Polar Ice Core Samples Research Articles

EBSD analysis of subgrain boundaries and dislocation slip systems in Antarctic and Greenland ice

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.

A new method based on low background instrumental neutron activation analysis for major, trace and ultra-trace element determination in atmospheric mineral dust from polar ice cores

Dust found in polar ice core samples present extremely low concentrations, in addition the availability of such samples is usually strictly limited. For these reasons the chemical and physical analysis of polar ice cores is an analytical challenge. In this work a new method based on low background instrumental neutron activation analysis (LB-INAA) for the multi-elemental characterization of the insoluble fraction of dust from polar ice cores is presented. Thanks to an accurate selection of the most proper materials and procedures it was possible to reach unprecedented analytical performances, suitable for ice core analyses. The method was applied to Antarctic ice core samples. Five samples of atmospheric dust (μg size) from ice sections of the Antarctic Talos Dome ice core were prepared and analyzed. A set of 37 elements was quantified, spanning from all the major elements (Na, Mg, Al, Si, K, Ca, Ti, Mn and Fe) to trace ones, including 10 (La, Ce, Nd, Sm, Eu, Tb, Ho, Tm, Yb and Lu) of the 14 natural occurring lanthanides. The detection limits are in the range of 10−13–10−6 g, improving previous results of 1–3 orders of magnitude depending on the element; uncertainties lies between 4% and 60%.

Low-background neutron activation analysis: a powerful tool for atmospheric mineral dust analysis in ice cores

The application of instrumental neutron activation analysis (INAA) for multi-elemental analysis of samples of extremely reduced mass such as dust samples extracted from ice cores requires specific efforts towards the development of a “low level counting” analytical technique. An analytical protocol specifically designed for this kind of samples, based on low-background INAA (LBNAA) is here presented. A first application of the method was successfully performed on samples from the new alpine firn core NextData-LYS12. Sub-ng detection limits were reached for many elements. According to this point the technique is also potentially suitable to be applied to polar ice core samples.

A method for analysis of vanillic acid in polar ice cores

Abstract. Biomass burning generates a wide range of organic compounds that are transported via aerosols to the polar ice sheets. Vanillic acid is a product of conifer lignin combustion, which has previously been observed in laboratory and ambient biomass burning aerosols. In this study a method was developed for analysis of vanillic acid in melted polar ice core samples. Vanillic acid was chromatographically separated using reversed-phase liquid chromatography (HPLC) and detected using electrospray ionization–triple quadrupole mass spectrometry (ESI-MS/MS). Using a 100 μL injection loop and analysis time of 4 min, we obtained a detection limit of 77 ppt (parts per trillion by mass) and an analytical precision of ±10%. Measurements of vanillic acid in Arctic ice core samples from the Siberian Akademii Nauk core are shown as an example application of the method.

Memory and the Anthropocene

The increase in carbon-dioxide emissions through the burning of fossil fuels and its effects of global warming has left a geological record, as shown by polar ice core samples that date from the mid-to-late-eighteenth century. This has prompted Paul J. Crutzen and Eugene F. Stoermer (in 2000 and 2002) to identify the end of the previous geological epoch, the Holocene, the warmer period of 10-12 millennia that succeeded the ice age of the Pleistocene. Crutzen and Stoermer have identified the A...

Complete determination of ice crystal orientation using Laue X-ray diffraction method

AbstractIce crystal orientation fabric data from ice cores contain important information concerning the internal structure and the flow behaviour of ice sheets. When ice cores are recovered from the Antarctic and Greenland ice sheets, crystal orientation measurements are performed immediately to obtain fundamental physical property information. In the past, we have measured thec-axis orientation of ice crystals by a manual optical method using a universal stage. This method is very time-consuming, involving tedious work in a cold laboratory. Recently, automated systems have been developed that enable measurement ofc-axis orientation, grain size and other microstructures. However, in order to detect the full crystal orientation of an ice crystal, we also need information on itsa-axis orientation. A variety of other crystal orientation measurement methods have previously been discussed, but some shortcomings for ice-core studies cannot be neglected. We have developed a crystal-orientation analysing device using the Laue X-ray diffraction method. As this device can measure the orientations of all crystal axes with high accuracy, it is possible to obtain new microstructure information on natural ice crystals. For the first time, we are able to quantify very low subgrain misorientation angles in polar ice-core samples, allowing us to investigate micro-deformation features of individual crystals. Here we discuss the analysis process, which is customized to measure standard ice thin sections, and show preliminary results.

