ICE CORE DATA ON CLIMATE AND COSMIC RAY CHANGES

Ice cores represent archives which contain unique information about a large variety of environmental parameters. Climatic information is stored in the form of stable isotopes, greenhouse gases and various chemical substances. The content of cosmogenic nuclides such as Be and Cl provide long-term records of the intensity of the cosmic ray flux and its modulation by solar activity and the geomagnetic dipole field. Cosmogenic nuclides are produced by the interaction of cosmic ray particles with the atmosphere. After production, these nuclides are transported and distributed within the environment, depending on their geochemical properties. Some of them are removed from the atmosphere by snow and incorporated into ice sheets and glaciers. The analysis of the Greenland ice cores GRIP and GISP2 are discussed in terms of climate and cosmic ray changes during the past 50’000 years. 1. ARCHIVE ICE Polar ice sheets are formed from snow. The snowflakes grow together to grains which slowly increase in size. Due to the pressure of the overlying new snow layers, the grains become more and more compacted and finally turn into ice. The consequence of this formation process is that the ice not only preserves all the atmospheric constituents such as aerosols and dust, it also contains air bubbles that enable to determine the atmospheric composition and in particular the reconstruction of greenhouse gases in the past. This unique property makes ice the only archive that virtually stores all the climate forcing factors (greenhouse gases, aerosols and volcanic dust, solar irradiance) except internal variability. Ice cores also contain information on the corresponding climate response (temperature, precipitation rate, wind speed, atmospheric circulation). Another important property of ice is that it flows. This can be seen in Fig. 1, which schematically depicts an ice-sheet. The ice slowly flows towards the margin of the ice sheet, where it partly melts and partly breaks up as icebergs. Under steady-state conditions, the ice lost in the ablation area is replaced by snow falling on the accumulation area where new layers are formed continuously. As a consequence of the horizontal movement of the ice, the annual layers become thinner with increasing depth, as indicated in Fig. 1. This leads to another special property of the archive ice. The depth–age relationship is nonlinear, which has the advantage that the uppermost part of the core is well resolved and the total time period covered is long (of the order of 10 years for polar ice cores). The disadvantage of this non-linear time-scale is, however, that dating ice is difficult and relies strongly on correct modeling of the ice-flow. The main ice sheets are situated in polar regions (Greenland, with a maximum thickness of approx. 3 km and Antarctica, with a thickness of up to 4 km). Smaller ice sheets at lower latitudes can only be found at high altitudes (Andes, Himalayas, Alps) [1]. There is a steadily growing number of parameters which can be measured in ice cores. It is beyond the scope of this paper to discuss all these parameters. In Table 1, a small selection of those related to climate forcing and climate response is given. BEDROCK ABLATION ABLATION ❄ ❄ ❄ ❄ ❄ ❄ ❄ ❄ ACCUMULATION ❄ ❄ ❄ ❄ ❄ ❄ ❄ ❄ ❄ Figure 1: Formation of an ice sheet. The snow falling in the accumulation region turns into ice that slowly flows towards the ablation area where it breaks up into ice-bergs or melts. As a consequence of the flow characteristics the thickness of annual layers decreases with increasing depth. Table 1. Climate parameters measured in ice cores. Parameter Proxy for CO2 CH4 SO4 Ash Be, Cl dO Borehole temperature Annual layer thickness Dust Anions / cations Greenhouse gases Greenhouse gases Volcanic eruptions Volcanic eruptions Solar activity Temperature Temperature Precipitation rate Wind speed Atmospheric circulation As an example, dO of the GRIP ice core is shown in Fig. 2. dO (relative deviation of the O/O ratio in ice from a standard in ‰) reflects mainly the temperature at which snow is formed.