Lecture 16

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Lecture 16Oxygen isotopes/Keplers laws

• Abiol 574

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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 18O as
  temperature decreases.

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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
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O isotopes during the last 3 m.y.
Kump et al., The Earth System, Fig. 14-4

• 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

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
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Eccentricity e b/a a 1/2 major axis
(semi-major axis) b 1/2 distance between foci
b
a
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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)
Perihelion
Aphelion
a
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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
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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