AMS carbon-14 dating of ice: progress and future prospects

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.

Radiometric dating of glacier ice is an essential tool where stratigraphic dating methods cannot be applied. This study focuses on Alpine glacier ice and presents a new sample preparation system for dating of glacier ice samples via radiocarbon (14C) dating of the microscopic particulate organic carbon (POC) fraction incorporated in the ice matrix. An adaptable, low-cost inline filtration-oxidation-unit (REFILOX) has been developed, which for the first time unifies all sample preparation steps from ice filtration to CO2quantification in one closed setup. A systematic14C investigation of modern European aerosol samples revealed that a POC combustion temperature of 340°C provides the best representation of the real sample age. A very low process blank of maximally 0.3±0.1 µgC now enables14C dating of high Alpine ice samples, where POC concentrations are generally low (typically 10–50 µgC/kg), in an ice sample mass range of 300–500 g. In a first successful application, the method was used to obtain age constraints for an ice core from the cold, high Alpine firn saddle Colle Gnifetti (Switzerland). Analysis of the bottom ice core sections revealed a basal age of 4171–3923 cal yr BP but also a so far enigmatic discontinuity in the age-depth relationship.

[1] Past variations in the concentration of the greenhouse gas CO2 are thought to have played a major role in controlling Earth's climate on pre-Quaternary and Quaternary timescales. To identify the contribution of CO2 to past climatic change requires accurate quantification of its content in the ancient atmosphere, and a number of proxies have been developed for this purpose (for a review see Royer et al. [2001a]). For the Late Quaternary, there is the unique opportunity to measure directly the composition of fossil air samples trapped in bubbles preserved in the polar ice sheets. Results from Antarctic ice cores reveal that the glacial-interglacial changes characterizing Quaternary climate were accompanied by variations in the atmospheric concentration of CO2 [Petit et al., 1999]. Although detection of phase relations between isotope-derived temperature estimates and trace gas concentration values is hampered by the difference in age of ice and air from the same ice sample, it is believed that CO2 lags glacial-interglacial temperature change and has acted as an amplifier of orbitally forced changes in temperature [Petit et al., 1999; Shackleton, 2000; Mudelsee, 2001]. [2] Detailed ice core data from the last glacial period indicate temperature-CO2 covariation on shorter, millennial, timescales [Stauffer et al., 1998]. However, such detailed studies are rarely feasible because the CO2 concentrations retrieved from the ice cores are inherently smoothed by diffusion during air enclosure, an effect controlled by ice accumulation rate. It is therefore not a surprise that ice cores drilled at low accumulation sites show little evidence of abrupt CO2 change during the last deglaciation [Monnin et al., 2001] and the Holocene [Indermühle et al., 1999]. A temporally detailed understanding of global carbon cycle dynamics over shorter, century to millennial timescales, might be achieved with sufficiently sensitive and reliable CO2 proxies. [3] Of the various CO2 proxies available, the carbon isotope composition of phytoplankton and the boron isotope composition of calcium carbonate shells of foraminfera have temporal resolutions suitable for detecting CO2 changes on 103–104 years [Royer et al., 2001a], but only the stomatal method has the capability to record century scale CO2 dynamics. Moreover, it is also arguably the most direct, with cellular differentiation taking place as the leaves sense CO2 in the environment. The CO2 signal was encoded in fossil leaf cuticles because as atmospheric CO2 concentrations increase, leaves of terrestrial vascular plants develop fewer stomatal pores for regulating gaseous exchange with the atmosphere [Woodward, 1987]. However, the recovery of quantitative information from this unique paleobotanical CO2 archive is still in its infancy and has yet to gain widespread critical acceptance. To achieve this goal, its performance deserves to be critically evaluated against empirical data drawn from field studies, laboratory experiments, ice cores and the plant fossil record. [4] An important criterion for any CO2 proxy is that the fidelity of the CO2 signal remains undiminished by changes in other features of the environment. Paleo-CO2 estimates from leaf fossils which utilize measurements of stomatal index (SI, defined as the percentage of leaf epidermal cells that are stomata) are likely to be secure. This is because observations on leaves of tree and shrub populations growing across natural climatic gradients, and controlled environment experiments, show SI is relatively insensitive to soil water supply, irradiance, atmospheric moisture and temperature [Beerling, 1999; Royer, 2001]. The stability of SI arises because it is a proportional quantity independent of leaf expansion; CO2 actually changes the number of epidermal cells that develop into stomatal pores [Woodward, 1987]. In fact, altering the growth atmospheric CO2 concentration is one of the few means of inducing a marked change in the SI of leaves within a given species. Stomatal density, in contrast, simply expresses the number of stomata per unit area of leaf, and is influenced by the climate-controlled degree of leaf expansion (see Figure 1). [5] If measurements of fossil leaf SI securely reflect past CO2 changes, how do the stomatal-based paleo-CO2 reconstructions compare with benchmark CO2 records reported from ice core studies? A key demonstration of the capacity of the technique to capture and retrieve the past CO2 history of the atmosphere derives from temporally detailed measurements on radiocarbon-dated Holocene fossils, made exclusively with leaves of the dwarf willow (Salix herbacea) [Rundgren and Beerling, 1999]. The resulting reconstruction