Although numerous cores covering a few centuries have been drilled, with many different analyses made, I will discuss just one other example of recent change here. Lead (Pb) occurs naturally in polar snow and ice only at the ng/kg level. However, it was shown that, in Greenland ice, the concentration had risen during the industrial era by about a factor 200, firstly due to metal smelting, and then due to the use of lead additives in petrol. Furthermore, as unleaded petrol was introduced in North America and then elsewhere, the concentration again decreased by an order of magnitude during the 1970s and 1980s (Boutron et al., 1991). My own group showed that even Antarctica was not immune, with concentrations (although always tiny compared to any water sample from Europe) increasing by a factor 5 or more, and then starting to decrease again. This story shows how the results of emissions in industrial areas reach the whole globe, but also how measures to clean such emissions really do have an effect.
Numerous other chemical measurements give us access to knowledge of other aspects of the environment over long time periods. Sea salt in ice cores arises from aerosol produced at the sea ice surface as well as from open water. In the interpretation I favour, the record of sodium ion concentrations tells us about past sea ice extent, a key variable for understanding climate. Not surprisingly, we therefore find higher concentrations in cold periods, and the Dome C sodium record (Wolff et al., 2006) can tell us about the relative timing of changes in sea ice, climate and CO2.
Sulfate concentrations, after correction for the part coming directly from sea salt, and bearing in mind a sporadic volcanic contribution, arise from the oxidation of dimethyl sulfide (DMS), which is produced by some marine organisms. Surprisingly, the flux of sulfate to the ice sheet varies only within narrow bounds over glacial-interglacial cycles, contradicting earlier work that suggested major changes in marine productivity in the relevant section of the Southern Ocean. If such changes occurred, they must have occurred further north than the section of ocean that Dome C effectively samples.