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1
Legrand, Michel. « Ice–core records of atmospheric sulphur ». Philosophical Transactions of the Royal Society of London. Series B : Biological Sciences 352, no 1350 (28 février 1997) : 241–50. http://dx.doi.org/10.1098/rstb.1997.0019.
Texte intégralRésumé :
Sulphate and methanesulphonate (MSA), the two major sulphur species trapped in polar ice, have been extensivelyh studied in Antarctic and Greenland ice cores spanning the last centuries, as well as the entire last climatic cycle. Data from the cores are used to investigate the past contribution of volcanic and biogenic emissions to the natural sulphur budget in high latitude regions of both Hemispheres. Sulphate concentrations in polar ice very often increased during one or two years after large volcanic eruptions. Sulphate records show that fossil fuel combustion has enhanced sulphate concentrations in Greenland snow by a factor of 4 since the beginning of this century, and that no similar trend has occurred in Antarctica. At present, sulphate in Antarctic snow is mainly marine and biogenic in origin and the rate of dimethyl sulphide (DMS) emissions may have been enhanced during pst developments of El Niño Southern Oscillations (ENSO). Marine biota and non–eruptive volcanic emissions represent the two main contributors to the natural high northern latitude sulphur budget. Whele these two sources have contributed equally to the natural sulphur budget of Greenland ice over the last 9000 years BP, non–eruptive volcanic emissions largely dominated the budget at the beginning of the Holocene. A general negative correlation is observed between surcace air temperatures of the Northern Hemisphere and Greenland snow MSA concentrations over the last two centuries. Positive sea–ice anomalies also seem to strengthen DMS emissions. A steady decrease of MSA is observed in Greenland snow layers deposited since 1945, which may either be related to decreasing DMS emissions from marine biota at high northern latitudes or a changing yield of MSA from DMS oxidation driven by modification of the oxidative capacity of the atmosphere in these regions. Slightly reduced MSA concentrations are obvserved in Greenland glacial ice with respect to interglacial levels. In contrast, sulphate and calcium levels are strongly enhanced during the ice age compared to the present day. These long–term variations in Greenland cores are opposite in sign to those revealed by Antarctic ice cores. Such a difference suggests that climate changes led to a quite different sulphur cycle response in the two Hemispheres.
2
Shoji, Hitoshi, Atau Mitani, Kohji Horita et Chester C. Langway. « Crystal growth rates in polar firn ». Annals of Glaciology 18 (1993) : 208–10. http://dx.doi.org/10.3189/s0260305500011526.
Texte intégralRésumé :
Continuous crystal-size measurements made on the G6 Antarctic ice core (100m deep) show enhanced growth rates above a depth of 30 m (Zone 1) and in the interval between 70 and 80 m (Zone 2). Crystal growth in Zone 1 most probably takes place by a process of sublimation and condensation. The higher growth rate in Zone 2 is most probably related to the pore close-off transformation process in which a non-uniform strain field is created to form air bubbles by plastic deformation and “cannibalization” of individual ice crystals.
3
Shoji, Hitoshi, Atau Mitani, Kohji Horita et Chester C. Langway. « Crystal growth rates in polar firn ». Annals of Glaciology 18 (1993) : 208–10. http://dx.doi.org/10.1017/s0260305500011526.
Texte intégralRésumé :
Continuous crystal-size measurements made on the G6 Antarctic ice core (100m deep) show enhanced growth rates above a depth of 30 m (Zone 1) and in the interval between 70 and 80 m (Zone 2). Crystal growth in Zone 1 most probably takes place by a process of sublimation and condensation. The higher growth rate in Zone 2 is most probably related to the pore close-off transformation process in which a non-uniform strain field is created to form air bubbles by plastic deformation and “cannibalization” of individual ice crystals.
4
Tornow, Carmen, Ekkehard Kührt, Stefan Kupper et Uwe Motschmann. « Interaction between gas and ice phase in the three periods of the solar nebula ». Proceedings of the International Astronomical Union 5, S263 (août 2009) : 50–54. http://dx.doi.org/10.1017/s1743921310001493.
Texte intégralRésumé :
AbstractWe simulate the chemical processes in the three evolution periods of the solar nebula, which are (i) the quasi-stationary prestellar cloud core, (ii) the gravitationally collapsing protostellar core, and (iii) the evolving gas-dust disk. Our purpose is to identify chemical parameters which reflect special aspects of the interactions between the gas and ice phase in the different periods, e.g. isotopic or molecular ratios. In this study we derive the D/H and 15N/14N ratio of selected compounds as well as the CO2/H2O ratio to measure the fraction of non-polar to polar ice in the grain mantles. The chosen ratios depend on the depletion-enrichment relation between the ice and gas phases driven by the thermal evolution in each period, especially during the collapse. Hence, we have made great efforts in order to derive realistic and compact hydrodynamic models to describe the evolutionary periods of the solar nebula.
5
Shen, Liang, Yongqin Liu, Tandong Yao, Ninglian Wang, Baiqing Xu, Nianzhi Jiao, Hongcan Liu, Yuguang Zhou, Xiaobo Liu et Yanan Wang. « Dyadobacter tibetensis sp. nov., isolated from glacial ice core ». International Journal of Systematic and Evolutionary Microbiology 63, Pt_10 (1 octobre 2013) : 3636–39. http://dx.doi.org/10.1099/ijs.0.050328-0.
Texte intégralRésumé :
A Gram-stain-negative, rod-shaped, aerobic, non-motile bacterium, designated Y620-1T, was isolated from a glacier on the Tibetan Plateau, China. The 16S rRNA gene sequence of the novel isolate shared 93.6–95.1 % similarity with type strains of species of the genus Dyadobacter . The major fatty acids of strain Y620-1T were summed feature 3 (C16 : 1ω7c and/or iso-C15 : 0 2-OH), iso-C15 : 0, C16 : 1ω5c and iso-C17 : 0 3-OH. The predominant isoprenoid quinone and polar lipid were MK-7 and phosphatidylethanolamine (PE), respectively. The DNA G+C content was 44.4±0.3 mol% (T m). Flexirubin-type pigment was produced. The novel isolate was classified in the genus Dyadobacter , but a number of phenotypic characteristics distinguished the novel isolate from type strains of species of the genus Dyadobacter . From these genotypic and phenotypic data, it is evident that strain Y620-1T represents a novel species of the genus Dyadobacter , for which the name Dyadobacter tibetensis sp. nov. is proposed. The type strain is Y620-1T ( = JCM 18589T = CGMCC 1.12215T).
6
Kerch, Johanna, Anja Diez, Ilka Weikusat et Olaf Eisen. « Deriving micro- to macro-scale seismic velocities from ice-core <i>c</i> ; axis orientations ». Cryosphere 12, no 5 (23 mai 2018) : 1715–34. http://dx.doi.org/10.5194/tc-12-1715-2018.
Texte intégralRésumé :
Abstract. One of the great challenges in glaciology is the ability to estimate the bulk ice anisotropy in ice sheets and glaciers, which is needed to improve our understanding of ice-sheet dynamics. We investigate the effect of crystal anisotropy on seismic velocities in glacier ice and revisit the framework which is based on fabric eigenvalues to derive approximate seismic velocities by exploiting the assumed symmetry. In contrast to previous studies, we calculate the seismic velocities using the exact c axis angles describing the orientations of the crystal ensemble in an ice-core sample. We apply this approach to fabric data sets from an alpine and a polar ice core. Our results provide a quantitative evaluation of the earlier approximative eigenvalue framework. For near-vertical incidence our results differ by up to 135 m s−1 for P-wave and 200 m s−1 for S-wave velocity compared to the earlier framework (estimated 1 % difference in average P-wave velocity at the bedrock for the short alpine ice core). We quantify the influence of shear-wave splitting at the bedrock as 45 m s−1 for the alpine ice core and 59 m s−1 for the polar ice core. At non-vertical incidence we obtain differences of up to 185 m s−1 for P-wave and 280 m s−1 for S-wave velocities. Additionally, our findings highlight the variation in seismic velocity at non-vertical incidence as a function of the horizontal azimuth of the seismic plane, which can be significant for non-symmetric orientation distributions and results in a strong azimuth-dependent shear-wave splitting of max. 281 m s−1 at some depths. For a given incidence angle and depth we estimated changes in phase velocity of almost 200 m s−1 for P wave and more than 200 m s−1 for S wave and shear-wave splitting under a rotating seismic plane. We assess for the first time the change in seismic anisotropy that can be expected on a short spatial (vertical) scale in a glacier due to strong variability in crystal-orientation fabric (±50 m s−1 per 10 cm). Our investigation of seismic anisotropy based on ice-core data contributes to advancing the interpretation of seismic data, with respect to extracting bulk information about crystal anisotropy, without having to drill an ice core and with special regard to future applications employing ultrasonic sounding.
7
Weikusat, Ilka, Ernst-Jan N. Kuiper, Gill M. Pennock, Sepp Kipfstuhl et Martyn R. Drury. « EBSD analysis of subgrain boundaries and dislocation slip systems in Antarctic and Greenland ice ». Solid Earth 8, no 5 (6 septembre 2017) : 883–98. http://dx.doi.org/10.5194/se-8-883-2017.
Texte intégralRésumé :
Abstract. Ice has a very high plastic anisotropy with easy dislocation glide on basal planes, while glide on non-basal planes is much harder. Basal glide involves dislocations with the Burgers vector b = 〈a〉, while glide on non-basal planes can involve dislocations with b = 〈a〉, b = [c], and b = 〈c + a〉. During the natural ductile flow of polar ice sheets, most of the deformation is expected to occur by basal slip accommodated by other processes, including non-basal slip and grain boundary processes. However, the importance of different accommodating processes is controversial. The recent application of micro-diffraction analysis methods to ice, such as X-ray Laue diffraction and electron backscattered diffraction (EBSD), has demonstrated that subgrain boundaries indicative of non-basal slip are present in naturally deformed ice, although so far the available data sets are limited. In this study we present an analysis of a large number of subgrain boundaries in ice core samples from one depth level from two deep ice cores from Antarctica (EPICA-DML deep ice core at 656 m of depth) and Greenland (NEEM deep ice core at 719 m of depth). EBSD provides information for the characterization of subgrain boundary types and on the dislocations that are likely to be present along the boundary. EBSD analyses, in combination with light microscopy measurements, are presented and interpreted in terms of the dislocation slip systems. The most common subgrain boundaries are indicative of basal 〈a〉 slip with an almost equal occurrence of subgrain boundaries indicative of prism [c] or 〈c + a〉 slip on prism and/or pyramidal planes. A few subgrain boundaries are indicative of prism 〈a〉 slip or slip of 〈a〉 screw dislocations on the basal plane. In addition to these classical polygonization processes that involve the recovery of dislocations into boundaries, alternative mechanisms are discussed for the formation of subgrain boundaries that are not related to the crystallography of the host grain.The finding that subgrain boundaries indicative of non-basal slip are as frequent as those indicating basal slip is surprising. Our evidence of frequent non-basal slip in naturally deformed polar ice core samples has important implications for discussions on ice about plasticity descriptions, rate-controlling processes which accommodate basal glide, and anisotropic ice flow descriptions of large ice masses with the wider perspective of sea level evolution.
8
NG, Felix, et T. H. Jacka. « A model of crystal-size evolution in polar ice masses ». Journal of Glaciology 60, no 221 (2014) : 463–77. http://dx.doi.org/10.3189/2014jog13j173.
