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Radiocarbon Dating of Glacier Ice: Overview, Optimisation, Validation and Potential

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Radiocarbon Dating of Glacier Ice: Overview, Optimisation, Validation and Potential The Cryosphere, 10, 3091–3105, 2016 www.the-cryosphere.net/10/3091/2016/ doi:10.5194/tc-10-3091-2016 © Author(s) 2016. CC Attribution 3.0 License. Radiocarbon dating of glacier ice: overview, optimisation, validation and potential Chiara Uglietti1,2,3, Alexander Zapf1,2,3,†, Theo Manuel Jenk1,3, Michael Sigl1,3, Sönke Szidat2,3, Gary Salazar2,3, and Margit Schwikowski1,2,3 1Laboratory of Environmental Chemistry, Paul Scherrer Institute, 5232 Villigen PSI, Switzerland 2Department of Chemistry and Biochemistry, University of Bern, 3012 Bern, Switzerland 3Oeschger Centre for Climate Change Research, University of Bern, 3012 Bern, Switzerland †deceased Correspondence to: Theo Manuel Jenk ([email protected]) Received: 8 July 2016 – Published in The Cryosphere Discuss.: 22 July 2016 Revised: 26 October 2016 – Accepted: 14 November 2016 – Published: 21 December 2016 Abstract. High-altitude glaciers and ice caps from midlat- 1 order of magnitude lower. The accuracy of the WIOC 14C itudes and tropical regions contain valuable signals of past method was validated by applying it to independently dated climatic and environmental conditions as well as human ac- ice. With this method, the deepest parts of the ice cores from tivities, but for a meaningful interpretation this information Colle Gnifetti and the Mt Ortles glacier in the European Alps, needs to be placed in a precise chronological context. For Illimani glacier in the Bolivian Andes, Tsambagarav ice cap dating the upper part of ice cores from such sites, several rel- in the Mongolian Altai, and Belukha glacier in the Siberian atively precise methods exist, but they fail in the older and Altai have been dated. In all cases a strong annual layer thin- deeper parts, where plastic deformation of the ice results in ning towards the bedrock was observed and the oldest ages strong annual layer thinning and a non-linear age–depth re- obtained were in the range of 10 000 years. WIOC 14C dat- lationship. If sufficient organic matter such as plant, wood ing was not only crucial for interpretation of the embedded or insect fragments were found, radiocarbon (14C) analysis environmental and climatic histories, but additionally gave a would have thus been the only option for a direct and ab- better insight into glacier flow dynamics close to the bedrock solute dating of deeper ice core sections. However such frag- and past glacier coverage. For this the availability of multiple ments are rarely found and, even then, they would not be very dating points in the deepest parts was essential, which is the likely to occur at the desired depth and resolution. About strength of the presented WIOC 14C dating method, allowing 10 years ago, a new, complementary dating tool was there- determination of absolute ages from principally every piece fore introduced by our group. It is based on extracting the of ice. µg-amounts of the water-insoluble organic carbon (WIOC) fraction of carbonaceous aerosols embedded in the ice ma- trix for subsequent 14C dating. Since then this new approach has been improved considerably by reducing the measure- 1 Introduction ment time and improving the overall precision. Samples with High-altitude glaciers and ice caps from midlatitudes and ∼ 10 µg WIOC mass can now be dated with reasonable un- tropical regions contain valuable signals of past climate and certainty of around 10–20 % (variable depending on sample atmospheric variability at regional and local scales and are age). This requires about 300 to 800 g of ice for WIOC con- located in areas with large biological diversity and inhab- centrations typically found in midlatitude and low-latitude ited by the majority of the world’s population. Midlatitude glacier ice. Dating polar ice with satisfactory age precision glaciers, for instance in the European Alps or in the Hi- is still not possible since WIOC concentrations are around malayas, are influenced, in particular, by the nearby anthro- Published by Copernicus Publications on behalf of the European Geosciences Union. 3092 C. Uglietti et al.: Radiocarbon dating of glacier ice: overview, optimisation, validation and potential pogenic pollution sources, thereby additionally preserving according to their refractory and optical properties. Elemen- the signature of human activities. This information can gen- tal carbon (EC) consists of highly polymerised substances erally be retrieved from glacier ice cores, but needs to be which are extremely refractory and light absorbent; therefore placed in a precise chronological context to allow meaning- this fraction is also called black carbon (BC) or soot (Ge- ful interpretation with respect to environmental and climatic lencsér, 2004; Hallquist et al., 2009). EC is merely derived changes. from the incomplete combustion of fossil fuels and biomass. Ice core dating is a sophisticated task and the most com- Organic carbon (OC) is formed by weakly