Glaciological Observations on the Northern Patagonian Ice Field

The 600-m-wide Río de la Colonia valley is flooded each summer by the outburst of an ice-dammed lake, a fact that has destroyed any hope of a permanent settlement (colonia) there (southeast side of NPIF, fig. 27). The Trimetrogon aerial survey first documented the location of the lake 2 km from the glacier terminus, on the right bank of the glacier. It is crossed by an arcuate terminal moraine, hence its name of Lago Arco. Landsat MSS images of 25 February 1976 and 8 March 1979 (table 8 and fig. 42, respectively) show a situation just after a flood, when dry land is downstream of the arcuate moraine. They also show that a second lake, 9 km from the glacier terminus on the left bank of Río de la Colonia, was partially drained on those dates; thus, this lake also contributes to the flooding. A Japanese-Chilean expedition headed by Professor Tanaka, which made the first ascent of Cerro (Volcán) Arenales in 1958, studied the flood problem (weekly "Ercilla" of 2 April and 21 May 1958: Tanaka, 1980).

Supraglacier debris produced by rockfalls and avalanches on two glaciers south of Cerro (Volcán) Arenales has been used to calculate the glacier surface velocity between two dates of Landsat MSS images (25 February 1976 and 8 March 1979). The surface velocity was about 200 m a^-1 on Glaciar Pared Norte, at its elbow near the right bank, and about 260 m a^-1 on Glaciar Pared Sur (fig. 27). Velocities can also be inferred from wave ogives forming at the foot of ice falls (Lliboutry, 1957). These data are of little interest as long as glacier thicknesses remain unknown.

Glaciar Soler is the only east outlet of the NPIF that has been thoroughly studied (Aniya and Naruse, 1987; Casassa, 1987; Naruse, 1987). At the foot of its ice fall, wave ogives show that the surface velocity in the middle of the glacier is 300 m a^-1. It decreases progressively downstream on the tongue. At 1.5 km from the ice fall, the surface velocity is 200 m a^-1. At that point, the glacier is 1,590 m wide and has a maximum thickness of 575 m. Bottom sliding probably accounts for one-third of the surface velocity, whereas near the terminus, where the ice is thinner, it accounts for 90 percent. At the equilibrium line, found at 1,350 m in the ice-fall area, the Glaciar Soler discharges about 1x10⁸ m³ a^-1 of ice from an accumulation area of 36.4 km².

The drift in the proglacial area and in ice-cored moraines consists mostly of well-rounded gravel and boulders, which the glacier has picked up from an outwash plain (Aniya, 1987). Similar forms have been observed, sometimes akin to eskers, in the Central Andes (Lliboutry, 1958).

On the west side of the NPIF, interest has focused on Glaciar San Rafael, which is one of the fastest flowing ice streams in the world. It has a calving ice cliff 3 km long and 30-70 m high above the tidal Laguna San Rafael (a circular lagoon about 15 km in diameter that is limited by a Holocene moraine (Heusser, 1960), which was reached again during the "Little Ice Age"). Velocities at the terminus were measured in the summer of 1984 by Kondo and Yamada (1988). More detailed investigations, including bathymetry and monitoring of calving rates, were done by Warren and others (1995) in February 1991 and February 1992. Lastly, velocities more than 17 m d^-1 were determined in October 1994 by Rignot and others (1996a, b). They used a National Aeronautics and Space Administration imaging radar (SIR-C) on board the space shuttle at an altitude of 175 km that employed a radar-interferometry technique.

