Class 19. Paleoceanography William Wilcock OCEAN/ESS 410

Learning Goals Understand how  18 O is defined Understand why  18 O decreases with decreasing temperature in ice sheets Understand what causes  18 O 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 / 10 3 yr Bioturbation in most settings –3-10 cm Resolution –10 3 to 10 5 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 1.Create Ions 2.Accelerate Ions 3.Select Ions based on velocity (electric and magnetic forces cancel out for selected velocity) 4.2 nd magnetic field separates ions based on charge/mass ratio 5.Detector

Equations for Mass Spectrometer Velocity selection stage –Electrostatic force F E = qE Where q is charge and E is electric field –Magnetic force F B1 = qvB Where v is velocity and B 1 is magnetic field –Selection (no bending) when F E = F B1 or v=E/B 1 Charge to mass ratio separation –Acceleration from magnetic field F B2 = ma = qvB 2 or a=qvB 2 /m –Centripetal force a = v 2 /r = qvB 2 /m or r = mv/(qB 2 ) r increase with mass of ion

Oxygen Isotopes Stable Isotopes % 16 O 0.037% 17 O 0.204% 18 O 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  18 O = -9 to -11‰

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

 18 O in ice cores Modern Ice Averages  18 O SMOW = -25‰ but it depends on location During Ice Ages it was colder and therefore ice is lighter (  18 O more negative)  18 O, ‰

Hydrogen Fractionation Also Occurs

Antarctic Ice Core Records Temperature comes from Oxygen and Hydrogen isotopes

Oxygen Isotopes Foraminifera Calcium Carbonate skeletons for Foraminifera form with a  18 O value that is offset from water by an amount dependent on temperature (some variation between species)

Average  18 O record of foraminifera in sediments Ice Volume (  18 O 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

Effect of Ice Volume on  18 O

 18 O of present day surface waters

Isotope record of ice ages

Lisiecki and Raymo stack of  18 O in deep water benthic foraminifera in 57 cores

Fourier Transform

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