Carbonaceous Particles Stored in Ice: A Tool to Determine Their Sources and to Date Ice Cores from Mid- and Low-Latitude Glaciers by Ultra-Sensitive Radiocarbon AMS Measurements

Thursday, 8 September 2005

This presentation is part of: Poster Session II

Carbonaceous Particles Stored in Ice: A Tool to Determine Their Sources and to Date Ice Cores from Mid- and Low-Latitude Glaciers by Ultra-Sensitive Radiocarbon AMS Measurements

Theo M. Jenk¹, Heinz W. Gäggeler¹, Soenke Szidat¹, Margit Schwikowski², Lukas Wacker³, and Hans-Arno Synal². (1) Laboratory of Environmental Chemistry and Radiochemistry, University of Berne, Freiestrasse 3, Bern, Switzerland, (2) Paul Scherrer Institut (PSI), Villigen, Switzerland, (3) Institute for Particle Physics, ETH Zürich, ETH Hönggerberg, Zürich, Switzerland

Carbonaceous particles from combustion processes influence the climate due to a direct effect of changing the radiation balance by absorbing and scattering light and an indirect effect by acting as cloud condensation nuclei. Because of their mean atmospheric residence time of about one week only, they are strongly correlated to their sources. In mid-latitudes we are close to the main sources of fossil fuel combusting, whereas low-latitudes fall in the region of mainly biomass burning sources e.g. from tropical forest or savanna fires. This work is based on a method to separate the organic carbon fraction (OC) and the elemental carbon fraction (EC) from carbonaceous particles stored in ice by thermal combustion, combined with a method to perform ¹⁴C AMS measurements of these fractions on the level of a few micrograms of carbon. To be applied for ice samples, both methods had to be slightly modified with respect to sample decontamination, sample handling and the target preparation for AMS. The advantage of combining these two methods lies in the possibility of a more sophisticated reconstruction of biogenic and anthropogenic emissions than by a simple OC / EC determination only. If we go back in time, the human impact of burning fossil fuels is strongly reduced, which also can be shown in ice cores. Thus, in old ice from cores retrieved from mid- and low-latitude glaciers the OC fraction tends to reach approximately 100% biogenic origin and therefore may serve as tool for radiocarbon dating.

Usually, the bottom part of mid- and low-latitude glaciers bears large dating uncertainties of ± 1000 yrs or even more. This is a result of strong layer thinning (leading to an absence of visible annual layers in the chemistry and stable isotope (δ¹⁸O and δD) record) and the geometry of the bedrock which makes models fail in these depths. A more accurate dating technique such as the one described is therefore strongly needed. Method and first results are presented here.

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