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Class 19. Paleoceanography William Wilcock

OCEAN/ESS 410. Class 19. Paleoceanography William Wilcock. Learning Goals. Understand how δ 18 O is defined Understand why δ 18 O decreases with decreasing temperature in ice sheets Understand what causes δ 18 O in foraminifera to vary and how it can be used to infer past climate.

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Class 19. Paleoceanography William Wilcock

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  1. OCEAN/ESS 410 Class 19. PaleoceanographyWilliam Wilcock

  2. Learning Goals • Understand how δ18O is defined • Understand why δ18O decreases with decreasing temperature in ice sheets • Understand what causes δ18O in foraminifera to vary and how it can be used to infer past climate.

  3. Paleoclimate • Ice cores 123,000 years Greenland, 800,000 years Antarctica Temperature & air bubbles Sub annual resolution • Tree Rings Continuous for a few thousand years (older with radiocarbon dating) • Corals Continuous for a few hundred years (older with dating) • Sediments >100 Million years but not in 1 core and preservation of fossils effectively limits it to significantly less.

  4. Time resolution of sediment record • Typical deep sea sedimentation rates • 0.1 to 3 cm / 103 yr • Bioturbation in most settings • 3-10 cm • Resolution • 103 to 105 years • Changes over shorter term cannot be resolved in a sediment core

  5. Dating Sediments • Absolute • Radiometric (14-C, 230-Th/U, K-Ar) • Relative from cross-correlation • Paleomagnetic • Fossil record • Lithology • Time on a rubber band

  6. Bainbridge (Sector) Mass Spectrometer • Create Ions • Accelerate Ions • Select Ions based on velocity (electric and magnetic forces cancel out for selected velocity) • 2nd magnetic field separates ions based on charge/mass ratio • Detector

  7. Equations for Mass Spectrometer • Velocity selection stage • Electrostatic force • FE = qE Where q is charge and E is electric field • Magnetic force • FB1 = qvB • Where v is velocity and B1 is magnetic field • Selection (no bending) when FE = FB1 or v=E/B1 • Charge to mass ratio separation • Acceleration from magnetic field • FB2 = ma = qvB2 or a=qvB2/m • Centripetal force • a = v2/r = qvB2/m or r = mv/(qB2) • r increase with mass of ion

  8. Oxygen Isotopes Stable Isotopes 99.759% 16O 0.037% 17O 0.204% 18O The lighter isotopes is preferentially incorporated into vapor, slightly more so at lower temperatures, and the heavier isotope is preferentially incorporated into rain. Standard = SMOW (Standard Mean Ocean Water) Water vapor in equilibrium with SMOW had δ18O = -9 to -11‰

  9. d18O of precipitation – Latitude Dependence • Most evaporation occurs at low latitudes and most precipitation at high latitudes. • Vapor forming with the equator with d18O = -9‰ will always precipitate rain with more 18O and the remaining vapor will get progressivly lighter in 18O as it moves to higher latitudes. • This process is known as fractionation

  10. δ18O in ice cores • Modern Ice Averages δ18OSMOW = -25‰ but it depends on location • During Ice Ages it was colder and therefore ice is lighter (δ18O more negative) δ18O, ‰

  11. Hydrogen Fractionation Also Occurs

  12. Antarctic Ice Core Records Temperature comes from Oxygen and Hydrogen isotopes

  13. Oxygen Isotopes Foraminifera Calcium Carbonate skeletons for Foraminifera form with a δ18O value that is offset from water by an amount dependent on temperature (some variation between species)

  14. Average δ18O record of foraminifera in sediments • Ice Volume (δ18O of oceans increases when more isotopically light ice is locked up on the continents) - ⅔ of variation (calibrate with deep sea foraminifera) • Temperature - ⅓ of variation

  15. Effect of Ice Volume on d18O

  16. δ18O of present day surface waters

  17. Isotope record of ice ages

  18. Lisiecki and Raymo stack of δ18O in deep water benthic foraminifera in 57 cores

  19. Fourier Transform

  20. Fourier Transform Versus Time • 100 kyr eccentricity period important now. • 41 kyr obliquity important in the past

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