Comment on “Next-Generation Ice Core Technology Reveals True Minimum Natural Levels of Lead (Pb) in the Atmosphere: Insights From the Black Death” by More et al

Abstract

More et al. (2017, https://doi.org/10.1002/2017GH000064) state that lead (Pb) concentrations in ice core from the high-altitude site Colle Gnifetti in Switzerland, reflecting atmospheric deposition over the past two millennia, have been consistently high because of human technological activity. They posit that abrupt departures toward smaller concentrations are caused by economic disruptions resulting from bubonic plague in the fourteenth and fifteenth centuries (plus a brief late nineteenth century economic depression), and by a decline in atmospheric injection of industrial Pb in the most recent few decades. Concentrations of Pb reported over those short anomalous intervals are very small, at least as small as in most pre-industrial Antarctic ice. More et al. believe that these intervals reflect natural, pre-anthropogenic levels. Their interpretation conflicts with a body of scientific information about the geochemical behavior and sources of Pb and how Pb is deposited by the atmosphere. To evaluate the validity of the interpretation of More et al. it is important to know (a) whether Pb in the ice is accompanied by other trace metals known to be present in natural sources; and (b) whether the isotopic composition of Pb in the ice is consistent with mined Pb or Pb from other, natural sources.