Texte intégralRésumé :
AbstractIn the deep ice cores drilled at the GRIP, NGRIP and GISP2 sites in Greenland and at Byrd Station and the summit of Law Dome in Antarctica, the mean crystal size increases with depth in the shallow subsurface and reaches steady values at intermediate depth. This behaviour has been attributed to the competition between grain-boundary migration driven crystal growth and crystal polygonization, but the effects of changing crystal dislocation density and non-equiaxed crystal shape in this competition are uncertain. We study these effects with a simple model. It describes how the mean height and width of crystals evolve as they flatten under vertical compression, and as crystal growth and polygonization compete. The polygonization rate is assumed to be proportional to the mean dislocation density across crystals. Migration recrystallization, which can affect crystal growth via strain-induced grain boundary migration but whose impact on the mean crystal size is difficult to quantify for ice at present, is not accounted for. When applied to the five ice-core sites, the model simulates the observed crystal-size profiles well down to the bottom of their steady regions, although the match for Law Dome is less satisfactory. Polygonization rate factors retrieved for the sites range from 10–5 to 10–2 a–1. We conclude that since crystal size and dislocation density evolve in a strongly coupled manner, consistent modelling requires multiple differential equations to track both of these variables. Future ice-core analysis should also determine crystal size in all three principal directions.
9
Sigl, M., T. M. Jenk, T. Kellerhals, S. Szidat, H. W. Gäggeler, L. Wacker, H. A. Synal et al. « Towards radiocarbon dating of ice cores ». Journal of Glaciology 55, no 194 (2009) : 985–96. http://dx.doi.org/10.3189/002214309790794922.
Texte intégralRésumé :
AbstractA recently developed dating method for glacier ice, based on the analysis of radiocarbon in carbonaceous aerosol particles, is thoroughly investigated. We discuss the potential of this method to achieve a reliable dating using examples from a mid- and a low-latitude ice core. Two series of samples from Colle Gnifetti (4450 m a.s.l., Swiss Alps) and Nevado Illimani (6300 m a.s.l., Bolivian Andes) demonstrate that the 14C ages deduced from the water-insoluble organic carbon fraction represent the age of the ice. Sample sizes ranged between 7 and 100 μg carbon. For validation we compare our results with those from independent dating. This new method is thought to have major implications for dating non-polar ice cores in the future, as it provides complementary age information for time periods not accessible with common dating techniques.
10
Schwikowski, Margit, Sabina Brütsch, Gino Casassa et Andrés Rivera. « A potential high-elevation ice-core site at Hielo Patagόnico Sur ». Annals of Glaciology 43 (2006) : 8–13. http://dx.doi.org/10.3189/172756406781812014.
Texte intégralRésumé :
AbstractThe Patagonia icefields constitute a unique location in the Southern Hemisphere for obtaining non-polar paleo-records from ice cores south of 45°S. Nevertheless, no ice-core record with meaningful paleoclimate information has yet been obtained from Patagonia. This deficiency is due to extremely harsh field conditions, and to the fact that the main plateaus of both Hielo Patagónico Norte (HPN; northern Patagonia icefield) and Hielo Patagónico Sur (HPS; southern Patagonia icefield) are strongly affected by meltwater percolation. In order to explore the suitability of high-elevation glacier sites at HPS as paleoclimate archives, three shallow firn cores were retrieved covering the altitude range 1543−2300 ma.s.l. The glaciochemical records from the two lower sites confirm the presence of superimposed ice, a clear sign of meltwater formation and percolation. In the core from 2300 m, the glaciochemical signature appears to be preserved, indicating that no significant melting occurred. Although there might be problems associated with wind erosion and extreme melt events, there is good potential for well-preserved paleo-records within glaciers in the Patagonia icefields located higher than 2300 m.
11
Elsässer, C., D. Wagenbach, I. Levin, A. Stanzick, M. Christl, A. Wallner, S. Kipfstuhl, I. K. Seierstad, H. Wershofen et J. Dibb. « Simulating ice core <sup>10</sup>Be on the glacial–interglacial timescale ». Climate of the Past 11, no 2 (3 février 2015) : 115–33. http://dx.doi.org/10.5194/cp-11-115-2015.
Texte intégralRésumé :
Abstract. 10Be ice core measurements are an important tool for paleoclimate research, e.g., allowing for the reconstruction of past solar activity or changes in the geomagnetic dipole field. However, especially on multi-millennial timescales, the share of production and climate-induced variations of respective 10Be ice core records is still up for debate. Here we present the first quantitative climatological model of the 10Be ice concentration up to the glacial–interglacial timescale. The model approach is composed of (i) a coarse resolution global atmospheric transport model and (ii) a local 10Be air–firn transfer model. Extensive global-scale observational data of short-lived radionuclides as well as new polar 10Be snow-pit measurements are used for model calibration and validation. Being specifically configured for 10Be in polar ice, this tool thus allows for a straightforward investigation of production- and non-production-related modulation of this nuclide. We find that the polar 10Be ice concentration does not immediately record the globally mixed cosmogenic production signal. Using geomagnetic modulation and revised Greenland snow accumulation rate changes as model input, we simulate the observed Greenland Summit (GRIP and GISP2) 10Be ice core records over the last 75 kyr (on the GICC05modelext timescale). We show that our basic model is capable of reproducing the largest portion of the observed 10Be changes. However, model–measurement differences exhibit multi-millennial trends (differences up to 87% in case of normalized to the Holocene records) which call for closer investigation. Focusing on the (12–37) b2k (before the year AD 2000) period, mean model–measurement differences of 30% cannot be attributed to production changes. However, unconsidered climate-induced changes could likely explain the model–measurement mismatch. In fact, the 10Be ice concentration is very sensitive to snow accumulation changes. Here the reconstructed Greenland Summit (GRIP) snow accumulation rate record would require revision of +28% to solely account for the (12–37) b2k model–measurement differences.
12
Wolff, Eric, et Renato Spahni. « Methane and nitrous oxide in the ice core record ». Philosophical Transactions of the Royal Society A : Mathematical, Physical and Engineering Sciences 365, no 1856 (18 mai 2007) : 1775–92. http://dx.doi.org/10.1098/rsta.2007.2044.
Texte intégralRésumé :
Polar ice cores contain, in trapped air bubbles, an archive of the concentrations of stable atmospheric gases. Of the major non-CO 2 greenhouse gases, methane is measured quite routinely, while nitrous oxide is more challenging, with some artefacts occurring in the ice and so far limited interpretation. In the recent past, the ice cores provide the only direct measure of the changes that have occurred during the industrial period; they show that the current concentration of methane in the atmosphere is far outside the range experienced in the last 650 000 years; nitrous oxide is also elevated above its natural levels. There is controversy about whether changes in the pre-industrial Holocene are natural or anthropogenic in origin. Changes in wetland emissions are generally cited as the main cause of the large glacial–interglacial change in methane. However, changing sinks must also be considered, and the impact of possible newly described sources evaluated. Recent isotopic data appear to finally rule out any major impact of clathrate releases on methane at these time-scales. Any explanation must take into account that, at the rapid Dansgaard–Oeschger warmings of the last glacial period, methane rose by around half its glacial–interglacial range in only a few decades. The recent EPICA Dome C (Antarctica) record shows that methane tracked climate over the last 650 000 years, with lower methane concentrations in glacials than interglacials, and lower concentrations in cooler interglacials than in warmer ones. Nitrous oxide also shows Dansgaard–Oeschger and glacial–interglacial periodicity, but the pattern is less clear.
13
Elsässer, C., D. Wagenbach, I. Levin, A. Stanzick, M. Christl, A. Wallner, S. Kipfstuhl, I. K. Seierstad, H. Wershofen et J. Dibb. « Simulating ice core <sup>10</sup>Be on the glacial–interglacial timescale ». Climate of the Past Discussions 10, no 1 (26 février 2014) : 761–808. http://dx.doi.org/10.5194/cpd-10-761-2014.
Texte intégralRésumé :
Abstract. 10Be ice core measurements are an important tool for paleoclimate research, e.g. allowing for the reconstruction of past solar activity or variation in the natural 14C production rate. However, especially on multi-millennial timescales, the share of production and climate induced variations of respective 10Be ice core records is still up to debate. Here we present the first quantitative climatological model of the 10Be ice concentration up to the glacial–interglacial timescale. The model approach is composed of (i) a coarse resolution global atmospheric transport model and (ii) a local 10Be air–firn-transfer model. Extensive global-scale observational data of short-lived radionuclides as well as new polar 10Be snow pit measurements are used for model calibration and validation. Being specifically configured for polar 10Be, this tool thus allows for a straight-forward investigation of production and non-production related modulation of this nuclide. We find that the polar 10Be ice concentration does not record a globally mixed cosmogenic production signal. In fact, the geomagnetic modulation of Greenland 10Be is up to 50% lower than in case of the global atmospheric 10Be inventory. Using geomagnetic modulation and revised Greenland snow accumulation rate changes as model input we simulate the observed Greenland Summit (GRIP and GISP2) 10Be ice core records over the last 75 kyr (on the GICC05modelext timescale). We show that our basic model is capable to reproduce the largest portion of the observed 10Be changes. However, model-measurements differences exhibit multi-millennial oscillations with amplitudes up to 87% of the mean observed Holocene 10Be concentration. Focusing on the (12–37) kyr b2k (before the year 2000 AD) period, mean model-measurements differences of 30% cannot be imputed to production changes. However, unconsidered climate-induced changes could likely explain the model shortcomings. In fact, the 10Be ice concentration is very sensitive to snow accumulation changes. Here the reconstructed Greenland Summit (GRIP) snow accumulation rate record would require revision of +28% to solely account for the (12–37) kyr b2k measurements-model differences.
14
Konrad, Hannes, Pascal Bohleber, Dietmar Wagenbach, Christian Vincent et Olaf Eisen. « Determining the age distribution of Colle Gnifetti, Monte Rosa, Swiss Alps, by combining ice cores, ground-penetrating radar and a simple flow model ». Journal of Glaciology 59, no 213 (2013) : 179–89. http://dx.doi.org/10.3189/2013jog12j072.
Texte intégralRésumé :
AbstractIce cores from cold Alpine glaciers may provide unique paleoclimate information from non-polar latitudes. We explore the three-dimensional internal age distribution of the small cold glacier saddle (Colle Gnifetti, Monte Rosa, Italy/Switzerland) to compare the age/depth relations from four local deep ice cores. Tracking isochronous reflection horizons detected by ground-penetrating radar (GPR) among the core locations reveals consistent dating up to 80 years BP. This approach is confined to recent ages, due to the lack of clear reflections below the firn/ice transition. We attempt to overcome this limitation by including a two-dimensional flow model adapted to the GPR-derived surface accumulation and ice thickness distribution. Modeled and GPR isochrones are compared, indicating agreement in shape but featuring a potential offset of 0–3.5 m. The modeled isochrones are interpolated to the core array with ages assigned according to the ice-core datings. The resulting age distribution is consistent up to 110 years BP, with age uncertainties increasing from 7 to >80 years in the lower half of the ice. This combination of methods is novel for Alpine sites and may be adapted for spatial extrapolation of ice properties other than age.
15
Curran, Mark A. J., Tas D. Van Ommen et Vin Morgan. « Seasonal characteristics of the major ions in the high-accumulation Dome Summit South ice core, Law Dome, Antarctica ». Annals of Glaciology 27 (1998) : 385–90. http://dx.doi.org/10.3189/1998aog27-1-385-390.
