23-Ice Ages and Climate

23-Ice Ages and Climate - Past climates: clues Geological...

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Unformatted text preview: Past climates: clues Geological markers Ice retreat, tree rings and carbon dating Stable isotopes Past climates: time scales and controls 10-100 million years: Plate tectonics 10-1000 thousand years: Milankovitch cycles 10-100 years Greenhouse gases Solar output Ocean circulation Feedbacks Ice Ages and Climate Next lecture: Oil There is abundant evidence of ice sheets that have covered large parts of North America in the not-too- distant past. Here is evidence of two different glacial episodes in Ontario, Canada. An ice sheet has polished this rock surface flat and left its characteristic striations. The polished rock is a metamorphosed glacial till, as can be easily recognized from the great variation in clast sizes. The rock is 2.2 billion years old. By looking at carbon isotopes of organic matter left behind by the retreating ice sheet, the timing of glacial retreat has been worked out. The blue areas were once covered by glacial meltwater lakes (not all at the same time); release of these lakes caused tremendous floods. These are gigantic ripple marks caused by the huge escape of water when glacial lake Missoula emptied out during the last glacial retreat. They are so big that its very difficult to recognize whats going on on the ground. http://earthobservatory.nasa.gov/Features/Paleoclimatology_CloseUp/ Recent events can be dated with the aid of carbon isotopes and tree rings. The radioactive isotope Carbon-14 has a 5730-year half life, and is produced by cosmic rays hitting the upper atmosphere. Tree rings record distinctive patterns based on annual rainfall, fires, etc. These two dating methods are used to double-check each other. Past climates: clues Geological markers Ice retreat, tree rings and carbon dating Stable isotopes Past climates: time scales and controls 10-100 million years: Plate tectonics 10-1000 thousand years: Milankovitch cycles 10-100 years Greenhouse gases Solar output Ocean circulation Feedbacks Ice Ages and Climate Next lecture: Oil http://earthobservatory.nasa.gov/Features/Paleoclimatology_OxygenBalance/ There are three stable isotopes of oxygen, 16 O, 17 O, and 18 O, which are present in different proportions...
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23-Ice Ages and Climate - Past climates: clues Geological...

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