Baker, Ian

Iliescu, Daniel

Obbard, Rachel

Chang, Hui

Bostick, Benjamin

Daghlian, Charles

English

Viljakainen, Anna

VTT Research System

Digimarkkinointi vahvistaa asiakasuskollisuutta

Dartmouth Digital Commons (Dartmouth College)

Dartmouth College Dartmouth Digital Commons Open Dartmouth: Published works by Dartmouth faculty Faculty Work 1-1-2005 Microstructural characterization of ice cores Ian Baker Dartmouth College Daniel Iliescu Dartmouth College Rachel Obbard Dartmouth College Hui Chang Dartmouth College Benjamin Bostick Columbia University See next page for additional authors Follow this and additional works at: https://digitalcommons.dartmouth.edu/facoa  Part of the Glaciology Commons Dartmouth Digital Commons Citation Baker, Ian; Iliescu, Daniel; Obbard, Rachel; Chang, Hui; Bostick, Benjamin; and Daghlian, Charles, "Microstructural characterization of ice cores" (2005). Open Dartmouth: Published works by Dartmouth faculty. 64. https://digitalcommons.dartmouth.edu/facoa/64 This Article is brought to you for free and open access by the Faculty Work at Dartmouth Digital Commons. It has been accepted for inclusion in Open Dartmouth: Published works by Dartmouth faculty by an authorized administrator of Dartmouth Digital Commons. For more information, please contact dartmouthdigitalcommons@groups.dartmouth.edu. Authors Ian Baker, Daniel Iliescu, Rachel Obbard, Hui Chang, Benjamin Bostick, and Charles Daghlian This article is available at Dartmouth Digital Commons: https://digitalcommons.dartmouth.edu/facoa/64 Microstructural characterization of ice coresIan BAKER,1 Daniel ILIESCU,1 Rachel OBBARD,1 Hui CHANG,1Benjamin BOSTICK,2 Charles P. DAGHLIAN31Thayer School of Engineering, Dartmouth College, Hanover, NH 03755-8000, USAE-mail: Ian.Baker@Dartmouth.edu2Department of Earth Sciences, Dartmouth College, Hanover, NH 03755-3571, USA3Rippel Electron Microscope Facility, Dartmouth College, Hanover, NH 03755-3810, USAABSTRACT. In this paper, we outline the use of Raman spectroscopy coupled with scanning confocaloptical microscopy for determining the microstructural location of impurities in ice-core specimens. Wealso demonstrate how the orientations of grains and the misorientations across grain boundaries can bedetermined to high precision for ice polycrystals using either selected area channeling patterns orelectron backscatter patterns in a scanning electron microscope.INTRODUCTIONThe precision and accuracy of the techniques used to deriveinformation on the concentrations of soluble ions and gasesin ice cores are remarkable. By coupling this informationwith annual-layering and stable-isotope data, researchershave produced depth–age correlated data on the concen-trations of particles, trace gases and soluble impurities forseveral polar ice cores (Legrand and Mayewski, 1997).Although there are still some questions about how accur-ately the impurity and gas concentrations measured in icecores reflect the concentrations in freshly deposited snow –due to snow/firn interactions with the atmosphere prior topore closure and to possible subsequent migration ordiffusion in the ice – ice-core studies have contributedgreatly to our understanding of paleoclimate and, hence, toforeseeing future climate change.By comparison, the microstructural location of impuritiesin ice has only recently been the focus of research (see, e.g.,Fukazawa and others, 1998; Wolff and others, 1988; Bakerand others, 2003; Barnes and others, 2003). The concen-tration and microstructural location of impurities in ice coresare both important since both can have significant impactson the mechanical properties, recrystallization, graingrowth, electrical conductivity, diffusion and texture (fabric)development of ice. The microstructural location of impu-rities could also very well influence the post-depositionalmovement of the impurities in ice cores, affecting theinterpretation of the ice-core record.In previous studies, we used energy-dispersive spectro-scopy (EDS) in a scanning