Report of Glaciological Work on Project Snow Cornice in 1949
American Alpine Club Research Fund
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14. REPORT OF GLACIOLOGICAL WORK ON PROJECT SNOW CORNICE IN 1949
Robert P. Sharp
IT was on the initial reconnaissance flight of the 1949 season that pilot Maury King tilted the wing of the Norseman to give us a clear view of the Seward firn field. There below in solitary splendor was the windsock marking the site of our camp and cache of the previous year. One look was enough to show that the past winter had been a most prosperous one for the Seward. The windsock rose a bare seven feet above the surface, yet we knew only too well that it was attached to the end of a 21-foot pole. A subsequent excavation to uncover equipment cached at the foot of the pole showed the 1948-49 snow blanket to be 14.5 feet thick on June 20th. At an average density of 0.44, this is equivalent to 6.38 feet or 76.5 inches of water. From a number of considerations, too detailed to recount, the total precipitation from 1 July 1948 to 30 June 1949 is estimated at 80 inches. For the same period Yakutat recorded 160 inches, and these figures give some measure of the maritimity of Yakutat compared to the continentality of the Seward firn field, a scant 60 miles north.
The change in density of the 1948-49 firn layer was traced during the summer and showed a gradual increase from 0.44 on June 20th to 0.54 on August 29th. Since this layer became isothermal at 0° C. in early July, the density increase could not have been due solely to refreezing of meltwater. Compaction-settling and crystal growth, perhaps facilitated by local pressure melting and vapor transfer, seem to have played a significant role.
The excess of accumulation over wastage for 1948-49 on the firn field was approximately 67 to 68 inches of water. The significance of this figure becomes apparent when it is compared with the surpluses from previous years, namely, 23.5 inches in 1947-48,17.5 inches in 1946-47, and about 30 inches in 1945-46. The fact that Malaspina Glacier, the principal dissipation area of this system, was still largely covered by snow in late August indicates that wastage during the 1948-49 season was grossly subnormal. Although evaluation of the field data is incomplete, it seems likely that the budget year 1948-49, with greatly increased income and sharply reduced expenditures, was exceedingly prosperous for the Seward-Malaspina system.
Rates of gross ablation were determined at six stations on Seward firn field and at two localities near the south edge of Malaspina Glacier. These were supplemented by daily readings at the Airstrip Camp on the firn field and by a few hourly observations with an ablatometer. Average gross ablation at all stations on the Seward was 0.55 inches of firn per day. The water equivalent would be roughly half this value. The daily maximum recorded was 1.3 inches of firn, and the hourly maximum 0.3 inches. Gross ablation at the Malaspina stations averaged 2.53 to 2.64 inches of ice of approximate density 0.9. On a water equivalent basis, gross ablation near the south edge of Malaspina Glacier was roughly nine times that on the Seward firn.
In 1948 the winter’s chilled layer had been entirely dissipated before our thermal recording apparatus, kindly loaned by the National Bureau of Standards, could be installed. A more successful program of temperature observations was carried out in 1949. On June 28th the top of the winter’s chilled layer lay 12 to 14 feet below the surface, and the bottom was between 42 and 51 feet deep. The lowest temperature,—1.1° C., was recorded at a depth of 21 feet. Changes in the temperature regime were traced by daily observations until the chilled layer was completely dissipated between July 7th and 8th. Irregularities developed in the temperature curve during the amelioration period suggest that the firn is warmed principally by meltwater percolating downward from the surface and moving laterally along stratified layers. These temperature data have already proved useful in calculating total annual precipitation and in evaluating the causes of increase in firn density. They are also necessary for calculations of net ablation from gross ablation.
Thermal studies of diurnal crust development on the firn surface gave useful data on the rate of crustal development and on temperature gradients within the firn and in the layer of air just above. Among other things, it was discovered that the greatest thickness of crust was attained between 0600 and 0700 hours. It appears likely that the history of diurnal crusts reproduces on a small scale some aspects of the development and deterioration of the annual chilled layer.
Determinations of meltwater circulation in the firn were also more successful in 1949 than in 1948. The daily meltwater wave percolated downward at six to eight inches per hour, so the collecting pan 12 feet deep recorded its maximum flow nearly 24 hours later than pans close to the surface. The daily peak of meltwater flow builds up rapidly and tapers off gradually. The largest volume measured was 1.1 cc per cm2 of the collecting pan. This flow was recorded by a pan at a depth of six feet. Horizontal strata in the firn appear to exert considerable control on meltwater circulation.
Gravity profiles were run across Institute Glacier and across the eastern basin of Seward firn field with a gravity meter kindly loaned by the Frost Geophysical Corporation. A broad firn field is nearly ideal for gravity studies, and two satisfactory profiles were produced. Seismic reflections were also obtained along these same profiles by the geophysical party (see cut). A combination of the seismic data and the gravity profiles gives a good picture of the depth and configuration of the subglacial rock floor. Maximum thickness of ice on the Institute Glacier profile is about 1000 feet. At the Airstrip Camp on Seward firn field the ice is close to 2000 feet thick. Owing to difficulties with the thermal boring apparatus used to “drill” shot holes, not so many reflections were obtained as originally hoped. Nonetheless, the data from seismic studies appear reliable and will prove most useful, although more would have been desirable.
This type of operation is bound to have its disappointments. In 1949 the radar provided our biggest source of unhappiness. Many echoes were obtained, but they were too variable to permit any one echo to be identified as that coming from the subglacial rock floor. Further study of the data may possibly permit some conclusions, but at present writing the outlook is not encouraging. We are not yet ready to abandon the radar work, but the method remains unproved. It is possible that the poor results of 1949 are to be attributed more to improper equipment than to faulty procedure.
Dr. Bader carried on structural and crystallographic studies in a research camp at the southern edge of Malaspina Glacier. Much of his time was devoted to developing and perfecting equipment and procedures for future work, but even so he was able to gather a considerable mass of data which should yield worth-while results when properly compiled and analysed.
As usual, we of the scientific staff are immeasurably indebted to Director Walter Wood and his colleagues. It seems appropriate that the men responsible for the above-described scientific work should be listed here. They are Dr. Henri Bader and Laurence H. Nobles, who worked on glaciological matters; S. Norman Domenico and John R. Reese, geophysicists; and Bernard O. Steenson and Fred A. Gross, radar operators. Special thanks are extended to Alan Bruce Robertson for his willing assistance at the Malaspina Glacier camp. The continuing interest of the American Alpine Club in this work is most gratifying.