Revista Chilena de Historia Natural 67: 369 - 383, 1994 The quaternary glaciation of Chile: a review
La glaciaci6n cuaternaria de Chile: una revision
CHALMERS M. CLAPPERTON Department of Geography, University of Aberdeen, Aberdeen AB9 2UF, Scotland
ABSTRACT
A review of literature on the Quaternary glaciations of Chile shows that repeated advances occurred in the central and southern Andes. However, radiocarbon dated sequences have been obtained only for the last glaciation in the Chilean Lakes Region and in the Paine and Magallanes regions of Patagonia. These suggest that glacier lobes from the icefields advanced at least five times during the last glaciation. Reconstruction of palaeo-snowlines indicates that the climatic belts during the glacial interval were basically similar to the modem, but that effective precipitation was greater in the north (arid) of Chile, and that the humid westerlies in temperate latitudes were displaced 5-6° to the north. Snow line descended 800-1 OOOm. While glaciers in the Lakes Region reached their maximum extension at ca.20-21,000 yr BP, those in the south and north of the country may have been more extensive at ca.27,000 yr BP, due to the greater humidity available. Some glacier lobes from the southemmost icefields of Chile were still expanded at ca.l2,000 yr BP and may have readvanced during the interval I 2,000-9000 yr BP.
Key words: Southern Andes, glacier dynamics, paleo-snowlines, paleolakes.
RESUMEN
Una revision de la literatura sob re glaciaciones cuaternarias en Chile muestra, que en Ios Andes del centro y sur, han ocurrido repetidamente avances de glaciares. Sin embargo, secuencias fechadas radiocarbonicamente solamente se han obtenido para la ultima glaciacion en la Region de Ios Lagos yen !as regiones del Paine y Magallanes, en la Patagonia. Ellas sugieren que Ios lobulos glaciares de Ios campos de hielo avanzaron a! menos cinco veces durante la ultima glaciacion. La reconstruccion de !as paleolfneas de nieves indican que Ios cinturones climaticos durante la ectad glacial fueron basicamente similares a Ios actuales, pero la precipitacion efectiva fue mayor en el norte (ári do) de Chile, y que cl cinturon de Ios oestes de !as latitudes templadas se desplazo 5-6° a! norte. La lfnea de nieves descendio 800-1000 m. Mientras que Ios glaciares de la Region de Ios Lagos alcanzaron su extension maxima hace 20-21,000 años atras, aquellos del sur y norte del pafs pueden haber sido más extensos ea. de 27,000 afios, debido a la mayor humedad disponible. Algunos lobulos glaciares de Ios campos de hielo del extremo sur de Chile estaban aún expandidos hace 12,000 afios A.P. y pueden haber reavanzado entre 12,000 y 9,000 afios A.P.
Palabras clave: Andes del sur, dinamica glaciar, paleolfneas de nieve, paleolagos. INTRODUCTION This paper reviews the state of knowledge about past glacier fluctuations throughout Observations on Quaternary glaciation in the Chilean Andes in an attempt to expose various parts of Chile have been made since problems that should be tackled to give a Darwin's visit in the 19th. Century. The better understanding of Quaternary Magellan region was commonly visited by glaciation in Chile, and its contribution to European scientists on their way to and studies of global climatic change. from Antarctica and this led to short reports about glacial features in the area (e.g. Nordeqskjld 1898, Quensel 1910). Topography, climate and modern snowline Subsequently, some classic works on Quaternary glaciation and glaciology in As in most of the Andes, the large-scale southernmost South America laid the topography of Chile has evolved primarily foundations for an understanding of the through late Cenozoic tectonics and glacial history of the southern Andes volcanic activity (Clapperton 1993a); (Caldenius1932; Briiggen 1950, Auer1956, denudational processes have shaped the Mercer 1976, Rabassa and Clapperton smaller-scale relief according to the 1990). climatic regime. 370 CLAPPERTON
