Transcript Chapter 7 Back into the Icehouse

Back into the Icehouse:
The Last 55 Million Years
Chi-jung Wu
Global Climate Change since 55
Million Years
Examine evidence showing when this
greenhouse-icehouse cooling occurred.
This cooling can be explained by :
– BLAG spreading rate hypothesis
– Uplift weathering hypothesis
– The ocean heat transport hypothesis
Investigate factors that will determine the
slow changes in future climate over
tectonic time scales.
Global cooling for 55 Myr
Gradual cooling during
the last 55 Myr is
demonstrated by the
initiation of mountain
glaciation and of
continental-scale ice
sheets, and by a
progressive trend toward
cold-adapted vegetation
in both hemispheres.
Leaf outlines indicate temperature
Trees with smooth-edged leaves flourish today
in the tropics, while trees with more jaggededged leaves grow in colder climates.
Cooling in western North America
Temperature trends
estimated from the
outline shapes of
fossil leaves indicate
an erratic but
progressive cooling of
the middle latitudes of
the northern
hemisphere during the
last 55 Myr.
Measuring δ18O values
 18O( o oo) 
(18 O / 16 O) sample  (18 O / 16 O) s tan dard
(18 O / 16 O) s tan dard
T  16.9  4.2( 18Oc   18Ow )
•δ18OC= δ18O measured in the
calcite shells of
foraminifera
•δ18Ow= mean δ18O value of
ocean water when shells
formed
•4.2°c effect per mass balance
1/50× 50‰ change of
ocean water.
Long-term δ18O trend
 18Oc   18Ow  0.23T
Measurements of δ18O In
benthic foraminifera show an
erratic long-term trend toward
more positive values. From 55
to 40 Myr ago, the increase in
δ18O was caused by cooling of
the deep ocean. After 40 to 35
Myr ago, it reflects both further
cooling of the deep ocean and
formation of ice sheets.
Why Did Global Climate Cool over
the Last 55 Myr?
• Evaluating the BLAG spreading rate
hypothesis.
• Evaluating the Uplift Weathering
hypothesis.
– Extensive high terrain
– Unusual physical weathering
– Unusual chemical weathering
• Evaluating the Ocean Heat Transport
hypothesis.
Continental movements since
200 Myr ago
Changes in spreading rates
Relative production and consumption
of ocean crust vs. today
The average rate of
seafloor spreading
slowed until 15 Myr ago,
and has since sped up
again.
Adding in the effects of
generation of new crust
by volcanism at hot spots
away from plate margins
does not change this
basic trend.
Earth’s high topography
Uplift Weathering Hypothesis
India-Asia collision and Tibet
Unusual physical weathering
Himalayan sediments in the Indian Ocean
The rate of influx of
sediments from the
Himalayas and Tibet
to the deep India
Ocean has increased
almost tenfold since
40Myr ago.
Tibet and the monsoon
The fact that a plateau the size of Tibet in effect
creates its own weather, including the powerful
South Asian monsoon.
Chemical Weathering
Sediments suspended in river
Evaluating the Ocean Heat
Transport hypothesis.
Cause of Brief Tectonic-Scale Climate Change
Volcanic explosions and cooling
Each year about half of the remaining particles settle
out, and within a few years aerosol concentrations are
much reduced.
Understanding and Predicting
Tectonic Climate Change
Global cooling
produces more ice on
Earth (A), and the ice
increases rock
fragmentation in high
mountain terrain (B),
and near ice sheets (C).
Chemical weathering
of this fragmented
debris may cause
further cooling by
positive feedback.
Summary
All tectonic-scale process and feedbacks
operate at extremely slow rates, and the changes they
produce become evident only over millions of years.
Even though we are headed toward a colder future,
Earth’s climate won’t be getting there soon enough
for it to cause you or me any concern.
結論
此章節是在探討由過去五千五百萬年前由於板塊構造變
動所影響氣候的變化，而導致現今有冰山及冰原的存
在。又瞭解到板塊構造變動包括海底擴張、地表抬升
及風化作用皆是在很長時間(百萬年)下逐漸影響氣候
的變化，所以短時間的板塊構造變動(例如火山運動)
不會影響氣候在時間尺度上的大變動。