Texte intégralRésumé :
Seasonal cycles of the chemical species Na+, Κ+ , Mg2+, Ca2+, CH3SO3 (MSA) Cl− NO3 − and NO3 − in the Dome Summit South (DSS) ice core from Law Dome were measured for a number of epochs (AD 1809-15, 1821-31 1980-92) span-nine a total of 28 years. These preliminary trace-chemical patterns show that the DSS site is mainly affected by marine air. The main features found in the seasonal pattern of sea-salt concentrations (e.g. Na+, Cl− and Mg2+) were a winter peak and a summer minimum. The variations in sea salts are believed to reflect aerosol production and transport due to the level of storminess, and are less affected by sea-ice extent. The seasonal cycles of marine biogenic compounds, non-sea-salt SO4 2- and MSA are in good agreement. They show a characteristic summer maximum arid a winter minimum, due to variations in biological activity. While the main sources of nitrate in polar snow remain unclear, the seasonal signal, including sub-seasonal structure, at DSS resembles that found m the atmosphere at coastal Antarctic sites. However, the timing of the nitrate maximum is different in the ice-core record compared with the aerosol records. Overall, the results indicate that the DSS core, with sub-seasonal resolution, contains a sensitive record for investigating climate variability.
16
Curran, Mark A. J., Tas D. Van Ommen et Vin Morgan. « Seasonal characteristics of the major ions in the high-accumulation Dome Summit South ice core, Law Dome, Antarctica ». Annals of Glaciology 27 (1998) : 385–90. http://dx.doi.org/10.1017/s0260305500017778.
Texte intégralRésumé :
Seasonal cycles of the chemical species Na+, Κ+ , Mg2+, Ca2+, CH3SO3 (MSA) Cl− NO3 − and NO3 − in the Dome Summit South (DSS) ice core from Law Dome were measured for a number of epochs (AD 1809-15, 1821-31 1980-92) span-nine a total of 28 years. These preliminary trace-chemical patterns show that the DSS site is mainly affected by marine air. The main features found in the seasonal pattern of sea-salt concentrations (e.g. Na+, Cl− and Mg2+) were a winter peak and a summer minimum. The variations in sea salts are believed to reflect aerosol production and transport due to the level of storminess, and are less affected by sea-ice extent. The seasonal cycles of marine biogenic compounds, non-sea-salt SO4 2- and MSA are in good agreement. They show a characteristic summer maximum arid a winter minimum, due to variations in biological activity. While the main sources of nitrate in polar snow remain unclear, the seasonal signal, including sub-seasonal structure, at DSS resembles that found m the atmosphere at coastal Antarctic sites. However, the timing of the nitrate maximum is different in the ice-core record compared with the aerosol records. Overall, the results indicate that the DSS core, with sub-seasonal resolution, contains a sensitive record for investigating climate variability.
17
Chubarenko, Irina. « Physical processes behind interactions of microplastic particles with natural ice ». Environmental Research Communications 4, no 1 (1 janvier 2022) : 012001. http://dx.doi.org/10.1088/2515-7620/ac49a8.
Texte intégralRésumé :
Abstract Microplastic particles (MPs, <5 mm) are found in marine ice in larger quantities than in seawater, however, the distribution pattern within the ice cores is not consistent. To get insights into the most general physical processes behind interactions of ice and plastic particles in cool natural environments, information from academic and applied research is integrated and verified against available field observations. Non-polar molecules of common-market plastics are hydrophobic, so MPs are weak ice nucleators, are repelled from water and ice, and concentrate within air bubbles and brine channels. A large difference in thermal properties of ice and plastics favours the concentration of MPs at the ice surface during freeze/thaw cycles. Under low environmental temperatures, falling in polar regions below the glass / brittle-ductile transition temperatures of the common-use plastics, they become brittle. This might partially explain the absence of floating macroplastics in polar waters. Freshwater freezes at a temperature well below that of its maximum density, so the water column is stably stratified, and MPs eventually concentrate at the ice surface and in air bubbles. In contrast, below growing sea ice, mechanisms of suspension freezing under conditions of (thermal plus haline) convection should permanently entangle MPs into ice. During further sea ice growth and aging, MPs are repelled from water and ice into air bubbles, brine channels, and to the upper/lower boundaries of the ice column. Sea ice permeability, especially while melting periods, can re-distribute sub-millimeter MPs through the brine channels, thus potentially introducing the variability of contamination with time. In accord with field observations, analysis reveals several competing factors that influence the distribution of MPs in sea ice. A thorough sampling of the upper ice surface, prevention of brine leakage while sampling and handling, considering the ice structure while segmenting the ice core—these steps may be advantageous for further understanding the pattern of plastic contamination in natural ice.
18
Gautier, E., J. Savarino, J. Erbland, A. Lanciki et P. Possenti. « Variability of sulfate signal in ice core records based on five replicate cores ». Climate of the Past 12, no 1 (20 janvier 2016) : 103–13. http://dx.doi.org/10.5194/cp-12-103-2016.
Texte intégralRésumé :
Abstract. Current volcanic reconstructions based on ice core analysis have significantly improved over the past few decades by incorporating multiple-core analyses with a high temporal resolution from different parts of the polar regions into a composite common volcanic eruption record. Regional patterns of volcanic deposition are based on composite records, built from cores taken at both poles. However, in many cases only a single record at a given site is used for these reconstructions. This assumes that transport and regional meteorological patterns are the only source of the dispersion of the volcanic products. Here we evaluate the local-scale variability of a sulfate profile in a low-accumulation site (Dome C, Antarctica), in order to assess the representativeness of one core for such a reconstruction. We evaluate the variability with depth, statistical occurrence, and sulfate flux deposition variability of volcanic eruptions detected in five ice cores, drilled 1 m apart from each other. Local-scale variability, essentially attributed to snow drift and surface roughness at Dome C, can lead to a non-exhaustive record of volcanic events when a single core is used as the site reference, with a bulk probability of 30 % of missing volcanic events and close to 65 % uncertainty on one volcanic flux measurement (based on the standard deviation obtained from a five-core comparison). Averaging n records reduces the uncertainty of the deposited flux mean significantly (by a factor 1∕ √ n); in the case of five cores, the uncertainty of the mean flux can therefore be reduced to 29 %.
19
Gibson, John A. E., Kristina S. Paterson, Camille A. White et Kerrie M. Swadling. « Evidence for the continued existence of Abraxas Lake, Vestfold Hills, East Antarctica during the Last Glacial Maximum ». Antarctic Science 21, no 3 (16 février 2009) : 269–78. http://dx.doi.org/10.1017/s0954102009001801.
Texte intégralRésumé :
AbstractEvidence is provided from a sediment core from saline Abraxas Lake, Vestfold Hills, that indicates that the lake existed through the Last Glacial Maximum. It can therefore be concluded that at least part of the Vestfold Hills also remained ice-free through the Last Glacial Maximum, or at most was covered by a thin, non-erosive cold-based ice sheet. The evidence for the continued existence of Abraxas Lake includes a 14C date that significantly predates the Last Glacial Maximum (though this cannot be considered direct proof of the existence of the lake prior to the Last Glacial Maximum); the presence of saline porewater throughout the core, including in compacted sediments deposited during the glacial period, which implies that the lake obtained its salt prior to any Holocene marine highstand; and the occurrence of marine-derived fauna from the onset of significant biological activity late in the Pleistocene. The occurrence of ice-free land in the Vestfold Hills and similar oases suggests that the margin of the polar ice cap did not reach far beyond its current position at the Last Glacial Maximum, at least in regions now occupied by these oases.
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Gautier, E., J. Savarino, J. Erbland, A. Lanciki et P. Possenti. « Variability of sulfate signal in ice-core records based on five replicate cores ». Climate of the Past Discussions 11, no 4 (27 août 2015) : 3973–4002. http://dx.doi.org/10.5194/cpd-11-3973-2015.
Texte intégralRésumé :
Abstract. Current volcanic reconstructions based on ice core analysis have significantly improved over the last decades. Relying on limited and disparate sulfate profiles at first, they have progressively incorporated multi cores analysis with high temporal resolution from different parts of the Polar Regions. Regional patterns of volcanic deposition flux are now based on composite records, built from several cores taken at both poles. However, it is worth mentioning that most of the time only a single record at a given site is used for such reconstructions. This implicitly assumes that transport and regional meteorological patterns are the only source of the dispersion of the volcanic-products. In the present work, we evaluate the local scale variability of a sulfate profile in a low accumulation site (Dome C, Antarctica), in order to assess the representativeness of one core for such reconstruction. We evaluate the depth variability, statistical occurrence, and sulfate flux deposition variability of volcanic eruptions detected on 5 ice cores, drilled 1 m away from each other. Local scale variability, essentially attributed to snow drift and surface roughness at Dome C, can lead to a non-exhaustive record of volcanic events when a single core is used as the site reference with a bulk probability of 30 % of missing volcanic events and 60 % uncertainty on the volcanic flux estimation. Averaging multiple records almost erases the probability of missing volcanic events and can reduce by half the uncertainty pertaining to the deposition flux.
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Jónsdóttir, Ingibjörg Rósa, Sædís Ólafsdóttir et Áslaug Geirsdóttir. « Marine climate variability from Arnarfjörður, NW Iceland, during the Medieval Warm period and early/middle Little Ice Age ». Jökull 65, no 1 (15 décembre 2015) : 73–87. http://dx.doi.org/10.33799/jokull2015.65.073.
Texte intégralRésumé :
A high-resolution sedimentary record from the subarctic fjord Arnarfjörður in northwestern Iceland provides information on local changes in sea ice cover and a regional oceanographic climatic signal reflecting changes in the position of the Polar Front that separates the North Atlantic Current and the East Greenland Current. The 520 cm long sediment core spans approximately 2000 years and thus offers a multi-decadal time resolution during the Medieval Warm Period (MWP) and the early to middle part of the Little Ice Age (LIA). Approximately 150 years from the top of the core were lost during coring. The marine climate reconstruction is based on multi-proxy study with focus on benthic foraminiferal fauna allowing down-core bottom water temperature (BWT_{TF}) estimations based on the statistical transfer function approach. This first of the kind study from Arnarfjörður demonstrates significant variability in the benthic foraminiferal fauna dominated by Cibicides lobatulus, Cassidulina reniforme and Elphidium excavatum, BWT_{TF} variations of ∼3°C, fluctuating from ca. 1.5±1.1 °C to 4.5±0.6 °C. The data is in harmony with previously reported LIA characteristics from the region, which has been described as a period of high amplitude fluctuations, with non-stable conditions and cold bottom waters.
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Siggaard-Andersen, Marie-Louise, Jørgen Peder Steffensen et Hubertus Fischer. « Lithium in Greenland ice cores measured by ion chromatography ». Annals of Glaciology 35 (2002) : 243–49. http://dx.doi.org/10.3189/172756402781816483.
Texte intégralRésumé :
AbstractIon chromatography (IC) is a widely used technique for analyzing ice cores for ions like Na+, NH4+, K+, Mg2+, Ca2+, F–, MSA–, Cl–, NO2– and SO42– that are present in polar ice cores at ppb level. By using sample preconcentration and an optimized separation technique, we have been able to detect Li+ in ice-core samples in concentrations as low as 0.0001 μeq kg–1 or 0.7 ppt by IC. During routine analysis of ions in ice cores, the lithium content has been evaluated and recorded. the IC technique used in these measurements and some exemplary IC data from the Greenland Icecore Project (GRIP) and the North Greenland Icecore Project (NorthGRIP) ice cores are presented. By these data we introduce Li+ concentration as a new parameter in the analysis of ice cores. Like other ions, Li+ reflects climatic changes and shows seasonal cycles. on the basis of the geochemistry of lithium, we suggest that Li+ measured in the Greenland ice cores is derived from mineral dust. However, data from the NorthGRIP ice core representing the 8.2 kyr BPHolocene cold event show a strong Li+ signal that does not correlate with any other ionic component measured. This means that the lithium content in ice cores is a signal with its own pattern, which is not yet understood.
