An Extraction System for Radiocarbon Microanalysis of Dissolved Organic Carbon in Glacier Ice

Abstract

Alpine glaciers situated in mid- and low latitudes are valuable archives for paleoclimatology. They offer a continuous record of recent local climatic conditions in regions where the majority of humankind lived and still lives. For meaningful interpretation of an ice core from such an archive, accurate dating is essential. Usually, several complementary approaches are used to establish a depth-age relationship. The oldest part of the ice at the bottom of the ice core suffers annual layer thinning and is influenced by small-scale bedrock geometry, which limits the use of annual layer counting or the assignment of reference horizons for dating. Nuclear dating techniques overcome this restriction since they do not rely on the preservation of a resolvable stratigraphy by using the continuous record of the respective radioisotope. Radiocarbon is especially powerful for dating alpine glaciers because its half-life of 5730 years suitably allows it to cover the typical age range of these archives. Most important, glacier ice does contain minute amounts of carbon. While macrofossils can only be found by coincidence, organic aerosols deposited on the glacier offer the best source of contemporary carbon in glacier ice. Despite a large part of its chemical composition being unknown, organic carbon found in an ice sample can be operationally classified into a particulate fraction (POC) and a dissolved fraction (DOC). Radiocarbon dating of POC has proven to be very successful and is a routine application by now. The major limitation of this technique is the low POC concentration found especially in pre-industrial and polar ice samples. Therefore, the DOC fraction promises even better suitability for dating thanks to its by a factor of 5 to 10 higher concentrations. Nevertheless, a straightforward analysis of DOC is hampered by its vulnerability to contamination. DOC consists in large part of mono- and dicarboxylic acids - compounds that can easily be taken up from the surrounding gas phase during sample preparation or which are dissolved from surfaces in contact with the liquid sample. Hence it is vital to ensure ultra-clean sample preparation with a low and stable procedural blank for reliable radiocarbon analysis of DOC from glacier ice. In this work, we developed an extraction system for DOC from glacier ice samples. To meet the requirements of ultra-clean and effcient carbon extraction, the complete sample treatment is performed in inert gas conditions and only dedicated materials are chosen for the individual components of the setup. Ice samples are pre-cleaned and melted in a melting vessel. POC is separated from the liquid sample by filtration during the transfer to the photo-reactor. The sample is acidified and inorganic carbon is degassed from the solution. A minimal invasive photo-oxidation method is applied by means of external UV irradiation of the sample. This converts the DOC to CO2, which is degassed, cleaned and captured in cryogenic traps. The CO2 is quantified to determine the initial DOC concentration and is sampled to glass vials. With state-of-the-art accelerator mass spectrometry, the gaseous CO2 sample is directly analysed for its radiocarbon content to yield the age of the ice sample. Following a detailed description of the extraction system hardware and its operation protocol, we show the results of its extensive characterisation. The setup can process ice samples of up to 400 g mass. Within 45 min of irradiation time, oxalic acid was oxidised and recovered as CO2 with an efficiency of (85 ± 7)%. Most important, thanks to the stringent working conditions we achieved a low overall procedural blank of mblank = (3.5±0.6) µg C with F14Cblank = 0.65±0.04. This allows for the reliable measurement of ice samples with carbon concentrations as low as 33 µg C/kg ice, if we require the minimal sample mass to be larger than three times the blank mass. Thus by now, the method provides the anticipated effciency and accuracy to analyse DO14C of ice samples from alpine glaciers. As a side product of the method, also POC is extracted. We found that the procedural blank for this method is similar to the standard method for PO14C analysis. Therefore, this setup can be used to analyse both organic carbon fractions from only one ice sample. We validated this new method with well-dated ice samples from Juvfonne ice patch in Norway. Six samples from three different ice blocks were analysed for DO14C and PO14C. Within the uncertainties and the sample-to-sample variability most F14C results from both organic carbon fractions agree with each other and with the reference samples from the same ice blocks. In contrast to previous studies that proposed a possible in-situ DO14C production in glacier ice, we did not find such a bias. Thus, we conclude that radiocarbon microanalysis with DOC from glacier ice is both technically feasible and physically meaningful and can now contribute to future cryospheric science.