GISP2 major ion concentrations in parts per billion (ppb) by mass. Blanks in the H2O2 data indicate samples that were not analyzed. Blanks in other chemical species represent samples below detection limits. Firn samples were cleaned in the freezer by hand-scraping and placed in a pre-cleaned container. The pieces were kept frozen until analyzed. Samples below 60 meters were cleaned by rinsing with pure (milli-Q) water immediately prior to melting, also in a pre-cleaned container. The data has not been edited to remove possibly contaminated samples.