Pre-industrial Metal Anomalies in Ice Cores: A Simplified Reassessment of Windborne Soil Dust Contribution and Volcanic Activity during the Last Glaciation

Abstract

A major issue regarding pre-industrial accumulation of heavy metals such as Cu, Pb, Zn, or Hg in ice cores relates to the source(s) of the metals. We investigate the two possibilities that have been generally advocated for the origin of metal anomalies: (1) increased amounts of windborne dust during particularly cold periods; and (2) volcanic activity. We analyze the time span 35,000–14,000 yrs BP, which is characterized by extreme temperature deviations (^T: –2.1º to –9.8ºC). Our findings show no definitive relationships between climate, dust, and metal contents, at least during the colder episodes. Thus, although dust deposition undoubtedly adds metals to the ice, it not clear whether climate can fully account for a direct relationship between these variables. In order to study the volcanic hypothesis, we analyze the case under a double perspective: (1) large explosive episodes related to acid magmatism; and (2) quieter mafic volcanic activity. Although the first one can introduce huge volumes of metal-rich aerosols to the stratosphere during single, catastrophic events, the latter is more continuous, accumulative, and far richer in sulfur. However, inasmuch as metals build volatile species with sulfur and halides, the chloride-rich character of the acid volcanism could easily compensate for its deficiency in sulfur. Thus, we suggest that the volcanic activity, via quiescent degassing and eruptive episodes, may account for an important part of the heavy metal contents present in the ice cores.