Discontinuous Flow, Ice Texture, and Dirt Content in the Basal Layers of the Devon Island Ice Cap

Abstract

ABSTRACT. Surface-to-bedrock cores obtained with a CRREL thermal drill we re taken in 1972 and 1973 from the top of the Devon Island ice cap. There are very pronounced variations in oxygen isotope, microparticle concentration, and ice texture in the lowermost 5 m of the core. There is a section of isotopically cold, very fin e bubbly ice with high mic ro-particle concentrations between 2.6 and 4 .4 m above the bed, considered to represe nt the Last l ee Age. There is coarse, isotopically warm, clean ice above and below this. F o r 1. 2 m a bove the bed, the ice is finer again with high micro-particle concentrations but it shows very low bubble concentration and is isotopically the warmest in the core. While the broad variations are common to both cores, in detail there are signifi ca nt variations despite the fa ct that the cores were taken only 27 m apart. The variations, when analysed statisticall y, show that at least 25-30 % of the originally continuous profile is missing from each core. Faulting within the near-bedrock ice may be responsible for some of the effect but bubble fabric also gives evid e nce for irregular non-Iaminar flow. Because of the strong relationship between crystal size and micro-particle co ncentrations in the Devon Isla nd cores, it is suggested that the fine-grained nature of dirty layers in the Antarctic and Greenland ice sheets is due to the effect of the dirt inclusions a nd not of shearing. Steep isotopic gradients in the Devon Isla nd cores are shown to be evidence for possible shearing, which does not effect any change in the crystal texture. C lear ice near the bed is consid ered a tectonic feature, but the lack of effect on its bed by the ice cap confirms the non-erosional nature of an ice cap froz en to its bed .In terms of paleoclimatic history, it m eans that, because of bedrock effects, ice caps of intermediate d epth (i.e. < 400 m ) ca n give continuous informatio n only over the last approximate 5000 years. Between 5000 a nd 10000 B.P. the time seri es becomes sli ghtly discontinuous and beyond 10000 B.P. so discontinuous as to allow only broad climatic inferences to b e drawn. RES UME. Ecoulemellts discOlltillllS, texture de la glace et teneur en debris dall s les niveaux de base de la calotte glaciairede l'lle D evon. On a preleve avec une sonde thermique CRREL des carottes continues surface-bedrock en 1972 et 1973 depuis le sommet de la calotte glaciaire de l'Ile Devon. Il y a des variations tres prononcees en isotopes de l' oxygene, en concentration e n micro-particul es e t en texture de la glace dans les cinq derniers m etres a u bas de la carotte. II y a un e section d'une glace isotopiquement fro ide, tres finement bulleuse, rich e en mi cro-pa rticules e ntre 2,6 et 4,4 m au d ess us du lit, que l'o n considere comme representant le Dernier Age G lac iaire. Au d ess us et en dessous, on trouve une glace propre, a gros grains, isotopiquement chaud e. Sur 1,2 m au dessus du lit, la glace est d e nouvea u a grains plus fin s avec un e forte concentration en bulles ...

Grain Growth in Polar Ice: II. Application

“…The numbers in Table I are the measured volume fraction of particles and the weighted average particle radius that gives the calculated total drag. Koerner and Fisher (1979) did not report the particle-size distribution in Devon Island ice, but our assumed values of r in Table I are reasonable or over-estimate drag anE concord with the data reported. Equation (I) is evaluated in the final column of Table I. 426 a.. We have ignored Aitken particles in this analysis.…”

“…Then even if our estimates of the effects of microparticles are low by a full order of magnitude, grain growth is still slowed by less than 2% at both sites and falls well within the "normal growth" regions of Figures 1 and 2. It was observed by Koerner and Fisher (1979) that the downward decrease in grain-size across the HoloceneWisconsinan boundary correlates strongly at several polar The modal microparticle radius grain radius R, and the ratio Notice that Pp / PI (%) is also particles.…”

“…As examples of the effect of microparticles on grain growth, consider data from the Devon Island Ice Cap of Arctic Canada (Koerner and Fisher, 1979) and from Dome C, East Antarctica (personal communication from E. MosleyThompson, 1985), two very different sites on polar ice caps. The data necessary to evaluate Equation (I) are listed in Table I, both for Wisconsinan and Holocene ice.…”

“…In the light of the known effect of microparticles on grain growth, Koerner and Fisher (1979) advanced the reasonable hypothesis that the high concentrations of microparticles in Wisconsinan ice cause the small grain-sizes observed there. From Table I it is clear, however, that this hypothesis cannot be correct.…”

The microstructure of meteoric ice from Vostok, Antarctica

“…Indeed, grain sizes are often found to be smaller in visually dirty ice than in clean ice of the same age (Gow and Williamson, 1976;Duval and Lorius, 1980). Initially, a causal relationship was drawn between the increase in particle concentration and the decrease in grain size across the Holocene-Wisconsin boundary in several ice cores (Koerner and Fisher, 1979). Later, however, two separate groups (Duval and Lorius, 1980;Alley and others, 1986b) compared the intrinsic driving force for grain growth with the drag force from particles in ice.…”