Extending approximately north-south primary climatic systems affecting Chile. across latitudes ca.l7°S-56°S, some In the south (lats.56°S-42°S) the constantly 4200km, the Chilean Andes contain a wet westerly system, with only slight number of different tectonic segments with changes in precipitation amount seasonally, distinctive topography. For example, brings totals in excess of 5000mm to sea between latitudes l8°S and 27°S Quater- level stations (Miller 1976). Such high nary-Recent volcanism has created the annual precipitation explains the presence highest relief, with more than 40 mountains of large icefields in southern Chile, with rising above 5500m altitude (Fig.l a); but outlet glaciers descending to sea level the mean topographic elevation is (Fig. le). ca.4000m. South of Nevada Ojos del Salado Between 42°S-31 os lat., in the so-called (6893m) at latitude 27°S, the Andes are 'Mediterranean-type' climatic zone, winter devoid of active volcanoes until latitude precipitation is brought by the seasonal 33° 27'S. Nevertheless, underplating of the movement northwards of the Polar Front continental edge by a buoyant part of the and associated cyclonic depressions. This N azca ocean crus tal plate has raised this nourishes glaciers and snowfields where segment of the Andes above a mean altitude altitude, topography and exposure provide of 4000m, culminating in Cerro Aconcagua suitable accumulation sites in these latitu- (6960m). des; but many parts of the mountains even Volcanic massifs appear again at latitude above 4000m altitude lose their snow cover 33°S, where Volcan Tupungatito (5628m) in summer because of high insolation and and Volcan Maipo (5264m) are two of the the relatively low total of mean annual highest, but a dramatic change in topo- precipitation (250mm). graphic elevation occurs at latitude 37°S, North of latitude ca.32°S mean annual south of which only Cerro Valentin (4058m) precipitation falls below 300mm, and comes at latitude c.46°50' S rises above 4000m, mainly in the austral winter as occasional and fewer than ten summits exceed 3000m. outbursts from cyclonic storms extend Here, thinner crust and the subduction northwards. Some precipitation may come regime have created a segment of highly to this part of the Chilean Andes in the fractured terrain with abundant volcanism; austral summer if convectional storms on but the mean altitude of the Andean crest is the Puno-Altiplano extend westwards (in- only ca.l800m. Many volcanic peaks rise vierno boliviano), but the annual amount above 2000m, as in Volcan Villarica from this source is generally insignificant. (2940m) and Volcan Osorno (2660m), both In this region, dominated by anticyclonic of which support small icecaps (Fig.lb). atmospheric circulation, intense radiation South of latitude 4 7°S, the volcanic systems and evaporation combined with low are more widely spaced but the mean precipitation totals cause the permanent topographic elevation rises to ea. 2500m snowline to lie above 6000m. Thus even and much of this is covered with the where mountains rise above the mean level Patagonian icefields (Fig.l c). of the atmospheric 0°C isotherm, commonly The climate of the Chilean Andes varies at ea. 4900-4700m in these latitudes (Fox from hyper-arid tropical in the north 1993), permanent snow and ice can not (lat.l rS-27°S) to hyper-humid temperate survive because of aridity and high at latitu.de ca.50°S. The precipitation insolation. Even the highest massif, which regime clearly controls local and regional rises to 6893m in Nevada Ojos del Salado, snowline altitudes and determines the has a patchy distribution of permanent distribution of modern glaciers. Fig. 2 snow, and glaciers no longer exist. illustrates the annual precipitation cha- Moreover, although the summit of racteristics for stations in different climatic Llullaillaco volcano culminates above regions of Chile and Fig. 3 shows the 6700m, it has no glaciers and supports only dramatic fall in snowline altitude from a small permanent snowpatch (Messerli et ca.6000m in the north to ca.250m in the al. 1993). Not only does the snowline south. This is due to the effect of the three altitude rise from east to west in these THE QUATERNARY GLACIATION OF CHILE 371
...
,..
,..
,. .
.,.
FiR. l.a Fig. lb
ARGENTINA
PATAGONIA
ATLANTIC OCEAN ,.
,..
