Dating using impurity mesurements

When dating an ice core by counting annual layers, one can use data of any kind that has an annual cycle. The variation in isotopic composition (δ18O and δD) of the ice reflects the annual temperature cycle and is the most widely used parameter for annual layer counting in ice cores (read more about this here), but this approach cannot be used for the older parts of ice cores or from ice cores from sites with low annual snow accumulation.

The dust content and the concentration of many chemical impurities in the ice also show seasonal variations and can therefore be used for annual layer counting. The advantage is that the impurities are unaffected by diffusion and can be used to identify annual layers in ice of any age, and that high-resolution measurements of ice impurities produce several parallel data series that can be used for dating, thereby making the annual layer identification process more robust.

The data for annual layer identification using impurity measurements most often come from so-called Continuous Flow Analysis of ice cores, where a stick of ice cut from an ice core is melted and analysed for a range of impurities. Many of these impurities exhibit regular concentration variations during the year that can be used for annual layer identification. An example is the dust content of the ice. Dust is brought to Greenland with the wind and deposits on the ice sheet. A part of the dust is soluble and dust is for example responsible for the main part of the Calcium ions found in the ice, while the insoluble dust particles are detected using light scattering. The measurements show that during interglacial climate conditions (similar to the current climate), the dust content peaks every year in spring when storms bring in relatively large amounts of dust. The Sodium concentration in contrast, peaks in winter, while the concentrations of Nitrate and Ammonium peak in summer. Using the measurements of these impurities and their relative timing, it is possible to identify the annual layers with a very high confidence. During glacial times, the timing differences disappear and all impurity records with an annual signal seems to peak at the same time in winter/spring.

Impurity dating

NGRIP impurity data obtained by Continuous Flow Analysis. From Rasmussen et al., Journ. Geophys Res., 2006.

The plot shows an example from the dating of the NorthGRIP ice core. The section shown here is from the Younger Dryas, a period with cold conditions at the very end of the last glacial, approximately 12,000 years ago. The annual layer thickness is about 3 centimeters. The lower 4 curves show concentrations of Calcium, Nitrate, Sodium, and Sulphate ions in the ice, the blue curve shows the conductivity of melted ice samples, the brown curve shows the dust content, and the black curve is a greyscale record of the visible layers in the ice (high values corresponding to pale "milky" layers in the ice, and low values representing clear ice). The blue and dark green thin curves show data series that have been mathematically resolution enhanced. All these data series exhibit annual variations and have been used for annual layer counting. Vertical grey lines mark the annual layers. The "open bar" at 1502.39 m marks a feature that could be a thin annual layer only partially resolved by the records. It is therefore regarded as "an uncertain annual layer".

Data like these form the basis for a recent effort to construct a new time scale for the Greenland ice cores, the so-called Greenland Ice Core Chronology 2005, or short GICC05. More data examples are available in the papers describing the GICC05 time scale. Also, three examples of longer data sections are provided here as .eps or .pdf files. The plots can be viewed on the screen or printed on a wide-format printer (full size is width 88 cm, height 16 cm).
- Example of data from the Holocene (present warm period), NGRIP depth interval 1463 - 1466 m, age approximately 11,200 years. Download as eps / pdf.
- Example of data from the Younger Dryas (cold period), NGRIP depth interval 1503.2 - 1504.4 m, age approximately 12,100 years. Download as eps / pdf.
- Example of data from the Bølling (rather warm period), NGRIP depth interval 1582.2 - 1584.5 m, age approximately 14,300 years. Download as eps / pdf.

Read more
- how impurities are measured in the ice cores
- other uses of impurity records
- about the GICC05 time scale, which is based mainly on annual layer counting using impurity data