No abstract

Abstract. The recovery of a 1.5 million yr long ice core from Antarctica represents a keystone of our understanding of Quaternary climate, the progression of glaciation over this time period and the role of greenhouse gas cycles in this progression. Here we tackle the question of where such ice may still be found in the Antarctic ice sheet. We can show that such old ice is most likely to exist in the plateau area of the East Antarctic ice sheet (EAIS) without stratigraphic disturbance and should be able to be recovered after careful presite selection studies. Based on a simple ice and heat flow model and glaciological observations, we conclude that positions in the vicinity of major domes and saddle position on the East Antarctic Plateau will most likely have such old ice in store and represent the best study areas for dedicated reconnaissance studies in the near future. In contrast to previous ice core drill site selections, however, we strongly suggest significantly reduced ice thickness to avoid bottom melting. For example for the geothermal heat flux and accumulation conditions at Dome C, an ice thickness lower than but close to about 2500 m would be required to find 1.5 Myr old ice (i.e., more than 700 m less than at the current EPICA Dome C drill site). Within this constraint, the resolution of an Oldest-Ice record and the distance of such old ice to the bedrock should be maximized to avoid ice flow disturbances, Published by Copernicus Publications on behalf of the European Geosciences Union. H. Fischer et al.:Where to find 1.5 million yr old ice for the IPICS "Oldest-Ice" ice core for example, by finding locations with minimum geothermal heat flux. As the geothermal heat flux is largely unknown for the EAIS, this parameter has to be carefully determined beforehand. In addition, detailed bedrock topography and ice flow history has to be reconstructed for candidates of an Oldest-Ice ice coring site. Finally, we argue strongly for rapid access drilling before any full, deep ice coring activity commences to bring datable samples to the surface and to allow an age check of the oldest ice.

ABSTRACT. Many observations regarding grain growth in ice sheets are glaciologically interesting but imperfectly understood. Here we develop the theory of grain growth in ice th,at is not deforming rapidly, and in the succeeding paper we use this theory to explain observations from glacial ice. In the absence of significant strain energy, the driving force for grain growth arises from grain-boundary curvature. Grain growth is slowed by the interaction of grain boundaries with extrinsic materials (microparticles, bubbles, and dissolved impurities) . If the driving force for growth is not large enough to cause boundaries to separate from an extrinsic material, then the grain-boundary velocity is determined by the velocity characteristic of the extrinsic material (low-velocity regime). If the driving force is large enough to cause separation, then boundaries migrate more rapidly than the extrinsic material (high-velocity regime) but the net driving force is reduced through transient pinning by the extrinsic material. Polar ice is typically in the low-velocity regime relative to dissolved impurities and the high-velocity regime relative to microparticles and bubbles. Cross-sectional area of grains is predicted to increase linearly with time under most but not all circumstances. RESUME. Croissance des grains dans la glace poiaire: I. Theorie. De nombreuses observations sur la croissance des grains dans les calottes polaires sont interessantes du point de vue glaciologique mais pas encore totalement elucidees. Nous developpons ici une theorie de croissance des grains dans une glace it deformation faible et dans I'article suivant nous l'utilisons pour expliquer les observations obtenues sur ce type de glace. En I'absence d'energie de deformation notable, la cause de croissance du grain provient de sa courbure de surface. La croissance du grain est diminue par l'interaction de sa surface avec des materiaux extrinseques (microparticules, bulles, et impuretes dissoutes). Si la force majeure de croissance n'est pas suffisante pour separer la surface du materiel extrinseque, alors la vitesse de la limite du grain est determinee par la vitesse caracteristique du materiau extrinseque (regime de vitesse lente). Au contraire SYMBOLS USED AND VALUES OF CONSTANTSGrain-boundary impurity concentration Eutectic composition for impurity-ice system Ice-lattice impurity concentration si cette force est suffisante pour assurer la separation, alors les limites migrent plus rapidement que le materiau (regime de fortes vitesses) mais la force principale est reduite it cause du pincement dO au materiau extrinseque. La glace polaire se situe typiquement dans un regime de basse vitesse vis it vis des impuretes dissoutes et dans un regime de fortes vitesses quant aux microparticules et aux bulles. On arrive it la conclusion qu'une section droite de grain s'accrOIt lineairement en fonction du temps dans la plupart des situations.

Micro-earthquakes have been monitored at two locations on Ice Stream Β and one on Ice Stream C using a seismographic array built specifically for that purpose. Subglacial micro-earthquakes arc 20 times more abundant beneath Ice Stream C than beneath Ice Stream B, despite the 100 times more rapid movement of Ice Stream B. Triangulation shows the foci beneath Ice Stream C, like those beneath Ice Stream B, to be within a few meters of the base of the ice, presumably within the uppermost part of the bed, and fault-plane analysis indicates slips on horizontal planes at about a 30° angle to the presumed direction of formerly active flow. Source parameters, computed from spectra of the arrivals, confirmed that the speed of slip is three orders of magnitude faster beneath Ice Stream C than beneath Ice Stream Β which means that a five orders-of-magnitude greater fraction of the velocity of Ice Stream C is contributed by the faulting, although that fraction is still small. We attribute the difference in activity beneath the two ice streams to the loss of dilatancy in the till beneath Ice Stream C in the process that led to its stagnation.

