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The Debris-Laden Ice at the Bottom of the Greenland Ice Sheet

  • Susan Herron (a1), Hoar (a1) and Chester C. Langway (a1)

Abstract

The Camp Century, Greenland, ice core was recovered from a bore hole which extended 1 375 m from the surface of the Greenland ice sheet to the ice/sub-ice interface. The bottom 15.7 m of the core contain over 300 alternating bands of clear and debris-laden ice. The size of the included debris ranges from particles less than 2 μm in diameter to particle aggregates which are a maximum of 3 cm in diameter: the average debris concentration is 0.24º º by weight. The debris size, concentration, and composition indicate that the debris originates from the till-like material directly below the debris-laden ice. The total gas concentration averages 51 ml/kg ice compared to the average of 101 ml/kg ice for the top 1 340 m. The gas composition of debris-bearing ice has apparently been modified by the oxidation of methane as reflected by traces of methane, high CO2 levels, and low O2 levels with respect to atmospheric air. Argon, which is not affected by the oxidation, shows an enrichment in samples with lower gas concentrations. Both the low gas concentrations in the debris-laden zone and the argon enrichment may be explained by the downward diffusion of gases from bubbly glacier ice into an originally bubble-free zone of refrozen debris-laden ice. Ice texture and ice-fabric analyses reveal extremely fine-grained ice and highly preferred crystal orientation in the lowermost 10 m of the core, indicating a zone of high deformation.

Résumé

Une carotte de glace provenant de Camp Century. Groënland, a été extraite d'un forage qui atteignit 1 375 m depuis la surface de la calotte groëlandaise jusqu'à l'interface glace/lit. Les 15.7 mètres du bas de la cartte contiennent plus de 300 bandes alternées de glace claire et chargée de matière. La dimension des matériaux inclus va de particules de moins de 2 μm en diamètre jusqu'à des agrégats qui out jusqu'à 3 cm de diamètre: la concentration moyenne en sédiments est de 0.24° ° en poids. La dimension, la concentration et la composition des sédiments indiquent qu'ils proviennent de dépόts de type morainique existant directement en-dessous de la glace chargée. La concentration totale en gaz est en moyenne de 51 ml/kg de glace, teneur à comparer avec la moyenne de 101 ml/kg pour la glace du sommet à 1 340 m. La composition en gaz des sédiments chargeant la glace a été apparemment modifiée par l'oxydation du méthane comme il ressort des traces de méthane, de haute teneur en CO2 et de faible concentration en O2 par rapport à la composition de l'air atmosphérique. L'argon que l'oxydation n'affecte pas enrichit les échantillons qui ont la plus faible teneur en gaz. La faible teneur en gaz dans le zone chargée de matériaux et son enrichissement en argon peuventétre expliqués par la diffusion vers le bas des gaz issus de la glace bulleuse du glacier vers la zone originellement dépourvue de bulles de la glace regelée chargée de sédiments. Les analyses de texture de la glace et d'orientation des axes optiques révèlent une glace à grains très fins et des orientations cristallines préférentielles très marquées dans les to derniers mètres de la carotte, prouvant que eette zone est le siège de nombreuses contranintes qui peuvent être dues aux irrégularités du lit.

Zusammenfassung

Der Eiskern von Camp Century in Grönland wurde aus einem Bohrloch eingebracht, das von der Oberfläche des grönländischen Inlandeises bis zur Grenzfläche zwischen Eis und Untergrund in 1 375 m Tiefe reichte. Die letzten 15.7 m des Kerns enthalten über 300 Bänder, die weehselweise aus klarem und schuttbeladenem Eis bestchen. Die Grösse des eingeschlossenen Schuttes reicht von Partikeln mit Durchmessern kleiner als 2 µm bis zu Partikelansammlungen. deren Durchmesser bis maximal 3 cm beträgt: Die mittlere Schuttkonzentration liegt bei 0.24º ° des Gewichtes, Grösse. Konzentration und Zusammensetzung des Schuttes lassen erkennen, dass dieser aus dem geschiebeähnlichen Material unmittelbar unter dem schuttbeladenen Eis stammt. Die gesamte Konzentration an Gasen beträgt im Mittel 51 ml pro kg Eis, verglichen mit dem mittleren Gasgehalt von 101 ml pro kg Eis für die obersten 1 340 m. Die Gaszusammensetzung im schuttdurchsetzten Eis wurde offensichtlich durch die Oxydation von Methan verändert, was sich durch Spuren von Methan, hohen CO2- und niedrigen O2-Gehalt, verglichen mit atmosphärischer Luft äussert. Argon, das durch die Oxydation nieht beeinflusst wird, zeigt eine Anreicherung in Proben mit geringem Gasgehalt. Sowohl die niedrige Gaskonzentration in der schuttbeladenen Zone wie die Argonanreicherung lassen sich durch die Gasdiffusion aus blasenreichem Gletschereis abwärts in eine ursprünglich blasenfreie Zone wiedergefrorenen, schuttbeladenen Eises erklären. Analysen von Eistextur und -gefüge zeigen extrem feinkörniges Eis und hoch anisotrope Kristallorientierung in den untersien 10 m des Kerns; dies deutet auf eine Zone hoher Druck-konzentration, möglicherweise infolge von Unregelmässigkeiten des Felsbettes.

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The Debris-Laden Ice at the Bottom of the Greenland Ice Sheet

  • Susan Herron (a1), Hoar (a1) and Chester C. Langway (a1)

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