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Pickard, Heidi M., Alison S. Criscitiello, Christine Spencer, Martin J. Sharp, Derek C. G. Muir, Amila O. De Silva et Cora J. Young. « Continuous non-marine inputs of per- and polyfluoroalkyl substances to the High Arctic : a multi-decadal temporal record ». Atmospheric Chemistry and Physics 18, no 7 (13 avril 2018) : 5045–58. http://dx.doi.org/10.5194/acp-18-5045-2018.
Texte intégralRésumé :
Abstract. Perfluoroalkyl acids (PFAAs) are persistent, in some cases, bioaccumulative compounds found ubiquitously within the environment. They can be formed from the atmospheric oxidation of volatile precursor compounds and undergo long-range transport (LRT) through the atmosphere and ocean to remote locations. Ice caps preserve a temporal record of PFAA deposition making them useful in studying the atmospheric trends in LRT of PFAAs in polar or mountainous regions, as well as in understanding major pollutant sources and production changes over time. A 15 m ice core representing 38 years of deposition (1977–2015) was collected from the Devon Ice Cap in Nunavut, providing us with the first multi-decadal temporal ice record in PFAA deposition to the Arctic. Ice core samples were concentrated using solid phase extraction and analyzed by liquid and ion chromatography methods. Both perfluoroalkyl carboxylic acids (PFCAs) and perfluoroalkyl sulfonic acids (PFSAs) were detected in the samples, with fluxes ranging from < LOD to 141 ng m−2 yr−1. Our results demonstrate that the PFCAs and perfluorooctane sulfonate (PFOS) have continuous and increasing deposition on the Devon Ice Cap, despite recent North American and international regulations and phase-outs. We propose that this is the result of on-going manufacture, use and emissions of these compounds, their precursors and other newly unidentified compounds in regions outside of North America. By modelling air mass transport densities, and comparing temporal trends in deposition with production changes of possible sources, we find that Eurasian sources, particularly from Continental Asia, are large contributors to the global pollutants impacting the Devon Ice Cap. Comparison of PFAAs to their precursors and correlations of PFCA pairs showed that deposition of PFAAs is dominated by atmospheric formation from volatile precursor sources. Major ion analysis confirmed that marine aerosol inputs are unimportant to the long-range transport mechanisms of these compounds. Assessments of deposition, homologue profiles, ion tracers, air mass transport models, and production and regulation trends allow us to characterize the PFAA depositional profile on the Devon Ice Cap and further understand the LRT mechanisms of these persistent pollutants.
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Kobashi, T., T. Ikeda-Fukazawa, M. Suwa, J. Schwander, T. Kameda, J. Lundin, A. Hori, M. Döring et M. Leuenberger. « Post bubble-closeoff fractionation of gases in polar firn and ice cores : effects of accumulation rate on permeation through overloading pressure ». Atmospheric Chemistry and Physics Discussions 15, no 11 (11 juin 2015) : 15711–53. http://dx.doi.org/10.5194/acpd-15-15711-2015.
Texte intégralRésumé :
Abstract. Gases in ice cores are invaluable archives of past environmental changes (e.g., the past atmosphere). However, gas fractionation processes after bubble closure in the firn are poorly understood, although increasing evidence indicates preferential leakages of smaller molecules (e.g., neon, oxygen, and argon) from the closed bubbles through ice crystals. These fractionation processes are believed to be responsible for the observed millennial δO2/N2 variations in ice cores, linking ice core chronologies with orbital parameters. Herein, we found that δAr/N2 at decadal resolution on the gas age scale in the GISP2 ice core has a significant negative correlation with accumulation rate over the past 6000 years. Furthermore, the precise temperature and accumulation rate records over the past 4000 years are found to have nearly equal effects on δAr/N2 with sensitivities of 0.72 ± 0.1 ‰ °C−1 and −0.58 ± 0.09 ‰ (0.01 m ice yr−1)−1, respectively. To understand the fractionation processes, we applied a permeation model to "microbubbles (< 1 % of air content in the Vostok ice core)" and "normal bubbles" in the firn. The model indicates that δAr/N2 in the microbubbles is negatively correlated with the accumulation rate as found in the observation, due to changes in overloading pressure. Colder (warmer) temperatures in the firn induce more (less) depletions in δAr/N2. The microbubbles are so depleted in δAr/N2 at the bubble closeoff depth that they dominate the total δAr/N2 changes in spite of their smaller volumes. The model also indicates that δAr/N2 of GISP2 and NGRIP should have experienced several permil of depletion during the storage 14 years after coring. Further understanding of the δAr/N2 and δO2/N2 fractionation processes in the firn may lead to a new proxy for the past temperature and accumulation rate.
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Witrant, E., P. Martinerie, C. Hogan, J. C. Laube, K. Kawamura, E. Capron, S. A. Montzka et al. « A new multi-gas constrained model of trace gas non-homogeneous transport in firn : evaluation and behaviour at eleven polar sites ». Atmospheric Chemistry and Physics 12, no 23 (4 décembre 2012) : 11465–83. http://dx.doi.org/10.5194/acp-12-11465-2012.
Texte intégralRésumé :
Abstract. Insoluble trace gases are trapped in polar ice at the firn-ice transition, at approximately 50 to 100 m below the surface, depending primarily on the site temperature and snow accumulation. Models of trace gas transport in polar firn are used to relate firn air and ice core records of trace gases to their atmospheric history. We propose a new model based on the following contributions. First, the firn air transport model is revised in a poromechanics framework with emphasis on the non-homogeneous properties and the treatment of gravitational settling. We then derive a nonlinear least square multi-gas optimisation scheme to calculate the effective firn diffusivity (automatic diffusivity tuning). The improvements gained by the multi-gas approach are investigated (up to ten gases for a single site are included in the optimisation process). We apply the model to four Arctic (Devon Island, NEEM, North GRIP, Summit) and seven Antarctic (DE08, Berkner Island, Siple Dome, Dronning Maud Land, South Pole, Dome C, Vostok) sites and calculate their respective depth-dependent diffusivity profiles. Among these different sites, a relationship is inferred between the snow accumulation rate and an increasing thickness of the lock-in zone defined from the isotopic composition of molecular nitrogen in firn air (denoted δ15N). It is associated with a reduced diffusivity value and an increased ratio of advective to diffusive flux in deep firn, which is particularly important at high accumulation rate sites. This has implications for the understanding of δ15N of N2 records in ice cores, in relation with past variations of the snow accumulation rate. As the snow accumulation rate is clearly a primary control on the thickness of the lock-in zone, our new approach that allows for the estimation of the lock-in zone width as a function of accumulation may lead to a better constraint on the age difference between the ice and entrapped gases.
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Rhodes, Rachael H., Xavier Faïn, Edward J. Brook, Joseph R. McConnell, Olivia J. Maselli, Michael Sigl, Jon Edwards et al. « Local artifacts in ice core methane records caused by layered bubble trapping and in situ production : a multi-site investigation ». Climate of the Past 12, no 4 (26 avril 2016) : 1061–77. http://dx.doi.org/10.5194/cp-12-1061-2016.
Texte intégralRésumé :
Abstract. Advances in trace gas analysis allow localised, non-atmospheric features to be resolved in ice cores, superimposed on the coherent atmospheric signal. These high-frequency signals could not have survived the low-pass filter effect that gas diffusion in the firn exerts on the atmospheric history and therefore do not result from changes in the atmospheric composition at the ice sheet surface. Using continuous methane (CH4) records obtained from five polar ice cores, we characterise these non-atmospheric signals and explore their origin. Isolated samples, enriched in CH4 in the Tunu13 (Greenland) record are linked to the presence of melt layers. Melting can enrich the methane concentration due to a solubility effect, but we find that an additional in situ process is required to generate the full magnitude of these anomalies. Furthermore, in all the ice cores studied there is evidence of reproducible, decimetre-scale CH4 variability. Through a series of tests, we demonstrate that this is an artifact of layered bubble trapping in a heterogeneous-density firn column; we use the term “trapping signal” for this phenomenon. The peak-to-peak amplitude of the trapping signal is typically 5 ppb, but may exceed 40 ppb. Signal magnitude increases with atmospheric CH4 growth rate and seasonal density contrast, and decreases with accumulation rate. Significant annual periodicity is present in the CH4 variability of two Greenland ice cores, suggesting that layered gas trapping at these sites is controlled by regular, seasonal variations in the physical properties of the firn. Future analytical campaigns should anticipate high-frequency artifacts at high-melt ice core sites or during time periods with high atmospheric CH4 growth rate in order to avoid misinterpretation of such features as past changes in atmospheric composition.
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Witrant, E., P. Martinerie, C. Hogan, J. C. Laube, K. Kawamura, E. Capron, S. A. Montzka et al. « A new multi-gas constrained model of trace gas non-homogeneous transport in firn : evaluation and behavior at eleven polar sites ». Atmospheric Chemistry and Physics Discussions 11, no 8 (16 août 2011) : 23029–80. http://dx.doi.org/10.5194/acpd-11-23029-2011.
Texte intégralRésumé :
Abstract. Insoluble trace gases are trapped in polar ice at the firn-ice transition, at approximately 50 to 100 m below the surface, depending primarily on the site temperature and snow accumulation. Due to the different time scales for snow accumulation versus diffusion of gases through the snowpack, age differences between gases and the ice in which they are "trapped" can be large; e.g. several thousand years in central Antarctica (a low snow accumulation area). Models of trace gas diffusion in polar firn are used to relate firn air and ice core records of trace gases to their atmospheric history. We propose a new diffusion model based on the following contributions. First, the airflow transport model is revised in a poromechanics framework with specific emphasis on the non-homogeneous properties (convective layer, depth-dependent diffusivity and lock-in zone) and an almost-stagnant behavior described by Darcy's law (gravity effect). We then derive a non-linear least square multi-gas optimization scheme to calculate the effective firn diffusivity (automatic diffusivity tuning). The improvements associated with the additional constraints gained by the multi-gas approach are investigated (up to eleven gases for a single site are included in the optimization process). The model is applied to measured data from four Arctic (Devon Island, NEEM, North GRIP, Summit) and seven Antarctic (DE08, Berkner Island, Siple Dome, Dronning Maud Land, South Pole, Dome C, Vostok) sites and the depth-dependent diffusivity profiles are calculated. Among these different sites, a relationship between an increasing thickness of the lock-in zone defined from the isotopic composition of molecular nitrogen in firn air (denoted δ15N) and the snow accumulation rate is obtained, in accordance with observations. It is associated with reduced diffusivity depth-gradients in deep firn, which decreases gas density depth-gradients, at high accumulation rate sites. This has implications for the understanding of δ15N of N2 records in ice cores, in relation with past variations of the snow accumulation rate. Although the extent of layering is clearly a primary control on the thickness of the lock-in zone, our new approach that allows calculation of an estimated lock-in depth may lead to a better constraint on the age difference between the ice and entrapped gases.
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Kobashi, T., T. Ikeda-Fukazawa, M. Suwa, J. Schwander, T. Kameda, J. Lundin, A. Hori, H. Motoyama, M. Döring et M. Leuenberger. « Post-bubble close-off fractionation of gases in polar firn and ice cores : effects of accumulation rate on permeation through overloading pressure ». Atmospheric Chemistry and Physics 15, no 24 (16 décembre 2015) : 13895–914. http://dx.doi.org/10.5194/acp-15-13895-2015.
