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Immediate report of Victoria University of Wellington Antarctic Expedition 1989-90: VUWAE 34
OPTICAL PROPERTIES OF SEA ICE (K132)
OPTICAL PROPERTIES OF SEA ICE (K132)
Abstract
In determining the effect of enhanced UV levels on Antarctic life, it is important to know the UV radiance under the sea ice where most Antarctic organisms live. The underice radiance is controlled by the transmission of the ice and here we report the results of these measurements. The transmission is largest early in the season, and drops by a factor of 10 by mid-November. This result implies that life under the ice has always experienced its major UV dose in October, a dose that can rise by a factor of 10 during the presence of the deepest ozone holes observed.
Results
The 1989/90 field season was particularly successful for event K132. Of particular interest were the measurements we made of the transparency of sea ice to UV radiation. It was found to be even more transparent early in the season than we had estimated. Since the ozone depletion also occurs early, our results imply a ten-fold increase in the total yearly UV radiation dose under the ice during periods of deep holes such as 1987 or 1989.
This season's programme was a natural extension of our work during 1985/86 and 1986/87, which has already led to 5 publications in international journals, including Nature and Science. Our entirely new optical technique, in conjunction with our theoretical modelling of sea ice, has led to new insight into the details of the light field in and under the ice. Throughout we have monitored the structure of the ice with various physical measurements. This year they included temperature, salinity (brine volume), density, and heat capacity, which have been of considerable importance in understanding the optical behaviour of the ice and its seasonal changes. The significance of this research derives from the fact that light is both essential and potentially harmful to life under the ice.
We show in Figures 1-5 the main results of this year's work. In Fig. 1 the fraction of radiation that is transmitted through the ice te plotted against the wavelength. It can be seen that the harmful UV bands from 250 to 400 nm (UV-B) are in a region of relatively high transmittance. The time dependence is displayed in Fig. 2 and clearly seen is higher transmission early in the season. The largest value was on our first day of taking data, 26 October, at 350 nm. On the basis of results in the visible part of the spectrum we earlier estimated a transmittance of 5% in early October, falling by about a factor of 100 by mid-December. This prediction is seen to be rather close to our measurements, except that any reasonable extrapolation of our data to early October yields more than 5% transmission. It is of course in this early part of the season that UV damage is likely to affect the algae living under and in the ice. A paper on this aspect of our results has been accepted by Nature.
Optical data were also collected in the visible part of the spectrum, and similar time dependence effects were found. We expect to interpret all of these data to separate effects of absorption by algae from the intrinsic sea ice optical behaviour, but as this will require detailed computer modelling of light scattering in the ice, it will not be completed until the end of 1990.
The depth dependent profiles of temperature, salinity, and density of the ice are shown in Fig. 3. In both the salinity and the density there are changes in the surface layer as the ice temperature rises. These changes signal the draining of brine from this layer, which leaves behind a high density of air bubbles. It is scattering by these bubbles that leads to the increased turbidity of the ice and the decrease in transmission at all wavelengths observed in Fig. 2.
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Shown in Fig. 4 is the underice radiance at 300 nm calculated using our measured transmission values and the surface radiance measured in 1988 at Palmer Station. These are the only surface radiance measurements available to our knowledge. It is interesting to note that even before the appearance of the hole the net yearly dose received by ice algae at the base of the ice, given by the area under the curves in Fig. 4, was dominated by the large flux in the early spring. The significant point here is that although the surface radiance is larger in the summer, the developing turbidity of the ice shields the ice algae late in the season. The figure clearly shows that there is a factor of four enhancement in the UV radiance under the ice during October 1968, resulting from the coincidence of the ozone hole and the period of high sea ice transparency. During times of deeper holes, such as 1987 and 1989, the underice UV enhancement in October would be expected to be a factor of 10 higher.
We have also commenced a programme to study the thermal behaviour of sea ice, and to that end we deployed a thermistor array to continuously monitor the temperature at eight depths. This measurement was only a partial success, for we found that the thermistors were directly heated by sunlight to an extent that we could not follow the subtle heat waves that we wished to study. However, larger effects from the sun heating the ice could be clearly measured. The interpretation of the array data will require a knowledge of the heat capacity of sea ice, and as rather little has been done to directly measure this important parameter we developed a simple scheme to make these measurements. The heat capacity results and a comparison with theoretical predictions are shown in Fig. 5. These, along with the array data, win give the degree of solar heating at various depths, which can then be compared with predictions based on the optical models that we have developed.
Acknowledgements
We would like to thank the staff of the Physics and Engineering workshop and Dave Gilmore for design and construction of some of the equipment used in this work. We also would like to thank the Antarctic Division of DSIR for logistic support and the New Zealand University Giants Committee for some financial support. We are grateful to Leigh Johnson for developing the heat capacity equipment.
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Figure 1 Net transmission of the ice vs wavelength in the UV and visible on the 9 November 1989.
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Figure 2 Time evolution of the transmission of the ice (a) at the UV wavelengths 250 nm (●), 280 nm (∆), 300 nm (O), 350 nm (▲), and 400 nm (□), and (b) at visible wavelengths of 500 nm (O), 600 nm (□), and 700 nm (∆).
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Figure 3 Depth dependent profiles for (a) temperature, (b) salinity, and (c) density of sea ice. Triangles and circles correspond to data of the 3 and 13 November respectively. In the case of the temperature the plot shows the highest values recorded up to the date in question which were recorded on the 3 and 9 November.
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Figure 4 The estimated 300 nm flux at the base of the sea ice at Palmer Station during the 1988 season (upper curve), and before the appearance of the ozone hole (tower curve). The dashed lines indicate seasonal periods for which we have no direct measurements of the ice transmission. The net yearly dose is given by the areas under the curves.
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Figure 5 The heat capacity of sea ice taken from depths of 0-50 mm (●), 339-380 mm (■), and 570-620 mm (▲). The salinities were 1.32, 0.84, and 0.62% respectively. The solid lines are the predictions based on the assumption that the ice-brine mixture is at all times in thermal equilibrium.