Over its last 18 km, Glaciar San Rafael has a very regular slope of 5.7 percent. A rule of thumb estimation of its mean thickness is 11.3 m/0.057200 m. Nevertheless, it probably exceeds 300 m in thickness along its axis, where a fast ice stream is found. However, the transversal profile of the bottom of Laguna San Rafael near the calving front is concave, and has a maximum depth of 272 m at the middle. In this ice stream, surface velocities are about 3.0 m d^-1 all along a 12-km distance. Only about 1.0 m d^-1 can be explained by ice deformation, and about 2.0 m d^-1 comes from sliding. Such a large sliding velocity has never been observed over such a long distance in temperate mountain glaciers, even at the climax of the melting season. On the last 6 km, the ice stream speeds up, progressively but dramatically, reaching 17.0+-0.2 m d^-1 at the calving front. Such a large velocity has only been observed during some glacier surges, but in this case, it is not a surge; the velocities were about the same in 1984, 1991, 1992, and 1994. The flux of ice that is calved amounts to 2,170,000 m³ d^-1 (Warren and others, 1995). Glaciar San Rafael can well provide this amount of flux continuously on an annual basis because it corresponds to an ice layer about 1.35 m thick over the entire accumulation area (585 km²), several times less than the mean mass balance in this area. The acceleration area 6 km from the front is probably where the bed becomes lower than the surface of Laguna San Rafael, and the water pressure in the subglacier cavities begins to increase.

Glaciological Observations on the Southern Patagonian Ice Field

Among the west-calving glaciers of the SPIF, Glaciar Brüggen (Pío XI) is the only one where velocities have been measured (Rivera and others, 1997). At four points (S3 to S6) in its central part, at 1.4 to 2.7 km from its calving front in Fiordo Eyre, the mean velocity ranged from 15.2 to 36.8 m d^-1 on 14-17 November 1995. These velocities are faster than at Glaciar San Rafael, but they oscillate much more. No precipitation fell during these 4 days, and air temperatures were higher on 15-16 November 1995. Therefore, the doubling of the sliding velocity seems to have been caused by surface melting. Meltwater reaching the bottom probably does not drain off easily.

Glaciers calving into east lakes do not flow as fast (Naruse and others, 1992). In November 1990, the velocity of Glaciar Upsala at about 4 km from the front was 3.5-3.7 m d^-1, and the velocity of Glaciar (Perito) Moreno at 4.5 km from the calving front was about 2.0 m d^-1.

A puzzling feature on the SPIF is the presence of looping bands of volcanic tephra that were discovered from the Trimetrogon aerial photographic surveys (figs. 45-48). We now know that the tephra originates from Cerro (Volcán) Lautaro (lat 49°01'S., long 73°33'W.; 3,380 m). The explorers of Patagonia during the years 1867 to 1878 were told by natives on the Argentine side that an active volcano was present within the ice field. On the Chilean side, Lord Thomas Brassey, on board his yacht Sunbeam along Canal Messier, observed a fall of tephra in 1876, and the officers of the American corvette Omaha reported in 1878 that they had seen an active volcano within the ice field, which they called Humboldt (Martinic, 1982). In recent times, Cerro (Volcán) Lautaro has had five documented eruptions:

1. 1933 (seen by Reichert (1947); compare Shipton (1963, p. 38-39) and Agostini (1945, p. 24))
2. January 1945 (documented by Trimetrogon survey flight, aerial photograph 556-L-124)
3. January 1960 (reference in Shipton (1963, p. 73))
4. 1972 (evidence reported by farmers to C. Marangunic, oral commun.)
5. June 1978 (evidence reported by farmers to C. Marangunic, oral commun.)

Figure 45.--Trimetrogon (oblique) aerial photograph showing three bands of tephra on Glaciar Chico.	Figure 46.--Trimetrogon (oblique) aerial photograph showing three transverse bands of tephra on Glaciar Viedma.	Figure 47.--Trimetrogon (oblique) aerial photograph showing three bands of tephra on Glaciar Occidental and, flowing to the right, Glaciar Greve.	Figure 48.--Trimetrogon (vertical) aerial photograph showing several thin tephra layers on Glaciar Pascua.

