Paleoclimate proxies, Part II: Vostok ice core activity
Background information:
The Vostok ice core was drilled at a Russian research station high on a dome of glacial ice in Antarctica
by an international team of researchers. It provides valuable paleoclimate data going back more than
400,000 years. One way of estimating temperature from the ice core data is to examine isotopic
variations in hydrogen. Water, of course, is a molecule made of two atoms of hydrogen and one atom of
oxygen. Like many other elements, hydrogen comes in
neutron-heavy and neutron-light versions (isotopes).
Normal hydrogen has one proton only, but some
hydrogen has a neutron as well, making 2
H, also known
as “deuterium,” and usually abbreviated as “D.”
The signature of ancient temperature can be found in
the ratio of heavier to lighter versions of hydrogen.
When it is warmer out, there is more energy to power
evaporation of heavy (D-containing) water molecules
from the ocean. This means the ocean gets isotopically
“lighter” during warm times, and the water that
evaporates and later condenses into clouds and falls as
precipitation (snow, in Vostok’s case) gets isotopically
“heavier.” During cold times, the opposite is true: there
is not enough energy to boost heavy water molecules out of the ocean as vapor, so the ocean gets
isotopically heavier, and the snow (made from the water molecules that succeeded in evaporating) gets
isotopically more “lightweight.”
We measure the ratio of D to H using a machine called a mass spectrometer. We can then describe the
“mix” of D and H in a sample of water (or ice) with a single number, called the δD (pronounced “deltadeuterium”).
The notation of δD is defined:
δD = (((D/H of a sample)/ (D/H of a standard)) – 1) x 1000
(The “standard” is “Vienna Standard Mean Ocean Water” (vSMOW), which is not seawater that has salt
in it, but rather water that evaporates from seawater, without any salt coming with it.)
The thing to remember about δD is that when δD decreases, it means that H is increasing in proportion
to D – that the overall mix of the two isotopes is getting skewed toward the lightweight isotope. So:
lower δDs mean more H and less D; this indicates a lower temperature (because there was less energy
available to evaporate water molecules containing D atoms from the ocean). When δD is higher, it
means there is more D and less H; this indicates a higher temperature (with more available energy to
evaporate water molecules containing D atoms).
C. Bentley annotations on Google Earth map
The unit we use to express δD is “per mil,” which basically means “out of a thousand,” in the same way
that “per cent” means “out of a hundred.” Per mil is written ‰, which is just like the % symbol with one
more “0,” just as a thousand (1000) has one more “0” than a hundred (100).
Instructions:
Open the spreadsheet distributed by your instructor.
The formula for calculating temperature from δD is:
Temperature (° C) = -55.5 + (δD + 440) / 6
Enter this formula into the final column of the spreadsheet (highlighted in yellow) to calculate the
temperature. This will automatically draw a graph for you.
Examine the graph and use it and the graph from the previous activity to answer the following
questions:
1. Which geological epochs are included in the sampled time period?
2. According to the coiling direction of the sampled forams, when was the temperature hotter?
When was it colder?
3. What famous event was happening in Earth history during this interval of time?
4. According to this data set, when did that event end?
5. What is the total range (maximum minus minimum) of paleotemperatures recorded by the data
set?
6. Would the temperatures during the warmer times at Vostok have felt “warm” by human
standards?
7. How does this data set compare to the data set you examined for foram coiling direction?
(Compare the two graphs) What do they have in common? How do they differ?
8. What might explain the differences you observed between the two data sets?
