Ice core impurities - sea ice and burning trees
Many impurities are measured at high resolution in the ice cores. The sources of some of these, like dust and calcium or volcanic sulphate, are well-understood, while other impurity records are harder to interpret. For example, concentrations of ions of Sodium (Na+), Chloride (Cl-), and Ammonium (NH4+) have been measured in centimeter resolution.
Both Na+ and Cl- mainly derive from sea salt, but Na+ also has other sources such as mineral dust. The concentrations of Na+ and Cl- vary strongly across the abrupt climatic shifts in the glacial period (the so-called D-O events) with approximately five times higher concentrations during the colder periods. Na+ and Cl- is released from freezing sea water when sea ice forms, and the concentration variations are thought to be a result of changes in sea ice extent in the North Atlantic combined with a change in the atmospheric circulation pattern, but the interpretation is not straightforward.
The ice core content of NH4+ is interpreted as an indicator of the emissions from soil and vegetation, and from biomass burning, for example from forest fires on the North American continent. In the Holocene, NH4+ shows a clear annual summer peak, whereas concentrations are generally vanishing during winter.
Ice Core Drilling Projects