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World Glacier Inventory

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World Glacier Inventory World Glacier Inventory - Inventaire mondial des Glaciers (Proceedings of the Riedeialp Workshop, September 1978; Actes de l'Atelier de Riederalp, septembre 1978): IAHS-AISH Publ. no. 126,1980. East-west and north-south snow line gradients in the northern Patagonian Andes, Argentina J. Rabassa, S. Rubulis and A. Brandani Abstract The snow line has been identified on the Argentine side of the northern Patagonian Andes. A definite trend is observed in both the east-west and the north-south snow line gradients. The snow line rises abruptly from the west (Chile) to the east (Patagonia), having progressively higher positions in the landscape since it remains above the summit of the higher peaks of the Sub-Andean ranges and Patagonian volcanoes. Similarly, the snow line descends steadily, although not so markedly, from north to south, favouring the conservation of cirque and valley glaciers at lower latitudes. The east-west gradients are caused by the interference of the humid winds from the Pacific with the Andean Cordillera. The north-south gradient is related to the lowering of mean annual temperatures away from the equator and also to the lower elevation of Andean peaks. For certain latitudes, present snow line gradients are compared with those from the Pleistocene. Gradients est-ouest et nord-sud de la limite des neiges persistantes dans les Andes patagoniennes septentrionales Résumé. On a identifié la limite des neiges persistantes sur le versant argentin des Andes patagoniennes septentrionales. On observe un trend bien défini pour chacun des gradients est-ouest et nord-sud de la limite des neiges persistantes. La limite s'élève brusquement de l'ouest (Chili) vers l'est (Patagonie), occupant des positions de plus en plus élevées dans le paysage, puisqu'elle demeure au-dessus des sommets des contreforts des Andes et des volcans de Patagonie. De manière analogue, la limite des neiges persistantes s'abaisse de façon continue, mais toutefois moins rapidement, du nord vers le sud, favorisant la conservation des cirques glaciaires et des glaciers de vallée à des latitudes inférieures. Les gradients est-ouest sont causés par l'interférence des vents humides venant du Pacifique avec la Cordillère des Andes. Le gradient nord-sud est lié à l'abaissement des températures annuelles moyennes lorsque l'on s'éloigne de l'équateur ainsi qu'à l'altitude moins élevée des sommets andins. Pour certaines latitudes, les gradients de la limite actuelle des neiges permanentes sont comparés avec ceux du pleistocene. INTRODUCTION The significance of snow line position in relation to the climate and glaciation of a region, both past and present, has been stated by numerous researchers. A few papers have dealt with the Pleistocene and the modern snow line of South America, mainly in Peru, Chile, Bolivia and northern Argentina (Tricart, 1965; Hastenrath, 1967, 1971a, 1971b). There are even fewer data for the Argentine side of the northern Patagonian Andes, between latitudes 39°S and 42°S, with the exception of the papers by Wilhelmy (1957), Flint and Fidalgo (1964) and Auer (1970). Field work performed during the 1978 Southern Hemisphere summer for the World Glacier Inventory provided the opportunity for a detailed, although not exhaustive, survey of the snow line position of this section of the Patagonian Cordillera of Argentina; see Fig. 1 for the location of the area studied. The aim of this paper is to present these data, explain the observed east-west and north-south gradients, and compare them with Pleistocene climatic snow line. THE NORTHERN PATAGONIAN ANDES BETWEEN 39°S AND 42°S South of latitude 37° 30'S, the Andes present a significant decrease in their summit height: the elevation of the peaks drops dramatically from 5000 m to less than 1 2 J. Rabassa, S. Rubulis and A. Brandani FIGURE 1. Location map. 3000 m. This is due to the passage from the Mesozoic-Tertiary structures of the Andean Geosyncline, to the older (Variscan) structures of the northern Patagonian Andes. A general summit altitude of 2300-2600 m is exceeded only by several Upper Tertiary