Christian Bégin

Dendrogeochemical distinction between geogenic and anthropogenic emissions of metals and gases near a copper smelter

For countries where metal resources represent an important economic sector, one of the main challenges of environmental research is to distinguish between natural and anthropogenic accumulations of potentially toxic metals in mining districts. The present work aims to evaluate a new environmental monitoring tool combining dendrochronology with natural (Ca, Ca/Mn, δ13C) and anthropogenic (Cd, Pb, 206Pb/207Pb, 208Pb/206Pb) geochemical tracers in tree rings in such a region. We compare spruce trees sampled at a control site near Hudson Bay, with those sampled near the Horne smelter active since 1928 in Rouyn-Noranda. The first effect of smelter emissions is detected by the tree-ring carbon isotope records. The δ13C values obtained on trees near the smelter show major changes immediately after 1928. This is due to the presence of atmospheric SO2 which generates a rapid response of the foliar system. The Ca/Mn ratios in tree-ring pairs of 1936–1937 and younger suggest a SO2-related soil acidification. The concentrations in Cd and Pb show a major increase starting in 1944 which coincides with a decrease of the 206Pb/207Pb ratios. The smelter activities likely generated this increase and the apparent delay of 14 years may have been generated mainly by the residence time of metals in airborne particulates, the buffering effect of the soils and, to a lesser extent, perhaps by mobility of heavy metals in tree stems. The 206Pb/207Pb and 208Pb/206Pb ratios indicate that the growth rings contain at least three types of Pb: natural, derived from the mineral soil horizons; industrial, from coal burning urban pollution; and mining, typical of the volcanogenic massive sulphides treated at the Horne smelter. This new combination of natural and anthropogenic tracers allows recognition of the succession of atmospheric and pedogeochemical changes related to industrial activities in the Rouyn-Noranda mining area.

Lead concentrations and isotope ratios in the exchangeable fraction: tracing soil contamination near a copper smelter

A total of 75 soil samples were collected from podzolic soils at eight sites in northwestern Quebec in order to compare results in contaminated locations near the Horne Copper smelter in Rouyn-Noranda with those near Hudson Bay, 800 km further north, and a priori devoid of pollution. Lead concentrations and Pb isotopic compositions were determined on soil samples leached by 0.25 M HCl. Lead is enriched in the surface organic horizons at all sites in Rouyn-Noranda. Its concentration decreases abruptly from the surface organic horizons to the underlying mineral horizons. The 206Pb/207Pb ratios are low in surface organic horizons, and they increase sharply in the mineral horizons. Along a NE–SW transect, the highest average Pb concentration (869 ppm) in organic horizons is found at the test site, 9 km from the smelter. The lowest average of 39 ppm Pb in surface organic layers is accompanied by the highest average 206Pb/207Pb ratio (1.15) at site 5, 116 km NE of the Horne smelter. At the test site, the highest Pb concentration of 1414 ppm is also accompanied by the lowest 206Pb/207Pb ratio (0.98), which is close to the isotopic composition of Noranda galena (206Pb/207Pb=0.92). Both Pb concentration and isotopic composition indicate that the Horne smelter is the main source of Pb contamination of soils in the Rouyn-Noranda region. The two soil profiles from near Hudson Bay display much lower Pb concentration and different trends of isotope ratios with depth.

Are industrial SO2 emissions reducing CO2 uptake by the boreal forest

Although the boreal forest has been proposed to accommodate increasing amounts of CO2, the large-scale response of photosynthesis to widespread noxious gases is largely unknown. Here we present carbon isotope ratios for ring series from trees subjected to different levels of SO2 emitted from a copper smelter. Our results indicate that noxious gases drastically lower the ability of trees to capture CO2 in the region of the smelter, and that this ability may also be reduced in an extensive region of the Canadian boreal forest undergoing effects from diffuse pollution. This raises concerns about the proposed increased capacity of boreal forests to sequester excess anthropogenic CO2.

Is wood pre-treatment essential for tree-ring nitrogen concentration and isotope analysis?