electron microscope (SEM) toexamine a number of ice-core specimens from the Green-land Ice Sheet Project 2 (GISP2) core and found thatalthough impurities were located throughout the ice, Na andCl segregation occurred (occasionally with other elements)to the grain boundaries (GBs) and triple junctions (Cullenand Baker, 2001; Baker and Cullen, 2003; Baker and others,2003; Obbard and others, 2003). While EDS is useful foridentifying the elements present in ice, it cannot identifycompounds. For example, we commonly see sodium,potassium and chlorine X-ray fluorescence peaks in thesame spectra, but we cannot tell whether they indicateNaCl, KCl or dissociated Na+ and Cl– ions. And, while EDScan reveal magnesium, sulfur and oxygen peaks together inice, it is not possible to determine from EDS that MgSO4(rather than MgS) is present, partially due to the ubiquity ofthe oxygen peak in ice. Further, it is not possible to detectammonium ions or nitrates in ice using EDS since the X-rayfluorescence yields of light elements, such as nitrogen, arelow, and the nitrogen X-ray peak sits adjacent to theomnipresent large oxygen X-ray fluorescence peak from ice,and is easily obscured by it (Cullen and Baker, 2001; Obbardand others, 2003).This paper presents preliminary observations of impuritiesin GISP2 ice using Raman spectroscopy on a confocalscanning optical microscope. Unlike EDS, Raman spectro-scopy can identify ammonium, nitrates and sulfates as wellas other molecules and chemical species. Further, it ispossible to determine whether the Raman shift peaks, forexample, arise from nitric acid, sodium nitrate or potassiumnitrate.In addition, we demonstrate the use of two SEM methods,selected area channeling patterns (SACPs) and electronbackscatter patterns (EBSPs), for determining the orientationsof grains and, hence, for determining the misorientationsacross GBs in ice. Apart from their higher spatial andangular resolution, one advantage of obtaining crystalorientations in ice using SACPs and EBSPs, compared tothe traditional Rigsby universal stage, is that all threecrystallographic axes are determined, whereas the Rigsbystage only allows the c axis to be determined. The latter isinsufficient for distinguishing between the three main typesof recrystallization in ice, i.e. rotational recrystallization,nucleation and growth of new grains, and strain-inducedboundary migration (SIBM). Polygonization or rotationalrecrystallization, the formation of low-angle sub-boundariesby dislocations undergoing recovery, leads to only smallchanges in texture or fabric. Nucleation and growth of newgrains changes the texture significantly, while SIBM maychange the microstructure locally, but does not generallyaffect the overall texture (Jacka, 1984; Jacka and Maccag-nan, 1984; Budd and Jacka, 1989; de la Chapelle andothers, 1998). Using SACPs and EBSPs, it will be possible toclearly distinguish between these phenomena.The ultimate goal of the work is to develop and usetechniques to determine the microstructural location of allimpurities in natural ice and, if the impurities havesegregated to the GBs, determine if there are preferencesfor the impurities to segregate to some GBs and not toothers.Annals of Glaciology 42 2005 441SPECIMEN PREPARATIONSpecimens were prepared using previously established pro-cedures (Cullen and Baker, 2000), i.e. the ice was carefullyshaved with a razor blade, at –208C under a High EfficiencyParticle Air (HEPA)-filtered, laminar-flow hood followingstandard clean-room practices, into thin plates typically30mm15mm 2mm thick, which are flat, smooth andfree of scratches. Specimens were then frozen onto a brassmount and transported at temperatures below –208C.Raman spectroscopic analysis of the ice specimensutilized a Thermo Electron Nicolet Almega XR dispersiveRaman spectrometer with a 532 nm frequency doubledNd:YVO diode-pumped solid-state laser coupled to aconfocal scanning optical microscope. The system has anintegrated video camera that can be used to image the icespecimens