and Quaternary volcanoes which grew on top of the 'Gipfelflur': Volcan Domuyo (4709 m), Volcan Copahue (2980 m), Volcan Tromen (3978 m), Volcan Lanin (3776 m) and Monte Tronador (3554 m), and others in Chile. The northern Patagonian Andes, south of 39°S, are a group of relatively low ranges north-south oriented, which were heavily glaciated during Pleistocene times. Glaciers flowed away from the ice cap by means of numerous transverse valleys that had previously been fluvially eroded. Most of these valleys are now occupied by lakes of glacial origin. A great number of peaks still have glaciers. Most of these glaciers have not been studied or even visited by scientists or explorers. Moreover, most of them are still unnamed. A preliminary list of more than 200 glaciers of varied nature has been presented as our contribution to the World Glacier Inventory (Rabassa et ah, 1978). A summary of some of the data obtained is given in Table 1. Snow line gradients in Argentina TABLE 1. Summary data from the glacier inventory of the northern Patagonian Andes (235 glaciers) Latitudinal distributior i Size distribution Sector No. % Area [km2 ] No. % 39°00' - 39°20'S 31 13.2 0.010-0.031 46 19.6 39°20' 39°40'S 45 19.1 0.031 - 0.062 37 15.7 39 40 40°00'S 62 26.4 0.062-0.124 40 17.0 40°00'-40°20'S 8 3.4 0.125-0.250 38 16.2 40°20' - 40°40'S 12 5.1 0.25-0.50 32 13.6 40°40'-41°00'S 0 0.0 0.50-1.00 18 7.7 41°00'-41°20'S 46 19.6 1.00-2.00 11 4.7 41°20'-41°40'S 6 2.6 2.00 - 4.00 6 2.6 41°40'-42°00'S 16 6.8 4.00 - 8.00 3 1.3 42°00' - 42°20'S 3 1.3 8.00-16.00 3 1.3 42°20'-42°40'S 6 2.6 16.00-32.00 1 0.4 Total 235 100.0 Total 235 100.0 Length and width distribution Mean length Mean width Size class [km] No. % No. % 0.10-0.20 84 35.7 13 5.5 0.20 - 0.40 82 34.9 66 28.1 0.40 - 0.80 28 11.9 84 35.7 0.80-1.60 18 7.7 58 24.7 1.60 - 3.20 11 4.7 13 5.5 3.20 - 6.40 10 4.3 0 0.0 6.40-12.80 2 0.9 1 0.4 12.80-25.60 0 0.0 0 0.0 Total 235 100.0 235 100.0 Classification Median glacier elevation First 2 digits No. % Contour [m] No. % 30 1 0.4 1400-1500 1 0.4 43 11 4.7 1500-1600 1 0.4 53 4 1.7 1600-1700 19 8.1 63 12 5.1 1700-1800 35 14.9 64 50 21.3 1800-1900 34 14.5 65 14 6.0 1900-2000 49 20.9 67 4 1.7 2000-2100 43 18.3 68 18 7.7 2100-2200 16 6.8 74 16 6.8 2200 - 2300 17 7.2 75 30 12.8 2300 - 2400 8 3.4 77 2 0.9 2400 - 2500 6 2.6 78 72 30.6 2500 - 2600 3 1.3 79 1 0.4 2600 - 2700 1 0.4 2700 - 2800 1 0.4 2800 - 2900 0 0.0 2900 - 3000 0 0.0 3000-3100 0 0.0 3100-3200 0 0.0 3200 - 3300 1 0.4 Total 235 100.0 Total 235 100.0 4 J. Rabassa, S. Rubulis and A. Brandani TABLE 1 continued Orientation Quadrant No. % N 5 2.1 NE 32 13.6 E 65 27.7 SE 49 20.9 S 59 25.1 sw 14 6.0 8 3.4 NwW 3 1.3 Total 235 100.0 The climate of the region is temperate, humid to perhumid. The following data have been obtained from De Aparicio (1958). The mean annual precipitation is more than 3000 mm at the international border; it becomes gradually drier to the east but the area studied lies totally within the 800 mm isohyet. Precipitation is concentrated in winter, the rainfall then being four times higher than during the summer. Snow storms are frequent, more than 20 per year. The whole area is rain-shadowed by the Andean ranges. The humid winds from the Pacific Ocean drop most of their moisture on the western slopes of the Andes (Chile) as they rise over the mountain chains. The westernmost peaks of the Argentine side still receive some of the rain but the eastern sides of the ranges and the lowlands of Patagonia become progressively drier. A strong rain gradient is evident from the meteorological data of our own stations at Mt Tronador: 1900 mm/year at Pampa Linda Station and 1450 mm/year at Lake Mascardi Station, the latter located 15 km to the east at the same latitude. This gradient is similar throughout the region under study. The temperature is moderate, the whole area being enclosed by the 6°C annual isotherm. Most of the region lies within the 2°C July isotherm, but the 0°C July isotherm is located more than 300 km to the south. METHODOLOGY Porter (1975) enumerated the reasons that make a strict comparison of present and past snow line data between different regions of the world very difficult.
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