The latest compilations of surface mass balance, mean annual surface temperature, and elevation for the Antarctic ice sheet, derived from data sets of approximately 1500, 700, and 105 points, respectively, have been used to obtain areally integrated means for 24 ice drainage systems and 329 grid point values covering the whole ice sheet. Monthly summaries of remotely sensed sea ice data for 1973–1976 have been used to obtain mean annual distance to open ocean. Linear and second‐order regression analyses of surface balance on (1) temperature for the entire ice sheet, (2) elevation for the conterminous grounded ice, and (3) distance to the open ocean for the ice shelves (including ice rises and attached islands) have been made using both system means and grid point values. These analyses show correlation coefficients of between 0.63 and 0.81, and they provide bases for descriptive models of the present Antarctic ice sheet, as well as for predictive models of the response of the ice sheet to air temperature changes and variations in meridional mass and energy transfers. Extrapolation to other ice sheets, past and present, may be possible but should be made cautiously. Linear models are recommended for paleoclimatic reconstructions of ice sheets in upper mid‐latitudes, and second‐order models are recommended for those in high latitudes. System means are shown to be reliable for these purposes. Incidental results are new estimates of the mean annual surface temperature for the whole ice sheet (−36°C) and mean surface elevation for the conterminous grounded ice (2290 m).

ABSTRACT. Grain growth observed in polar ice that is not deforming rapidly can be accounted for if concentrations and distributions of extrinsic materials (microparticles, bubbles, and dissolved impurities) are characterized fully. Dissolved impurities segregate to grain boundaries and slow grain growth in all cold glacial ice. The high concentration of soluble impurities in Wisconsinan ice from the Dome C (Antarctica) ice core (and perhaps other ice cores) probably causes the small grain-sizes observed in that ice. Microparticles have little effect on grain growth in ordinary ice . In ice layers that appear dirty owing to concentrations of volcanic tephra (such as in the Byrd Station (Antarctica) ice core) or of morainal material, microparticles reduce grain-growth rates significantly. The relatively high vapor pressure of ice allows rapid growth and high mobility of intergranular necks, so grain growth in firn is limited by boundary migration rather than by neck growth. Bubbles formed by pore close-off at the firn-ice transition are less mobile than grain boundaries, causing bubble-boundary separation whenever geometric constraints are satisfied; however, such separation reduces grain-growth rates by only about 10%. The observed linear increase of grain area with time is thus predicted by theory, but the growth rate depends on soluble-impurity concentrations as well as on temperature. RESUME. Croissance des grains dans la glace polaire: ll. Applications. On peut rendre compte de la croissance des grains observee dans la glace polaire a deformation lente par une bonne description des concentrations et repartitions des materiaux extrinseques (microparticules, bulles et impuretes dissoutes). Les impuretes dissoutes se rassemblent aux frontieres des grains pour en diminuer la croissance dans toute glace froide. La forte concentration des impuretes solubles dans la glace du Wisconsin au DOme C (Antarctique), et peut Mre d'autres carottages, est la cause probable de la faible taille des grains observee dans ce type de glace. Les microparticules n'ont qu'un faible effet sur la croissance des grains pour de la glace ordinaire. Dans des couches de glace qui apparaissent sales par suite de la presence de tephras volcaniques (comme dans la carotte de glace de la Byrd Station, Antarctique) ou bien chargees en materiau morainique, les microparticules reduisent notablement les tau x de croissance des grains. La relativement forte pression de vapeur de la glace permet une croissance rapide et une grande mobilite des appendices intergranulaires, de sorte que la croissance du grain dans le neve est limitee par les migrations a la frontiere plutOt que par la croissance des appendices. Les bulles formees par le fermeture des pores lors de la transition neve,lace sont moins mobiles que les frontieres des grains; en causant des frontieres de separation de bulles partout oil les conditions geometriques sont satisfaites; cependant une telle separation reduit la vitesse croissance des grains d'environ 10% seulement. La croissance lin...

The recovery of a 1.5 Myr long ice core from Antarctica represents a keystone to our understanding of Quaternary climate, the progression of glaciation over this time period and the role of greenhouse gas cycles in this progression. Here we show that such old ice is most likely to exist in the plateau area of the East Antarctic Ice Sheet (EAIS) without stratigraphic disturbance and should be able to be recovered after careful pre-site selection studies. Based on a simple ice and heat flow model and glaciological observations, we conclude that positions in the vicinity of major domes and saddle positions on the East Antarctic Plateau will most likely have such old ice in store and represent the best study areas for dedicated reconnaissance studies in the near future. In contrast to previous ice core drill site selections, we strongly argue for significantly reduced ice thickness to avoid bottom melting, while at the same time maximizing the resolution and the distance of such old ice to the bedrock. For example for the geothermal heat flux and accumulation conditions at Dome C, an ice thickness lower than 2500 m would be required to find 1.5 Myr old ice. However, the final choice is strongly dependent on the local geothermal heat flux, which is largely unknown for the EAIS and has to be determined beforehand. In addition, the detailed bedrock topography and ice flow history for candidates of an Oldest Ice ice coring site has to be reconstructed. Finally, we argue strongly for rapid access drilling before any full deep ice coring activity commences to bring datable samples to the surface and to allow an age check of the oldest ice

Many observations regarding grain growth in ice sheets are glaciologically interesting but imperfectly understood. Here we develop the theory of grain growth in ice that is not deforming rapidly, and in the succeeding paper we use this theory to explain observations from glacial ice. In the absence of significant strain energy, the driving force for grain growth arises from grain-boundary curvature. Grain growth is slowed by the interaction of grain boundaries with extrinsic materials (microparticles, bubbles, and dissolved impurities). If the driving force for growth is not large enough to cause boundaries to separate from an extrinsic material, then the grain-boundary velocity is determined by the velocity characteristic of the extrinsic material (low-velocity regime). If the driving force is large enough to cause separation, then boundaries migrate more rapidly than the extrinsic material (high-velocity regime) but the net driving force is reduced through transient pinning by the extrinsic material. Polar ice is typically in the low-velocity regime relative to dissolved impurities and the high-velocity regime relative to microparticles and bubbles. Cross-sectional area of grains is predicted to increase linearly with time under most but not all circumstances.