Reconstruction of pollution sources over the last century in Europe : Isotopic analysis of Sulphur, Nitrogen and Oxygen contained in the ICE MEMORY ice core from the Col du Dôme (Mt Blanc).

Alexis Lamothe (2020-2023)

Director : Joël Savarino
Funding : ministerial grant

Summary :
The 20th century will remain as the anthropocene century. This period, characterized by the upheaval introduced by the massive and large-scale use of fossil fuels, leaves and will leave deep environmental scars from which we are not sure that humanity as we know it will recover. The environmental and ecological emergency is there, in front of our eyes. What actions must we take to reduce our impacts ? What effects will these actions have ? On what timescale ? To answer these fundamental questions, which require decisions with far-reaching environmental, social and economic consequences, it is necessary and imperative to have the best and most complete information possible so that decisions can be taken with the highest level of certainty. While modelling based on a set of deterministic, statistical and random processes that we know (or assume to know) makes it possible to draw up probable future trajectories, the study of past archives revealing the responses of the Earth system to internal or external solicitations is still the best way to apprehend the consequences of our actions. The present project aims at extracting from a glaciological archive the upheavals that the industrial revolution produced on the chemical composition of aerosols on the scale of the Alpine region and the European continent. Using an ice core taken in 2016 from the Col du Dôme glacier, Mont Blanc (4250 m, mean annual temperature -12°C, (Preunkert et al., 2000) within the framework of the IceMemory project, the temporal dynamics of changes in the environment near the site will be established. Using tracers from natural and anthropogenic cycles (sulphate, nitrate, ammonium aerosols), we will reveal for the first time the S, N, O isotopic compositions of the different sources of emissions linked to these biogeochemical tracers, from the beginning of the 20th century to the present day. This work will make it possible to establish a source-isotope composition relationship, which is the basis for the use of isotope tracers. While this type of work has already begun on Greenland ice (Hastings et al., 2009 ; Felix and Elliott, 2013), the remoteness of the site from pollution sources significantly complicates the attribution of natural and anthropogenic sources to the aerosol budget due to the complex interactions between transport from different source zones, gas-aerosol conversions and wet and dry deposition processes. At the other end of the spectrum, inventories focusing on source-to-source isotopic compositions (Walters et al., 2015 ; Felix et al., 2012 ; Smirnoff et al., 2012 ; Felix et al, 2013) present heterogeneities and variabilities (e.g. for vehicles : type of engine, rpm, type of fuel, age of the vehicle, etc.) that are difficult to take into account in impact studies (Widory, 2007 ; Nanus et al., 2018 ; Chang et al., 2016). The approach here is to be midway between these two extremes, i.e. close enough to the sources to characterize them well and far enough away to smooth out local heterogeneities. Moreover, the proximity of the study site to the major European economic centres will provide answers directly related to our uses and practices. This spatial level, from regional to continental, is the relevant link for the implementation of effective environmental policies, as close as possible to societal concerns. This project is part of the international IceMemory programme, a project carried by the UGA Foundation as a founding member and sponsored by UNESCO. IceMemory is based on the observation that past information preserved in high mountain glaciers is an invaluable resource for deciphering the functioning of our environment, and is the only paleo-environmental archive with a direct link to the atmosphere. The atmosphere is the arm of environmental change. It is in this compartment that most of the environmental changes on a decadal scale (temperature, precipitation, air quality, luminosity, energy redistribution) pass through. However, these precious archives are in danger, not from a sudden disappearance induced by global warming, but from repeated and prolonged summer melting leading to percolation of melt water, mixing of snow layers and the definitive loss of the annual resolution of the snow layers and the physico-chemical information they contain.

Keywords : anthropocene, biogeochemical cycles, mass spectrometry, Alps, continuous flow analysis