Application of LA–ICP–MS in polar ice core studies

The direct determination of element signatures in polar ice core samples from Greenland by laser ablation with subsequent inductively coupled plasma mass spectrometry analysis has been investigated. A cryogenic sample chamber enables the element determination in ice directly from the solid (frozen) state. A procedure was developed to analyse up to 38 elements (traces: Mg, Al, Fe, Zn, Cd, Pb and rare earth elements; minor constituents: Na) in ice samples from Greenland with a previously unachievable spatial resolution of 4 mm along the core axis. This resolution is helpful to detect seasonal variations of element concentration in thin annual layers of deep ice. We report operating conditions and analytical performance of the experimental set up, the improvement of signal stability by (17)OH internal standardisation and application of a desolvation unit. Calibration of the system was performed with frozen multielement standard solutions along a special preparation procedure. Detection limits for the tracers Na, Mg (sea salt), Al (mineral dust) and Zn (anthropogenic source) are 0.1-1 microg kg(-1). Best detection limits in the range of 0.001-0.01 microg kg(-1 )were reached for Co, Pb and all rare earth elements. To validate the method, frozen standard reference materials were measured. The recovery is about +/-10%. Greenland ice core samples from different ages were analysed with the new technique. The results obtained by laser ablation were compared with values from solution analysis, available published data and the particle content. Most elements have shown good correlation with the particle content in the Greenland samples; however, differences could be seen between the values obtained by laser ablation and solution bulk analysis after a tri-acid digestion. The influence of particles is discussed. The high spatially resolved 2D mapping of element concentrations shows strong inhomogeneities along the core axis most probably due to seasonal variations of element deposition.

First sulfur isotope measurements in central Greenland ice cores along the preindustrial and industrial periods

Sulfur isotopes of sulfate have been measured in a discontinuous set of polar ice core samples from Summit, central Greenland, covering the preindustrial (from the fourteenth to the eighteenth century) and industrial (from 1872 to 1969 A.D.) periods. Results have been used to estimate the different source contributions to the deposited sulfate and their evolution along the last centuries. They indicate that the preindustrial background sulfate budget is slightly dominated on a year‐round average by marine biogenic emissions, amounting to close to half of the non‐sea‐salt sulfate (49%). The second contribution is provided by continental sources of secondary sulfate, including background volcanism and, to a lesser extent, continental biota (44% of the non‐sea‐salt sulfate). Sulfur emitted by relatively weak eruptions is found to be largely depleted in 34S compared to bulk volcanic S, suggesting an efficient washout of the heavier isotope during the tropospheric transport. The impact of human‐driven emissions on the sulfate deposited in central Greenland ice is visible in isotope data as early as 1870 A.D. The isotopic signature of anthropogenic sulfur deposited during the twentieth century is found to be constant (δ34S ≈ +3.0 ± 1.5‰), regardless of the changes of dominant source regions and emission processes. This signature is slightly but measurably lighter than the one reported for Arctic haze pollution events.

Analysis of ions in polar ice core samples by use of large injection volumes in ion chromatography

In the present study a non-suppressed ion chromatography system with conductivity detection was tested in terms of sampling effects, the effects on the ion separation efficiency and analysis detection limits to find optimum conditions for the determination of chloride, nitrate, sulfate, sodium, ammonium, potassium, calcium and magnesium ions in polar ice core samples.