Worked Example

Isotope ratios are vital in the determination of historical temperatures using proxy measurements. Use the Ice Core Extraction and Analysis Learning Tool to calculate the temperature in 850 A.D., according to the Crete, Greenland ice core.

First, navigate to the 'How Are They Analyzed?' page, click on 'Determine the temperature of a Greenland ice core' and choose the ice core from Crete, Greenland. Select the 850 A.D. Layer from the 'Choose Layer' drop-down menu and scan the sample.

After the scan has been completed, record the intensities at 44 m/z and 46 m/z. Do you remember from the Ice Core and Isotope Ratio Mass Spectrometry Learning Tool why the intensities are at 44 m/z and 46 m/z? Recall that the water molecules are reacted with carbon dioxide, which gives the carbon dioxide molecules the same 18O/16O ratio as the ratio found in the water molecules. Then, the carbon dioxide sample is analyzed with the mass spectrometer, producing the peaks at 44 m/z and 46 m/z, which correspond to the intensities of 16O and 18O, respectively, in the carbon dioxide molecules. It is important to recognize that the intensity at 44 m/z corresponds to the 16O abundance and that the intensity at 46 m/z corresponds to the 18O abundance.

Note that though the 18O/16O ratio is not exactly equal to the ratio of 46CO2/44CO2, the probability of having a CO2 molecule formed of a 13C, an 16O, and an 17O molecule is relatively small compared to the probability of having a CO2 molecule formed from 12C, an 16O, and an 18O. Since the focus is on 18O/16O ratios, the assumption that 44 m/z corresponds to the 16O abundance and that the intensity at 46 m/z corresponds to the 18O abundance is reasonable.

Using Equation 1 on the next page of the Learning Tool, calculate the 18O/16O ratio. Note: The exact intensity values which you obtained may be slightly different than the intensity values used in this calculation. Next, use Equation 2 to calculate ∂18O. Remember that the ∂18O value relates the 18O/16O ratio to temperature. The 18O/16O standard (the Vienna Standard Mean Ocean Water) is 2.005 × 10-3.   Finally, use the graph on the next page of the Learning Tool to estimate the temperature when the ∂18O value is -35.26. Use the 'Evaluate Single Point Tool' on the top left corner of the graph to more accurately determine the temperature indicated by the ∂18O value of -35.26. The temperature indicated by the ice core is approximately -31.75 °C, corresponding to the average temperature in Greenland in 850 A.D.