LAKE VANDA AN ANTARCTIC LAKE, A SOLAR ENERGY TRAP
Lake Vanda is shown to be a natural example of the trapping and storing of solar energy by a salt water density gradient. The bottom of this lake (218 ft) is maintained at 25°C despite a mean annual air temperature of about −20°C, the solar heating being limited to the short Antarctic summer.
Lake Vanda (Lat. 77° 35′S, Long. 161° 39′E) is permanently covered with about 12 ft of ice and is five miles long and a mile wide. It occupies a depression in the lowest part of the Wright dry valley, Victoria Land, Antarctica. In the summer of 1360/61 an American biological party discovered that its bottom waters were warm and highly saline. (1)
The authors studied the physics and chemistry of the lake and the geology of the surrounding area during the first half of December, 1961. This communication deals with the heat balance of the lake.
Twelve holes were drilled at 1000 ft intervals along two lines, one along the length of the lake and the other across its widest part. Water temperatures were measure to ± 0.05°C at 5 ft intervals, using a copper-constantan thermocouple and a potentiometer. The thermocouple was checked against a reversing thermometer of the type used in oceanography. Density and chemical composition were measured at 5 ft intervals through one hold in the central part of the lake. The lower region of the lake below 160 ft is strongly density-stratified saline water which we consider to be non-convective. The in situ density increases from 1.007 g/cm3 at 160 ft to 1.100 g/cm3 at 215 ft.page 2
intervals. 2N and 2S are at positions 1000 ft north and 1000 ft south respectively of position 2.
The temperature gradient in the soft mud at the lake bottom was measured near the centre of the lake by driving in a geothermal gradient probe. This probe consisted of 76 copper constantan couples connected in series and mounted on a wooden rod with their junctions 25 cm. apart. The probe was calibrated by placing it at the 170 ft level in the lake where the gradient was known from the thermal profile measurements. This calibration agreed with that calculated theoretically to within ten per cent. Two feet below the bottom of the lake the temperature was found to be decreasing downward with a gradient of 0.04°C per foot. Heat is thus being conducted downwards from the lake. Assuming the thermal conductivity of the mud salt water mixture at the lake bottom to be similar to that of water, the annual heat flow would be about 50 cal/cm2/yr).
Bolometer measurements of the solar energy flux were made through five holes under a variety of cloud conditions and at depths up to 50 ft below water level.
The heat balance of the lake presents some interesting problems. The temperature gradient at the 165 ft level is 0.37°C/foot. This means that the non-convective lower part of the lake is losing heat at 550 cal/cm2/yr by conduction upwards. To this must be added the 50 cal/cm2/yr that is being lost downward by conduction through the bottom of the lake.
|1.||Biological activity within or at the base of the lake.|
|3.||Hot springs in lake bottom.|
|4.||Abnormally high geothermal gradient under lake.|
|5.||Radiant energy from the sun penetrating into the lake and being absorbed.|
According to Zobell et al. (2), biological activity would not produce more than an extremely small part of the heat required even under the most favourable conditions.
Coring of the bottom yielded no solid soluble salts. The bottom waters of the lake contain in solution calcium chloride with smaller amounts of sodium and potassium chlorides. Even if these salts did occur on the bottom of the lake, consideration of their heats of solution (3) rules out any appreciable heating, from this source. There is no evidence for any other chemical reactions that are producing appreciable heat.
There is no geological evidence for hot springs in the area surrounding the lake. The temperature gradient is positive at all depths in all profiles measured. In the lower half of the lake the temperature versus depth profile is remarkably similar (± 0.1°C) over an area at least 3000 ft by 2000 ft (Table I) whereas hot spring areas are characteristically non-uniform in lakes.
The possibility of a high geothermal gradient is ruled out, since the temperature gradient probe showed that heat is being conducted out through the bottom of the lake. Other evidence is that the temperature gradient decreases from 160 ft downward. This page 3 is not a salinity effect since the thermal conductivity of water decreases as the salt concentration increases (4), and implies that the heat flow increases as one goes upwards.
Thus we are left with radiant energy from the sun as the only possible heat source. No data is available on the total annual radiant solar energy flux at Lake Vanda, but it is Known (5) that the solar energy flux at Scott Base, 80 miles due east of Late Vanda, is 91,000 cals/cm2/yr. The fraction of the incident solar energy which penetrates the twelve feet of ice was determined with a bolometer. With a cloudless sky, five per cent, and with a cloudy sky, eight per cent of the radiant energy flux enters the water below the ice. Measurements in the upper 50 ft of the lake showed that 45 ft of water reduced this energy flux to one half. By adopting a six per cent value at the bottom of the ice and assuming that the water maintains the same attenuation value, the quantity of energy reaching the 165 ft level would be550 cals/cm2/yr, which is in reasonable agreement with the 600 cals/cm2/yr, calculated above as the heat loss from the lake water below 165 ft.
As additional evidence of a solar heating mechanism, the temperature gradient below 160 ft has been calculated using the above data and is compared with the observed data in Table II.
It is seen that above 190 ft the temperature gradient versus depth profile is as would be expected from a solar heating mechanism. At 195 ft the temperature gradient falls off more rapidly than is predicted from theory, but it was observed while sampling that the water below 195 ft is brown in colour.
It is interesting to speculate on the construction of similar installations for the trapping and storing of solar energy in the more temperate latitudes. The heat could then be used for the heating of buildings or for evaporation processes such as salt production from sea water. Some experimentation along these lines has been carried out in Israel (6).
The authors wish to thank Mr C. J. Banwell and Mr I.G. Donaldson and other member of the staff of the D.S.I.R. for helpful advice and for the loan of equipment; also Dr Angino of the University of Kansas, the United States Navy and U.S.A.R.P. personnel without page 4 whose cooperation the expedition would not have been possible.
(1) Armitage, K.B. and House, H.B., Limnology and Oceanography 7, 36 (1962).
(2) Zobell, C.E., Sisler, F.D. and Oppenheimer, C.H. J. of Sed. Pet. 23, 13 (1963).
(3) Handbook of Chemistry and Physics. Chemical Rubber Publishing Co., Cleveland, Ohio, U.S.A. (1954).
(4) Dorsey, N.E. "Properties of Ordinary Water Substance". Reinhold Publishing Corporation, New York (1940).
(5) Thompson, D.C. and Macdonald, W.J.P. Dent. of Sci. & Ind. Res. Bulletin 140, T. Hatherton ed., N.Z. Government Printer, Wellington, New Zealand (1961).
(6) Tabor, H. United Nations Conference on New Sources of Energy. E/Conf. 35/8/47 (1961).