Lecture 17-- Oxygen isotopes and climate/Kepler

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Lecture 17-- Oxygen isotopes and climate/Keplers
laws

• Meteo 466

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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.

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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.

? 1000
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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
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Oxygen isotopes and paleoclimate
so cloud water becomes progressively more
depleted in H218O as it moves poleward
Precipitation favors H218O
and snow and ice are depleted in H218O
relative to H216O.
Evaporation favors H216O
H218O
H218O
Ice
Land
H216O, H218O
Ocean
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Oxygen isotopes and paleoclimate

• As climate cools, marine carbonates record an
  increase in d18O.
• Warming yeilds a decrease in d18O of marine
  carbonates.

JOIDES Resolution
Scientists examining core from the ocean floor.
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Long-term oxygen isotope record
From K. K. Turekian, Global Environmental Change,
1996
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Drake passage

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

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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 0.8-0.9
  Ma
• - 41,000 yrs prior to this time
• - 100,000 yrs after this time

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O isotopesthe last 900 k.y.

• Dominant period is 100,000 yrs during this time
• Note the sawtooth pattern..

after Bassinot et al. 1994
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Ice Age Cycles 100,000 years between ice
ages Smaller cycles also recorded
every 41,000 years, 19,000 - 23,000
years This was the dominant period prior to
900 Ma
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Milutin Milankovitch, Serbian mathematician 1924
--he suggested solar energy changes and seasonal
contrasts varied with small variations in Earths
orbit He proposed these energy and seasonal
changes led to climate variations
NOAA
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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)
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Keplers 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
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Ellipse Combined distances to two fixed points
(foci) is fixed
r
r
r r 2a
a

• The Sun is at one focus

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Aphelion Point in orbit furthest from the sun
Earth (not to scale!)
ra
ra aphelion distance
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Aphelion Point in orbit furthest from the
sun Perihelion Point in orbit closest to the sun
Earth
rp
rp perihelion distance
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Eccentricity e b/a so, b ae a 1/2 major
axis (semi-major axis) b 1/2 distance between
foci
b
a
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Keplers Second Law
2nd law A line joining the Earth to the Sun
sweeps out equal areas in equal times
Corollary Planets move fastest when they are
closest to the Sun
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Keplers Third Law

• 3rd law The square of a planets period, P, is
  proportional to the cube of its semi-major axis,
  a
• Periodthe time it takes for the planet to go
  around the Sun (i.e., the planets year)
• If P is in Earth years and a is in A.U., then
• P2 a3

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Other characteristics of Earths orbit vary as
well. The three factors that affect climate are ?
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Eccentricity (orbit shape) 100,000
yrs 400,000 yrs Obliquity (tilt--21.5 to
24.5o) 41,000 yrs Precession (wobble)
19,000 yrs 23,000 yrs
http//www.geo.lsa.umich.edu/crlb/COURSES/205/Lec
20/lec20.html
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Q What makes eccentricity vary?A The
gravitational pull of the other planets

• The pull of another
• planet is strongest
• when the planets
• are close together

• The net result of
• all the mutual inter-
• actions between
• planets is to vary the
• eccentricities of their
• orbits

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Eccentricity Variations

• Current value 0.017
• Range 0-0.06
• Period(s) 100,000 yrs
• 400,000 yrs

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Unfiltered Orbital Element Variations
0.06
65o N solar insolation
Imbrie et al., Milankovitch and Climate, Part 1,
1984
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Q What makes the obliquity and precession
vary?A First, consider a better known example
Example a top

• Gravity exerts a torque
• --i.e., a force that acts
• perpendicular to the spin
• axis of the top
• This causes the top to
• precess and nutate

g
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Q What makes the obliquity and precession
vary?A i) The pull of the Sun and the Moon on
Earths equatorial bulge
N
g
g
Equator

• The Moons torque on
• the Earth is about twice
• as strong as the Suns

S
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Q What makes the obliquity and precession
vary?A ii) Also, the tilting of Earths orbital
plane
N
?
N
S
?

• Tilting of the orbital plane is like
• a dinner plate rolling on a table
• If the Earth was perfectly spherical,
• its spin axis would always point in
• the same direction but it would make
• a different angle with its orbital plane
• as the plane moved around

S
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Obliquity Variations

• Current value 23.5o
• Range 22o-24.5o
• Period 41,000 yrs

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Precession Variations

• Range 0-360o
• Current value Perihelion occurs on Jan. 3
• ? North pole is pointed almost directly away
  from the Sun at perihelion
• Periods 19,000 yrs
• 23,000 yrs

Today
Actual precession period is 26,000 yrs, but the
orienta- tion of Earths orbit is varying, too
(precession of perihelion)
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Which star is the North Star today?
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Which star was the North Star at the opposite
side of the cycle?
Polaris
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Vega
Polaris
Actually, Vega would have been the North Star
more like 13,000 years ago
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Unfiltered Orbital Element Variations
0.06
65o N solar insolation
Imbrie et al., Milankovitch and Climate, Part 1,
1984
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Ref Imbrie et al., 1984
Eccentricity
Obliquity
Precession
Filtered Orbital Element Variations
800 kA
Today
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• Optimal Conditions for Glaciation
• Low obliquity (low seasonal contrast)
• High eccentricity and NH summers during aphelion
  (cold summers in the north)
• Milankovitchs key insight
• Ice and snow are not completely melted during
  very cold summers.
• (Most land is in the Northern Hemisphere.)

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• Optimal Conditions for Deglaciation
• High obliquity (high seasonal contrast)
• High eccentricity and NH summers during
  perihelion (hot summers in the north)

11,000 yrs ago
Today
N
?
S
Optimal for glaciation
Optimal for deglaciation
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NH Insolation vs. Time
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O isotopesthe last 900,000 yrs
Peak NH summertime insolation
after Bassinot et al. 1994
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Big Mystery of the ice ages Why is the
eccentricity cycle so prominent? The change in
annual average solar insolation is small (0.5),
but this cycle records by far the largest climate
change Two possible explanations 1) The
eccentricity cycle modulates the effects of
precession (no change in insolation when e
0) 2) Some process or processes amplify the
temperature change. This could take place by a
positive feedback loop