Landsat 3 RBV (30368-13450-D, Path 248, Row 94; 30368-13453-B, Path 248, Row 94) and MSS (30368-13450, band 5; Path 248, Row 94) images taken on 8 March 1979 show the tephra airfall pattern extending 110 km south of the volcano (figs. 49-51). The pattern covers a wider area when seen in MSS band 5 (visible) than when seen in MSS band 7 (near-infrared). The location of the tephra poses intriguing questions. The first is how, with the exception of a small spot east-southeast of Cerro (Volcán) Lautaro, tephra covers the southwest slope of the volcano and extends to the south in a region where the prevailing wind direction is almost always from the southwest or northwest. Considering the fact that Cerro (Volcán) Arenales to the north had tephra on its southwest flank on the same day (fig. 42), two possible explanations are that (1) both eruptions, at two volcanoes 205 km apart, took place at about the same period during a northerly to northeasterly wind direction, or (2) westerly winds have blown off any posteruption deposit of snow on windward slopes and only uncovered the tephra layer on that side of the volcanoes.

Figure 49.--Landsat 3 RBV image of the Southern Patagonian Ice Field on 8 March 1979.	Figure 50.--Landsat 3 RBV image of the central part of the Southern Patagonian Ice Field taken on 8 March 1979.	Figure 51.--Landsat MSS false-color composite image of the Southern Patagonian Ice Field.	Figure 52.--The second tephra band of the three observed within layers in depression crevasses on Glaciar Viedma during the reconnaissance made in late February 1994.

The second question is how tephra, at first disseminated over a wide area, becomes concentrated into narrow bands. Figure 52 shows that a tephra layer is formed, becomes embedded within the ice, and appears in the ablation zone as a dark layer in the ice. Next, rain and running meltwater wash the tephra from the ice surface, and only a narrow belt contiguous to the emergence line is left. I called this kind of banding "sedimentary bands" (Lliboutry, 1957). According to Williams (1976), this sequence probably takes place on the outlets of Vatnajökull (Iceland), where similar layers of tephra are observed.

All the outlet glaciers around Cerro (Volcán) Lautaro exhibited three tephra bands in 1945 (figs. 45-47) (Lliboutry, 1957, figs. 32 and 41). The bands may be recognized on satellite images and might be used in the future to calculate (1) ice velocity and (2) the interval of time between the big eruptions that produced the tephra layers. Because southerly winds are almost unknown in the area, the presence of the three characteristic tephra layers north of Cerro (Volcán) Lautaro indicates that ice flows toward the north. Conversely, the finer bands found on Glaciar Pascua (figs. 29, 48) cannot have formed from the same eruptions because they have a quite different pattern. Perhaps they represent tephra whose origin is from Cerro (Volcán) Arenales?

In piston cores recovered from the bottom of Lago Argentino, near Glaciar (Perito) Moreno (fig. 30), three layers of tephra have been discovered (del Valle and others, 1995). They probably do not correspond to the three bands of tephra just discussed. First, the two upper bands are much thicker (12 cm and 9 cm, respectively) than the third one (1 cm). Second, if we assume a constant rate of sedimentation, the same as the one that has been accurately determined during the last 71 years, the three big explosive eruptions should have taken place in A.D. 1200, 1520, and 1620. The tephra layers that are observed today at the surface of the SPIF are obviously younger. In my opinion, they were ejected during the 19th century.

Mass and Energy Balances of Glaciers

In southern Patagonia, precipitation is equally distributed throughout the year, without the maximum in winter that is observed farther to the north. Whether the precipitation falls as rain or snow depends on the air temperature, which ranges in general between 2°C and 13°C at sea level. The 0°C isotherm varies accordingly between 700 m and 2,500 m. During summer, rain is more frequent on the ice fields than snowfall, and when snow does fall, it is very wet and heavy.

For the preceding reason, and not only because of melting, annual mass balances in the accumulation area, although almost unknown, are certainly much lower than the 6 to 7.5 m of precipitation. At 1,296 m above sea level on Glaciar San Rafael in the NPIF, coring down to 37.6 m revealed the mean mass balance to be 3.45 m of water equivalent per year (Yamada, 1987; Yamada and others, 1987), whereas the equilibrium line has been established to be about 250 m lower--at least at 1,050 m (Club Andino Bariloche, 1954). Thus, the activity coefficient should be at least 1.5 m per 100 m of difference in elevation. Many ice crusts and ice layers were found in the cores. At the end of November 1985, the firn had a density of 0.50 near the surface and turned into impermeable ice at a depth of 26.7 m. Over this ice was an aquifer 2.8 m thick, as well as a "water-soaked layer" (owing to capillarity) 5 m thick.

