Human activities are impacting our environment and, in particular, the composition of the atmosphere, water, and plants. However, it is sometimes difficult to distinguish the "natural" signature from the one induced by industrial and urban activities. The latest analytical developments allow the fine and precise measurement of isotopic signatures of elements whose content or isotopic composition may be changed by modern industrial activities, such as the transition metals (Ni, Hg, Se, and Zn), lithium, and helium. This approach opens up original and new avenues of inquiry, including the determination of the sources of aerosols, of the elements carried by rivers, the effects of bioaccumulation, the phytoavailability of metals, and allows the refinement of the determination of anthropogenic fluxes.
In this context, a new tracer is being developed at CRPG : isotopic variations (over time and space) of atmospheric helium. This inert gas is not retained in the atmosphere due to its low mass. The isotopic composition of helium in the atmospheric reservoir results from a balance between its losses and sources. At the human scale, the losses (escaping into space) are considered negligible, but a significant flux of 4He is generated by the degassing of the continental crust due to the use of fossil carbon (coal and oil). Conversely, it is also possible that the nuclear industry has caused large fluxes of 3He. Thus, the isotopic composition of atmospheric helium may have varied from a few parts per thousand at the start of the industrial era, with significant spatial disparities. The idea is to develop a tracer of anthropogenic sources that is independent of chemical species, for which there may be significant uncertainties. It is possible that the isotopic composition of helium in air shows geographical variation (sources of radiogenic 4He in temperate and tropical regions, with a leak of He at the poles). To this end, our current projects (ANR VIHA II and ERC NOGAT) will document the helium in air sampled from the equator to both poles at a spacing of latitude of approximately 500 km (this profile has already been created for the southern hemisphere and is in progress for the northern hemisphere). We will also characterise the temporal variations by the analysis of air collected over a decade. Finally, we will study the oldest preserved air, which is trapped in metal containers (e.g., bocce balls). The expected changes in the isotopic ratio of atmospheric helium are small and are not detected by current spectrometer standards. We have developed, over the past two years, a new analytical line to quickly switch between standard and sample (stable isotope technique). Also, a new dedicated analyser, a mass spectrometer in dual collection (Helix SFT Thermo MC), will be delivered in October 2011.
Jenifer Mabry and the new SFT mass sectrometer for helium isotopic composition of air and map of sampling locations.
Lichens have also shown a strong potential to quantify the anthropogenic sources of aerosols, the recent evolution of the composition of the atmosphere, and the impact of human activities. Thus, for 10 years, we have been using lichens in the Messina region to document temporal trends of atmospheric aerosols. We now want to conduct laboratory experiments to determine the reaction kinetics of metals during their bio-accumulation. It is necessary to "soak" the lichens for a given time in a solution enriched with metals of known isotopic composition. The different compartments of the lichen are then separated and analysed, as are the residual solutions. Preliminary results suggest very fast kinetic adsorption for Pb, Hg, Se and Zn in the lichen species Evernia, indicating a rapid diffusion of ions from the solution to the lichens. Subsequently, we intend to extend these experiments to other types of plants, such as cereal plants and hyper-accumulator species.
Regarding the hydrosphere (rivers and groundwater), an important task is to better quantify the human impact on the cycles of metals (Zn, Pb) and on elements such as lithium. In this context, it is necessary to clarify the isotopic signatures that are specific to industrial, urban, and agricultural activities. We propose to achieve this clarification through the study of streams or rivers flowing through urban areas (the Seine in Paris), agricultural areas (the Madon), and highly industrialised areas (the Fensch). These studies will be undertaken in collaboration with laboratories or work groups who have been studying these polluted areas for many years (LEM, IPGP, ZAM, and GISFI).