Texte intégralRésumé :
Abstract. Gases in ice cores are invaluable archives of past environmental changes (e.g., the past atmosphere). However, gas fractionation processes after bubble closure in the firn are poorly understood, although increasing evidence indicates preferential leakages of smaller molecules (e.g., neon, oxygen, and argon) from the closed bubbles through the ice matrix. These fractionation processes are believed to be responsible for the observed millennial δO2/N2 variations in ice cores, linking ice core chronologies with orbital parameters. In this study, we investigated high-resolution δAr/N2 of the GISP2 (Greenland Ice Sheet Project 2), NGRIP (North Greenland Ice Core Project), and Dome Fuji ice cores for the past few thousand years. We find that δAr/N2 at multidecadal resolution on the "gas-age scale" in the GISP2 ice core has a significant negative correlation with accumulation rate and a positive correlation with air contents over the past 6000 years, indicating that changes in overloading pressure induced δAr/N2 fractionation in the firn. Furthermore, the GISP2 temperature and accumulation rate for the last 4000 years have nearly equal effects on δAr/N2 with sensitivities of 0.72 ± 0.1 ‰ °C−1 and −0.58 ± 0.09 ‰ (0.01 m ice year−1)−1, respectively. To understand the fractionation processes, we applied a permeation model for two different processes of bubble pressure build-up in the firn, "pressure sensitive process" (e.g., microbubbles: 0.3–3 % of air contents) with a greater sensitivity to overloading pressures and "normal bubble process". The model indicates that δAr/N2 in the bubbles under the pressure sensitive process are negatively correlated with the accumulation rate due to changes in overloading pressure. On the other hand, the normal bubbles experience only limited depletion (< 0.5 ‰) in the firn. Colder temperatures in the firn induce more depletion in δAr/N2 through thicker firn. The pressure sensitive bubbles are so depleted in δAr/N2 at the bubble close-off depth that they dominate the total δAr/N2 changes in spite of their smaller air contents. The model also indicates that δAr/N2 of ice cores should have experienced several per mil of depletion during the storage 14–18 years after coring. Further understanding of the δAr/N2 fractionation processes in the firn, combined with nitrogen and argon isotope data, may lead to a new proxy for the past temperature and accumulation rate.
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Llorens, Maria-Gema, Albert Griera, Paul D. Bons, Ilka Weikusat, David J. Prior, Enrique Gomez-Rivas, Tamara de Riese, Ivone Jimenez-Munt, Daniel García-Castellanos et Ricardo A. Lebensohn. « Can changes in deformation regimes be inferred from crystallographic preferred orientations in polar ice ? » Cryosphere 16, no 5 (25 mai 2022) : 2009–24. http://dx.doi.org/10.5194/tc-16-2009-2022.
Texte intégralRésumé :
Abstract. Creep due to ice flow is generally thought to be the main cause for the formation of crystallographic preferred orientations (CPOs) in polycrystalline anisotropic ice. However, linking the development of CPOs to the ice flow history requires a proper understanding of the ice aggregate's microstructural response to flow transitions. In this contribution the influence of ice deformation history on the CPO development is investigated by means of full-field numerical simulations at the microscale. We simulate the CPO evolution of polycrystalline ice under combinations of two consecutive deformation events up to high strain, using the code VPFFT (visco-plastic fast Fourier transform algorithm) within ELLE. A volume of ice is first deformed under coaxial boundary conditions, which results in a CPO. The sample is then subjected to different boundary conditions (coaxial or non-coaxial) in order to observe how the deformation regime switch impacts the CPO. The model results indicate that the second flow event tends to destroy the first, inherited fabric with a range of transitional fabrics. However, the transition is slow when crystallographic axes are critically oriented with respect to the second imposed regime. Therefore, interpretations of past deformation events from observed CPOs must be carried out with caution, particularly in areas with complex deformation histories.
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Upadhyaya, Kalpana, Venkatesh Gude, Golam Mohiuddin et Rao V. S. Nandiraju. « A new family of four-ring bent-core nematic liquid crystals with highly polar transverse and end groups ». Beilstein Journal of Organic Chemistry 9 (7 janvier 2013) : 26–35. http://dx.doi.org/10.3762/bjoc.9.4.
Texte intégralRésumé :
Non-symmetrically substituted four-ring achiral bent-core compounds with polar substituents, i.e.., chloro in the bent or transverse direction in the central core and cyano in the lateral direction at one terminal end of the molecule, are designed and synthesized. These molecules possess an alkoxy chain attached at only one end of the bent-core molecule. The molecular structure characterization is consistent with data from elemental and spectroscopic analysis. The materials thermal behaviour and phase characterization have been investigated by differential scanning calorimetry and polarizing microscopy. All the compounds exhibit a wide-ranging monotropic nematic phase.
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Langway, C. C., et K. Goto-Azuma. « Temporal Variations in the Deep Ice-Core Chemistry Record from Dye 3, Greenland (Abstract) ». Annals of Glaciology 10 (1988) : 209. http://dx.doi.org/10.3189/s0260305500004559.
Texte intégralRésumé :
Measurements of the chemical constituents in polar-snow deposits translate into chronological records representing a history of atmospheric composition for the periods involved. The 2037 m deep continuous and undisturbed ice core recovered at Dye 3, Greenland between 1979 and 1981 contains a temporal record of sequential snow deposits for the past 9 × 104 a B.P. (Dansgaard and others 1985). The upper 90 m of the deep core were unsuitable for chemistry studies, but stratigraphic continuity with present-day accumulation was obtained by hand-excavating a 5.4 m deep pit and augering two shallow cores to 138 and 113 m depths. The pit and shallow cores represent the last two centuries of snow precipitation.To date, over 6000 individual samples of the pit, shallow and deep ice cores have been measured by ion chromatography for Cl−, NO3−, and SO42− in the field and laboratory (Herron and Langway 1985, Finkel and Langway 1985, Finkel and others 1986), under clean-room conditions. All pit and shallow-core samples were prepared in a continuous sequence of eight samples per year, as identified by other stable and radioactive isotope-dating methods. The deep ice-core samples were selected and prepared from core intervals spaced over the 2037 m profile from time units which showed evidence of abrupt or transitory periods in climate change or volcanic disturbances, as defined by stable isotopes (Dansgaard and others 1985), atmospheric gases (Oeschger and others 1985) and dust (Hammer and others 1985).Approximately 1700 new measurements from the Dye 3 samples are included in this study. Variability in the chemical constituents and their concentration levels is present and meaningful on a short-term and long-term basis. The time units measured represent seasons, years, decades, centuries and longer geological periods. Particular attention is given to two new high- and low-frequency detailed chronological data sets from (1) a continuous 26 m core profile, representing 3000 years, extending from the Holocene/Wisconsin boundary back into the late Wisconsin and (2) measurements made on 106 samples spaced every 2 m over the Wisconsin-age ice from 1786 to 2008 m.
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Langway, C. C., et K. Goto-Azuma. « Temporal Variations in the Deep Ice-Core Chemistry Record from Dye 3, Greenland (Abstract) ». Annals of Glaciology 10 (1988) : 209. http://dx.doi.org/10.1017/s0260305500004559.
Texte intégralRésumé :
Measurements of the chemical constituents in polar-snow deposits translate into chronological records representing a history of atmospheric composition for the periods involved. The 2037 m deep continuous and undisturbed ice core recovered at Dye 3, Greenland between 1979 and 1981 contains a temporal record of sequential snow deposits for the past 9 × 104 a B.P. (Dansgaard and others 1985). The upper 90 m of the deep core were unsuitable for chemistry studies, but stratigraphic continuity with present-day accumulation was obtained by hand-excavating a 5.4 m deep pit and augering two shallow cores to 138 and 113 m depths. The pit and shallow cores represent the last two centuries of snow precipitation. To date, over 6000 individual samples of the pit, shallow and deep ice cores have been measured by ion chromatography for Cl−, NO3 −, and SO4 2− in the field and laboratory (Herron and Langway 1985, Finkel and Langway 1985, Finkel and others 1986), under clean-room conditions. All pit and shallow-core samples were prepared in a continuous sequence of eight samples per year, as identified by other stable and radioactive isotope-dating methods. The deep ice-core samples were selected and prepared from core intervals spaced over the 2037 m profile from time units which showed evidence of abrupt or transitory periods in climate change or volcanic disturbances, as defined by stable isotopes (Dansgaard and others 1985), atmospheric gases (Oeschger and others 1985) and dust (Hammer and others 1985). Approximately 1700 new measurements from the Dye 3 samples are included in this study. Variability in the chemical constituents and their concentration levels is present and meaningful on a short-term and long-term basis. The time units measured represent seasons, years, decades, centuries and longer geological periods. Particular attention is given to two new high- and low-frequency detailed chronological data sets from (1) a continuous 26 m core profile, representing 3000 years, extending from the Holocene/Wisconsin boundary back into the late Wisconsin and (2) measurements made on 106 samples spaced every 2 m over the Wisconsin-age ice from 1786 to 2008 m.
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Festi, Daniela, Luca Carturan, Werner Kofler, Giancarlo dalla Fontana, Fabrizio de Blasi, Federico Cazorzi, Edith Bucher, Volkmar Mair, Paolo Gabrielli et Klaus Oeggl. « Linking pollen deposition and snow accumulation on the Alto dell'Ortles glacier (South Tyrol, Italy) for sub-seasonal dating of a firn temperate core ». Cryosphere 11, no 2 (13 avril 2017) : 937–48. http://dx.doi.org/10.5194/tc-11-937-2017.
Texte intégralRésumé :
Abstract. Dating of ice cores from temperate non-polar glaciers is challenging and often problematic. However, a proper timescale is essential for a correct interpretation of the proxies measured in the cores. Here, we introduce a new method developed to obtain a sub-seasonal timescale relying on statistically measured similarities between pollen spectra obtained from core samples and daily airborne pollen monitoring samples collected in the same area. This approach was developed on a 10 m core retrieved from the temperate-firn portion of Alto dell'Ortles glacier (Eastern Italian Alps), for which a 5-year annual/seasonal timescale already exists. The aim was to considerably improve this timescale, reaching the highest possible temporal resolution and testing the efficiency and limits of pollen as a chronological tool. A test of the new timescale was performed by comparing our results to the output (date of layer formation) of the mass balance model EISModel, during the period encompassed by the timescale. The correspondence of the results supports the new sub-seasonal timescale based on pollen analysis. This comparison also allows us to draw important conclusions on the post-depositional effects of meltwater percolation on the pollen content of the firn core as well as on the climatic interpretation of the pollen signal.
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Erhardt, Tobias, Matthias Bigler, Urs Federer, Gideon Gfeller, Daiana Leuenberger, Olivia Stowasser, Regine Röthlisberger et al. « High-resolution aerosol concentration data from the Greenland NorthGRIP and NEEM deep ice cores ». Earth System Science Data 14, no 3 (16 mars 2022) : 1215–31. http://dx.doi.org/10.5194/essd-14-1215-2022.
