Geochemical perspectives from the past: understanding the natural enrichment of Cd in pre-industrial and pre-anthropogenic aerosols using polar ice and peat cores from remote locations

Cadmium (Cd) is a potentially toxic chalcophile element with profound health consequences when organisms are exposed to elevated concentrations. Cadmium is emitted to the atmosphere through various industrial processes (metallurgical smelting, coal combustion), but it is also derived from natural sources (volcanic emissions, wind-borne soil particles). In contemporary air samples collected within urban areas across the globe, Cd enrichment factors (EF) relative to the Upper Continental Crust (UCC) are often up to 100, and are typically interpreted as reflecting inputs exclusively from anthropogenic activities.

This presentation reports on the range of Cd EF values in aerosol archives from various locations representing up to 15,500 years of atmospheric Cd deposition, including: (i) Polar ice collected at Devon Island (Nunavut, Canada), and; (ii) Peat from ombrotrophic bogs collected at Etang de la Gruère (Jura Mountains, Switzerland), Birch Mountain (BMW) and Caribou Mountain Wildlands (CMW) (Alberta, Canada), and Drizzle Bog (DB) (British Columbia, Canada). The ice samples were melted and acidified, and the peat samples digested, with double sub-boiled concentrated nitric acid, analyzed using SF-ICP-MS and ICP-QMS, respectively. All analyses were undertaken in metal-free, ultraclean laboratories. Age-dating of ice was obtained using electrical conductivity and oxygen isotopes, and the peat samples with conventional ¹⁴C AMS and ²¹⁰Pb. Cadmium concentrations were normalized to the conservative lithophile reference elements (Sc, Ti, and Th) to calculate the EF.

Regardless of the reference element and the compilation selected for the UCC, the EF in old and ancient samples was often very high: up to 4000 in ice and up to 400 in peat bogs. These profound enrichments of Cd and dramatic variation pre-dating anthropogenic activities suggest that either natural emissions of Cd were not constant, or that non-crustal sources of Cd have been underestimated. 

In the ice cores, the period with the lowest and most constant Cd concentrations was found between 2,500 and 4,500 years BP (0.92 ± 0.13 pg g^-1), with an average EF_Sc of 107. Samples from the early and middle Holocene (4,500 to 12,000 years BP) contained higher concentrations of Cd (2.43±0.50 pg g^-1), with an average EF_Sc of 255. In the Swiss bog, the Cd EF_Th ranged from 9 to 800, and was more stable in the period between 9,000 to 11,000 years BP (14±5). In the pre-industrial layers of peat bogs from remote locations in Canada, the EF_Th values averaged 30 (DB), 54 (BMW) and 167 (CMW).

These variations in Cd enrichment levels likely reflect differences in contributions from natural sources: volcanic activity, deposition of fine airborne soil particles, and natural forest fires. The biological uptake and recycling of Cd observed in some terrestrial plants and corresponding enrichment in humus may also play a role, but these potential contributions have received limited attention. The very high EF found in contemporary aerosols, recent peat layers and modern snow samples should be interpreted with caution pending improved understanding of the sources of natural aerosol Cd enrichments, and associated processes.