Trace elements in South America aerosol during 20th century inferred from a Nevado Illimani ice core, Eastern Bolivian Andes (6350 m asl)

[摘要] A 137 m ice core drilled in 1999 from Eastern Bolivian Andes at the summit ofNevado Illimani (16º 37' S, 67º 46' W, 6350 m asl) was analyzed at high temporal resolution,allowing a characterization of trace elements in Andean aerosol trapped in the ice duringthe 20th century. The upper 50 m of the ice core were dated by multi-proxy analysis of stableisotopes (d¹⁸O andd²H),¹³⁷Cs and Ca⁺² content, electrical conductivity, and insolublemicroparticle content, together with reference historical horizons from atmosphericnuclear tests and known volcanic eruptions. This 50 m section corresponds to a record ofenvironmental variations spanning about 80 years from 1919 to 1999. It was cut in 744sub-samples under laminar flow in a clean bench, which were analyzed by Ion Chromatographyfor major ionic concentration, by a particle counter for insoluble aerosol content, and byInductively Coupled Plasma Mass Spectrometry (ICP-MS) for the concentration of 45chemical species from Li to U. This paper focuses on results of trace element concentrationsmeasured by ICP-MS. The high temporal resolution used in the analyses allowed classifyingsamples as belonging to dry or wet seasons. During wet season elemental concentrations arelow and samples show high crustal enrichment factors. During dry seasons the situation isopposite, with high elemental concentrations and low crustal enrichments. For example, withsalt lakes as main sources in the region, average Li concentration during the 20th century is0.035 and 0.90 ng g^-1 for wet and dry seasons, respectively. Illimani average seasonalconcentration ranges cover the spectrum of elemental concentration measurements at anotherAndean ice core site (Sajama) for most soil-related elements. Regional crustal dust load in thedeposits was found to be overwhelming during dry season, obfuscating the contribution ofbiomass burning material. Marked temporal trends from the onset of 20th century to morerecent years were identified for the concentrations of several trace species of anthropic origin,especially for Cu, As, Zn, Cd, Co, Ni and Cr. Among these elements, Cu shows average wetseason crustal enrichment factors above 10³, while the others range between10² to about 5x10². P and K show moderate average wet season enrichment factors, suggesting an impact ofnatural biogenic emissions from the Amazon Basin. Pb has multiple anthropic sources in theregion, from mining activities in the beginning of 20th century to automotive fuel after 1950s.From the large number of samples analyzed from Illimani, it was possible to derive aneffective chemical characterization of the deposited background Andean soil dust aerosolduring 20th century.

[学科分类] 大气科学