Tracing volcanic eruptions and anthropogenic pollution in tree rings: preliminary results from synchrotron-light chemical analyses

We analysed larch trees from high elevation in northern Italy for Hg with a Lumex RA-915M Mercury Analyzer with pyrolyzer PYRO-915+) and for S, Zn, Cu, Ni, Pb (by synchrotron light X-ray fluorescence at beamline i18 Diamond Light Source, UK). Hg analyses were performed on 11 single rings per sample centred on the year of a known volcanic eruption. The in-situ synchrotron analyses were carried out on longer timeseries of up to 150 years with a maximum replication of 5 samples. We detected significant peaks of volatile elements in tree rings matching periods of repeated volcanic eruptions with high volcanic explosivity indices (VEI 5-6, out of a maximum of 8). A good example is shown by increased Zr, Cu, S, Pb occurring during the decade 1875-1886 corresponding to eruptions of the volcanoes Askja (Iceland, year 1875, VEI 5), Vesuvius (Italy, 1875), Cotopaxi (Ecuador, 1877, VEI 4), Krakatoa (Indonesia, 1883 and 1884, VEI 6) and Okataina (New Zeland, 1886, VEI 5). Furthermore, during the last 3 decades of the XX century, we observed a monotonic long-term increase of the elements Pb, Ni, Zn and Cu in association with a significant reduction of the tree-ring widths. We consider the chemical shifts and the abrupt growth reduction as reflecting anthropogenic pollution mainly related to a sustained and increasing use of fossil fuel, in particular leaded gasoline. Notably, the chemical contamination in the analysed wood decreases significantly in the first decade of the XXI century, when unleaded gasoline was substituted the Pb-rich fuels. Both these approaches seems promising in detecting hidden effects of volcanic or pollution activities when other tree-ring traits do not show any other peculiar feauture.