Texte intégralRésumé :
Abstract. Records of chemical impurities from ice cores enable us to reconstruct the past deposition of aerosols onto polar ice sheets and alpine glaciers. Through this they allow us to gain insight into changes of the source, transport and deposition processes that ultimately determine the deposition flux at the coring location. However, the low concentrations of the aerosol species in the ice and the resulting high risk of contamination pose a formidable analytical challenge, especially if long, continuous and highly resolved records are needed. Continuous flow analysis, CFA, the continuous melting, decontamination and analysis of ice-core samples has mostly overcome this issue and has quickly become the de facto standard to obtain high-resolution aerosol records from ice cores after its inception at the University of Bern in the mid-1990s. Here, we present continuous records of calcium (Ca2+), sodium (Na+), ammonium (NH4+), nitrate (NO3-) and electrolytic conductivity at 1 mm depth resolution from the NGRIP (North Greenland Ice Core Project) and NEEM (North Greenland Eemian Ice Drilling) ice cores produced by the Bern Continuous Flow Analysis group in the years 2000 to 2011 (Erhardt et al., 2021). Both of the records were previously used in a number of studies but were never published in full 1 mm resolution. Alongside the 1 mm datasets we provide decadal averages, a detailed description of the methods, relevant references, an assessment of the quality of the data and its usable resolution. Along the way we will also give some historical context on the development of the Bern CFA system. The data is available in full 1 mm and 10-year-averaged resolution on PANGAEA (https://doi.org/10.1594/PANGAEA.935838, Erhardt et al., 2021)
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Heaton, Timothy J., Peter Köhler, Martin Butzin, Edouard Bard, Ron W. Reimer, William E. N. Austin, Christopher Bronk Ramsey et al. « Marine20—The Marine Radiocarbon Age Calibration Curve (0–55,000 cal BP) ». Radiocarbon 62, no 4 (août 2020) : 779–820. http://dx.doi.org/10.1017/rdc.2020.68.
Texte intégralRésumé :
ABSTRACTThe concentration of radiocarbon (14C) differs between ocean and atmosphere. Radiocarbon determinations from samples which obtained their 14C in the marine environment therefore need a marine-specific calibration curve and cannot be calibrated directly against the atmospheric-based IntCal20 curve. This paper presents Marine20, an update to the internationally agreed marine radiocarbon age calibration curve that provides a non-polar global-average marine record of radiocarbon from 0–55 cal kBP and serves as a baseline for regional oceanic variation. Marine20 is intended for calibration of marine radiocarbon samples from non-polar regions; it is not suitable for calibration in polar regions where variability in sea ice extent, ocean upwelling and air-sea gas exchange may have caused larger changes to concentrations of marine radiocarbon. The Marine20 curve is based upon 500 simulations with an ocean/atmosphere/biosphere box-model of the global carbon cycle that has been forced by posterior realizations of our Northern Hemispheric atmospheric IntCal20 14C curve and reconstructed changes in CO2 obtained from ice core data. These forcings enable us to incorporate carbon cycle dynamics and temporal changes in the atmospheric 14C level. The box-model simulations of the global-average marine radiocarbon reservoir age are similar to those of a more complex three-dimensional ocean general circulation model. However, simplicity and speed of the box model allow us to use a Monte Carlo approach to rigorously propagate the uncertainty in both the historic concentration of atmospheric 14C and other key parameters of the carbon cycle through to our final Marine20 calibration curve. This robust propagation of uncertainty is fundamental to providing reliable precision for the radiocarbon age calibration of marine based samples. We make a first step towards deconvolving the contributions of different processes to the total uncertainty; discuss the main differences of Marine20 from the previous age calibration curve Marine13; and identify the limitations of our approach together with key areas for further work. The updated values for ΔR, the regional marine radiocarbon reservoir age corrections required to calibrate against Marine20, can be found at the data base http://calib.org/marine/.
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Winton, V. Holly L., Alison Ming, Nicolas Caillon, Lisa Hauge, Anna E. Jones, Joel Savarino, Xin Yang et Markus M. Frey. « Deposition, recycling, and archival of nitrate stable isotopes between the air–snow interface : comparison between Dronning Maud Land and Dome C, Antarctica ». Atmospheric Chemistry and Physics 20, no 9 (15 mai 2020) : 5861–85. http://dx.doi.org/10.5194/acp-20-5861-2020.
Texte intégralRésumé :
Abstract. The nitrogen stable isotopic composition in nitrate (δ15N-NO3-) measured in ice cores from low-snow-accumulation regions in East Antarctica has the potential to provide constraints on past ultraviolet (UV) radiation and thereby total column ozone (TCO) due to the sensitivity of nitrate (NO3-) photolysis to UV radiation. However, understanding the transfer of reactive nitrogen at the air–snow interface in polar regions is paramount for the interpretation of ice core records of δ15N-NO3- and NO3- mass concentrations. As NO3- undergoes a number of post-depositional processes before it is archived in ice cores, site-specific observations of δ15N-NO3- and air–snow transfer modelling are necessary to understand and quantify the complex photochemical processes at play. As part of the Isotopic Constraints on Past Ozone Layer Thickness in Polar Ice (ISOL-ICE) project, we report new measurements of NO3- mass concentration and δ15N-NO3- in the atmosphere, skin layer (operationally defined as the top 5 mm of the snowpack), and snow pit depth profiles at Kohnen Station, Dronning Maud Land (DML), Antarctica. We compare the results to previous studies and new data, presented here, from Dome C on the East Antarctic Plateau. Additionally, we apply the conceptual 1D model of TRansfer of Atmospheric Nitrate Stable Isotopes To the Snow (TRANSITS) to assess the impact of NO3- recycling on δ15N-NO3- and NO3- mass concentrations archived in snow and firn. We find clear evidence of NO3- photolysis at DML and confirmation of previous theoretical, field, and laboratory studies that UV photolysis is driving NO3- recycling and redistribution at DML. Firstly, strong denitrification of the snowpack is observed through the δ15N-NO3- signature, which evolves from the enriched snowpack (−3 ‰ to 100 ‰), to the skin layer (−20 ‰ to 3 ‰), to the depleted atmosphere (−50 ‰ to −20 ‰), corresponding to mass loss of NO3- from the snowpack. Based on the TRANSITS model, we find that NO3- is recycled two times, on average, before it is archived in the snowpack below 15 cm and within 0.75 years (i.e. below the photic zone). Mean annual archived δ15N-NO3- and NO3- mass concentration values are 50 ‰ and 60 ng g−1, respectively, at the DML site. We report an e-folding depth (light attenuation) of 2–5 cm for the DML site, which is considerably lower than Dome C. A reduced photolytic loss of NO3- at DML results in less enrichment of δ15N-NO3- than at Dome C mainly due to the shallower e-folding depth but also due to the higher snow accumulation rate based on TRANSITS-modelled sensitivities. Even at a relatively low snow accumulation rate of 6 cm yr−1 (water equivalent; w.e.), the snow accumulation rate at DML is great enough to preserve the seasonal cycle of NO3- mass concentration and δ15N-NO3-, in contrast to Dome C where the depth profiles are smoothed due to longer exposure of surface snow layers to incoming UV radiation before burial. TRANSITS sensitivity analysis of δ15N-NO3- at DML highlights that the dominant factors controlling the archived δ15N-NO3- signature are the e-folding depth and snow accumulation rate, with a smaller role from changes in the snowfall timing and TCO. Mean TRANSITS model sensitivities of archived δ15N-NO3- at the DML site are 100 ‰ for an e-folding depth change of 8 cm, 110 ‰ for an annual snow accumulation rate change of 8.5 cm yr−1 w.e., 10 ‰ for a change in the dominant snow deposition season between winter and summer, and 10 ‰ for a TCO change of 100 DU (Dobson units). Here we set the framework for the interpretation of a 1000-year ice core record of δ15N-NO3- from DML. Ice core δ15N-NO3- records at DML will be less sensitive to changes in UV than at Dome C; however the higher snow accumulation rate and more accurate dating at DML allows for higher-resolution δ15N-NO3- records.
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Hide, Raymond. « Forecasting short-term changes in the Earth's rotation ». Symposium - International Astronomical Union 128 (1988) : 287–88. http://dx.doi.org/10.1017/s007418090011962x.
Texte intégralRésumé :
Summary of PosterIt has long been appreciated that atmospheric motions must contribute to the excitation of fluctuations in the Earth's rotation (Munk and MacDonald 1960, Lambeck 1980, Rochester 1984) but the exploitation of modern meteorological data, collected largely to meet the demands of daily global weather forecasting, in the routine evaluation of angular momentum exchange between the atmosphere and the solid Earth was not initiated until comparatively recently (Hide et al. 1980). This procedure constitutes a necessary step towards the accurate separation of these features of the observed non-tidal changes in the length of day and polar motion and that are of meteorological origin from those that must be attributed to other geophysical processes, such as angular momentum transfer between the solid Earth and other fluid regions of the Earth (liquid metallic core, oceans, etc.), and to changes in the inertia tensor of the solid Earth associated with earthquakes, melting of ice, etc.
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Münch, Thomas, et Thomas Laepple. « What climate signal is contained in decadal- to centennial-scale isotope variations from Antarctic ice cores ? » Climate of the Past 14, no 12 (20 décembre 2018) : 2053–70. http://dx.doi.org/10.5194/cp-14-2053-2018.
Texte intégralRésumé :
Abstract. Ice-core-based records of isotopic composition are a proxy for past temperatures and can thus provide information on polar climate variability over a large range of timescales. However, individual isotope records are affected by a multitude of processes that may mask the true temperature variability. The relative magnitude of climate and non-climate contributions is expected to vary as a function of timescale, and thus it is crucial to determine those temporal scales on which the actual signal dominates the noise. At present, there are no reliable estimates of this timescale dependence of the signal-to-noise ratio (SNR). Here, we present a simple method that applies spectral analyses to stable-isotope data from multiple cores to estimate the SNR, and the signal and noise variability, as a function of timescale. The method builds on separating the contributions from a common signal and from local variations and includes a correction for the effects of diffusion and time uncertainty. We apply our approach to firn-core arrays from Dronning Maud Land (DML) in East Antarctica and from the West Antarctic Ice Sheet (WAIS). For DML and decadal to multi-centennial timescales, we find an increase in the SNR by nearly 1 order of magnitude (∼0.2 at decadal and ∼1.0 at multi-centennial scales). The estimated spectrum of climate variability also shows increasing variability towards longer timescales, contrary to what is traditionally inferred from single records in this region. In contrast, the inferred variability spectrum for WAIS stays close to constant over decadal to centennial timescales, and the results even suggest a decrease in SNR over this range of timescales. We speculate that these differences between DML and WAIS are related to differences in the spatial and temporal scales of the isotope signal, highlighting the potentially more homogeneous atmospheric conditions on the Antarctic Plateau in contrast to the marine-influenced conditions on WAIS. In general, our approach provides a methodological basis for separating local proxy variability from coherent climate variations, which is applicable to a large set of palaeoclimate records.
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Oteiza, Patricia I., Alejandra G. Erlejman, Sandra V. Verstraeten, Carl L. Keen et César G. Fraga. « Flavonoid-membrane Interactions : A Protective Role of Flavonoids at the Membrane Surface ? » Clinical and Developmental Immunology 12, no 1 (2005) : 19–25. http://dx.doi.org/10.1080/10446670410001722168.
Texte intégralRésumé :
Flavonoids can exert beneficial health effects through multiple mechanisms. In this paper, we address the important, although not fully understood, capacity of flavonoids to interact with cell membranes. The interactions of polyphenols with bilayers include: (a) the partition of the more non-polar compounds in the hydrophobic interior of the membrane, and (b) the formation of hydrogen bonds between the polar head groups of lipids and the more hydrophilic flavonoids at the membrane interface. The consequences of these interactions are discussed. The induction of changes in membrane physical properties can affect the rates of membrane lipid and protein oxidation. The partition of certain flavonoids in the hydrophobic core can result in a chain breaking antioxidant activity. We suggest that interactions of polyphenols at the surface of bilayers through hydrogen bonding, can act to reduce the access of deleterious molecules (i.e. oxidants), thus protecting the structure and function of membranes.
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Uglietti, Chiara, Alexander Zapf, Theo Manuel Jenk, Michael Sigl, Sönke Szidat, Gary Salazar et Margit Schwikowski. « Radiocarbon dating of glacier ice : overview, optimisation, validation and potential ». Cryosphere 10, no 6 (21 décembre 2016) : 3091–105. http://dx.doi.org/10.5194/tc-10-3091-2016.