Fig. 1: General topography of the Chilean Andes L______J00km ... and location of places mentioned in the text. Topograffa general de Ios Andes de Chile y ubicaci6n de Ios Fig. l.c lugares mencionados en el texto 372 CLAPPERTON
400 A 400 B SP 350 350
300 300
250 250 "'11.) 11.) .. :::"' u 11.) ~ 200 ~ 200 Ev ::i 150 ::i 150
100 100
50 ,_ .... , 50 '/ ------PA 0 0 JFMAMJJASOND JFMAMJJASOND
Fig. 2: Mean monthly precipitation for selected stations representing the different climatic regions of Chile (after Miller, 1976). A. Stations south of 45°S: SP: San Pedro (47°43'S, 74°55'W) EV: Evangelistas (52°24'S, 75°06'W) PA: Punta Arenas (53°10'S, 70°54'W) B. Stations north of 45°S: PM: Puerto Montt (41°28'S, 72°57'W) Val: Valdivia (39°48'S, 73°14'W) CON: Concepci6n (36°40'S, 73°03'W) VAL: Valparafso (33°01'S, 71°38'W) LS: La Serena (29°54'S, 71°15'W)
Precipitaciones medias mensuales de estaciones seleccionadas como representativas de diferentes regiones climticas de Chile (de acuerdo a Miller, 1976). A. Estaciones al sur de 45"S: SP: San Pedro (47"43'S, 74"55'W) EV: Evangelistas (52"24'S. 75"06'W) PA: Punta Arenas (53"10'S, 70"54'W) B.Estaciones al norte de 45"S: PM: Puerto Montt (41"28'S, 72"57'W) VAL: Valdivia (39"48'S, 73"14'W) CON: Concepci6n (36"40'S, 73"03'W) latitudes, but a clear gradient rises also 2500m until the region of humid westerlies from north to south between latitudes 30°S at ca.Iat.45°S. South of here, myriads of and 35°S; a 'snowline' does not exist here glaciers are scattered throughout the since even summits exceeding 6500m are Chilean cordillera, including the North and ice-free (Fig.3). South Patagonian Icefields and extensive Glaciers are largely absent in the Chilean icefields on Isla Santa Ines and in the Cor- Andes north of 33°S due to increasing dillera Darwin of Isla Gran de de Tierra del aridity, although some small ice fields are Fuego (Fig.lc). present ,locally as far north as 30°S on All of the modern icefields were more massifs exceeding 5000m, as on Cerro Doiia extensive during the late Quaternary, while Ana and Cerro del Volcan. Farther south, icecaps and valley glaciers formed over higher seasonal (westerly) precipitation wide areas that no longer contain permanent permits substantial icefields and valley snow. glaciers where summits are above ca.4000m, and these were recorded systematically for the first time by Lliboutry The quaternary icefields of Chile (1956). South of lat.37°S, where the Andean crest is almost 2000m lower, There are few published papers reporting glaciers exist only on summits exceeding detailed morphological and chronological THE QUATERNARY GLACIATION OF HCILE 373
evidence of Quaternary glacier extent in Tatio geysers (lats.22°10'-22°2l 'S). They the northern and central Andes of Chile, noted that one of the most extensive whereas a considerable literature exists on moraines terminates at about 4280m and the glaciation of southern Chile. Thus the considered it to be of Quaternary age. The following sections review the nature of the most recent work on Quaternary glaciation published data using a regional approach, in northern Chile has been by Grosjean et dealing in turn with the northern, central al. (1991), Messerli et al.,(1993) and Fox and southern parts of the Chilean Andes. (1993). i. The Northern Andes of Chile: Briiggen Fox (1993) estimated the modern and late (l950,p.212) referred to glacial features on Pleistocene snow line altitudes of the Cen- Volcano Toconce at lat.22°S, and tral Andes (5-28°S) on the basis of glacial considered that Pleistocene glaciers had features (cirques and moraines) interpreted descended to 3900m when the snowline from satellite imagery. This study dropped to 5000m; this was a remarkable concluded that in the latitude of discovery for such a hyper-arid area. northernmost Chile (l7-20°S), the modern Supporting observations were made snow line rises from ca.5200m in the eastern subsequently by Hollingworth and Guest cordillera of Bolivia to 5600m in the (1967) in a brief description of moraines Chilean Andes. At latitudes 23-2rS, on the flanks of Volcano Toconce and snowline rises steeply east to west across adjacent volcanoes between Linzor and the Andes from 5200-6000m (Fig.4) in the
Height (km) Ojos del Salado 7 ~ Aconcagua
4_ Tupungato. 6
5
4
3
2
•s 200 300 400 so•