The preceding observations confirm that the ice fields of Patagonia are entirely at the melting point (temperate), except at the highest summits. They are quite similar in their size, ELA, and velocities to the temperate ice fields of southern Alaska (Bagley, St. Elias Mountains, and Juneau Ice Fields). They have less dissymmetry between their sides, however, because the summits are about 1,000 m lower.

On the SPIF at 2,680 m above sea level, Aristarain and Delmas (1993) found a mean mass balance of only 1.2 m water equivalent per year by coring to a depth of 13.7 m, whereas the equilibrium line on Glaciar Moreno nearby is at 1,150 m. At this elevation, many ice crusts and some ice layers continue to be found, which testifies to frequent episodes of melting. Such large differences in accumulation come from the persistent and very strong easterly wind, which causes dry winter snow to drift. Consequently, accumulation is not a constantly increasing function of elevation. On the other hand, the highest peaks and needles that remain most of the year within the clouds are capped by fantastic mushrooms of ice, rime, and snow and have overhangs that can exceed 10 m. They have caused the rock climbing attempts of many first-class alpinists to end in failure.

Measured negative balances in the ablation zone are more consistent. On Glaciares (Perito) Moreno and Upsala, the balance is -12.5 m of ice a^-1 at 350 m, whereas it is zero at 1,150 m (Naruse and others, 1995; Skvarca, Satow, and others, 1995). Thus, the mean balance gradient in the ablation zone is 1.56 m a^-1 of ice per 100 m of elevation, a very high value. The very negative balances of the ablation zone and calving at the front together explain the high values of the accumulation area ratio (AAR).

Values of the AAR on the SPIF have been determined from Landsat TM satellite imagery by Aniya and others (1996). The values span a wide range because of local factors (even deleting one very low value, 0.25, which might be an error). For tidewater glaciers (13 values in total), the AAR ranges between 0.65 and 0.97 and has 0.854 as a mean. The values close to one, denoting that almost all the discharge of ice disappears by calving, explain why some fjords of the Pacific coast, such as Fiordo Falcón (fig. 29), are crowded with icebergs and growlers and are closed even to small boats. For the other glaciers (14 in total, including Glaciares Ofhidro, Bernardo, and Occidental (fig. 29), which do not reach the sea as indicated in the table in Aniya and others (1996)), the AAR ranges between 0.58 and 0.87 and has 0.736 as a mean. On the NPIF, the mean value of the AAR is only 0.63 (Aniya, 1988). In the European Alps at the beginning of the century, the AAR was 0.75 according to Penck and Brückner (Lliboutry, 1965, p. 444), but it is much less today, often less than 0.60.

Table 10.-- Energy balances on ablation zones of the Patagonian Andes

[Given in megajoules per square meter per day (MJ m^-2 d^-1). Based on data from Ohata and others (1985), Fukami and Naruse (1987), and Takeuchi and others (1995). Asterisk (*) indicates a kind of föhn blows on some days]

Energy balances have been determined by Ohata and others (1985) and by Fukami and Naruse (1987) on two ablation zones of the NPIF; later, by Takeuchi and others (1995) on two ablation zones of the SPIF. Measurements were done at low elevations, during periods of 2 to 14 days, in summer. Given the very large variability of the different terms of the balance (excepting the energy emitted in the thermal infrared by melting ice), this sampling cannot be extrapolated to the whole ablation season nor to the whole ablation zone. The results are very interesting, however, and are given in table 10.

On leeward Glaciares Soler and (Perito) Moreno, a kind of föhn blows on some days (indicated by asterisk in table 10). This is a wind that has warmed by losing a large percentage of its moisture on the windward side of the mountain. Its dew point remains more than 0°C, however, and thus, it conveys latent heat to the melting ice surface. On Glaciar San Rafael, a katabatic wind at the surface blows westward, in the opposite direction of the wind at higher altitude (Inoue, 1987). This air has been cooled by contact with the glacier, but when it reaches the site of the measurements, it has been warmed by adiabatic compression.