which are mounted on a home-built, Peltiermodule-based, cold stage and held at –208C in a dry nitrogengas environment. Care has to be taken so that light from themicroscope objective does not cause localized melting of theice. The sampling volume for Raman spectroscopy is roughly11 2 mm, i.e. similar to that for EDS of ice.Two different SEM techniques were used for analysis ofthe uncoated ice specimens, which were maintained at –70to –1208C using home-made cold stages. First, SACPs wereobtained using backscattered electrons on a Zeiss DSM 962SEM operated at 15 kV. Second, EBSPs were obtained usingeither a Zeiss SEM operated at 15 kV or an environmental FEIfield emission gun (FEG) XL30 SEM operated at either 15 or20 kV. For the EBSP work, the cold stage had a movable sledwith a face inclined at 708 on which the ice was mounted;the 708 angle is needed to maximize the backscatteredelectron yield. EBSPs were acquired and automaticallyindexed using a HKL, Inc. Channel 5 Orientation ImagingSystem. It is worth noting that while the Zeiss SEM was run athigh vacuum (510–4 Pa), the FEI FEG SEM was run in low-vacuum mode at a pressure of 13 Pa.In acquiring both SACPs and EBSPs, the topography of thespecimen surface is critical. While sublimation is desiredbecause it reduces or even eliminates surface charging, it alsohas the adverse effect of roughening the surface of the ice.Hence, all the results below were obtained by very carefullybalancing temperature, probe current and exposure time.RESULTSMicrochemical analysisFigure 1 shows a Raman spectrum from a triple junction in1551m GISP2 ice. NaNO3 was found, for the first time, inboth the GBs and triple junctions. As noted above, it wouldnot have been possible to identify NaNO3 using EDS. Adownside of Raman spectroscopy is that it is impossible toidentify dissociated ions of elements such as Na, K, Ca, Aland Cl, which are easily identified with EDS. Thus, EDS andRaman spectroscopy can be viewed as complementarytechniques for examining the microstructural location ofimpurities in ice.Orientation determinationSEM images are formed by rastering a focused electron probeover a specimen and, typically, collecting either the back-scattered, secondary or absorbed electron signal and using iteither to modulate the intensity of the electron beam in acathode ray tube that is rastered in synchronization with theelectron beam on the specimen, or after digitization, to forman electronic image. SACPs are obtained in a SEM byadjusting the lens currents so that the crossover from thesecond condenser lens is in the back focal plane of theobjective lens (see Fig. 2a). This results in a parallel electronbeam emerging from the objective lens, rather than thefocused probe used for imaging. In addition, the first set ofscan coils is turned off so that instead of a focused electronprobe scanning the specimen (as in imaging), a parallelelectron beam rocks, in a specified solid angle, about aselected area of 20–50 mm diameter on the specimen surface(see Fig. 2a). In the Zeiss DSM 962 SEM used here, and someother SEMs, these two actions occur automatically byswitching to ‘SACP mode’. As the electron beam is rockedback and forth, the number of backscattered electronscaptured by a detector located above the specimen fluctuateswith the angle of incidence. For a particular set of latticeplanes that are close to parallel to the incident beamdirection, the backscattered signal exhibits a higher, al-most-constant value for angles between –B and +B, whereB is the Bragg angle, and sharp drops when the inclination ofthe beam becomes slightly larger than |B| (Joy and others,2006). In the latter orientations, the electrons are channeledinto the specimen, traveling parallel to the lattice planes withlittle scattering, and consequently the electron backscattersignal exhibits a minimum. Even small deviations of theFig. 