Texte intégralRésumé :
Abstract. High-altitude glaciers and ice caps from midlatitudes and tropical regions contain valuable signals of past climatic and environmental conditions as well as human activities, but for a meaningful interpretation this information needs to be placed in a precise chronological context. For dating the upper part of ice cores from such sites, several relatively precise methods exist, but they fail in the older and deeper parts, where plastic deformation of the ice results in strong annual layer thinning and a non-linear age–depth relationship. If sufficient organic matter such as plant, wood or insect fragments were found, radiocarbon (14C) analysis would have thus been the only option for a direct and absolute dating of deeper ice core sections. However such fragments are rarely found and, even then, they would not be very likely to occur at the desired depth and resolution. About 10 years ago, a new, complementary dating tool was therefore introduced by our group. It is based on extracting the µg-amounts of the water-insoluble organic carbon (WIOC) fraction of carbonaceous aerosols embedded in the ice matrix for subsequent 14C dating. Since then this new approach has been improved considerably by reducing the measurement time and improving the overall precision. Samples with ∼ 10 µg WIOC mass can now be dated with reasonable uncertainty of around 10–20 % (variable depending on sample age). This requires about 300 to 800 g of ice for WIOC concentrations typically found in midlatitude and low-latitude glacier ice. Dating polar ice with satisfactory age precision is still not possible since WIOC concentrations are around 1 order of magnitude lower. The accuracy of the WIOC 14C method was validated by applying it to independently dated ice. With this method, the deepest parts of the ice cores from Colle Gnifetti and the Mt Ortles glacier in the European Alps, Illimani glacier in the Bolivian Andes, Tsambagarav ice cap in the Mongolian Altai, and Belukha glacier in the Siberian Altai have been dated. In all cases a strong annual layer thinning towards the bedrock was observed and the oldest ages obtained were in the range of 10 000 years. WIOC 14C dating was not only crucial for interpretation of the embedded environmental and climatic histories, but additionally gave a better insight into glacier flow dynamics close to the bedrock and past glacier coverage. For this the availability of multiple dating points in the deepest parts was essential, which is the strength of the presented WIOC 14C dating method, allowing determination of absolute ages from principally every piece of ice.
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Rashid, Harunur, Qian Qian Lu, Min Zeng, Yang Wang et Zhao Wu Zhang. « Sea-Surface Characteristics of the Newfoundland Basin of the Northwest Atlantic Ocean during the Last 145,000 Years : A Study Based on the Sedimentological and Paleontological Proxies ». Applied Sciences 11, no 8 (8 avril 2021) : 3343. http://dx.doi.org/10.3390/app11083343.
Texte intégralRésumé :
Dramatic changes occur in the sea-surface characteristics (i.e., temperature and salinity) and freshwater input due to the interaction of cold and fresh Labrador Current and warm and salty North Atlantic Current (NAC) on the southeast Grand Banks. As a result, the biological productivity and seasonal stratification of the upper water masses are intensified. Such changes must have been more dramatic during the glacial times due to the penetration of the Polar and Arctic fronts and southward migration of the Gulf Stream/NAC. However, the extent to which such changes impacted the sea-surface characteristics in the Newfoundland Basin is poorly known. We report changes in the sea-surface characteristics using a piston core (Hu9007-08) collected from the Milne seamount during the last 145,000 years. Heinrich layers H1, H2, H4, and H5 and H11 within the MIS3 and at the penultimate deglaciation were identified by the ice-rafted detritus (IRD) and Neogloboquadrina pachyderma peaks and lighter oxygen isotopes. Rapid turnover by the foraminiferal species with distinct depth habitats and ecological niches in the mixed-layer and thermocline suggests an interplay between the polar and subpolar water masses during the Heinrich and non-Heinrich periods. Only two North Atlantic-wide cooling events, C24 and C21, in which the latter event linked to the minor IRD event during the marine isotope stage (MIS) 5 in Hu90-08, compared to the eight events in the eastern subpolar gyre (e.g., ODP site 984). Millennial-scale N. pachyderma variability in the western subpolar gyre appears to be absent in the eastern subpolar gyre during the MIS3 suggesting the occasional presence of salty and warm water by the NAC inflow, implying a different climate state between the western and eastern subpolar gyre. Although T. quinqueloba data are fragmentary, there are differences between the western and eastern subpolar gyre in addition to the differences within the western subpolar gyre during MIS5 that might imply a variable influence by the subpolar water. This finding suggests that the influence by the NAC outweighs the impact of cold and fresh polar water in the northern northwest Atlantic during the MIS5.
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Mulvaney, R., D. A. Peel et A. P. Reid. « 26-Year High Resolution Profile Of Major Anions In Snow From Coats Land, Antarctica ». Annals of Glaciology 14 (1990) : 349. http://dx.doi.org/10.3189/s0260305500009198.
Texte intégralRésumé :
In January 1987, an 7.8 m core, with an age at the bottom of 26 years, was collected from a site approximately 150 km inland from Halley Station (77°02.2′S, 22°32′W; altitude 1862 m a.s.l.); 10 m temperature ≈ −30°C; accumulation rate ≈ 14 g cm−2 a−1). The site lies some 140 km from the coast of the Weddell Sea and within the area bounded in winter by the polar vortex.The core has been analysed at a frequency of ≈28 samples per accumulation year for sulphate, nitrate and chloride, and has been dated stratigraphically from the clear seasonal cycles in non sea salt sulphate (Mulvaney and Peel, 1988). With this resolution it is possible to examine the seasonal pattern of deposition of chemical species and their phase relationships.Of particular interest is the possibility that ice cores may preserve evidence for disturbances in tropospheric chemistry, associated with the recent spring-time depletion of stratospheric ozone. It has been proposed that this is accompanied by a denitrification of the stratosphere during the winter months, implying enhanced levels of NOX in the late winter/spring troposphere and in precipitation. Our data reveal a strong seasonal signal in nitrate deposition, apparently peaking in spring. Similar behaviour has been reported by Wagenbach and others (1988) for nitrate in the atmospheric aerosol, in a 3-year sequence (1983–86) from Georg von Neumayer Station (70°S, 8°W). There does not appear to be any evidence in our data of an increase in spring-time nitrate deposition since the appearance of the Antarctic ozone hole in 1978.
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Mulvaney, R., D. A. Peel et A. P. Reid. « 26-Year High Resolution Profile Of Major Anions In Snow From Coats Land, Antarctica ». Annals of Glaciology 14 (1990) : 349. http://dx.doi.org/10.1017/s0260305500009198.
Texte intégralRésumé :
In January 1987, an 7.8 m core, with an age at the bottom of 26 years, was collected from a site approximately 150 km inland from Halley Station (77°02.2′S, 22°32′W; altitude 1862 m a.s.l.); 10 m temperature ≈ −30°C; accumulation rate ≈ 14 g cm−2 a−1). The site lies some 140 km from the coast of the Weddell Sea and within the area bounded in winter by the polar vortex. The core has been analysed at a frequency of ≈28 samples per accumulation year for sulphate, nitrate and chloride, and has been dated stratigraphically from the clear seasonal cycles in non sea salt sulphate (Mulvaney and Peel, 1988). With this resolution it is possible to examine the seasonal pattern of deposition of chemical species and their phase relationships. Of particular interest is the possibility that ice cores may preserve evidence for disturbances in tropospheric chemistry, associated with the recent spring-time depletion of stratospheric ozone. It has been proposed that this is accompanied by a denitrification of the stratosphere during the winter months, implying enhanced levels of NOX in the late winter/spring troposphere and in precipitation. Our data reveal a strong seasonal signal in nitrate deposition, apparently peaking in spring. Similar behaviour has been reported by Wagenbach and others (1988) for nitrate in the atmospheric aerosol, in a 3-year sequence (1983–86) from Georg von Neumayer Station (70°S, 8°W). There does not appear to be any evidence in our data of an increase in spring-time nitrate deposition since the appearance of the Antarctic ozone hole in 1978.
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Salvi, Lavish, Chetan Kumar Dubey, Kapil Sharma, Devendra Nagar, Monika Meghani, Saloni Goyal, Jagdish Chandra Nagar et Archana Sharma. « A Synthesis, Properties and Application as a Possible Drug Delivery Systems Dendrimers – A Review ». Asian Journal of Pharmaceutical Research and Development 8, no 2 (13 avril 2020) : 107–13. http://dx.doi.org/10.22270/ajprd.v8i2.676.
Texte intégralRésumé :
Dendrimer is derived from the Greek word “dendron” which is used for tree and from the Greek suffix “mer” (segment) which describes the synthetic, three-dimensional molecules having branching parts.“A dendrimer is generally described as a macromolecule, which is characterized by its dendritic and hyper branched 3D structure that offers a high degree of surface functionality and versatility.” Dendrimers possess three distinguishable architectural components i.e. an interior core, interior layer(generations) composed of repeating units radially attached to the interior core, and exterior (terminal functionality) attached to outermost interior generation (Fig. 1). The higher generation dendrimers, due to their globular structure, occupy a smaller hydrodynamic volume compared to the corresponding linear polymers. The dendritic structure is characterized by layer between each generation.Dendrimers are generally prepared using either a divergent method or a convergent one. There is a fundamental difference between these two construction concepts. Dendrimers shows the various properties such as structure, shape, aqueous solubility, non-polar solubility, &architecture. Dendrimer can be classification on the basis of their shape, structure, branching, solubility, chirality and attachment. Dendrimer can be differentiated on the basis of their shape, end functional groups and internal cavities.
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Mosley-Thompson, Ellen, Lonnie G. Thompson, John Paskievitch et Pieter M. Grootes. « Shallow-Core Analysis and Pit Studies at Siple Station, Antarctica : Implications for Extraction of a 500 Year Proxy Climate Record (Abstract) ». Annals of Glaciology 10 (1988) : 212. http://dx.doi.org/10.1017/s0260305500004584.
