The aim of this post is not just general interest, but specifically to provide an informal overview of ice cores for my Earth Surface Processes and Environments students (if you are one of them, the material here complements the lecture on Tuesday 15th October. Further notes and the lecture slides are available on Udo).
Ice coring is an extremely useful tool which allows us to reconstruct temperatures, atmospheric and oceanic conditions, volcanism and other events which occurred in the distant past. Without ice cores we would not have such a good grasp upon the timing of glacial and interglacial cycles, we would not have temperature records spanning the last 800,000 years, and the predictions we make about future climate change would be far less reliable.
But what exactly are they? Ice cores are cylinders of ice removed from a glacier or ice sheet by drilling downwards from the surface. This means they represent a vertical profile of the internal structure of the mass of ice. This is useful because the vertical profile of an ice mass contains a stratigraphy, much like a sedimentary record, since ice is deposited with an annual periodicity. Glacier ice forms when snow precipitates onto a glacier surface, is buried by further snowfall and compacted into ice. The thickness of ice layers can therefore tell us something about the rate of deposition in a given year.
Snow and ice are comprised of water molecules; however, not all water molecules are equal. Different isotopes of oxygen (molecules with the same number of protons and electrons, but different numbers of neutrons) exist in the ocean and atmosphere. The major isotopes are 18O and 16O. 18O has a greater atomic weight than 16O and more energy is therefore required to evaporate water molecules containing 18O from the hydrosphere into the atmosphere, and therefore the ratio of these molecules in the air provides a proxy for temperature. Atmospheric water molecules are precipitated to form glacier ice. The warmer the global temperature, the more ‘heavy’ 18O molecules are preserved when ice accumulates on glaciers and ice sheets. Thanks to careful calibration, we can measure the oxygen isotope ratios and provide a value for average temperatures for specific years in the distant past. Its also useful to know that isotopes of other elements can also be used in the same way – deuterium, for example is a ‘heavy’ form of hydrogen, measurements of which can also be used as a temperature proxy.
There are some complicating factors with isotope proxies, however, since it is not only temperature which influences their rate of deposition. For example, heavy isotopes are more readily precipitated and therefore travel less distance from their site of evaporation before they are deposited, meaning there is a negative gradient of heavy isotope concentration with distance inland from an ocean source. This is known as the continental effect. Secondly, altitude has a similar effect, since heavier isotopes will condense and precipitate at lower altitudes than lighter isotopes. Since ice can flow and therefore move location post-deposition, this can complicate palaeotemperature reconstructions. Furthermore, isotopes can diffuse out of layers of ice over long time scales, so records become somewhat less reliable with age. There is also a seasonal signal (due to temperature changes between summer and winter) in the ratio of preserved isotopes.
Along with snow, various impurities are also laid down via atmospheric deposition. These impurities can include particulate organic matter, atmospheric pollutants, and volcanic ash. Specific events, such as the volcanic eruption of Vesuvius in AD79 can be identified in ice cores due to the specific mineralogy and composition of sediment deposited in the aftermath of the event. This is useful for dating ice cores, as will be discussed shortly. Events such as nuclear bomb tests and the Chernobyl incident can also be identified in ice cores due to the radioactive minerals preserved in glacier ice.
Relative dating of ice is therefore relatively simple – we can often identify layers by counting up or down the ice core from a point which represents an event with a known date – for example we know that three layers of ice above the AD79 Pinatubo eruption represents ice deposited in AD84. However, this requires knowledge of the location within an ice core of specific events, which is not always the case or can be laborious and slow to find. Therefore other, pre-analysed ice cores are often used to help ‘map’ a new ice core, or identify the known events along its profile. However, this assumes common conditions of depositions for both the standard and the new ice cores. Furthermore, identifying annual accumulation layers can be difficult in very old ice due to ice being compressed by the vast weight of overlying ice, or deformed by shear and stress forces in the body of a glacier and ice sheet. Ice cores can also be compared to oceanic sediment cores, which record events in much the same way as glacier ice.
Absolute dating of ice cores requires elements with a known and measurable half life (usually carbon) to be extracted from specific layers and dated. C and Cl have radio-isotopes which are commonly used for absolute dating of ice cores. The problem with this is that a lot of ice has to be melted to attain large enough samples of the gases for accurate dating.
The longest ice cores successfully extracted to date are over 2km in length and were sourced from Antarctica. These cores have provided us with temperature records going back 800,000 years. This data revealed the remarkable synchronicity between variations in the earth’s orbit around the sun with changes in earth’s global average temperature and helped to uncover the crucial ice-albedo feedback mechanism for global climate regulation. Without these data, we would not know the timing or forcings of the great ice ages, glacial and interglacial periods, or shorter timescale climate changes. They illustrate the effects of changing solar activity, of oceanic circulation patterns, and are recording twentieth and twenty-first century anthropogenic climate change as we speak. Ice core data has been invaluable in attributing contemporary climatic change to human activity, in particular because it has elucidated precisely the effect of a range of natural forces, and shown them to be insufficient to explain recent global temperatures.
Ice cores, therefore, are an extremely important tool in climate science.