Transcript U3A-ClimChange03 6995KB Oct 18 2012

CLIMATE CHANGE
THE GREAT DEBATE
Session 3
Advancing Franz Josef Glacier in 1996, New Zealand
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
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
1960’s 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
Palaeographic reconstruction of relative positions of
Australia and Antarctica at 36 Ma
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
Polar view of northern continents showing maximum
extent of glacial ice (after Bowen)
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
Map shows
extent of various
glacier advances
during the
Quaternary
(after Kiernan)
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
Dolerite pebbles with rinds between 25 and 35 mm thick
typical of tills deposited during the Moore Glaciation
Dolerite boulder with weathering rind more than 100 mm
thick typical of deposits of the Linda Glaciation
CLIMATE CHANGE DURING THE
QUATERNARY 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
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
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
Eccentricity a 100,000
year cycle
Eccentricity
Precession a 20,000
year cycle
Tilt (Obliquity) a 40,000
year cycle
(Precession)
(Obliquity)
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
EVIDENCE IN SUPPORT OF
MILANKOVICH 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