Radiocarbon microanalysis on ice impurities for dating of Alpine glaciers

Abstract

The main objective of this thesis is the deployment of radiometric dating on Alpine ice archives, where conventional, stratigraphical dating techniques fail. To this end a 14C analysis of particulate and, in a novel attempt, also of dissolved organic carbon contained in the ice matrix is developed. Aimed at minimizing the required ice sample size, the designed ice processing line allows for handling of up to 700 g ice and is shown to involve blank levels small enough to ensure reliable 14C analyses down to a modern carbon fraction of fm = 0.33 at a minimum sample mass of about 20 µg carbon. Deployment of the system at the high altitude drilling area Colle Gnifetti (Monte Rosa, 4 550 m.a.s.l.) and, in contrast, to an ice core at the low altitude Vadret dal Corvatsch ice cap (Upper Engadin, 3 433 m.a.s.l.) reveals a quite significant 14C excess, not explained by any blank contribution and in contrast to the expected age range. The hypothesis that in situ produced 14C might contribute to the 14C content of dissolved organic carbon reveals strong short comings in applying this carbon fraction for radiometric dating of glacier ice, but also the new possibility to infer past accumulation changes. The particulate fraction, mostly unaffected by excess 14C, allows to connect an upper age limit of (7.5 ± 0.2) ka with the base of the Vadret dal Corvatsch, suggesting a minimal glacier extent during that time. For the Colle Gnifetti drill sites a tentative upper age constrain indicates pre-Holocene ice remains in the basal layer, while an enhanced 14C profile in the dissolved organic carbon fraction at core bottom suggests a large in situ produced 14C contribution most likely as a result of low accumulation during much colder periods.