shows a pattern of CO2 accumulation in the atmospheric reservoir over 7000 yrs that closely tracks the Taylor Dome Antarctic ice core CO2 record, although the stomatal record shows a greater amplitude [Indermühle et al., 1999] (see Figure 1). Both records also reveal minor fluctuations during the so-called Medieval Warm Period and the Little Ice Age climatic oscillations, as well as the more recent exponential rise in CO2 due to fossil fuel burning and deforestation (see Figure 1). [6] For the Holocene, the CO2 patterns compare favourably, but terrestrial high-resolution CO2 reconstructions [Beerling et al., 1995; McElwain et al., 2002; Wagner et al., 2002; Rundgren and Björck, 2003] through millennial-scale climate variations of the last deglaciation have revealed interesting new information that is absent from ice cores. They show that the steady rise in CO2 during deglaciation was apparently interrupted by an abrupt fall in CO2 coinciding with the beginning of the Younger Dryas stadial (see Figure 1). In addition, a temporary CO2 decline is registered at the time of the Preboreal oscillation, an early Holocene cooling event. Measurements on the Dome C Antarctic ice core indicate a more gradual deglacial CO2 increase [Monnin et al., 2001] without the relatively high-amplitude changes suggested by stomatal data. The trends in the two sets of data are, however, almost identical (see Figure 1). This discrepancy can partly be accounted for by the smoothing of ice core CO2 records caused by diffusion. The age distribution of enclosed air at the Dome C site, for example, lies between 200 and 550 yrs [Monnin et al., 2001], which is an order of magnitude larger than in a single sample of fossil leaves (30–40 yrs). If the fossil leaves are accurately portraying Lateglacial global carbon cycle dynamics, they suggest a higher sensitivity to climate than previously realized (see Figure 1) and demand a coherent explanation from the modelling community. [7] Other early Holocene and Lateglacial records [Wagner et al., 1999; McElwain et al., 2002] have reproduced similar CO2 patterns, indicating self consistency in the approach both between species and sites. Some stomatal-based records however have reconstructed atmospheric CO2 values higher (maximum 40 ppmv) than those obtained in ice core studies [Wagner et al., 1999, McElwain et al., 2002; Wagner et al., 2002]. The overestimations in these studies may relate to the use of fossil leaf assemblages containing a mixture of closely related species. Leaf SI responds to CO2 in a strongly species-specific manner [Royer et al., 2001a]; even closely related species capable of hybridising with each other differ in their CO2 responsiveness [Rundgren and Björck, 2003]. Additionally, studies involving fossil Betula leaves may be compromised by developing calibration functions with trees of very restricted genotypic diversity [Birks et al., 1999]. [8] However, how important are such mismatches relative to the performance of the other three leading paleo-CO2 proxies? And should they be allowed undermine stomatal-derived paleo-CO2 estimates? The paleosol CO2 barometer has typical errors of ±300–500 ppmv and is unsuitable for tracking ice core CO2 variations because of the time required for the formation of soil carbonates (103–104 yrs) [Cerling, 1992]. Atmospheric CO2 records derived from the carbon isotope composition of alkenones track the CO2 variations of the last glacial-interglacial seen in ice cores, but significant (c. 20 ppmv or more) mismatches are evident [Jasper et al., 1994]. As recently shown by Pagani et al. [2002], accurate CO2 reconstruction by this approach requires parallel estimates of marine phosphate concentration. CO2 records based on the boron isotopic method are likely to be influenced by changes in upwelling [Palmer and Pearson, 2003], and its accuracy is called into question because it suggests that whole-ocean pH was stable over the last glacial-interglacial cycle [Anderson and Archer, 2002]. [9] An implicit assumption for paleo-CO2 proxies involving the biota is that the growth of an organism in responses to its environment is the same on ecological and evolutionary timescales. For vascular land plants such as S. herbacea it is probably a quite reasonable assumption because atmospheric CO2 values reconstructed with fossil leaves dating back to the last interglacial (the Eemian, 130–115 kyrs BP) [Rundgren and Bennike, 2002] are directly comparable with CO2 data reported from the Vostok Antarctic ice core [Petit et al., 1999]. On a multimillion year timescale, the relationship between SI and CO2 exhibited by modern Ginkgo trees has been validated by analyses of changes in the SI of early Paleogene fossil Ginkgo leaves and independent CO2 estimates from paleosols spanning some 3 Myr [Beerling and Royer, 2002]. Strong arguments exist therefore to expect a similarity between the phenotypic and genotypic responses of plants to past histories of CO2; arguments which are critical to establishing the credibility of stomatal-based paleo-CO2 estimates. [10] The elegance of the stomatal paleo-CO2 proxy is that it rests on a simple inverse correlation between CO2 and stomatal formation which is underpinned by a gene involved in the signal transduction pathway controlling stomatal numbers at elevated CO2 [Gray et al., 2000]. Encoded into the leaf fossil record therefore is a rich archive detailing how the CO2 content of the ancient atmosphere has varied. Over the last decade, surprisingly rapid progress has been made in recognizing and recovering this valuable source of CO2 information. It has, for example, provided clues to the causes of mass extinction events [McElwain et al., 1999] and new constraints on radiative forcing by CO2 in the Tertiary [Royer et al., 2001b]. With the capacity to record millennial and century-scale CO2 changes, the stomatal approach to paleo-CO2 estimation offers the potential to identify new undetected rapid reorganizations of the global carbon cycle, as already suggested for the last deglaciation (see Figure 1).

