Title: CLIMATE CHANGE
1CLIMATE CHANGE
THE GREAT DEBATE
Session 3
2Advancing Franz Josef Glacier in 1996, New Zealand
3QUATERNARY 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
4TASMANIA 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
5Palaeographic reconstruction of relative
positions of Australia and Antarctica at 36 Ma
6NORTHERN 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
7Polar view of northern continents showing maximum
extent of glacial ice (after Bowen)
8TASMANIAN 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
9Map shows extent of various glacier
advances during the Quaternary (after Kiernan)
10RELATIVE 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
11Dolerite pebbles with rinds between 25 and 35 mm
thick typical of tills deposited during the Moore
Glaciation
12Dolerite boulder with weathering rind more than
100 mm thick typical of deposits of the Linda
Glaciation
13CLIMATE 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
14THEORY 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
15EARTH 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
16Eccentricity - 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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18MILANKOVICH 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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20EVIDENCE 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