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Firn Densification: An Empirical Model

  • Michael M. Herron (a1) and Chester C. Langway (a1)
Abstract

An empirical model of firn densification from the surface to the zone of pore close-off has been constructed. Fundamental rate equations have been derived for the first two stages of densification. In the first stage, for densities less than 0.55 Mg m−3, the densification rate is proportional to the mean annual accumulation times the term (ρ iρ), where ρ is the density of the snow and ρ i is the density of pure ice. The densification rate in the second stage, where 0.55 Mg m −3 < ρ < 0.8 Mg m−3, is proportional to the square root of the accumulation rate and to (ρi− ρ). Depth–density and depth–age calculations from this model are compared with observation. Model accumulation rates are within about 20% of values obtained by other techniques. It is suggested that depth intervals of constant density in some Antarctic cores may represent a synchronous event in the 1880 ’s when ten times the normal accumulation fell within a year or two.

Résumé

On a construit un modèle empirique de la densification du névé depuis la surface jusqu 'à la zone de fermeture des pores. Dans le premier stade, pour des densités inférieures à 0,55 Mg⁄m−3, la vitesse de densification est proportionnelleau produit de l’accumulation moyenne annuelle par le terme (ρ iρ), où ρ est la densité de la neige et ρ i la densité de la glace pure. La vitesse de densification dans le second stade, lorsque 0,55 Mg m−3 < ρ < 0,8 Mg m −3, est proportionnelle au produit de la racine carrée du taux d ’accumulation par (ρ iρ). Des valeurs de la densité selon la profondeur, et de la densité selon l ’âge sont calculées à partir de ce modéle et comparées avec l ’observation. Les vitesses d ’accumulation tirées du modéle restent à 20% prés voisines des valeurs obtenues par d ’autres techniques. On suggére que des tronçons de densité constante trouvés dans des carottes prelevées en Antarctique peuvent représenter un épisode vers les années 1880 où il est tombé dix fois plus de neige que la normale en un ou deux ans.

Zusammenfassung

Für die Verdichtung des Firns von der Ober-fläche bis zur Zone des Porenschlusses wurde ein empirisches Modell entwickelt. Für die beiden ersten Stufen der Verdichtung werden fundamentale Geschwindigkeitsgleichungen hergeleitet. In der ersten Stufe, für Dichten von weniger als 0,55 Mg m−3, ist die Verdichtungsrate proportional dem Produkt aus der mittleren jährlichen Akkumulation und dem Ausdruck (ρ iρ), wobei ρ die Dichte des Schnees und ρ i die des reinen Eises ist. Die Verdichtungsrate in der zweiten Stufe, wo 0,55 Mg m−3 <ρ < 0,8 Mg m −3 gilt, ist proportional zur Quadratwurzel aus der Akkumulationsrate, multipliziert mit (ρ iρ). Berechnungen der Dichte und des Alters mit der Tiefe aus diesem Modell werden mit Beobachtungen verglichen. Die Modell-Akkumulationsraten stimmen auf etwa 20% mit Werten überein, die mit anderen Methoden gewonnen wurden. Die Annahme erscheint gerechtfertigt, dass Intervalle konstanter Dichte in einigen antarktischen Eiskernen aus einern gleichzeitigen Ereignis in den 80er Jahren des letzten Jahrhunderts stammen, bei dem innerhalb eines Jahres oder zweier die Akkumulation zehnmal grösser war als sonst.

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References
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Journal of Glaciology
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