Sigma infinity has a pure ice background of 9 microsiemens/m, and responds to the chemistry of the ice. The only components known to cause an increased sigma infinity are acidity, ammonium and chloride (Wolff et al., 1995; Moore, et al., 1994). A calibration is given in Wolff et al., (1995). During warm periods, the DEP is mainly a record of acidity, while during cold (glacial) periods it is responding mainly to ammonium and/or chloride.