The release of carbon into the atmosphere due to the activities of humans has caused an increase in concentration as well as a change in the isotopic composition of atmospheric carbon dioxide. CO2 derived from fossil fuel combustion and from biomass destruction have δ13C values of ∼−25‰ (compared to the atmospheric value of ∼−7‰) and are thus depleted in 13C. We have measured δ13C of CO2 separated from air trapped in bubbles in ice samples from an ice core taken at Siple Station in Antarctica, in which it has been possible to demonstrate the atmospheric increase of CO2 (ref. 1) and methane2 with high time resolution. The isotopic results, together with the CO2 record from the same ice core, yield information on the sources of excess carbon dioxide and provide a data base for testing the consistency of global carbon cycle models.

Mechanical behavior and deformation mechanisms of AZ31 Mg alloy at liquid nitrogen temperature

The paper is concerned with the fracture toughness and microfractures of a sheet molding compound polyester composite at room and low temperatures. Fracture toughness tests were performed by using compact tension specimens of the composite at room temperature and liquid nitrogen temperature, 77K. Acoustic emission signals were monitored during the fracture toughness tests. The microscopic observation of the fractured surfaces and the spectrum analysis of the acoustic emission signals were made in order to obtain a reasonable explanation of the fracture mechanism. The results are summarized as follows:(1) The load-crack mouth displacement curves at liquid nitrogen and room temperatures became nonlinear at about the same load level and the maximum load observed at liquid nitrogen temperature increased by about three times that observed at room temperature. The acoustic emission activity for the specimen tested at liquid nitrogen temperature was higher than that at room temperature.(2) The fracture toughness KAE at liquid nitrogen temperature, obtained as the stress intensity factor which corresponds to the onset of abrupt increase of the accumulated acoustic emission energy, was extremely larger than that at room temperature.(3) At room temperature the fracture toughness KAE was in good agreement with the fracture toughness KQ obtained by the 5% offset procedure of ASTM E399, and at liquid nitrogen temperature KAE was larger than KQ.(4) From the microscopic observation, it was found that the fracture of a sheet molding compound composite accompanies the microfractures of four types, i.e. fiber breakage, fiber debonding, resin cracking and delamination.(5) The spectrum analysis indicated that the acoustic emission signals could be classified into three types for the specimens tested at room temperature and four types for the specimens tested at liquid nitrogen temperature. An attempt was made to assign each type of the frequency spectra to the microfracture, and the fracture mechanism of the SMC composite was discussed.

