MARTIN O. JEFFRIES, Geophysical Institute, University of Alaska-Fairbanks, Fairbanks, Alaska 99775-7320
Sea -ice thickness plays a key role in ocean-atmosphere interactions (exchanges of heat, mass, and momentum) and thus regional and global climate variability. The thickness of the sea-ice cover is determined by thermodynamic (freezing, melting) and dynamic (deformation) processes. Information on some of these processes can be obtained from the analysis of the ice-crystal texture and the stable isotopic composition of ice cores.
The ice-crystal texture and stable isotope composition of sea ice depend on the growth mechanism and the conditions under which it occurs. In calm conditions, congelation ice, which grows at the base of an ice floe as heat is conducted upward to the surface, develops a columnar texture of elongated crystals. In turbulent conditions, such as at the surface of a wind- and wave-roughened sea, rapid freezing creates frazil ice crystals that have a granular texture. Snow ice, which forms after seawater has flooded the ice/snow interface, also has a granular texture. Because of their similar textures, frazil ice and snow ice are identified on the basis of differences in stable isotopic composition (Lange et al. 1990).
Congelation ice, frazil ice, and snow ice are the most common types observed in antarctic sea-ice floes. The figure illustrates the amounts in which they occurred in four ice cores obtained from the same first-year ice floe (number 247-95). The amount of each ice type differs in each core, and the cores have different lengths. This variability is a common characteristic of antarctic sea ice and reflects the complex history of the floe, which had thickened by different combinations of thermodynamic and dynamic processes as a number of smaller floes agglomerated over time and under changing conditions.
Based on measurements made at 151 drill holes, floe 247-95 had a mean thickness of 0.43 meters (m) and range of 0.2-0.75 m. The four ice cores had a mean length of 0.425 m and range of 0.21-0.66 m. The cores, then, are reasonably representative of the thickness variability of the floe. Assuming that the ice types observed in the cores are also representative of the entire floe, the contribution of each ice type to the development of the floe is determined from the combined composition of the four cores.
There are two different methods for obtaining a measure of the contribution of ice types to the development of the ice cover: the average method and the absolute method, as illustrated in the figure. The average method adds the percentage amount of a particular ice type in each core and then divides that total by the number of cores to give an average percentage for that ice type. The absolute method adds the total amount (in meters) of a particular ice type measured in each core and then divides that total by the total length of the cores (in meters) to give an absolute percentage for that ice type.
While working aboard the R/V Nathaniel B. Palmer during four cruises in the Ross, Amundsen, and Bellingshausen Seas in May, June, August, September, and October 1993-1995, the composition of a total of 338 first-year ice cores with a total length of 253.71 m has been analyzed. Here, only the 282 cores (total length 219.09 m) obtained from the outer pack ice on the deep ocean north of the continental shelf are considered. The cores from the inner pack ice on the continental shelf are excluded because they contained an anomalously large quantity of congelation ice (approximately 65 percent; Jeffries and Adolphs 1997).
Whether one looks at a single floe (figure) or the entire winter pack-ice cover, the method of calculating the ice-core composition does not make a significant difference to the results, in this particular instance; the absolute and average amounts of each ice type are almost the same. For the set of 282 ice cores, the absolute amounts of snow ice, frazil ice, and congelation ice are 29, 37, and 31 percent, respectively. The average amounts of each ice type are 30, 34, and 35 percent, respectively. Minor ice types that are of no consequence to this analysis make up the remaining 1-3 percent.
Whether one considers the absolute or the average amount of ice, the preponderence of frazil ice is consistent with observations in the Weddell Sea and the east antarctic pack ice (Lange and Eicken 1991; Worby et al. in press). The amount of frazil ice is smaller than elsewhere, however, because snow ice makes a greater contribution to pack-ice development than has been reported in the other antarctic pack-ice zones. The significant difference in congelation ice amounts on the deep ocean and the continental shelf suggests that these zones may have very different pack-ice regimes, as determined by atmospheric and oceanographic forces (Jeffries and Adolphs 1997).
The method that is chosen to calculate the contribution of each ice type may depend on the purpose of the study and the preference of the investigator, but I recommend that the absolute method be adopted as the standard, because it provides a better measure of the contribution of a particular ice type to the total volume or mass of the ice cover. Adoption of a standard would also make it easier to compare observations made by different investigators in different sea-ice zones. Failing this, I recommend that all authors at least clearly identify the method that they have used to describe the composition of their cores.
This work was made possible by National Science Foundation grants OPP 91-17721 and OPP 93-16767. I thank the many people who worked long hours with me on ice floes and in the science freezer aboard the ship. Captain Joe Borkowski, the mates, and crew of the R/V Nathaniel B. Palmer contributed to the success and enjoyment of the cruises.
Jeffries, M.O., and U. Adolphs. 1997. Early winter snow and ice thickness distribution, ice structure and development of the western Ross Sea pack ice between the ice edge and the Ross Ice Shelf. Antarctic Science , 9(2), 188-200.
Lange, M.A., and H. Eicken. 1991. Textural characteristics of sea ice and the major mechanisms of ice growth in the Weddell Sea. Annals of Glaciology , 15, 210-215.
Lange, M.A., P. Schlosser, S.F. Ackley, P. Wadhams, and G.S. Dieckmann. 1990. 18O concentrations in sea ice of the Weddell Sea, Antarctica. Journal of Glaciology , 36(124), 315-323.
Worby, A.P., R.A. Massom, I. Allison, V.I. Lytle, and P. Heil. In press. East antarctic sea ice: A review of its structure, properties and drift. In M.O. Jeffries (Ed.), Antarctic sea ice physical processes, interactions and variability (Antarctic Research Series, Vol. 74). Washington, D.C.: American Geophysical Union.