Fig. 3: Schematic representation of the average topography along the Chilean Andes from 18°-51 os, and an estimate of the modern glacial Equilibrium Line Altitude (thick line). Based on Fox ( 1993 ), Messerli et al.(1993) and Lliboutry (1956). Representaci6n esquemtica de la topografia media a lo largo de Ios Andes chile nos de 18x a 51 xS, y una estimaci6n de la Altitud de la Linea de Equilibrio de Ios glaciares en la actualidad (linea gruesa). Basado en Fox (1993), Messerl1 et al. ( 1993) y Lliboutry (1956). 374 CLAPPERTON
69'W 68'W 67'W 66'W
Lowest late ~ s Ple_;J"'o"ne.nowh~ 52--I :ikm 2 ] 4 ~
c 0 > u
® 69"W 68'W 66'W
~ 20' .lj 4 ·ca < c 0 .,.,.0 -0 ;;; ., :! ; 0 .c ~u 0 ~ u I u• _, 0 30' 26°· 27°5 0 0 uu u 0: f1g. -+A & B: L'umatc' olmoJcrn anJ late Pleistocene snowline gradients across W-E transects in the Central Andes at latitudes 24-25°5 40"-.Jf/. and 26-27°5 (after Fox 1993). Estimaci6n de gradientes de lineas de nieves actuates y del Pleistoceno tardfo a travs de transectos W-E en Ios Andes Centrales alas latitudes 24-25xS y 26-27xS (de acuerdo a Fox 1993). so· region of maximum aridity. Since the 9o· w so· 7o· altitude of the mean annual ooc isotherm is at ca.4400m over the southern Altiplano- Puna, the snow line lies 1000-1500m higher. Fig. 4C: Estimate of snow line altitude along the This means that although the topography is Andes (after Foxl993). The thick arrows indicate high enough to support glaciers, there is precipitation-bearing winds which affect the inadequate precipitation. Chilean Andes. From the mean altitude of 5665 re- Estimaci6n de !as altitudes de la linea de nieves a lo largo de constructed glaciers on the southern Alti- Ios Andes (de acuerdo a Fox 1993). Las flechas gruesas indican Ios vientos que traen las precipitaciones que afectan plano and Puna region, Fox ( 1993) Ios Andes chilenos. estimated late Pleistocene snowline altitudes of ca.3800m in the east and 4400- 4600m in the west at latitudes 17-20°S; at snowline was depressed more in the west latitudes 23-27°S, the snowline gradient than in the east, from which he inferred a was much steeper, rising east-west from northward migration of the moist westerlies 3800-5200m (Fig.4). These data indicate from 29°S (modern limit) to 25°S. that the tropical easterly atmospheric Fox's results imply a late Quaternary circulation dominated the climate of these snowline depression in the arid Andes of latitudes during the late Quaternary, as now. northern Chile of ca.800m and he inferred This contradicts the conclusion of that glaciers reached their maximal extent Hastenrath ( 1971) that the late Pleistocene in this region when precipitation was 75% THE QUATERNARY GLACIATION OF HCILE 375 less than now. This conclusion is remark- Atacama (Fig.l). Here, even the highest able since it implies the development of mountain, Volcan Llullaillaco (6739m), has large glaciers with a mean annual preci- only a permanent firn patch under the pitation of only ea. 70mm, and in view of present climate. This region appears to have strong insolation and evaporation at such been too dry during the last global glaciation latitudes, the validity of such a view is maximum for extensive glaciers and doubtful. This amount of precipitation palaeolakes, but may have been affected by decrease may well have occurred at the extreme geocryogenic conditions. It height of the last global glaciation appears that late Quaternary glaciers maximum when arid conditions prevailed nourished by westerly precipitation in many parts of tropical and sub-tropical developed only as far north as the latitude South America (Clapperton 1993b), but is of La Serena (30°S). unlikely to have coincided with the maximal Radiocarbon and thermoluminescence extent of glaciation in the Altiplano-Puna dating of palaeolake sediments suggests region. that precipitation increased during the late- Grosjean et al. (1991) and Messerli et glacial interval17.000-15.000 yr BP when al.(1993) mapped glacial and periglacial a shallow lake 5-lOm higher than the (geocryogenic) features and palaeolake modern one developed in the Laguna Leija shorelines in the Chilean Andes basin (23°4 7' S; 4300m alt.). Although immediately northeast and east of Salar de temperatures were still low, these higher
West East 7000 Ceno A...._,a Cordillera (69S9) 6500 delosAndes
Ceno Iuacal 6000 (6180) RIO ACONCAGUA 5500 DRAINAGE RIOMENDOZA DRAINAGE