Long‐term deformation tests on ice from the Greenland Ice Core Project (GRIP) deep ice core show that ice from the different climate zones in the ice core has flow properties correlated with the concentrations of impurities in the sample. The deformation tests are performed by uniaxial unconfined compression at −16°C with an octahedral compression stress of 3 bars. The ice samples are compressed for ½ to 3 years until the tertiary strain rate is reached. It is believed that by the end all downhole flow conditions are forgotten and that the ice sample has settled in a state determined by the applied stress and temperature conditions. All samples are tested under the same stress and temperature conditions so the resulting deformation rates and final ice crystal size and fabrics can only differ due to varying impurity concentrations. The results show that ice from cold climatic periods with high concentrations of impurities deforms more slowly than ice from warm climatic periods in compression. When tertiary creep is reached, the crystal size is smaller in the cold ice than in the warm. The ice from warmer climatic periods with lower concentrations of impurities deforms at a factor of 2–3 times more rapidly in compression. The tertiary steady state crystal size is increased by 50% and the ice crystals have oriented more favorably for the applied compression in the warm ice, which is believed to be the reason why the strain rates are greater here than in the cold ice. In the bottom 200 m of the GRIP ice core, zones are observed with folds on the scale of 1–8 cm. An investigation of the ice layers in and around the folds shows that the layers are composed of ice from different climatic zones. The folding is believed to result from the different flow and rheological properties of the layers involved in the folding structures.

Abstract. Firn and polar ice cores enclosing trace gas species offer a unique archive to study changes in the past atmosphere and in terrestrial/marine source regions. Here we present a new online technique for ice core and air samples to measure a suite of isotope ratios and mixing ratios of trace gas species on a single sample. Isotope ratios are determined on methane, nitrous oxide and xenon with reproducibilities for ice core samples of 0.15‰ for δ13C–CH4, 0.22‰ for δ15N–N2O, 0.34‰ for δ18O–N2O, and 0.05‰ per mass difference for δ136Xe for typical concentrations of glacial ice. Mixing ratios are determined on methane, nitrous oxide, xenon, ethane, propane, methyl chloride and dichlorodifluoromethane with reproducibilities of 7 ppb for CH4, 3 ppb for N2O, 70 ppt for C2H6, 70 ppt for C3H8, 20 ppt for CH3Cl, and 2 ppt for CCl2F2. However, the blank contribution for C2H6 and C3H8 is large in view of the measured values for Antarctic ice samples. The system consists of a vacuum extraction device, a preconcentration unit and a gas chromatograph coupled to an isotope ratio mass spectrometer. CH4 is combusted to CO2 prior to detection while we bypass the oven for all other species. The highly automated system uses only ~ 160 g of ice, equivalent to ~ 16 mL air, which is less than previous methods. The measurement of this large suite of parameters on a single ice sample is new and key to understanding phase relationships of parameters which are usually not measured together. A multi-parameter data set is also key to understand in situ production processes of organic species in the ice, a critical issue observed in many organic trace gases. Novel is the determination of xenon isotope ratios using doubly charged Xe ions. The attained precision for δ136Xe is suitable to correct the isotopic ratios and mixing ratios for gravitational firn diffusion effects, with the benefit that this information is derived from the same sample. Lastly, anomalies in the Xe mixing ratio, δXe/air, can be used to detect melt layers.

Abstract. High-alpine glaciers are valuable archives of past climatic and environmental conditions. The interpretation of the preserved signal requires a precise chronology. Radiocarbon (14C) dating of the water-insoluble organic carbon (WIOC) fraction has become an important dating tool to constrain the age of ice cores from mid-latitude and low-latitude glaciers. However, in some cases this method is restricted by the low WIOC concentration in the ice. In this work, we report first 14C dating results using the dissolved organic carbon (DOC) fraction, which is present at concentrations of at least a factor of 2 higher than the WIOC fraction. We evaluated this new approach by comparison to the established WIO14C dating based on parallel ice core sample sections from four different Eurasian glaciers covering an age range of several hundred to around 20 000 years; 14C dating of the two fractions yielded comparable ages, with WIO14C revealing a slight, barely significant, systematic offset towards older ages comparable in magnitude with the analytical uncertainty. We attribute this offset to two effects of about equal size but opposite in direction: (i) in-situ-produced 14C contributing to the DOC resulting in a bias towards younger ages and (ii) incompletely removed carbonates from particulate mineral dust (14C-depleted) contributing to the WIOC fraction with a bias towards older ages. The estimated amount of in-situ-produced 14C in the DOC fraction is smaller than the analytical uncertainty for most samples. Nevertheless, under extreme conditions, such as very high altitude and/or low snow accumulation rates, DO14C dating results need to be interpreted cautiously. While during DOC extraction the removal of inorganic carbon is monitored for completeness, the removal for WIOC samples was so far only assumed to be quantitative, at least for ice samples containing average levels of mineral dust. Here we estimated an average removal efficiency of 98±2 %, resulting in a small offset of the order of the current analytical uncertainty. Future optimization of the removal procedure has the potential to improve the accuracy and precision of WIO14C dating. With this study we demonstrate that using the DOC fraction for 14C dating not only is a valuable alternative to the use of WIOC but also benefits from a reduced required ice mass of typically ∼250 g to achieve comparable precision of around ±200 years. This approach thus has the potential of pushing radiocarbon dating of ice forward even to remote regions where the carbon content in the ice is particularly low.

