LAKE VANDA

Lake Vanda, a permanently ice covered lake in Wright Valley near the centre of the dry valley system of Victoria Land, Antarctica, is 15 miles east of the Polar Ice Plateau and 30 miles west of the Ross Sea. The climate is cold and extremely arid with frequent strong winds. Four moraines ere distinguished near the lake by the trend of their ridges and by their degree of weathering. Weathering that is attributed to the effect of salts blown in from the sea entirely destroyed surface granite on the oldest moraine. None of the Moraines were deposited by glaciers from the Polar Ice Plateau. Scree formation has planed steep moraines into even slopes that extend down for 200 ft to the flat bottom of the lake. Lag gravels with wind facetted stones cover all except the steepest slopes, and two square miles of gneiss at the east end of the lake has been worn into sharp ridges by salt weathering and wind erosion.

Old lake terraces, developed only in steep scree, show that the lake once stood 150 ft higher than at present. The development of the terraces in scree is attributed to the persistence of the "edge crack", a kind of frost polygon that follows the steepest part of the lake shore and is absent where the lake is gentle and the shore is ice covered by normal frost polygons.

The ice on the lake is about 12 ft thick. From the average thickness of the ice above and below lake water level and from the height of frozen pools it is estimated that about a foot of the ice surface is evaporated each year, and about the same thickness added to the bottom by freezing. Water equal to a thickness of about three feet forms within the ice and the edge of the ice melts back over the shallow parts of the lake during the summer. The observed lowering of water level and the previous summer and winter lake levels indicate that the total annual evaporation took place during sunny days with strong westerly winds. Evaporation is replenished by the Onyx River which flows west from the Wilson Piedmont Glacier for a few months in summer.

The water temperature at the bottom of the lake at 217 ft is 25°C, about 47°C higher than the mean annual temperature at the lake surface. The temperature difference is attributed to solar heating. From the observed temperature gradients in the strongly density-stratified lower part of the lake, from the downward heat flow through the bottom of the lake, and from the observed attenuation of solar energy through the ice and upper lake water it is inferred that the annual radiant heat flux per cm² at the lake surface is about 100,000 calories, a value close to that recorded at Scott Base. The effectiveness of solar heating is attributed to the lake water being almost as clear as pure sea water, and to the ice being clear with negligible snow cover.

Samples at 5 ft intervals were tested for chlorinity on the ground, and 30 samples taken back to New Zealand for more complete analysis. Chlorinity increases downwards at an irregular rate, over half the chlorinity being within 25 ft of the lake bottom. The maximum chlorinity is 83,500 parts per million and the average if the water were mixed about 1,000. The bottom concentration and the irregularities in downward increase in chlorinity are attributed to the lake having risen and having been evaporated down several times in the past. The "chlorinity" age of the lake, determined by assuming past annual chlorinity inflow at its present value, is about 30,000 years.

The major constituents of the water are calcium, sodium, magnesium, and potassium as cations, and chlorine, bicarbonate, and sulphide as anions. As equivalent parts expressed as a percentage of total cations, sodium decreases from 30 at the surface to 15 from near the lake bottom, potassium from 5 to less than 1, and bicarbonate from about 15 to 0. Calcium remains about constant. Sulphide is page 2 mostly below 170 ft and does not exceed 7%. The major changes in composition are attributed to sodium bicarbonate having crystallized out on the lake shore, and having been blown away.

From chlorinity ages and from the degree of weathering of the moraines it is inferred that the oldest moraine is considerably more than 10,000 years old, and that Wright Valley was not filled by a through glacier from the Polar Ice Plateau during the last Glaciation. Wright Valley and the other dry valleys of Victoria Land are considered to pre-date the last Glaciation and to contain moraines that represent an appreciable part of the Pleistocene Period.