Ice core impurities of volcanic origin
When large volcanic eruptions occur, huge quantities of acids are introduced into the atmosphere and distributed over vast areas. In ice cores, volcanic layers can sometimes be identified by volcanic ash particles (tephra), but more often the layers only contain elevated concentrations of acids seen in the ice cores as mainly sulphate and sometimes fluoride. In Greenland the nearby Icelandic and Alaskan volcanoes have a strong imprint in the ice cores. However, if the volcanic eruptions are strong enough to inject acids into the stratosphere, also volcanoes from the tropics or even the southern hemisphere can be identified in Greenland. Examples of well-known historic volcanoes that have left a clear fingerprint in all Greenland ice cores are Tambora, Indonesia (1815), Krakatau, Indonesia (1883), Katmai, Alaska (1912), El Chichón, Mexico (1982), and Mount Pinatubo, Philippines (1991).
All volcanic eruptions have large impact locally in the region where they take place, but the largest volcanic events are also impacting global climate by injecting large amounts of small particles (so-called aerosols) into the stratosphere thereby partially shielding the Earth from solar radiation and reducing temperatures. For example, the 1991 Mount Pinatubo eruption led to a decrease in northern hemisphere average temperatures of 0.5-0.6°C. The impact of volcanoes on past climate is constrained from the long records of past volcanism reconstructed from Greenland and Antarctic ice cores.
Acid and tephras from volcanic eruptions are also used for dating purposes and for linking ice cores with other climate archives. Read more about this application of volcanic records in the ice core dating section.