1. Raman spectrum from the triple junction shown from1551m GISP2 ice, showing the presence of NaNO3. The ‘library’NaNO3 spectrum is shown for comparison.Baker and others: Microstructural characterization of ice cores442rocking beam from this channeling condition cause thenumber of backscattered electrons to increase. It is thedifference between this weak channeling signal and thestronger signal within the 2B range that produces the Kikuchiband. The Kikuchi bands can be envisioned as being rigidlyattached to the crystalline lattice of the specimen, so theymove if the specimen is tilted or rotated, but not if it istranslated. Figure 3 shows an example of a pattern from ice.Note that second-order, i.e. (240), Kikuchi lines can be seenoutside the (1 20) Kikuchi lines, indicating good patternquality. Indexing is performed using the fact that the width ofeach band is proportional to (double) the Bragg angle for theappropriate set of planes, while the angle between the bandsmatches the angle between the corresponding lattice planes.The ice crystal from which the pattern was obtained had its[001] direction inclined at 3.28 to the incident electron beam.EBSP formation is also straightforward. The convergentscanning electron probe, used in imaging mode, is simplystopped at a point of interest on the highly tilted icespecimen. The electrons are inelastically scattered in alldirections and subsequently diffracted by crystal planesaccording to Bragg’s law. For each set of planes that satisfyBragg’s law, the diffracted electrons form a pair of oppositewide-angle cones that intersect a phosphor screen in front ofthe specimen as parallel pairs of Kikuchi lines. The resultingEBSP on the phosphor screen is recorded using a charge-coupled device (CCD) camera (see Fig. 2b). Figure 4 showsexamples from ice.Figure 4a was obtained using the Zeiss SEM, which has atungsten filament and operates at a chamber vacuum of510–4 Pa. Note the high quality of the pattern: third-orderlines can be observed on some Kikuchi lines (see theexamples arrowed). Figure 4b was obtained using the FEIFEG SEM run in environmental mode at a chamber vacuumof 13 Pa. The lateral resolution on the Zeiss SEM is 0.5 mm,whereas the Nordlys detector on the FEI FEG SEM provides alateral resolution of 20nm. The pattern quality on the FEGSEM under the low vacuum, while worse than the EBSPsobtained at high vacuum, is still remarkable. The patterns areindexed using the same approach as for SACPs, i.e. using thefact that the width of each band is proportional to the Braggangle for the appropriate set of planes, while the anglebetween the bands matches the angle between the corres-ponding lattice planes. Practically, a computer automaticallyindexes the EBSPs once the crystallographic parameters ofthe material being studied have been input. The orientationsof the two ice grains from which the EBSPs in Figure 4 wereobtained are shown as hexagons on the EBSPs. Theorientations can, of course, be shown on a stereogram,presented as Euler angles, Miller indices or in any other form.With the Nordlys detector on the FEI FEG SEM, up to 100patterns can, in principle, be acquired and indexed persecond. This allows orientation image maps to be con-structed by obtaining and indexing several tens of thousandsof EBSPs and color-coding the grains in a constructed imagebased on their orientation (Iliescu and others, 2004).Both SACPs and EBSPs enable grain orientations to bedetermined to an angular resolution of 0.18, hence themisorientations across GBs can be determined to highFig. 