Tree-ring nitrogen concentrations and isotope ratios (δ(15)N) are gaining in popularity for environmental research although their use is still debated because of nitrogen mobility in tree stems. Modern studies generally present results on wood that is pre-treated to remove soluble nitrogen compounds and to minimize the impact of radial translocation on tree-ring nitrogen environmental records. However, the necessity to use such pre-treatment has never been fully assessed. Here we compare the nitrogen concentrations and δ(15)N values of two wood preparation protocols applied to beech and red spruce tree rings for the removal of soluble compounds from ring pairs with non pre-treated tree rings. For both tree species, pre-treatment did not minimize the radial patterns of tree-ring nitrogen concentrations and the increasing concentration trends that are coincident with the heartwood-sapwood boundary. Therefore, even if the tree-ring nitrogen concentrations are slightly modified by pre-treatment, these concentrations are considered to reflect internal stem processes rather than environmental conditions in both species. The δ(15)N values were similar for untreated and pre-treated ring pairs, suggesting that wood pre-treatment did not substantially change the δ(15)N values and temporal trends in ring series. In addition, tree-ring δ(15)N series of untreated and pre-treated wood did not show any sign of influence of the heartwood-sapwood boundary in either tree species, indicating that nitrogen translocation did not generate significant isotopic fractionation. We therefore suggest that untreated ring δ(15)N values of beech and red spruce trees can be used for environmental research.

Combining tree-ring metal concentrations and lead, carbon and oxygen isotopes to reconstruct peri-urban atmospheric pollution

ABSTRACT In this study, we analysed the tree-ring metal concentrations and isotope ratios of five stands located in three contrasted settings to infer the diffuse air pollution history of the northern part of the Windsor–Québec City Corridor in eastern Canada. Tree-ring series show that the Cd and Zn accumulation rates were higher between 1960 and 1986 and that the long-term acidification of the soil (Ca/Al series) was likely induced by NO x and SO x deposition (δ15N and δ13C trends as proxy). The Pb concentrations and 206Pb/207Pb ratios indicate that the dominant source of lead from 1880 to the 1920s was the combustion of north-eastern American coal, which was succeeded by the combustion of leaded gasoline from the 1920s to the end of the 1980s. Our modelling approach allows separating the climatic and anthropogenic effects on the tree-ring δ13C and δ18O responses. Diffuse air pollution caused an enrichment in 13C in all stands and a decrease of the δ18O values only in three of the stands. This study indicates that dendrogeochemistry can show contrasted responses to environmental changes and that the combination of several independent indicators constitutes a powerful tool to reconstruct the air pollution history in the complex context of peri-urban regions.

Chapter 9 Air Quality Changes in an Urban Region as Inferred from Tree-Ring Stable Isotopes

Abstract This chapter constitutes the first assessment of δ 18 O values of stem cellulose as an indicator of stress for trees exposed to pollution, and of δ 13 C combined with δ 18 O values to evaluate the relative impact of changes in climatic conditions and in air quality in a peri-urban region. The results obtained near Montreal (Canada) indicate that δ 13 C and δ 18 O characteristics in pine ( Pinus spp.) trees record changes in air quality, whereas δ 13 C patterns clearly change in beech ( Fagus spp.) trees under stressed conditions. Therefore, dendro-isotopic analyses offer a potential method for detecting past changes in air quality and evaluating forest responses to pollution.

Bayesian multiproxy temperature reconstruction with black spruce ring widths and stable isotopes from the northern Quebec taiga

Northeastern North America has very few millennium-long, high-resolution climate proxy records. However, very recently, a new tree-ring dataset suitable for temperature reconstructions over the last millennium was developed in the northern Quebec taiga. This dataset is composed of one δ18O and six ring width chronologies. Until now, these chronologies have only been used in independent temperature reconstructions (from δ18O or ring width) showing some differences. Here, we added to the dataset a δ13C chronology and developed a significantly improved millennium-long multiproxy reconstruction (997–2006 CE) accounting for uncertainties with a Bayesian approach that evaluates the likelihood of each proxy model. We also undertook a methodological sensitivity analysis to assess the different responses of each proxy to abrupt forcings such as strong volcanic eruptions. Ring width showed a larger response to single eruptions and a larger cumulative impact of multiple eruptions during active volcanic periods, δ18O showed intermediate responses, and δ13C was mostly insensitive to volcanic eruptions. We conclude that all reconstructions based on a single proxy can be misleading because of the possible reduced or amplified responses to specific forcing agents.