Texte intégralRésumé :
Two cores, 302 and 132 m long, were drilled 1.5 km up-wind from Siple Station (75°55'S, 84°15'W) during the 1985–86 austral summer. These cores are expected to contain an annually resolvable 500 year record of atmospheric constituents. The entire length of these cores will be analyzed for insoluble particulate concentrations, liquid conductivity, and oxygen-isotope ratios. Anion concentrations (Cl−, SO4 2−) will be measured in selected sections. The extraction of a proxy climate history from these cores requires an assessment of the temporal quality and the spatial variability of the preserved record. This paper presents this assessment, which is based upon an extensive investigation of both pit and shallow-core (20 m) records. To complement the two deeper cores, eight 20 m cores were drilled and three pits were sampled. The pits were excavated up-wind of the drill site at distances between 0.5 and 1.0 km. In the central pit, 2.8 m deep, walls A and C (each 1 m wide) were positioned parallel to the prevailing wind, with wall B (4 m wide) perpendicular to the prevailing wind. The stratigraphy of wall B was mapped and continuous vertical profiles of samples were collected for microparticle concentrations (MPC), oxygen-isotopic ratios (δ18O), anion chemistry (AC) and liquid conductivity (LC). Three 20 m cores, each associated with a vertical profile of pit samples, were drilled 0.5 m behind wall B. In addition, part of one core was sampled for beta radioactivity in order to isolate the 1965–66 horizon which resulted from atmospheric thermonuclear testing. Pits 2 and 3 were both 2 m deep and consisted of two 1 m wide perpendicular walls. Each wall was sampled for MPC, δ18O, LC and AC, and one 20 m core was drilled behind each wall. The visible stratigraphy of both walls in pit 3 was mapped. Densities were measured in each pit. The results from the pit and shallow-core analyses lead to the following conclusions. The high frequency of storm events, often associated with winds in excess of 50 knots, leads to substantial drifting. The result is that stratigraphic features in the uppermost annual layer (e.g. crusts, mass-loss layers, hard layers, etc.) exhibit substantial vertical deviations and, on occasion, are laterally discontinuous. Substantial variation may occur over very short distances: e.g. a 2 cm thick melt feature pinched out over a distance of 10 cm. Interestingly, the stratigraphy in firn older than 1 year exhibits much less lateral variation. In fact, several distinct visible stratigraphic features can be correlated among most of the shallow cores and the two deeper cores. Examples will be presented. These features provide excellent time-stratigraphic markers for cross-core comparisons. Of special interest is the presence of a major melt feature, first reported by Swiss investigators in 1983. This feature, present at four different drill sites, allows time-stratigraphic correlation and yields nearly identical 10 year accumulation rates for all four sites. Such spatial continuity means that accumulation histories reconstructed from the two deeper cores should be representative of this area. The MPC measurements (particles per ml of water) are the lowest that have been measured. No measurements of atmospheric particulate concentrations are available, but the low concentrations probably result from the high annual accumulation (∼0.55 m a−1, water equivalent), which dilutes the atmospheric input signal of particulates preserved in the firn. Swiss investigators reported similar results for the dilution of volcanic acids (e.g. SO4 2−), which muted the conductivity signals in a 201 m Siple Station core. The low concentrations make particles difficult to use for dating Siple cores. On the other hand, the low background levels of particulates may make Siple Station an excellent site for monitoring major atmospheric turbidity events (e.g. prolonged periods of desertification or volcanic activity). The high annual accumulation leads to the excellent preservation of the δ18O annual signal. The annual δ18O signal ranges from 15‰ at the surface to 8‰ at 122 m, so it is probable that the annual δ18O signal will be preserved over the entire 500 year record, allowing nearly absolute dating of the core. The potential of a nearly absolute time-scale, coupled with detailed particulate, liquid-conductivity and δ18O histories, should provide the first proxy climate record from this area. This 500 year history will complement similar records anticipated from the Antarctic Peninsula (∼73° S) and that recently constructed from the James Ross Island ice cap. A more global climatic picture of the last 500 years may be obtained when these high-resolution Antarctic ice-core records are integrated with non-polar records (e.g. the Quelccaya ice cap, Peru, and the Dunde ice cap, China) of similar quality.
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Mosley-Thompson, Ellen, Lonnie G. Thompson, John Paskievitch et Pieter M. Grootes. « Shallow-Core Analysis and Pit Studies at Siple Station, Antarctica : Implications for Extraction of a 500 Year Proxy Climate Record (Abstract) ». Annals of Glaciology 10 (1988) : 212. http://dx.doi.org/10.3189/s0260305500004584.
Texte intégralRésumé :
Two cores, 302 and 132 m long, were drilled 1.5 km up-wind from Siple Station (75°55'S, 84°15'W) during the 1985–86 austral summer. These cores are expected to contain an annually resolvable 500 year record of atmospheric constituents. The entire length of these cores will be analyzed for insoluble particulate concentrations, liquid conductivity, and oxygen-isotope ratios. Anion concentrations (Cl−, SO42−) will be measured in selected sections. The extraction of a proxy climate history from these cores requires an assessment of the temporal quality and the spatial variability of the preserved record. This paper presents this assessment, which is based upon an extensive investigation of both pit and shallow-core (20 m) records.To complement the two deeper cores, eight 20 m cores were drilled and three pits were sampled. The pits were excavated up-wind of the drill site at distances between 0.5 and 1.0 km. In the central pit, 2.8 m deep, walls A and C (each 1 m wide) were positioned parallel to the prevailing wind, with wall B (4 m wide) perpendicular to the prevailing wind. The stratigraphy of wall B was mapped and continuous vertical profiles of samples were collected for microparticle concentrations (MPC), oxygen-isotopic ratios (δ18O), anion chemistry (AC) and liquid conductivity (LC). Three 20 m cores, each associated with a vertical profile of pit samples, were drilled 0.5 m behind wall B. In addition, part of one core was sampled for beta radioactivity in order to isolate the 1965–66 horizon which resulted from atmospheric thermonuclear testing. Pits 2 and 3 were both 2 m deep and consisted of two 1 m wide perpendicular walls. Each wall was sampled for MPC, δ18O, LC and AC, and one 20 m core was drilled behind each wall. The visible stratigraphy of both walls in pit 3 was mapped. Densities were measured in each pit. The results from the pit and shallow-core analyses lead to the following conclusions.The high frequency of storm events, often associated with winds in excess of 50 knots, leads to substantial drifting. The result is that stratigraphic features in the uppermost annual layer (e.g. crusts, mass-loss layers, hard layers, etc.) exhibit substantial vertical deviations and, on occasion, are laterally discontinuous. Substantial variation may occur over very short distances: e.g. a 2 cm thick melt feature pinched out over a distance of 10 cm. Interestingly, the stratigraphy in firn older than 1 year exhibits much less lateral variation. In fact, several distinct visible stratigraphic features can be correlated among most of the shallow cores and the two deeper cores. Examples will be presented. These features provide excellent time-stratigraphic markers for cross-core comparisons. Of special interest is the presence of a major melt feature, first reported by Swiss investigators in 1983. This feature, present at four different drill sites, allows time-stratigraphic correlation and yields nearly identical 10 year accumulation rates for all four sites. Such spatial continuity means that accumulation histories reconstructed from the two deeper cores should be representative of this area.The MPC measurements (particles per ml of water) are the lowest that have been measured. No measurements of atmospheric particulate concentrations are available, but the low concentrations probably result from the high annual accumulation (∼0.55 m a−1, water equivalent), which dilutes the atmospheric input signal of particulates preserved in the firn. Swiss investigators reported similar results for the dilution of volcanic acids (e.g. SO42−), which muted the conductivity signals in a 201 m Siple Station core. The low concentrations make particles difficult to use for dating Siple cores. On the other hand, the low background levels of particulates may make Siple Station an excellent site for monitoring major atmospheric turbidity events (e.g. prolonged periods of desertification or volcanic activity).The high annual accumulation leads to the excellent preservation of the δ18O annual signal. The annual δ18O signal ranges from 15‰ at the surface to 8‰ at 122 m, so it is probable that the annual δ18O signal will be preserved over the entire 500 year record, allowing nearly absolute dating of the core.The potential of a nearly absolute time-scale, coupled with detailed particulate, liquid-conductivity and δ18O histories, should provide the first proxy climate record from this area. This 500 year history will complement similar records anticipated from the Antarctic Peninsula (∼73° S) and that recently constructed from the James Ross Island ice cap. A more global climatic picture of the last 500 years may be obtained when these high-resolution Antarctic ice-core records are integrated with non-polar records (e.g. the Quelccaya ice cap, Peru, and the Dunde ice cap, China) of similar quality.
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FUJII, Yoshiyuki. « Polar ice core and palaeo-environment. » Journal of Geography (Chigaku Zasshi) 98, no 5 (1989) : 535–61. http://dx.doi.org/10.5026/jgeography.98.5_535.
Texte intégral
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Schoen, Deborah. « Learning From Polar Ice Core Research ». Environmental Science & ; Technology 33, no 7 (avril 1999) : 160A—163A. http://dx.doi.org/10.1021/es992771c.
Texte intégral
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Savarino, J., W. C. Vicars, M. Legrand, S. Preunkert, B. Jourdain, M. M. Frey, A. Kukui, N. Caillon et J. Gil Roca. « Oxygen isotope mass balance of atmospheric nitrate at Dome C, East Antarctica, during the OPALE campaign ». Atmospheric Chemistry and Physics Discussions 15, no 17 (7 septembre 2015) : 24041–83. http://dx.doi.org/10.5194/acpd-15-24041-2015.
Texte intégralRésumé :
Abstract. Variations in the stable oxygen isotope composition of atmospheric nitrate act as novel tools for studying oxidative processes taking place in the troposphere. They provide both qualitative and quantitative constraints on the pathways determining the fate of atmospheric nitrogen oxides (NO + NO2 = NOx). The unique and distinctive 17O-excess (Δ17O = δ17O − 0.52 × δ18O) of ozone, which is transferred to NOx via oxidation, is a particularly useful isotopic fingerprint in studies of NOx transformations. Constraining the propagation of 17O-excess within the NOx cycle is critical in polar areas where there exists the possibility of extending atmospheric investigations to the glacial/interglacial time scale using deep ice core records of nitrate. Here we present measurements of the comprehensive isotopic composition of atmospheric nitrate collected at Dome C (East Antarctic plateau) during the austral summer of 2011/12. Nitrate isotope analysis has been here combined for the first time with key precursors involved in nitrate production (NOx, O3, OH, HO2, RO2, etc.) and direct observations of the transferrable Δ17O of surface ozone, which was measured at Dome C throughout 2012 using our recently developed analytical approach. Assuming that nitrate is mainly produced in Antarctica in summer through the OH + NO2 pathway and using concurrent measurements of OH and NO2, we calculated a Δ17O signature for nitrate in the order of (21–22 ± 3) ‰. These values are lower than the measured values that ranged between 27 and 31 ‰. This discrepancy between expected and observed Δ17O(NO3−) values suggests the existence of an unknown process that contributes significantly to the atmospheric nitrate budget over this east Antarctic region.
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Savarino, Joël, William C. Vicars, Michel Legrand, Suzanne Preunkert, Bruno Jourdain, Markus M. Frey, Alexandre Kukui, Nicolas Caillon et Jaime Gil Roca. « Oxygen isotope mass balance of atmospheric nitrate at Dome C, East Antarctica, during the OPALE campaign ». Atmospheric Chemistry and Physics 16, no 4 (3 mars 2016) : 2659–73. http://dx.doi.org/10.5194/acp-16-2659-2016.
Texte intégralRésumé :
Abstract. Variations in the stable oxygen isotope composition of atmospheric nitrate act as novel tools for studying oxidative processes taking place in the troposphere. They provide both qualitative and quantitative constraints on the pathways determining the fate of atmospheric nitrogen oxides (NO + NO2 = NOx). The unique and distinctive 17O excess (Δ17O = δ17O − 0.52 × δ18O) of ozone, which is transferred to NOx via oxidation, is a particularly useful isotopic fingerprint in studies of NOx transformations. Constraining the propagation of 17O excess within the NOx cycle is critical in polar areas, where there exists the possibility of extending atmospheric investigations to the glacial–interglacial timescale using deep ice core records of nitrate. Here we present measurements of the comprehensive isotopic composition of atmospheric nitrate collected at Dome C (East Antarctic Plateau) during the austral summer of 2011/2012. Nitrate isotope analysis has been here combined for the first time with key precursors involved in nitrate production (NOx, O3, OH, HO2, RO2, etc.) and direct observations of the transferrable Δ17O of surface ozone, which was measured at Dome C throughout 2012 using our recently developed analytical approach. Assuming that nitrate is mainly produced in Antarctica in summer through the OH + NO2 pathway and using concurrent measurements of OH and NO2, we calculated a Δ17O signature for nitrate on the order of (21–22 ± 3) ‰. These values are lower than the measured values that ranged between 27 and 31 ‰. This discrepancy between expected and observed Δ17O(NO3−) values suggests the existence of an unknown process that contributes significantly to the atmospheric nitrate budget over this East Antarctic region. However, systematic errors or false isotopic balance transfer functions are not totally excluded.