Ice core dating using stable isotope data
Ice consists of water molecules made of atoms that come in versions with slightly different mass, so-called isotopes. Variations in the abundance of the heavy isotopes relative to the most common isotopes can be measured and are found to reflect the temperature variations through the year. For ice, the abundance of the heavy isotopes of Hydrogen and Oxygen is the δD and δ18O values, respectively (pronounced delta-Deuterium" and "delta-O-18 value"). The values are always given in per mille (‰) and are negative for glacier ice. Follow these links to learn more about how to measure the stable isotope ratios, stable isotopes as indicators of past temperatures and the δ notation or read more about how stable isotope data are used for dating below. In the following, dating using oxygen isotope data (δ18O) is described, although exactly the same techniques can be applied when using hydrogen isotope data (δD).
The annual cycle in δ18O is connected to local or regional temperature variations and is a very reliable indicator of the seasonal temperature cycle. The graph below shows how the isotopes correlate with the local temperature over a few years in the early 1990s at the GRIP drill site:
The clarity of the annual signal in the isotope data makes counting of annual layers in δ18O data one of the most accurate ways of dating ice cores. At least the upper parts of most Greenland ice cores have therefore been dated from thousands of δ18O samples that have been individually cut from the ice core, packed, and measured in a mass spectrometer (read more about the measurements here).
How far back in time the annual layers can be identified depends on the thickness of the layers, which again depends on the amount of annual snowfall, the accumulation, and how deep the layers have moved into the ice sheet. In cores from high-accumulation drill sites like the South Greenland DYE-3 ice core drilling (snowfall about 56 cm of ice equivalent per year), annual layers can still be identified from the stable isotope data about 8000 years back in time, while the annual layers are only barely distinguishable in δ18O data from NGRIP where the annual accumulation is 19 cm.
As the ice layers get older, the isotopes slowly move around and gradually weaken the annual signal. This process is called diffusion and sets the limit for far back in time annual layers can be identified using δ18O data.
Read more about
- diffusion of stable isotopes
- how the DYE-3 ice core has been dated using stable isotope data
- how stable isotope measurements are performed
- stable isotopes as indicators of past temperatures
- how annual layers are identified using impurity data