1. i) Oxygen isotopes and climate /Kepler’s laws
Chapter 14—Part 1
i) Oxygen isotopes and climate
/Kepler’s laws

2. How do we know how warm it was millions of years ago?
Ice cores: bubbles contain samples of the atmosphere that existed when the ice formed. (ancient pCO2)
Marine isotopes: oxygen isotopes in carbonate sediments from the deep ocean preserve a record of temperature.
The records indicate that glaciations advanced and retreated and that they did so frequently and in regular cycles.

3. Oxygen isotopes and paleoclimate
Oxygen has three stable isotopes: 16O, 17O, and 18O. (We only care about 16O and 18O.)
18O is heavier than 16O.
The amount of 18O compared to 16O is expressed using delta notation:
Fractionation: Natural processes tend to preferentially take up the lighter isotope, and preferentially leave behind the heavier isotope.
d18O ‰ = 18O/16O of sample -18O/16O of standard
18O/16O of standard
 1000

4. Oxygen isotopes and paleoclimate
Oxygen isotopes are fractionated during evaporation and precipitation of H2O
H216O evaporates more readily than H218O
H218O precipitates more readily than H216O
Oxygen isotopes are also fractionated by marine organisms that secrete CaCO3 shells. The organisms preferentially take up more 16O as temperature increases.
18O is heavier than 16O
H218O is heavier
than H216O

5. Oxygen isotopes and paleoclimate
Precipitation favors
H218O
…so cloud water becomes progressively more depleted in H218O as it moves poleward…
… and snow and ice are depleted in H218O relative to H216O.
Evaporation favors
H216O
H218O
H218O
Ice
Land
H216O, H218O
Ocean
Carbonate sediments in equilibrium
with ocean water record a d18O signal which reflects the d18O of seawater and the reaction of marine CaCO3 producers to temperature.
CaCO3

6. Oxygen isotopes and paleoclimate
As climate cools, marine carbonates record an increase in d18O.
Warming yields a decrease in d18O of marine carbonates.
JOIDES Resolution
Scientists examining core from the ocean floor.

7. Long-term oxygen isotope record Ice cap begins to form on Antarctica
around 35 Ma
This may be related
to the opening of
the Drake passage
between Antarctica
and S. America
From K. K. Turekian, Global Environmental
Change, 1996

8. Drake
passage
Once the Drake passage had formed, the
circum-Antarctic current prevented warm ocean
currents from reaching Antarctica

9. O isotopes during the last 3 m.y.
Kump et al., The Earth System, Fig. 14-4
Climatic cooling accelerated during the last 3 m.y.
Note that the cyclicity changes around Ma
− 41,000 yrs prior to this time
− 100,000 yrs after this time

10. O isotopes—the last 900 k.y.
Dominant period is ~100,000 yrs during this time
Note the “sawtooth” pattern..
after Bassinot et al. 1994

11. Ice Age Cycles: *This was the dominant period prior to
100,000 years between ice ages
Smaller cycles also recorded every
41,000 years*,
19, ,000 years
*This was the dominant period prior to
900 Ma

12. Milutin Milankovitch, Serbian mathematician
                      
Milutin Milankovitch,
Serbian mathematician
1924--he suggested solar energy changes and seasonal contrasts varied with small variations in Earth’s orbit
He proposed these energy and seasonal changes led to climate variations
NOAA

13. Before studying Milankovitch cycles, we need
to become familiar with the basic characteristics
of planetary orbits
Much of this was worked out in the 17th century
by Johannes Kepler (who observed the planets
using telescopes) and Isaac Newton (who
invented calculas)

14. Kepler’s Laws First law:
Planets travel around the sun in elliptical orbits
with the Sun at one focus r’ r
r’ + r = 2a
a = semi-major axis
(= 1 AU for Earth) a
Major axis
Minor axis

15. Ellipse:
Combined distances to two fixed points (foci) is fixed r’ r
r’ + r = 2a a
The Sun is at one focus

16. Aphelion
Point in orbit furthest from the sun
Earth (not to scale!) ra
ra = aphelion distance

17. Aphelion
Point in orbit furthest from the sun
Perihelion
Point in orbit closest to the sun
Earth rp
rp = perihelion distance

18. Eccentricity
e = b/a so b = ae
a = 1/2 major axis (semi-major axis)
b = 1/2 distance between foci b a

19. Eccentricity
e = b/a
a = 1/2 major axis
b = 1/2 distance between foci
Sun-Earth distances
Aphelion: a + ae = a(1 + e)
Perihelion: a – ae = a(1 – e) b a

20. Eccentricity
e = b/a
a = 1/2 major axis
b = 1/2 distance between foci
Sun-Earth distances
Aphelion: a(1 + e)
Perihelion: a(1 – e)
Today:
e = 0.017
Range:
0 to 0.06
Cycles: 100,000 yrs b a

21. Kepler’s Second Law
2nd law: A line joining the Earth to the Sun sweeps
out equal areas in equal times
Kump et al., The Earth System, Box Fig. 14-1
Corollary: Planets
move fastest when
they are closest to
the Sun

22. Kepler’s Third Law
3rd law: The square of a planet’s period, P, is proportional to the cube of its semi-major axis, a
Period—the time it takes for the planet to go around the Sun (i.e., the planet’s year)
If P is in Earth years and a is in A.U., then
P2 = a3

23. Other characteristics of Earth’s orbit vary as well.
The three factors that affect climate are 

24. Eccentricity (orbit shape) 100,000 yrs &400,000 yrs Obliquity
(tilt to 24.5o)
41,000 yrs
Precession
(wobble)
19,000 yrs
& 23,000 yrs