Stratigraphic studies in the snow and firn of the Greenland ice sheet

摘要

NOTE: Text or symbols not renderable in plain ASCII are indicated by [...]. Abstract is included in .pdf document.The Greenland ice sheet is treated as a monomineralic rock formation, primarily metamorphic, but with a sedimentary veneer of snow and firn. This sedimentary member is perennial above the firn line, and the classical methods of stratigraphy and sedimentation can be profitably applied to it.During a 4-year period 146 pit studies and 288 supplementary Rammsonde profiles were made along 1100 miles of over-snow traverse (Fig.1). Temperature, density, ram hardness, and grain size were measured in the strata exposed in each pit.Stratification of snow results from variations in the conditions of deposition and is emphasized by subsequent diagenesis. Summer layers are coarser-grained and have generally lower density and hardness values than winter layers; they may also show evidence of surface melt. The onset of fall is usually identified by an abrupt increase in density and hardness accompanied by a decrease in grain size. This stratigraphic discontinuity is used as the annual reference plane.Strata in the upper 10 to 20 meters compose a succession of annual sequences which are preserved in recognizable form for at least several decades. Correlation of annual layers between pits, spaced 10 to 25 miles apart along the traverse of Figure 1, gives a picture of annual accumulation during the past 5 to 20 years for western Greenland between 69 and 77°N. The control established by these data, together with information from earlier expeditions (primarily those of Koch-Wegener and DeQuervain) and from permanent coastal meteorological stations, have been used to make a map showing the distribution of gross annual accumulation, essentially the equivalent of annual precipitation, for the entire ice sheet (Fig. 30). In general, the accumulation contours follow the north-south trend of the coast lines, with extremes of less than 10 cm H2O in the northeast and more than 90 cm H2O per year in the south; the average for the ice sheet is 34 cm H2O per year. The zone of maximum precipitation lies close to the coast in two regions, one on the east coast between Angmagssalik and Scoresbysund, the other on the west coast between Upernavik and Thule.In addition to the existence of a useful stratigraphic record four diagenetic facies are recognized on the ice sheet.(1) The ablation facies extends from the outer edge, or terminus, of the glacier to the firn line. The firn line is the highest elevation to which the annual snow cover recedes during the melt season.(2) The soaked facies becomes wet throughout during the melting season and extents from the firn line to the saturation line, i.e., the uppermost limit of complete wetting. The saturation line is the highest altitude at which the 0°C isothermal surface penetrates to the melt surface of the previous summer.(3) The percolation facies is subjected to localized percolation of melt water from the surface without becoming wet throughout. Percolation can occur in snow and firn of sub-freezing temperatures with only the pipe-like percolation channels being at the melting point. A network of ice glands, lenses, and layers forms when refreezing occurs. This facies extends from the saturation line to the upper limit of surface melting, the dry-snow line. Negligible soaking and percolation occur above the dry-snow line.(4) The dry-snow facies includes all of the glacier lying above the dry-snow line, and negligible melting occurs in it.The saturation line can be identified by discontinuities in temperature, density, and ram hardness data, and it may also be located by examination of melt evidence in strata exposed on pit walls. It is as sharply defined as the firn line; but the dry-snow line, although determined by the same methods, is an ill-defined transition zone 10- to 20-miles wide.The facies represent a response to climate, therefore changes in the location of facies boundaries may be used as indicators of secular climatic change. Since facies are not restricted to the Greenland ice sheet, they provide the basis for a general classification of glaciers. This "facies classification" is areal in nature and gives a greater resolution of characteristics than Ahlmann's "geophysical classification." In particular, the "facies classification" permits subdivision of large glaciers which span the entire range of environments from temperate to polar. Ahlmann's useful distinction between temperate and polar glaciers takes on new meaning in the light of glacier facies. Thus, a temperate glacier exhibits only the two facies below the saturation line whereas one or both of the facies above the saturation line are present on polar glaciers. An attempt has been made to map the distribution of facies on the Greenland ice sheet (Fig. 48).The distribution of mean annual temperature on the ice sheet may be approximated by gradients with respect to altitude and latitude of 1°C/100m and 1°C per degree latitude respectively. The altitude gradient is controlled by strong outgoing radiation, producing deep inversions and katabatic winds. The katabatic winds are warmed adiabatically as they descend along the surface of the ice sheets and this is the primary control determining the temperature gradient along the snow surface. The latitude gradient is based on temperature measurements made above 2000 m on the ice sheets and on average values from meteorological stations spanning 20° of latitude on the west coast. A contour map of isotherms based on these gradients compares well with temperature values obtained from pits on the ice sheet. (Fig. 40).The densification of snow and firn is discussed for the case where melting is negligible. The assumption is that accumulation remains constant at a given location and, under this assumption, the depth-density curve is invariant with time as stated by Sorge's law. As a layer is buried it moves through a pressure gradient under steady-state conditions, and it is assumed that the decrease in pore space with increasing load is simply proportional to the pore spaces, i.e., [...] where [...] = specific volume of firn ([...] = firn density), [...] = specific volume of ice = 1.09 cm3/g, [...] = load at depth z below the snow surface and m = a constant which depends on the mechanism of densification. The depth-density equation obtained from equation 8 is [...] where K = [...], [...] = void ratio for snow of density [...], and [...] = void ratio for snow of density [...], [...] = density of snow when [...] = 0.The consequences of the assumption in equation 8 compare favorably with observation. A fundamental change in the mechanism of densification is recognized within 10 m of the snow surface. The concept of a "critical density" is introduced. Before the density of snow attains the critical value it is compacted primarily by packing of the grains. The critical density represents the maximum value obtainable by packing and further compaction must proceed by other mechanisms. The rate of change of volume with increasing load decreases by a factor of 4 when the critical density is exceeded. The same equations hold in the case where melt is not negligible but the rates of densification are higher.Bauer's (1955) estimate for the balance of the ice sheet is revised. Two corrections are applied: (1) the average annual accumulation value of 31 cm H2O originally estimated by Loewe (1936) is revised to 34 cm H2O as a result of this study; (2) the relative areas of ablation and accumulation zones in Greenland north of 76°N are more accurately defined. The net result is a slightly positive balance which is interpreted to mean that the Greenland ice sheet is essentially in equilibrium with present day climate.