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On the Rates of Growth of Grains and Crystals in South Polar Firn

  • Anthony J. Gow (a1)

Abstract

The size of firn crystals as a function of age has been investigated in thin sections to a depth of 49 m at the South Pole. Grain cross-sections increased in size from 0.24 mm2 at 0.1 m depth to 0.63 mm2 at 10 m. Crystals, as distinct from grains, increased in size from 0.18 to 0.43 mm2 over the same interval, implying that grains are generally composed of just one or two crystals rather than several as is frequently contended. The mean crystal cross-section increased linearly with the age of the firn at a rate of 0.0006 mm2 year−1; in 388 year old firn at 49 m the crystal size measured 0.63 mm2. Analysis of crystal-growth data from other locations in Antarctica and Greenland also revealed a strong linear relationship between the mean cross-sectional arcas (D 2) of crystals (in mm2) and their ages in years (t), i.e. . The fact that the temperature dependence of the crystal growth rate K can be expressed very satisfactorily in an equation of the form K = K 0 exp (E/RT) confirms predictions that crystal growth in firn is essentially analogous to grain growth in metallic and ceramic sinters. An extrapolation of available data indicates that crystal growth rates in dry firn could be expected to vary by two orders of magnitude (0.0003 to 0.03 mm2 year−1) over the temperature range −60° to −15°C. A method of utilizing crystal growth-mean annual temperature data to determine accumulation rates in snow is demonstrated.

Résumé

La grosseur des cristaux de névé en fonction de l’âge a été étudié en lames minces à une profondeur de 49 m au Pôle Sud. Les coupes transversales augmentent de 0,24 mm2 a 0,63 mm2 lorsqu’on part d’une profondeur de 0,1 m pour atteindre 10 m. Les cristaux se différenciant des grains croissent de 0,18 mm2 à 0,43 mm2 dans le même intervalle, les grains étant supposés généralement composés de un au deux cristaux à l’apposé de plusieurs comme c’est fréquemment dit. La coupe transversale de cristal moyen augmente de façon linéaire en fonction de l’âge du névé jusqu’à une quantité de 0,0006 mm2 an−1. Dans un névé datant de 388 ans à 49 m, l’épaisseur de cristal mesure 0,63 mm2. L’analyse des données de croissance de cristal d’autres emplacements en Antarctique et au Groenland révèle aussi un grand rapport linéaire entre les régions à coupes transversales moyennes (D 2) de cristaux (en mm2) et leur âge en années (t), . Le fait que la dépendance de température de la vitesse de croissance de cristal K peut aisément être exprimé par l’équation de la forme K = K 0 exp(E/RT) confirme les prédictions mentionnant la croissance de cristal dans le névé essentiellement analogue à la croissance de grain dans les travertins métalliques et céramiques. Une extrapolation de données disponibles indique que les quantités en croissance de cristal dans le névé sec peut être évalué variant à deux ordres de grandeur (0,0003 à 0,3 mm2 an−1) au-dessus de la température s’échelonnant entre −60° et −15°C. Une méthode utilisant les données de température annuelles moyennes de cristal est décrite pour déterminer les quantités d’accumulation dans la neige.

Zusammenfassung

Die Grösse von Firnkristallen in Abhängigkeit von ihrem Alter wurde am Südpol mit Dünnschliffen bis zu einer Tiefe von 49 m untersucht. Die Korn-Querschnitte nahmen von 0,24 mm2 in 0,1 m Tiefe bis 0,63 mm2 in 10 m Tiefe zu. Die Grösse von Kristallen, soweit sie sich von Körnern unterschieden, wuchs im selben Bereich von 0,18 mm2 bis 0,43 mm2; dabei zeigte sich, dass Körner im allgemeinen eher nur aus einem oder zwei Kristallen zusammengesetzt sind als aus mehreren, wie häufig behauptet wird. Der mittlere Kristall-Querschnitt nahm linear mit dem Alter des Firns in einem Ausmass von 0,0006 mm2 pro Jahr zu; in 388 Jahre altem Firn bei 49 m Tiefe betrug die Kristallgrösse 0,63 mm2. Die Analyse von Kristallwachstumsdaten von anderen Stellen in Antarktika und Grönland ergab ebenfalls eine streng lineare Beziehung zwischen den mittleren Querschnittsflächen (D 2) der Kristalle (in mm2) und ihrem Alter (t) in Jahren, nämlich: . Die Tatsache, dass die Temperaturabhängigkeit der Kristallwachstumsgeschwindigkeit K sehr befriedigend durch eine Gleichung von der Form K = K 0 exp(E/RT) ausgedrückt werden kann, bestätigt Voraussagen, dass das Kristallwachstum in Firn in wesentlich analog zum Kornwachstum in metallischen und keramischen Sintern verläuft. Eine Extrapolation verfügbarer Daten zeigt, dass Unterschiede der Kristallwachstumsgeschwindigkeit in trockenem Firn bis zu zwei Grössenordnungen (0,0003 bis 0,03 mm2 pro Jahr) im Temperaturbereich von −60° bis −15°C erwartet werden können. Es wird eine Methode aufgezeigt, mit der Daten des Kristallwachstums und der mittleren Jahrestemperatur zur Bestimmung des Schneeauftrags benutzt werden können.

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References

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