Back into the Icehouse: The Last 55 Million Years Chi-jung Wu.

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 jagged-edged 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.

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Measuring δ18O values

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•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

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.

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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 ratesRelative 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 HypothesisIndia-Asia collision and Tibet

Unusual physical weatheringHimalayan 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 WeatheringSediments suspended in river

Evaluating the Ocean Heat Transport hypothesis.

Cause of Brief Tectonic-Scale Climate ChangeVolcanic 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。