CLIMATE CHANGE

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CLIMATE CHANGE
THE GREAT DEBATE
Session 3
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Advancing Franz Josef Glacier in 1996, New Zealand
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QUATERNARY ICE AGE

• The name is a misnomer because the beginning of
  the Quaternary at 2.5 Ma marks the onset of ice
  sheet accumulation in the Northern Hemisphere
  because by then Greenland and the continents had
  drifted far enough north to accumulate glacial
  ice
• In Antarctica an ice sheet first accumulated
  much earlier at about 36 Ma. We do not know
  whether ice has been present in Antarctica
  continuously since that time

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TASMANIA AND ANTARCTICA

• At 36 Ma Tasmania was still very close to
  Antarctica because the Australian continent had
  only just separated due to continental drift
• A tillite was discovered in a borehole put down
  by the HEC near Lemonthyme Creek in the Forth
  River Valley. It was originally described as a
  Quaternary deposit in the 1960s but later work
  has shown that it had to be very much older. It
  shows that there was glacial ice in Tasmania
  (Lemonthyme Glaciation) at a time when it was
  still close to the Antarctic continent

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Palaeographic reconstruction of relative
positions of Australia and Antarctica at 36 Ma
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NORTHERN HEMISPHERE

• In the early 20th Century four glacial advances
  were recognized north of the Alps by Penck and
  Bruckner
• They were
• Youngest Würm Glaciation
• Riss Glaciation
• Mindel Glaciation
• Oldest Günz Glaciation
• For half a century this remained the accepted
  version of major ice advances in the Quaternary
  but their exact age was unknown

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Polar view of northern continents showing maximum
extent of glacial ice (after Bowen)
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TASMANIAN STORY

• From 1970 to 1990 Dr Eric Colhoun and a number
  of research students mapped the extent and nature
  of glacial landforms in Western Tasmania
• They were
• Youngest Margaret Glaciation
• Henty Glaciation
• Moore Glaciation
• Oldest Linda Glaciation
• The Margaret Glaciation was the least extensive
  and the Linda Glaciation the most extensive. The
  Linda Glaciation occurred more than 783 ka ago

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Map shows extent of various glacier
advances during the Quaternary (after Kiernan)
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RELATIVE DATING

• The ages of glacial deposits can rarely be
  determined directly by radiometric dating
• Relative dating is a group of techniques that
  enables you to group your tills into a number of
  distinct age categories
• Many Tasmanian tills contain pebbles or boulders
  of dolerite, a very common rock in the state
• These pebbles form distinct weathering rinds as
  the dolerite weathers from the outside inwards
  due to groundwater percolating through the
  deposit

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Dolerite pebbles with rinds between 25 and 35 mm
thick typical of tills deposited during the Moore
Glaciation
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Dolerite boulder with weathering rind more than
100 mm thick typical of deposits of the Linda
Glaciation
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CLIMATE CHANGE DURING THEQUATERNARY ICE AGE
-MILANKOVICH THEORY
It is an astronomical theory that attempts to
explain climate changes during the Ice Age in
terms of variations in the way the Earth orbits
around the Sun. It is also known as the Orbital
Theory
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THEORY DEVELOPMENT
Made possible because planetary movements had
been closely studied In 1842 orbital theory was
first proposed by French scientist
Adhemar Further developed by Scottish mechanic
James Croll in late 19th Century Final evolution
of the theory by Serbian scientist Milankovich in
first half of 20thth century
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EARTH ORBITAL VARIATIONS
Eccentricity measures the extent to which the
orbit departs from a perfect circle and has a
cycle of approx. 100,000 years Tilt is the angle
between the Earth's axis and its plane of
rotation and varies from 21.8 to 24.4 degrees.
Cycle is approx. 40,000 years Precession refers
to the changing time of year when the Earth is
closest to the Sun. Today perihelion is in early
January. Its cycle is approx. 20,000 years
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Eccentricity - a 100,000 year cycle
Eccentricity
Precession - a 20,000 year cycle
Tilt (Obliquity) - a 40,000 year cycle
(Precession)
(Obliquity)
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MILANKOVICH CALCULATIONS

• He made all his calculations by hand, a very
  laborious task because there were no computers in
  those days
• As can be seen in the last diagram, he tried to
  correlate his data with the four glacial advances
  that had been documented in both Europe and North
  America, but the result is not very convincing
• At that time there was no continuous geological
  record of the Quaternary and there was no way of
  dating Quaternary sediments

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EVIDENCE IN SUPPORT OFMILANKOVICH THEORY

• When Milankovich died in 1958 there was no
  convincing evidence to support the orbital theory
  of climate change
• Information about Quaternary climate change
  has since come from deep sea cores, lake cores,
  ice cores, stalagmites from caves and other
  sources and methods have been developed to date
  them. It has confirmed that Milankovich was
  essentially correct