case study.pdf - SECOND CASE STUDY The Orbit of Earth and...

Info icon This preview shows pages 1–2. Sign up to view the full content.

13 SECOND CASE STUDY The Orbit of Earth and Pleistocene Glacial Rhythms The first unequivocal evidence of a large Antarctic ice cap is the occurrence of ice-rafted detritus in deep-sea cores dated at 28 million years of age. (Ice-rafted detritus is an indicator that continental ice carrying material eroded from the continents was transported out to sea by icebergs.) Greenland was clearly glaciated 3 million years ago. A closer examination of the record of glaciation during the last 1.9 million years (the Pleistocene) reveals considerable climatic variability in the form of repeated ad- vances and retreats of the polar ice caps. The movement of ice over the land surface is an extremely destructive process, mechanically grinding the rock surface and carrying the gla- cial debris downstream. On land the history of glaciation is recorded by glacial tracks and scours and by the deposition of glacial debris at the front (terminal moraines) and sides of glaciers and ice caps. In North America and Europe four major groups of terminal or end moraines are preserved from the Pleistocene. Does this record suggest four major glacial ad- vances of the modern ice cap? Moraines give scientists only the evidence of the farthest extent of glaciation. A more recent glaciation would destroy the record of any previous ice age where the maximum extent of the ice was less than that of the most recent advance. Until 1950, the end moraines were the only record of the ice age, and only four major glaciations were identified. A much better record was derived from the deep sea with the invention of coring devices in 1950. The coring of oceanic sediments retrieved a continuous record of microscopic fossil shells composed of calcium carbonate (CaCO 3 ). This new-found record was examined in detail with an innovative chemical methodology based on isotopes. A chemical element is defined by the number of protons in its nucleus. The number of neu- trons in the nucleus can vary. Isotopes are ele- ments with different mass numbers defined by the total number of neutrons and protons. Be- cause isotopes have different masses, different physical and chemical processes may separate, or fractionate, them. For example, oxygen has three isotopes: oxygen-16, -17, and -18. In the process of evaporation from the surface of the ocean, water containing oxygen with the light- est isotope value is preferentially evaporated. Likewise, in precipitation there is a preference to rain out water with the heaviest isotope, oxygen-18, first. The atmosphere is, in a sense, distilling the oxygen isotopic composition of the moisture it transports through the processes of evaporation and precipitation. The moisture that is finally deposited on the ice caps is ex- tremely enriched in the light isotope, and there- fore, during times of extensive glaciation, the ocean becomes depleted in oxygen-16. The mi- croscopic organisms in the ocean that grow CaCO 3 shells preserve the oxygen isotopic com- position of the oceanic water in which they live and build their shells.
Image of page 1

Info icon This preview has intentionally blurred sections. Sign up to view the full version.

Image of page 2
This is the end of the preview. Sign up to access the rest of the document.
  • Winter '16
  • tim
  • Milankovitch cycles, atmospheric carbon dioxide, carbon dioxide concentration, pleistocene glacial rhythms

{[ snackBarMessage ]}

What students are saying

  • Left Quote Icon

    As a current student on this bumpy collegiate pathway, I stumbled upon Course Hero, where I can find study resources for nearly all my courses, get online help from tutors 24/7, and even share my old projects, papers, and lecture notes with other students.

    Student Picture

    Kiran Temple University Fox School of Business ‘17, Course Hero Intern

  • Left Quote Icon

    I cannot even describe how much Course Hero helped me this summer. It’s truly become something I can always rely on and help me. In the end, I was not only able to survive summer classes, but I was able to thrive thanks to Course Hero.

    Student Picture

    Dana University of Pennsylvania ‘17, Course Hero Intern

  • Left Quote Icon

    The ability to access any university’s resources through Course Hero proved invaluable in my case. I was behind on Tulane coursework and actually used UCLA’s materials to help me move forward and get everything together on time.

    Student Picture

    Jill Tulane University ‘16, Course Hero Intern