Dating ice cores from cold glaciers via radiocarbon is still a challenge, calling for systematic investigations. This work describes an approach towards extraction of the particulate organic carbon (POC) fraction from ice samples for AMS radiocarbon dating at VERA (Vienna Environmental Research Accelerator). We have developed a filtration procedure which extracts the particulate but saves the melt water for further measurements. Systematic measurements have shown that the filtration introduces a carbon contamination of 6.8 ± 1.2 μg carbon, which originates mainly from the quartz filters and the rinsing liquids used. Graphitization and AMS measurement of such small blank samples is possible, and the additional contribution to the background is insignificant (~0.5 μg C).
First measurements on realistic environmental ice samples were performed on 1 snow sample from the top of the Grenzgletscher (Colle Gnifetti, 4450 m asl), and 11 ice samples from the ablation zone (~2500 m asl) of this glacier in the Monte Rosa massif (Swiss Alps). The sample masses used were between 0.3 kg and 1.4 kg ice yielding between 20 and 310 μg carbon as POC.
Whereas the snow sample yielded a 14C content of 119.6 ± 1.4 pMC, an average 14C content of 73.9 ± 5.6 pMC is obtained for the set of 11 ice samples. The latter corresponds to an calibrated age range of 2100 BC to 900 AD (95.4 % confidence level). Scatter and outliers in the radiocarbon data suggest that presently a single 14C date of glacial POC has limited significance, but that an extended set of measurements is required. Possible reasons for these outliers will be discussed.
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