Chy 121 (Amar) -- Fall, 2007            Oxygen Isotopes and Climate         

        We have been discussing the definition of the average atomic weight:

AW = S_i f_i (AW)_i

where f_i is the fraction of atoms of a given isotope and (AW)_i is the atomic weight of that isotope.
Oxygen has three isotopes:

                16O            17O         18O
(AW)i         15.994915        16.999131     17.999160        amu
   fi        0.9976(1)        0.0004        0.0020(1)
AW =         15.96     +      0.0068   +     0.036      =   16.00 amu

                                                         (15.9994 is accepted value)

        The¹⁸O/¹⁶O ratio has turned out to be a very significant diagnostic of the climate and in particular the
ice ages of the past million years or so. The ¹⁸O/¹⁶O ratio is measured in sediments that have collected over
millions of years at the bottom of the ocean. The oxygen atoms are sequestered in the carbonate shells formed
by microscopic plankton called forams [Recall that carbonates like calcium carbonate are ionic compounds
consisting of cations like Ca²⁺ and oxyanions of CO₃^2-]. The current understanding of this effect is summarized
below (see Ice Ages: Solving the Mystery by John and Katherine Imbrie, Harvard University Press, 1979)*:

    1. During an ice age, more water is "stored" in glaciers and the lighter and more abundant ¹⁶O tends to be
        extracted preferentially from sea water and stored in the ice sheet.

    2. During such a period, the ¹⁸O abundance in seawater is slightly higher than normal and the CaCO₃
        sediments formed during these periods have more ¹⁸O in them than during warmer periods.

    3. By drilling into the ocean floor and taking core samples or "cores" that can be many meters long, geologists
        and paleontologists are able to sample this history. The cores are sliced into small samples that can be chemically
        prepared and then analysed for the ¹⁸O/¹⁶O ratio (using a mass spectrometer). TheStable Isotope Lab in the
        Geology Department at UMaine is one such facility.

    4. The attached plot has time on the vertical axis and a measure of the ¹⁸O/¹⁶O ratio on the horizontal axis.
        The actual variable used on the horizontal axis is:

        where PDB stands for Pee Dee Belemnite, a standard reference measurement for ¹⁸O/¹⁶O ratios in carbonates.
        Thus in periods of glaciation  when the ¹⁸O/¹⁶O ratio is a little larger we should have more positive values
        of this quantity (left hand side of graph). The data in the plot indicates that there is  a cycle of glaciation that
        occurs every 100,000 years or so and that we appear to be near the end of a period of deglaciation at the
        present time.

    [*Thanks to Susie Carter, MIT Geology Department and Kirk Maasch, University of Maine Geology
        Department for sources, helpful discussions, and clarifications]