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 onthe 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 inCanada 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.

Foraminifera that live in the ocean form shells made ofcalcite, 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 froma 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.

Oxygen Isotope Data

Measuring δ18O values (Mass spectrometer)

Modern tropical surface water at 250C have δ18O values near0o/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 Myrof cooling.

Taking all view on the Earth’s system is the best way to salve the question.