CHAPTER 7
Back into the Icehouse:
The Last 55 Million Year
speaker:林聖烈
Global Climate Change Since 55 Myr Age
•Evidence from Ice & Vegetation
•Oxygen Isotope Data
Why Did Globe Climate Cool over the
Last 55 Myr?
•Evaluating the BLAG Spreading Rate Hypothesis
•Evaluating the Uplift Weathering Hypothesis
•Evaluating the Ocean Heat Transport Hypothesis
Understanding &Predicting Tectonic
Climate Change
Evidence from ice & vegetation
In the southern hemisphere, no
evidence exists for ice on
Antarctica until 35Myr ago,
when ice-rafted debris was first
dropped in ocean sediments on
the nearby continental margin.
Today an ice sheet as mush as 4 km thick covers most of Antarctica
30Myr ago Nothofagus trees, like those living today at the southern tip of South
America, still existed in some parts of Antarctica.
Nothofagus
Warm-adapted breadfruit trees
Lived above the Arctic Circle in
Canada until 60 Myr ago
The landmasses surrounding the Arctic Ocean today are covered by scrubby tundra
vegetation grazed by caribou.
The reason for this relationship is not known, but the correlation with temperature
in the modern vegetation is so strong that climate scientists have used this
relationship to estimate past temperatures from assemblages of fossil leaves
preserved in sedimentary rocks.
One record derived from
leaf-margin evidence in western
North America shows an
ongoing cooling over 55Myr.
Although interrupted by small,
short-lived warmings, the cooling
trend resumes and reaches even
lower temperatures over time.
Oxygen Isotope Data
Foraminifera that live in the ocean form shells made of
calcite, or CaCO3.The oxygen in these shells consist
of two isotopes(18O and 16O) taken directly from the
ocean water.
When the foraminifera die and become part of the
permanent geologic record on the seafloor, they from
a history of changes in the relative abundance of
these two isotopes of oxygen in the oceans.
The temperature of ocean water is directly related to
The δ18O value recorded in the shells of foraminifera:
For each 4.2oC increase in temperature, the δ18O
ratio decreases by 1o/oo.
Measuring δ18O values (Mass spectrometer)
Modern tropical surface water at 250C have δ18O values near
0o/oo .Using the temperature/δ18O relationship just defined,
high-latitude surface waters at temperatures of 00C should have
δ18O values as positive as +5o/oo . Instead, these waters have
δ18O values that are not very different from those of tropical
surface waters.
Fractionation
Each step in this cycle
decreases the δ18O value
of the water vapor by 10 0/00
in relation to that of ocean
water left behind.
The heavier 18O isotope is
more easily removed form
the atmosphere.
Long-term δ18O trend
BLAG Hypothesis
Uplift weathering Hypothesis
Himalaya Mountains
Himalaya sediments in
the Indian Ocean
Tibet and the monsoon
Sediments suspended in rivers
Gateway Hypothesis
Locations of Earth’s volcanoes
Conclusion
We have evidence of climate change in vegetation, ice
and δ18O values show that Earth has cooled in the last
55 Myr.
So many hypothesis have developed now. But no one
hypothesis can explain the true fact about the last 55 Myr
of cooling.
Taking all view on the Earth’s system is the best way to
salve the question.