In the upper atmosphere several radioactive isotopes are produced when cosmic rays collide with atmospheric molecules at high speed. These isotopes are known as cosmogenic isotopes. The production rate of the cosmogenic isotopes depends on the strength of the cosmic radiation, which again varies with the strength of the Earth magnetic field and with the solar activity. Therefore, records of cosmogenic isotope production rates are invaluable for understanding the relation between past climate change, the Earth magnetic field, and variations in the solar activity. Currently, the exact influence of past and future variations in the solar activity on climate is much debated. The cosmogenic ice core profiles provide one of the key records to resolve this controversy.
The Earth magnetic field is shielding the Earth from charged cosmic particles such that a relatively strong magnetic field reduces the production of radiogenic isotopes. The solar wind is a stream of charged particles emitted from the Sun, which varies with the solar activity. The Earth reacts to the solar wind by increasing the strength of the shielding magnetic field. Therefore, higher solar activity results in stringer shielding and thus lower production of cosmogenic isotopes. The combined magnetic field from the Earth itself and the reaction to the solar wind constitutes the Earth magnetosphere, illustrated by an artists' view below in blue.
The abundance of cosmogenic isotopes in the ice cores therefore reflects past variations in both the strength of the Earth magnetic field and in the solar activity.
The most well-known of the cosmogenic isotopes is probably Carbon-14 (14C) which is widely applied for radiometric dating. However, the abundance of 14C in ice sheets is very low, and 14C-measurements can generally not be used for dating of ice cores. However, two other cosmogenic isotopes, namely Beryllium-10 (10Be) and Chlorine-36 (36Cl), are deposited in measurable quantities in the ice cores and records of isotopes are obtained.
Read more about how cosmogenic isotopes are used for dating.
Ice Core Drilling Projects