Glaciation

Class Notes - Glaciers

Isotopic Composition of Oxygen

    Oxygen consists of three isotopes. None of these isotopes decay by radioactivity so Oxygen is one of several so-called stable isotopes

    1. 16O : 99.763%
    2. 17O : 0.0375%
    3. 18O : 0.1995%

      Out of 1,000 atoms of Oxygen, how many should be O18?

      200
      20
      2
      0.2

      Water, for example consists of : H216O and H218O. Which of these two molecules of water has the highest density?

      H216O
      H218O

    These two isotopes differ in mass and in how the molecules containing them behave.

    More 18O is evaporated from the oceans when temperatures are high and less when temperatures are low. Therefore, more 18O is abundant in rainifall (precipitation) during warm time periods and less abundant during cold periods.

    Cold water is enriched in 18O. Evaporation of this cold water creates gases that are enriched in the lighter isotope 16O. Therefore continental glaciers should be enriched in 16O.

    Calcium carbonates deposited in tillite deposits are strongly depleted in 18O. Therefore, this material could have formed as the glacial ice melted.

      True
      False

    Look at Box 12.4 on page 320 of the text. Note that the bottom scale is in units of heavy oxygen ranging from -36 to -42 and that heavy oxygen is increasing to the left. 10-1 is greater than 10-2,

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    Copyright by John C. Butler, July 29, 1995

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