5000
g 4500 Jaciation) .]., 4000 ELA(tastg ---- < ~ell------3500 UtpdMa (1150) ...... 3000
2500 ~ ~ (m5) 2000 ~ RioO>Iomdo ; (l5QO) (2300) 1500 - 40 30 20 10 0 10 20 30 40 50 60 70 Distance (km)
Fig. 5: Transect across the Andes around latitude 33°S showing the glacial limits mapped along the Rio Mendoza in Argentina and the Rio Aconcagua in Chile (after Caviedesl972 and Espizual993). Transecto a traves de Ios Andes cerea de la latitud 33°S mostrando Ios limites de Ios glaciares mapeados a lo largo de Ios cursos de Ios rios Mendoza en Argentina y Aconcagua en Chile (de acuerdo a Caviedes 1972 y Espizua 1993). 376 CLAPPERTON
lake levels imply mean annual precipitation A key observation by Messerli et al.( 1993) of ca.300-400mm, which is 60-70% higher is that the moraine limit rises from 4300m than the modern value. A further increase at 22°S to 4900m at 25°S and that moraines in precipitation occurred after ca.IS.OOO yr are absent from Ojos del Salado at 28°S. BP, and warmer conditions prevailed. The This pattern suggests that late Quaternary level of Laguna Leija rose 25m above its precipitation was brought to the Andes of modern surface, implying an increase in northern Chile by the southward shift of an effective precipitation of ca.120% intensified tropical summer rainfall pattern (Messerli et al. 1993,p.122). from a northerly to northeasterly source, In the adjacent cordillera glacial moraines probably with double the modern annual extend to 4200m near Tatio geysers, but cloud cover. have not been dated. Nevertheless, Messerli ii. The Central Andes of Chile: This et al. (1993) assigned them to the late- section of the Andes corresponds to the glacial interval of increased precipitation volcanically inactive segment characterised and inferred that the snowline had descen- by wide areas above 4000m altitude. It lies ded to ca.4650m, indicating a depression between latitudes 30°-33°S and contains of approximately 1350m below the modern many summits above 5000m; Cerro value. The validity of this conclusion Aconcagua (6960m) is the highest. Preci- depends on the age of the moraines, pitation totals rise north to south from however, and they could be much older. An ca.200-300mm to over 1 OOOmm in the alternative interpretation is that they formed Aconcagua area as the influence of winter during a cold but humid interval prior to cyclonic circulation increases. Glacial the last global glaciation maximum. features in the area are widespread
...... : VI 0 u ..: ...J w 0 ..: lr w ...J ...J
Llanquihue (undif) Llanquihue Ill ,..,. 111"1 Llanquihue 11 "!rT "" Llanquihue I T"TT TT Santa Moria ..--r- Rio Llico -v--r Caracol (buried) Glacier ice 0 0 20km
Fig. 6: Limit of Quaternary glacial deposits around Lago Llanquihue in the Chilean lakes region, according to Porter (1981 ). Limite de Ios dep6sitos glaciales del Cuaternario alrededor del Lago Llanquihue en la Region de Ios Lagos de Chile, de acuerdo a Porter (1981 ). THE QUATERNARY GLACIATION OF CHILE 377
eastern side of Cerro Aconcagua has provided an interesting correlation with the Caviedes-Paskoff scheme (Espizua 1993). Five Quaternary moraine limits were identified (Fig.5), the oldest of which formed before the deposition of a tephra dated to ca.360.000 yr BP. This, the U spallata glaciation, shows that a trunk glacier flowed 80km from Cerro Aconcagua to terminate at 1850m altitude. Assuming the mountains have not risen or lowered substantially ( tectonically) since this gla- cial stage, a snow line lowering of ca.l430m Fig. 7: Glacial deposits of late Quaternary advances is implied. The other three stages, the to the west of Lago Llanquihue (near Puerto Varas). Horcones (2750m), the Penitentes (2500m) Dep6sitos glaciales de avances del Cuaternario tardfo en el and the Punta de Vacas (2350m) were oeste del Lago Lanquihue (cerea de Puerto Varas). estimated to indicate snowline lowering of ca.l 000, 1200 and 1250m respectively (Espizua 1993; p.l61). compared to the region immediately north, Comparing the moraine stages either side and have been described by Brtiggen ( 1950), of the divide in the central Andes, it is clear Borde (1966), Santana (1967), Paskoff that the snowline gradients in the second (1970), Caviedes (1972) and Caviedes and half of the Quaternary have always sloped Paskoff (1975). The