Archives of total mercury reconstructed with ice and snow from Greenland and the Canadian High Arctic

Scarisoara Ice Cave (Romania) hosts one of world’s largest and oldest underground glacier. While no studies were carried out on the existence of microorganisms in this cave’s ice block, our interest is to investigate the presence of microorganisms and their chronological distribution in the cave’s subterranean ice in relationship with past climatic changes. Samples were collected from ice layers of different age (from present to ~900 cal. yrs. BP), and the diversity of embedded microbial communities was assessed by classical cultivation and molecular techniques. The microorganisms from icesediments were cultivated at 4 °C and 15 °C, in the presence and absence of light. Epifluorescence microscopy analysis indicates the presence of autotrophic prokaryotes and eukaryotes in sunlightexposed ice and water samples. Total DNA was isolated from each ice sample and the bacterial and eukaryotic SSU-rRNA genes were amplified by PCR. The chemical composition and organic content of both deeply buried (>10 m inside the ice block) and surface (supra- glacial pond water) habitats were analyzed in relation to their age and organic composition. This study is the first to report on the presence of both prokaryotic and eukaryotic microorganisms in the subterranean ice block of Scarisoara Ice Cave, thriving in both organic-rich ice and clear ice layers. Phototrophic prokaryotes and eukaryotes were identified in sun-exposed recent ice. The composition of cold-adapted ice embedded microbiota varied with the habitat age and organic content, as resulting from dissimilarities in growth curve profiles at two different temperatures. The presence of bacteria and eukaryotes in all the analyzed samples was asserted by PCR amplification of SSU-rRNA gene fragments. These findings can be further used to reconstruct changes in the microbial diversity over the past approximately 5000 years, in correlation with climatic and environmental changes recorded by the ice block.

<p>The Beyond EPICA project for retrieving the Oldest Ice Core (1.5 Myr) in Antarctica aims at obtaining high resolution climate records and water isotopes will be one of the most important parameters investigated. Given the extremely thin nature of the annual ice core layers, as we get deep down to the core, analysis of such an ice core requires new adopted techniques on water isotopes with high accuracy and precision. Laser ablation (LA) is an established powerful technique used in various fields and it can also be applied in ice sampling serving a dual purpose: a.direct solid-gas transition and b. the smallest amount of sample possible is used for analysis and that makes LA a micro-distructive process. A new instrument which couples LA sampling with the established Cavity Ring Down Spectroscopy (CRDS) for water isotopic analysis is developed. This novel design will allow both fast gas phase sample collection directly from the ice sample and high quality water isotopic measurements. Particular focus was given in the LA system which consists of a High Energy femtosecond IR LASER and the optical elements that focus the LASER beam into the ice surface. The focusing lens system is placed inside a freezer, up above a motorized stage that accomodates the ice sample. An enclosure supplied with dry air flow was build around the optics and tested by the means of humidity experiments. Subsequent series of experiments with varying laser ablation parameters: pulse energy, repetition rate, ablation time, together with the ablated crater characterization allow the evaluation of LA efficiency in ice and thus the optimization of the parameters controlling the ablation mechanism. Understanding the LA mechanism will provide the knowledge to further develop the sampling procedure and efficiently control and guide the vaporized ice into a CRDS instrument for detection.</p>

