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The Chemistry of Ice-Shelf Brines

Published online by Cambridge University Press:  30 January 2017

A. T. Wilson  and
A. J. Heine
A. T. Wilson
Affiliation:
Chemistry Department, Victoria University of Wellington, Wellington, New Zealand
A. J. Heine
Affiliation:
Antarctic Division, D.S.I.R., Wellington, New Zealand
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Type
Correspondence
Information
Journal of Glaciology , Volume 5 , Issue 38 , 1964 , pp. 265 - 267
Copyright
Copyright © International Glaciological Society 1964

Sir,

Brine soaking of ice shelves has been reported in the Antarctic by Reference Stuart and BullStuart and Bull (1963) in the Ross Ice Shelf near Scott Base, and by Reference DubrovinDubrovin (1960) near Lazarev Station The phenomenon has also been reported in the Ward Hunt Ice Shelf, Ellesmere Island (Reference Marshall and BushnellMarshall, 1960; Reference Lyons and LeavittLyons and Leavitt, 1961) The purpose of this communication is to describe the chemistry of the brine from the Ross Ice Shelf.

The brine was sampled in the summer of 1962–63 from a hole one kilometre east of Scott Base and 1.5 km from the seaward edge of the ice shelf The ice core containing the brine was allowed to melt and the melt water was collected in a polythene bottle The samples were returned to New Zealand for analysis. The chemical analysis is shown in Table I, together with that of sea-water for comparison

Table I Chemical Composition of Cores from Hole Number 2

Depth below Depth below Chemical composition of melted core Minimum
Sample top of ice surface of temperature
number shelf brine Na+ K+ Na+/K+ SO 4-- Cl– Cl–/SO 4-- (Fig. I)
cm. cm. g./l. g./1. g./1. g./1. °C.
S 13 407–458 107–158 1.3 0.05 26 0.221 2.3 10.4 −9.3
S 14 458–505 158–205 1.7 0.06 28 0.316 3.3 10.4 −9.3
Sea-water (for comparison) 9.1 0.33 27.5 2.3 16.5 7.2 —

Fig. 1. Plot of Cl–/SO4-- ratio of residual brines in equilibrium with ice and crystallized salts as a function of temperature. Calculated from experimental data given by Reference Thompson and NelsonThompson and Nelson (1956)

It can be seen from the data presented in Table I that the sample is depleted in sulphate with respect to sea-water Analyses by Reference DittmarDittmar (1884) have shown that, apart from variations in salinity, sea-water has practically a constant composition all over the world.

The most probable origin for this brine is sea-water percolating through the cold firn of the ice shelf where ice separates from it, producing a brine considerably more saline than sea-water.

Reference Thompson and NelsonThompson and Nelson (1956) have studied the concentration of brines and the deposition of salts from sea-water under frigid conditions When sea-water is cooled, ice first separates together with small quantities of calcium carbonate, but except for becoming more concentrated, the chemical composition remains practically unchanged until a temperature of 8.2°C is reached, at which point hydrated sodium sulphate begins to crystallize As temperatures are lowered the composition changes rapidly as sodium sulphate is removed from solution.

From the Cl–/SO4-- ratio of the Ross Ice Shelf brine it is evident that the minimum temperature to which it has been subjected during its passage through the frozen firn of the ice shelf was −9.3° C This result is in agreement with temperature measurements at the same depth in the brine layer in a nearby thermohm station situated a horizontal distance of 400 m. to the north (unpublished results of A. J. Heine).

The density of this brine would have been 1.1 g./cm.3 (Reference Thompson and NelsonThompson and Nelson, 1956), the sca-water having lost three-quarters of its original water. This poses an interesting hydrostatic problem, for if the brine were to flow downward through the firn it would move to a region of higher temperature and thus ice would melt until the concentration of the brine was reduced to that appropriate to the ice-brine equilibrium at that temperature. We thus have an inverted density gradient imposed by a temperature gradient.

There are several reports of the occurrence of hydrated sodium sulphate on the ice, especially in regions of ablation (e.g. Reference DebenhamDebenham, 1952). This is probably the hydrated sodium sulphate which has crystallized from the freezing sea-water and which has moved up to the surface as the ice above it has ablated.

We thank Mr. W. J. H. Baillie for carrying out some of the analyses and the Victoria University of Wellington Grants Committee for financial assistance.

30 October 1963

References

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Figure 0

Table I Chemical Composition of Cores from Hole Number 2

Figure 1

Fig. 1. Plot of Cl–/SO4-- ratio of residual brines in equilibrium with ice and crystallized salts as a function of temperature. Calculated from experimental data given by Thompson and Nelson (1956)