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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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OCEAN/ESS 410 Class 19. PaleoceanographyWilliam Wilcock
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.
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.
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
Dating Sediments • Absolute • Radiometric (14-C, 230-Th/U, K-Ar) • Relative from cross-correlation • Paleomagnetic • Fossil record • Lithology • Time on a rubber band
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
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
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‰
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
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, ‰
Antarctic Ice Core Records Temperature comes from Oxygen and Hydrogen isotopes
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)
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
Lisiecki and Raymo stack of δ18O in deep water benthic foraminifera in 57 cores
Fourier Transform Versus Time • 100 kyr eccentricity period important now. • 41 kyr obliquity important in the past