As the intricacies of paleoclimate dynamics are explored, it is becoming understood that sea-ice variability can instigate, or contribute to, climate change instabilities commonly described as “tipping points”. Compared to ice sheets and circulating ocean currents, sea-ice is ephemeral and continental-scale changes to sea ice cover occur seasonally. Sea-ice greatly influences polar albedo, atmosphere-ocean gas exchange and vertical mixing of polar ocean masses. Major changes in sea ice distribution and thickness have been invoked as drivers of deglaciations as well as stadial climate variability described in Greenland climate records as “Dansgaard-Oeschger” cycles and described in Antarctic climate records as “Antarctic Isotopic Maxima”.The role of halogens in polar atmospheric chemistry has been studied intensively over the past few decades. This research has been driven by the role of bromine, primarily as gas-phase bromine monoxide (BrO), which exerts a key control on polar tropospheric ozone concentrations. Initial findings led to the discovery of boundary-layer self-catalyzing heterogeneous bromine reactions fed by sunlight and ozone, known as bromine explosions. First-year sea-ice and blowing snow have been identified as key components for this heterogeneous bromine recycling in the polar boundary layer. This understanding of polar halogen chemistry – supported by an expanding body of observations and modeling – has formed the basis for investigating quantitative links between halogen concentrations in the polar atmospheric boundary layer and sea-ice presence and/or extent.Despite the clear importance of sea-ice in paleoclimate research, the ice core community lacks a conservative and quantitative proxy for sea-ice extent. The most commonly applied proxy, methanesulphonic acid (MSA), is volatile and has not been demonstrated reliably for ice core records extending beyond the last few centuries. Sodium has also been applied to reconstruct sea-ice extent in a semi-quantitative manner although the effects of meteorological transport noise are significant. Contrary to a priori expectations, the halogens bromine and iodine appear to be stable in polar snow and ice over millennial timescales, addressing the temporal limitations of MSA records. Unfortunately, transport and meteorological variability influence sodium deposition as well as the deposition of halogens and the many other ionic impurities found in ice cores. The atmospheric chemistry of halogens is more complex than those of sodium or MSA due to the mixed-phase (gas and aerosol) nature of halogen photochemistry. Thus the application of halogen records in ice cores to sea-ice reconstruction overcomes some challenges posed by existing proxies, but also opens new challenges specific to halogens. Challenges common to all sea-ice proxies include the deconvolution of changes in emission source locations and changes in transport efficacy, particularly those occurring during climate transitions combining changes in sea-ice and atmospheric circulation, such as stadial/interstadial or glacial/interglacial climate variability.In this review, we describe the rationale and available evidence for linking the halogens bromine and iodine found in polar snow and ice to sea-ice extent. Reported measurements of bromine and iodine in polar snow and ice samples are critically discussed. We also consider aspects of halogen transport and retention in polar snow and ice that are still poorly understood. Overall, there is a growing body of evidence supporting the application of bromine to sea-ice reconstructions, and the use of iodine to reconstruct marine biological activity mediated in part by sea-ice extent. These halogens complement existing sea-ice proxies but most crucially, offer the capacity to greatly extend the temporal and spatial coverage of ice core-based sea-ice reconstructions. We identify knowledge gaps existing in the current understanding of spatial and temporal variability of halogen distributions in the polar regions. We suggest areas where polar halogen chemistry can contribute to a better understanding of the halogen records recovered from ice cores. Finally, we propose future steps for establishing reliable and constructive sea-ice reconstructions based on bromine and iodine as observed in snow and ice cores.

Screening of 1000-years old ice layers from the perennial ice block in Ghețarul de la Scarișoara Cave (Romania) revealed the presence of a diverse fungal community. The ice layers were deposited annually by freezing of percolating water containing debris from the surface. Using molecular techniques, based on DGGE fingerprinting of 18S rRNA gene fragments and sequencing, we detected fungi in presently-forming ( i.e. , 1-year old) and in 400 and 900 years old ice layers, respectively. The fungal community profiles in enriched cultures were relatively different compared to those derived from the corresponding environmental ice samples. The community profiles of fungi cultivated at 15°C were more complex compared to the DGGE profiles of fungi cultivated at 4°C. The fungal community was dominated by sequences belonging to the cryophilic yeast Mrakia stokesii in all ice samples. Another cryophilic fungus, Mrakia gelida , was only identified in recent ice samples. Sequences of more ubiquitous fungi Aureobasidium pullulans , Teberdinia hygrophila , Hyphoderma praetermissum , Leucosporidium yakuticum , Candida sp., Cercomonas sp., Thelebolus sp., alongside several yet uncultured fungi, were also identified

Hydrate formation in layers of gas-saturated amorphous ice