CHAPTER 7 Back into the Icehouse: The Last 55 Million Year speaker: 林 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 CaCO 3.The oxygen in these shells consist of two isotopes( 18 O and 16 O) 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 18 O value recorded in the shells of foraminifera: For each 4.2 o C increase in temperature, the 18 O ratio decreases by 1 o / oo.
Measuring 18 O values (Mass spectrometer)
Modern tropical surface water at 25 0 C have 18 O values near 0 o / oo.using the temperature/ 18 O relationship just defined, high-latitude surface waters at temperatures of 0 0 C should have 18 O values as positive as +5 o / oo. Instead, these waters have 18 O values that are not very different from those of tropical surface waters.
Fractionation Each step in this cycle decreases the 18 O value of the water vapor by 10 0 / 00 in relation to that of ocean water left behind. The heavier 18 O isotope is more easily removed form the atmosphere.
Long-term 18 O 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 18 O 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.