Laurent Janssens

Segregation of solutes and gases in experimental freezing of dilute solutions: implications for natural glacial systems

Low ionic strength waters containing significant calcium and bicarbonate are common in nature, but little literature exists on their behaviour during freezing. Modelling indicates that freezing-induced concentration of solutes (in a closed-system) would lead to progressive increase in calcite saturation index, despite rising partial pressure of CO2 (PCO2), but the consequences of CaCO3 precipitation for the distribution of matter between solid, liquid, and gas phases required experimental investigation. We studied the effects of variations in the rate of advance of an ice-water interface and in the initial degree of saturation for calcite on the behaviour of the system. Downward growth of ice in a 24-cm diameter cylindrical vessel was achieved at a constant linear rate of 3 or 8 mm/h by the progressive cooling of an overlying alcohol reservoir, and the expansion of volume accommodated by regular water sampling through side ports, together with a small expansion chamber. Initial air-saturated solutions (initial PCO2 in the range 10−3 to 10−3.2) were prepared to reflect a range from strongly undersaturated to supersaturated for calcite. Comparative blank experiments were run using deionized water. Ice growth led to enrichment in solutes at the ice-water interface and the creation of a diffusive boundary layer, calculated to be 0.6 mm thick, truncated below by convecting fluid. The first-formed ice (stage 1), was relatively solute-rich because of initial rapid ice nucleation. Where solutions were not strongly supersaturated for calcite this was followed by formation of a solute-poor (stage 2) ice. Ice-interface water segregation coefficients of stage 2 ice were calculated to be 0.0004–0.003 for various solute ions. The relative magnitude of segregation coefficients (Mg2+ > Ca2+ > Sr2+) is attributed to interstitial incorporation (coupled with HCO3−) in the ice lattice, and controlled by ion size. Air bubbles nucleated once nitrogen supersaturation had reached values of 2–2.5 in the boundary layer and were incorporated into ice. These gas inclusions had dissolved air compositions modified by the differential diffusion of O2, N2, and CO2 out of the boundary layer, an O2/N2 ratio of 0.4 being characteristic. Freezing of solutions strongly supersaturated for calcite led to formation of impure (stage 3) ice in which ions are incorporated in similar proportions to those of the parent aqueous solution. Stage 3 ice contains both solid CaCO3 and aqueous (solute-rich) inclusions, associated with an irregular ice-water interface. Gas inclusions were invariably rich in CO2, up to 63% by volume, yet represented only a small proportion of the CO2 generated as a by-product of CaCO3 precipitation. These data allow a better understanding of the expected chemical characteristics of ice that has formed from freezing of bulk water, including river icings, basal ice of glaciers, and local refrozen layers in firn and glacier ice. Generation of CO2-rich gas bubbles by re-freezing is a powerful mechanism for modification of CO2 compositions of bulk gaseous inclusions in ice.

Stable isotopes in the basal silty ice preserved in the Greenland Ice Sheet at summit; environmental implications

Modelling ice sheet behaviour in the context of climatic changes depends on initial and boundary conditions which can be better defined by studying the composition of basal ice. This study deals with basal ice reached by deep drilling at Summit in Central Greenland (GRIP core). The isotopic composition of this ice indicates that ice formed at the ground surface in the absence of the ice sheet largely contributed to its formation. The basal silty ice is a remnant of a growing stage of the ice sheet, possibly the original build up.

Very low oxygen concentration in basal ice from Summit, central Greenland

Oxygen concentration as low as 3% has been detected in the basal silty ice of the GRIP core. Such values were never observed in ice from ice sheets. They are most probably the consequence of organic matter oxidation in ice developed in a peaty deposit when the Greenland Ice Sheet was not present at the site. Flow-induced mixing has further incorporated this ice into glacier ice during the ice sheet build up. The part of the local ice component in the mixing process diminishes with the distance from the bed. This is the process which explains the oxygen profile.

Reconstruction of basal boundary conditions at the Greenland ice sheet margin from gas composition in the ice

Abstract Modelling the response of the Greenland Ice Sheet to a temperature increase is an essential step towards estimating the climatic changes that could affect a large area of the Northern Hemisphere in the near future and is heavily dependent on accurate evaluation of the boundary conditions at the ice-bedrock interface. Here we use a study of gas composition in the basal ice from West Greenland. This study shows that changes in basal thermal zones occur towards the border in the marginal zone of the ice sheet. First, slight melting at crystal boundaries and vein water squeezing occurs when the ice could still be below the pressure melting point. Then, where the melting point is reached and more meltwater is produced, sliding by regelation occurs. Closer to the margin, partial freezing and ice accretion takes place, the melting point being maintained at the bed because of the latent heat release. Pressure effects induce a certain amount of decoupling. Such changes in basal flow conditions can in turn promote ice sheet thinning at the margin. A temperature increase of the air along the edge of some ice sheet areas in West Greenland, where decoupling occurs, would thus give rise to higher ablation rates and would be likely to lead to fast ice retreat.