oxygen isotopes – 16O = 99.8% Analyze ratios on stable isotope mass spectrometer – 17O = 0.08% – 18O = 0.2% Measure the 2 most abundant isotopes [18O/ 16Osample - 18O/ 16Ostandard]δ O (‰) = 18 * 1000 18 O/ 16O standard PeeDee Belemnite (PDB) Standard = SMOW (Std Mean Ocean Water)
  4. 4. Oxygen Isotopes δ18O• Calcite shells (foraminifera) = CaCO3 Seawater: HC16O16O16O HC18O16O16O HC17O16O16O CaC16O16O16O CaC16O16O18ORatio in the foram CaCO3 reflects the ratio in the seawater
  5. 5. Oxygen Isotopes Ruddiman (App. 1)
  6. 6. Oxygen Isotopes• Differences in mass are great enough to make the atoms behave differently during physical and chemical processes• 1. Evaporation – 16O is lighter- evaporates more readily• 2. Incorporation into a solid – 16O greater vibrational energy- more difficult to incorporate into solid
  7. 7. Oxygen Isotopes• Evaporation Effect (Ice volume, salinity) – 16O preferentially evaporated – incorporated into rain and snow – Stored on land as ice – During glacials- more 16O removed from the ocean and stored on land – 18O/ 16O of remaining ocean becomes “heavier” (or higher δ18O) – Reservoir forams create shells from is enriched in 18O
  8. 8. Oxygen Isotopes• Temperature Effect – Differences in vibrational energies are more pronounced at lower temperatures – More discrimination against 16O when it is colder – Shells incorporate a greater % of 18O when it is cold (= heavier 18O/ 16O or δ18O)
  9. 9. δ O 18• Rule of Thumb values• Ice volume effect: – ~1 ‰ δ18O/~100 m sea level• Temperature effect: – ~ 1 ‰ δ18O/~ 4°C – 0.25 ‰ /°C
  10. 10. Oxygen Isotopes• During glacials: – More water evaporated from ocean and stored on land as ice • 16O preferentially stored in ice • δ18O of seawater (and shells) is heavier – Water is colder • More discrimination against 16O • δ18O of shells is heavier
  11. 11. Interglacial 16 O 18 OGlacial * * * Remaining seawater higher δ18O Ice sheet lower δ18O Shells higher δ18OSee KKC 14-3
  12. 12. Oxygen Isotopes• Two effects- work in the same direction• Ice Volume- more ice = – Increase δ18O of seawater (and shells)• Temperature effect- colder – Increase δ18O of shells – δ18Oforams • Higher during cool intervals and glacial • Lower during warm intervals and interglacials
  13. 13. Oxygen Isotopes• Advantages – Forams common in deep sea sediments • Large dataset- high resolution • Benthic forams = deep water • Planktonic forams = surface water – Can also measure on: • corals, molluscs, fish ootoliths, speleothems (carbonate) • lake sediments • Phosphates – animal teeth and bones • Ice cores
  14. 14. Oxygen Isotopes• Complications – Multiple effects (temperature, ice volume) – Salinity (local effect) Ruddiman (App. 1) - Vital effects- biological fractionation - Diagenetic alteration of carbonate - Rayleigh fractionation
  15. 15. Oxygen Isotopes• Calculating paleotemperature (assumptions) t (°C) = 16.9 – 4.2(δ18Oforams - δ18Oseawater) + 0.13(δ18Oforams - δ18Oseawater)2 Shackleton, 1974 Two unknowns: * temperature * δ18Oseawater for the past (ice volume)
  16. 16. Cenozoic Climate ChangeRecord from benthicforams = bottomwater temperaturesAssume- mostly icevolume effect cooler more iceZachos et al., 2000 Ice free temperatures
  17. 17. Pleistocene ClimateIG More ice CoolerG *Last glacial max (LGM) * Trend toward cooler and more ice during glacial * Increasing amplitude of variations * 41k vs. 100k world (~600ky) Raymo, 1994 Site 607- North Atlantic
  18. 18. Carbon Isotopes• Stable Carbon isotopes – 12C = 98.9% Analyze ratios on stable isotope mass spectrometer – 13C = 1.11% – 14C = trace (radioactive) Measure the 2 most abundant isotopes [13C/ 12Csample – 13C/ 12Cstandard] δ C= 13 * 1000 13 C/ 12C standard PeeDee Belemnite (PDB) Standard = SMOW (Std Mean Ocean Water)
  19. 19. Carbon Isotopes• 12 C and 13C are fractionated during photosynthesis – Extent of fractionation depends on photosynthetic pathway• 12 C is preferentially incorporated into organic matter (smaller) – C3 trees/shrubs ~ -21 to -28 ‰ – C4 grasses ~ -11 to -15 ‰ – Marine organic matter ~ -22 ‰• Reduced forms of carbon are strongly fractionated – CH4 ~ -50 to -60 ‰
  20. 20. Carbon Isotopes * Fractionation by pathway * Removal and burial of small percent ofMajorpathway 12 C enriched organic matter leaves seawater enrichedRuddimanApp. 2
  21. 21. Carbon Isotopes• Processes that impact δ13C – Global carbon mass balance – Aging of water masses (circulation) – Air-sea exchange – Productivity
  22. 22. Carbon Isotopes•Global carbon mass balance • LGM vs. Holocene • Deep water δ13C was 0.32 to 0.46 ‰ lower during the LGM than today • Suggests transfer of ~ 500 gigatons of terrestrial organic matter to the ocean Global average δ13C of glacial (G) and Iinterglacial (I) deepwaters Boyle, 1992
  23. 23. Carbon Isotopes • Global carbon mass balance Carbon shift in terrestrial soils and mammal teeth Expansion of C4 plants (~8-4 Ma)Cerling et al., 1993 North AmPakistan Paleosols - horse teeth enamel Mammal tooth enamel
  24. 24. Carbon Isotopes • Aging of water masses (circulation) • More decay = more release of 12C •Older water = lower δ13C δ13C of Σ CO2 (‰) Modern Pacific Ocean Modern Atlantic Ocean 0 1.0 0 0.4 0. 1.0 1 0. 2 5 0.4 0.7 0. 9 1Depth (km) Depth (km) RF 2 -0.3 1.0 2 3 -0.2.1 -0 0.8 N ADW 3 PBW 0.0 PBW AB 0.7 0.6 W 4 4 0. 1 0.2 0.5 5 5 6 6 40N 20N 0 20S 40S 60S 40S 20S 0 20N 40N 60N 80N Latitude Latitude redrawn from Kroopnick, 1985
  25. 25. Carbon Isotopes• Aging of water masses (circulation) • Circulation during LGM Ruddiman, Ch. 10
  26. 26. Carbon Isotopes • Air-sea exchange •Paleocene-Eocene Thermal Maximum (PETM)- has been attributed to methane release •Impact on atmospheric and marine HCO3- δ13CZachos et al., 2008
  27. 27. Carbon Isotopes•Productivity • Removal of 12C in surface waters (productivity) • Release of 13C in deep waters (decay) • Produces a surface to deep gradient. PACIFIC • Strangelove Ocean older younger Kroopnick, 1985
  28. 28. Carbon Isotopes• Productivity Organic • More 12C buried in matter organic-rich anoxic sediments • Leaves waters enriched in 13C so organic matter becomes heavier Ocean Anoxic Event 2- Late Cretaceous
  29. 29. Carbon Isotopes• Advantages – Less susceptible to diagenetic alteration – Easily measured along with δ18O – Global Carbon cycle- Productivity indicator – Chemostratigraphy• Complications – Multiple sources of variation – Vital effects

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