2. Set-up for (a) SACP collection and (b) EBSP collection.Fig. 3. SACP obtained from an ice single crystal in a SEMoperated at 15 kV. The black and white lines were added to helpthe eye pick out the indexed Kikuchi bands. The orientation ofthe crystal is such that its [001] direction is inclined at 3.28 to theincident electron beam.Baker and others: Microstructural characterization of ice cores 443accuracy. This will allow the segregation behavior at GBs tobe related to their misorientation. The advantage of usingSACPs is that the specimen is horizontal during patterncollection, which is the specimen orientation used for EDScollection. On the other hand, a problem with obtainingSACPs from ice is that sublimation leads to surface rough-ening, resulting in out-of-focus topographic contrast em-bedded in the Kikuchi lines, which can be observed inFigure 3 as the bright features in the pattern background.This is not a problem in EBSP formation, but a disadvantageof EBSPs compared with SACPs is that the specimen is tiltedto 708 for EBSP collection but has to be returned tohorizontal for EDS data collection.SUMMARYFor the first time, we have found NaNO3, using confocaloptical microscopy coupled with Raman spectroscopy, atthe GBs and triple junctions in ice from GISP2. We have alsoshown that it is possible to use two SEM-based techniques,SACPs and EBSPs, to determine the complete crystallo-graphic orientations of grains in ice to both higher angularand spatial resolution compared to the traditional Rigsbystage technique.ACKNOWLEDGEMENTSThis research was supported by US National ScienceFoundation (NSF) grant 0221120 and Army Research Office(ARO) grant DAAD 19-03-1-0110. The views and conclu-sions contained herein are those of the authors and shouldnot be interpreted as representing official policies, eitherexpressed or implied, of the NSF, ARO or the USGovernment. The authors would like to thank the threeanonymous reviewers for their efforts.REFERENCESBaker, I. and D. Cullen. 2003. SEM/EDS observations of impuritiesin polar ice: artifacts or not? J. Glaciol., 49(165), 184–190.Baker, I., D. Cullen and D. Iliescu. 2003. The microstructurelocation of impurities in ice. Can. J. Phys., 81, 1–9.Barnes, P.R.F., E. Wolff, D.C. Mallard and H.M. Mader. 2003. SEMstudies of the morphology and chemistry of polar ice. Microsc.Res. Techn., 62, 62–69.Budd, W.F. and T.H. Jacka. 1989. A review of ice rheology for icesheet modelling. Cold Reg. Sci. Technol., 16(2), 107–144.Cullen, D. and I. Baker. 2000. Correspondence. The chemistryof grain boundaries in Greenland ice. J. Glaciol., 46 (155),703–706.Cullen, D. and I. Baker. 2001. Observation of impurities in ice.Microsc. Res. Techn., 55, 198–207.De La Chapelle, S., O. Castelnau, V. Lipenkov and P. Duval. 1998.Dynamic recrystallization and texture development in ice asrevealed by the study of deep ice cores in Antarctica andGreenland. J. Geophys. Res., 103(B3), 5091–5105.Fukazawa, H., K. Sugiyama, S. Mae, H. Narita and T. Hondoh.1998. Acid ions at triple junction of Antarctic ice observed byRaman scattering. Geophys. Res. Lett., 25(15), 2845–2848.Iliescu, D., I. Baker and H. Chang. 2004. Determining theorientation of ice crystals using electron backscatter patterns.Microsc. Res. Techn., 63, 183–187.Jacka, T.H. 1984. Laboratory studies on relationships between icecrystal size and flow rate. Cold Reg. Sci. Technol., 10(1), 31–42.Jacka, T.H. and M. Maccagnan. 1984. Ice crystallographic andstrain rate changes with strain in compression and extension.Cold Reg. Sci. Technol., 8(3), 269–286.Joy, D.C., D.E. Newbury and D.E. Davidson. 2006. Electronchanneling patterns in the scanning electron microscope.J. Appl. Phys, 53(8), R81–R119.Legrand, M. and P. Mayewski. 1997. Glaciochemistry of polar icecores: a review. Rev. Geophys., 35(3), 219–243.Obbard, R., D. Iliescu, D. Cullen and I. Baker. 2003. SEM/EDScomparison of polar and seasonal temperate ice. Microsc. Res.Techn., 62, 49–61.Wolff, E.W., R. Mulvaney and K. Oates. 1988. The location ofimpurities in Antarctic ice. Ann. Glaciol., 11, 194–197.Fig. 4. EBSPs obtained from individual grains in polycrystallineice using 15 keV electrons. The indices indicate the major poleswhere the Kikuchi lines intersect. The inset schematics show theorientations of the grains from which the patterns were obtained.Pattern (a) was obtained in conventional SEM at a vacuum of5 x 10–4 Pa, while (b) was obtained in a FEG SEM run inenvironmental mode at a chamber vacuum of 13 Pa. Note thethird-order lines on some Kikuchi bands (arrowed).Baker and others: Microstructural characterization of ice cores444