Peat bogs in northern Alberta, Canada reveal decades of declining atmospheric Pb contamination

Abstract

Peat cores were collected from six bogs in northern Alberta to reconstruct changes in the atmospheric deposition of Pb, a valuable tracer of human activities. In each profile, the maximum Pb enrichment is found well below the surface. Radiometric age dating using three independent approaches (14C measurements of plant macrofossils combined with the atmospheric bomb pulse curve, plus 210Pb confirmed using the fallout radionuclides 137Cs and 241Am) showed that Pb contamination has been in decline for decades. Today, the surface layers of these bogs are comparable in composition to the “cleanest” peat samples ever found in the Northern Hemisphere, from a Swiss bog ~ 6000 to 9000 years old. The lack of contemporary Pb contamination in the Alberta bogs is testimony to successful international efforts of the past decades to reduce anthropogenic emissions of this potentially toxic metal to the atmosphere.

Reconstructing Past Rates of Atmospheric Dust Deposition in the Athabasca Bituminous Sands Region Using Peat Cores from Bogs

“…To describe the changing dust fluxes, mass accumulation rates (MAR) were calculated for ash content, AIA and ASA in six of the seven peat cores (Figure ), as the product of the peat accumulation rates and the relevant concentrations. The average rate of organic matter accumulation corresponding to the living layer of the six bogs is 1,084 g m −2 y −1 , and this is based upon the six primary peat cores (MIL‐W1, JPH4‐W1, McK‐W2, McM‐W3, ANZ‐W3, and UTK‐W2) and their six duplicate cores (see Shotyk et al ., , for discussion). The age‐depth model for SEB is incomplete, and therefore, the MAR could not be calculated for this peat core.…”

“…Thorium accumulation rates (μg m −2 y −1 ) 1900 to time of collection (a) and Th enrichment factor profiles (b) for MIL‐W1, JPH4‐W1, McK‐W2, McM‐W3, ANZ‐W3, and UTK‐W2. To calculate the Th accumulation rates of the living layer, the average accumulation rate of 1,084 g m −2 y −1 was used, as described in detail elsewhere (Shotyk et al ., ).…”

“…A XenParTec (XPT-C) Optical Particle Analysis System was used to determine particle size distribution profiles for selected AIA samples (Methods S6). The peat cores were dated using a combination of 14 C and 210 Pb as described briefly here (Methods S7) and in more detail elsewhere (Shotyk et al, 2016).…”

“…The studies undertaken in northern Alberta to address contemporary dust deposition, with a focus on the ABS region, include snowpack sampling (Shelfentook, ; Barrie & Kovalick, ), characterization of PM 2·5 and PM 10 (Wang et al ., , ), source assessments (Gosselin et al ., ; Watson et al ., ), biomonitoring using lichens (Addison & Puckett, ; Edgerton et al ., ; Graney et al ., ) and Sphagnum moss from bogs (Mullan‐Boudreau et al ., ), and air quality modeling (Davies, ). Retrospective analyses of atmospheric deposition of dust and trace metals are in their infancy (Shotyk et al ., , ). The Wood Buffalo Environmental Association,, now part of Alberta Environment and Parks, has its own monitoring program that tracks atmospheric emissions in the ABS region but presently measures particulate deposition exclusively in the PM 2·5 and PM 10 size ranges.…”

“…Accumulation rates are very useful tools to quantify changes in dust deposition; however, these results are dependent on the age-dating models they are derived from. The age-depth models used in this study are extremely robust, using Bayesian statistical modeling of a combination of 14 C and 210 Pb dates (Bronk Ramsey, 2008;Shotyk et al, 2016). Precision, however, varies with age; at 1900, the confidence intervals range from ±14 to 46 years between cores, whereas the modern layers range from 1 to 2 years.…”