Distribution and chemistry of micro-inclusions in the EPICA-DML deep ice core
Final Report Abstract
By the time this project started, little was known about microinclusions, their evolution, and interaction with other structures within the ice. Evidence for molecular diffusion through the ice lattice has been found in studies of nitrogen/oxygen ratios in air bubbles and air hydrates, but it was not clear whether the observed diffusion phenomena were particular to the bubble-hydrate transition zone or a general process throughout the ice sheet. The possibility was also cogitated that similar diffusive processes could cause changes in microinclusions, not only in situ, but also during the relaxation of stored ice cores. Thus, several questions related to microinclusions intrigued ice core scientists, including: • Do microinclusions carry important paleoclimatic information? • What is their nature? • Do they evolve in ice, and if yes, how? Is this evolution related to diffusive processes? • How do microinclusions affect the rheology of polar ice? • How do microinclusions affect the microstructure, anisotropy, and recrystallization of polar ice? In order to answer these questions, this project aimed at a systematic study of the nature, evolution, frequency and distribution of microinclusions (viz. bubbles and micro-bubbles, “black dots” and plate-like inclusions) throughout the EDML deep ice core (EDML: EPICA-DML; EPICA: European Project for Ice Coring in Antarctica; DML: Dronning Maud Land, Antarctica). We focused on the chemical signatures of microinclusions, their correlation with established chemical and isotopic records, their interactions with the ice microstructure, the relation between cloudy bands and the ice flow, and the influence of ice flow on the rearrangement of microinclusions. Methodologically, we employed a combination of special optical and spectroscopic methods for analyzing the EDML deep ice core, sometimes complemented by additional techniques. Two methods of optical analysis, especially developed for the study of ice cores, have been used in this project: Microstructure Mapping (µSM) and Ice-Core Linescanning (ICLS). The former is a portable technique of automated optical microscopy that provides a fast means of mapping in microscopic resolution (ca. 10^-6 m/pixel) the microstructure of fresh ice-core samples, while drilling is ongoing. The latter is a special scanner that can produce high-resolution digital images (ca 10^-4 m/pixel) of the whole core length. These optical techniques were combined with Micro-focus Raman Spectroscopy (µRS) for looking into the chemical nature of microscopic impurities within ice samples, without destroying them. Some particular samples have been investigated also with supplementary methods, in particular cryo-SEM combined with EDS and/or EBSD for elemental and structural analyses. One of the highlights of this project has been a painstaking and unparalleled high-resolution stratigraphic analysis of a deep ice core (EDML). Thanks to that analysis we have now a clear understanding of the distribution of “black dots” (BDs) in polar ice, their relation to paleoclimate, and their effects on the ice rheology, microstructure, anisotropy and recrystallization. In particular, we have confirmed the conjecture that cloudy bands owe their appearance and properties to a high concentration of BDs. From our Raman analyses we concluded, among other results, that the EDML core is characterized by high concentrations of sulfate microinclusions (in agreement with the atmospheric aerosol composition around the EDML site) and that cloudy bands (and glacial ice) are clearly enriched in quartz and silicate microinclusions, characteristic of cold periods. We have also determined that newly nucleated microbubbles and plate-like inclusions are enriched in oxygen (O2), in a manner somewhat similar to the enrichments reported for air hydrates in the bubble–hydrate transition zone of polar ice cores. These results indicate that gases dissolved or entrapped in ice cores may diffuse through the ice matrix during the period of storage, viz. within a period of years. These results are also compatible with the theory that dissolved oxygen diffuses through the ice matrix faster than dissolved nitrogen. Digital image analyses of bubbles and micro-inclusions have shown that a large number of microbubbles have probably been formed prior to “normal” bubbles, either in the upper firn or within snow crystals in the atmosphere. The total bubble number density (“normal” plus microbubbles) shows a general correlation with the paleo-temperature proxy δ18O and the dust concentration, implying that interglacial ice generally has fewer bubbles than glacial ice. On the small and micro-scale, we observed that bubble number and size are generally anti-correlated at all depths above the Bubble-Hydrate Transition (BHT) zone, in such a way that cloudy bands are clearly characterized by a larger number of bubbles (up to several fold) and conspicuously smaller bubble sizes. Therefore we regard impurities as a controlling factor for the formation, size and distribution of bubbles in glacial ice. Most interesting is the fact that the bubble size-number anticorrelation inverts within the BHT zone, to such an extent that cloudy bands virtually free of bubbles can be found in the lower part of the BHT zone. Besides yielding interesting and valuable results, this project opened a new field in German glaciology (ice-core micro-chemical physics), triggered a number of spin-offs, and fostered the career of several young researchers.
Publications
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(2008) The connectivity of crystallite agglomerates in low density firn at Kohnen station, Dronning Maud Land, Antarctica. Ann. Glaciol. 49: 114–120
Freitag, J., S. Kipfstuhl, S. H. Faria
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(2008) The symmetry group of the CAFFE model. J. Glaciol. 54(187): 643–645
Faria, S. H.
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(2009) Raman spectra of plate-like inclusions in the EPICA-DML (Antarctica) ice core. J. Glaciol. 55(189): 183–184
Nedelcu, A. F., S. H. Faria, W. F. Kuhs
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(2009) Subgrain boundaries and related microstructural features in EDML(Antarctica) deep ice core. J. Glaciol. 55(191): 461–472
Weikusat, I., S. Kipfstuhl, S. H. Faria, N. Azuma, A. Miyamoto
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(2009) The multidisciplinary ice core. Low Temp. Sci. 68: 35–37
Faria, S. H.
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(2009) The multiscale structure of Antarctica. Part I: inland ice. Low Temp. Sci. 68: 39–59
Faria, S. H., S. Kipfstuhl, N. Azuma, J. Freitag, I. Hamann, M. M. Murshed, W. F. Kuhs
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(2010) Distribution of air bubbles in the EDML and EDC ice cores from a new method of automatic image analysis. J. Glaciol. 56(196): 339–348
Ueltzhöffer, K. J., V. Bendel, J. Freitag, S. Kipfstuhl, D. Wagenbach, S. H. Faria, C. S. Garbe
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(2010) Polar ice structure and the integrity of ice-core paleoclimate records. Quat. Sci. Rev. 29(1): 338–351
Faria, S. H., J. Freitag, S. Kipfstuhl