refractory hy- mon approach is annual layer counting, which relies on sea- drocarbons of low to medium molecular weight. Whereas sonally fluctuating signals. A number of ice core parame- EC is generally insoluble in water, OC is further subdi- ters such as the stable isotope ratio of hydrogen or oxygen vided into water-soluble organic carbon (WSOC) and water- in the water (δ2H, δ18O), the concentration of trace compo- insoluble organic carbon (WIOC) (Szidat et al., 2004a). In nents (e.g. ammonium, mineral-dust-related trace elements, water samples the former is also known as dissolved or- black carbon) and the presence of melt layers may vary with ganic carbon (DOC) (Legrand et al., 2013; May et al., 2013). the seasons. To reduce uncertainty in layer counting, the OC is emitted directly as primary aerosol from a vast vari- timescale is additionally anchored with reference horizons ety of sources and emission processes, including mobilisa- like the radioactivity peak resulting from nuclear weapon tion of plant debris, pollen, vegetation waxes, microorgan- tests in the 1960s or tephra and aerosol layers caused by vol- isms, spores, the organic fraction of soil as well as emissions canic eruptions (Thompson et al., 1998, 2013; Preunkert et from biomass burning (e.g. forest fires) and anthropogenic al., 2000; Schwikowski, 2004; Eichler et al., 2009; Moore et processes (biomass burning and fossil fuel combustion), but al., 2012). An independent method is nuclear dating with the it is also formed in the atmosphere by oxidation of gaseous naturally occurring radioisotope 210Pb. Determined by the precursors, with the product referred to as secondary organic 210Pb half-life of 22.3 years and its atmospheric concentra- aerosol (Gelencsér, 2004; Gelencsér et al., 2007; Hallquist et tion, the time period accessible for dating is in the order of al., 2009). a century (Gäggeler et al., 1983; Eichler et al., 2000; Her- Carbonaceous aerosols are transported in the atmosphere ren et al., 2013). All these dating techniques fail in the older to high-alpine glaciers, where they may be deposited by both and deeper parts of glaciers, where plastic deformation of wet and dry deposition processes and finally embedded in the ice under the weight of the overlying mass results in hor- glacier ice (Lavanchy et al., 1999; Jenk et al., 2006; Legrand izontal ice flow, stretching annual layers continuously with and Puxbaum, 2007; McConnell et al., 2007; Kaspari et increasing depth. Correspondingly, the age–depth relation- al., 2011). Consequently, using carbonaceous aerosols allows ship of high-alpine glaciers is strongly non-linear (Jenk et al., any piece of ice to be dated, given that it contains sufficient 2009) and annual layers and also volcanic signals become un- carbon mass. The WSOC fraction (i.e. DOC) would be ideal detectable below a certain depth with the current spatial res- for dating, since it has the highest concentrations in ice. How- olution of most analytical methods. Glacier flow modelling ever, its extraction is complicated. It involves the outgassing can only give rough age estimates with large uncertainties of aqueous atmospheric CO2, removal of dissolved carbon- close to the bedrock of high-alpine glaciers (Lüthi and Funk, ates, wet oxidation of the organic compounds to CO2 un- 2001). Radiocarbon (14C) analysis has been the only option der inert gas and, finally, quantitative trapping of the evolved allowing direct and absolute dating of these deeper ice core CO2 (May et al., 2013). Since major contributors of DOC, sections in the rare cases when sufficient organic matter such like light carboxylic acids, are ubiquitous in the air, all these as plant, wood or insect fragments were found (Thompson steps are prone to contamination. Therefore, and because of et al., 1998, 2002). However, in glacier ice such findings are the reasons summarised next, WIOC was selected from the not only very seldom, but even when they occur, they do not different carbonaceous particle fractions as the most promis- allow for continuous or at least regular dating, which per se ing target for 14C dating. First, WIOC is mainly of biogenic limits the application of the 14C technique and its use for origin in pre-industrial times (Jenk et al., 2006) and, there- deriving a complete chronology based on absolutely dated fore, supposed to contain a contemporary 14C signal repre- layers. In the following we refer to the dating of ice with sentative of the age of the ice (Jenk et al., 2006; Steier et macrofossils as conventional 14C dating. al., 2006). Second, the average WIOC concentration in ice A new, complementary dating tool was therefore intro- is higher than the respective EC concentration, allowing for duced by our group about 10 years ago, based on extracting smaller ice samples and a potentially higher time resolution, the µg-amounts of the water-insoluble organic carbon frac- which consequently provides a better signal-to-noise ratio tion of carbonaceous aerosols embedded in the ice matrix (mainly determined by the overall blank) and smaller uncer- for 14C dating (Jenk et al., 2006, 2007).
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