latter study combined upwards from west to east, reflecting observations made along two transects into precipitation supply from the westerlies. the mountains, in the Rio Elqui basin Age determinations on the Penitentes drift (ca.30°S) and in the Rio Aconcagua basin using 230Thf2 32Th and U-series techniques (ca.33°S), and is the most informative. suggest that this glaciation occurred The only modern glacier in the Rio Elqui sometime before 40.000 yr BP, but it is not basin is on the summit of Cerro El Tapado closely dated. The Horcones drift was (5124m), where the modern snowline is assigned a latest Pleistocene age because it estimated to lie at ca.5000-5500m. Greater is composed of relatively unweathered till precipitation in the Rio Aconcagua basin and has fresh morphology, while the Alma- nourishes glaciers that descend in places to cenes moraine farther upvalley is believed 3400m where the snow line lies at ea. 4300m to date from the late-glacial interval14.000- (Lliboutry, 1956). 10.000 yr BP. In the Elqui basin (Fig.1), the limits of iii. The Southern Andes From 33-37°S two distinct glacial stages were identified much of the Andean crest lies above 4.000m at 31 OOm and 2500m altitude, the lower and was glaciated extensively during the being more weathered than the upper. In Quaternary, judging from the erosional the Rio Aconcagua basin, at least three forms evident on topographical maps. Even major moraine stages exist, at 2800m, where the mean altitude of the Andean 1600m and 1300m altitude, each lower one cordillera drops dramatically to ca.2000m showing a greater degree of weathering. south of ca.37°S, much of the range was Table 1 illustrates the correlation and glaciated during Quaternary cold intervals inferrred ages for moraines in the central since it lies in a zone of comparatively high Andes of Chile. These data suggest that glacier precipitation. However, very little infor- expansions in this part of the Andes were mation is available on the glacial sequence associated with a marked increase in precipi- of this region, possibly due to the effect of tation, possibly caused by an equatorward shift by very active Quaternary-Holocene vol- 5-6°lat. of the austral polar front. canism. Thus it is not until the Region de A more recent study of moraines in the Ios Lagos (Lakes Region) is reached at Rio Mendoza valley (Argentina) on the 378 CLAPPERTON
latitude 39°S that published studies of The climate of this region is currently Quaternary glacial sequences are available. dominated by the seasonal migration The abrupt beginning of a remarkable northwards in winter of the austral Polar series of glacially-deepened lake basins on Front, which largely accounts for mean either side of the Andes at this latitude is annual precipitation totals of ca.3000- striking and demands an explanation 4000mm in the mountains. Despite (Fig.1); yet no one has addressed the issue. abundant precipitation, the low terrain and Presumably they relate in some way to a generally mild temperatures inhibit the zone of high glacial-age precipitation development of ice caps and modern periodically nourishing Quaternary icecaps with glaciers are confined to mountains rising erosive outlet glaciers descending to the softer above 2500m (commonly Quaternary sedimentary strata of the Central Valley. volcanoes). It has been estimated that the The most informative papers published snowline in this region lies at ca.2200m in on the Quaternary glacial sequence of the the north and possibly as low as 1800m in Chilean Lakes region are those by Briiggen the south (Rabassa et al. 1980). The largest (1950), Weischet (1958,1964), Olivares icefields cap the summits of volcanic (1967), Lauer (1968), Laugenie (1971), massifs such as Lonquimay (2822m), Mercer (1976), Porter (1981) and Heusser Llaima (3214m), Villarica (2840m), Lanin ( 1990). Rabassa and Clapperton ( 1990) and (3776m), Osorno (2661 m) and Tronador Clapperton ( 1993a) have provided (3554m), but there may be as many as 1000 comprehensive overviews. small glaciers scattered along the Andean
N t
-, Moraine system ~ Ice contact face ...... Gullied drift limit p Early last Qlaciallon I Late last Qlaciation n Late last Qlaclatlan m c14kaBP? nz: c 12ka BP lr c 12-9 ka BP 'lZI Holocene 8· 5 Radiocarbon dates (KaBP)
0 20km
Fig. 8: Schematic map showing the distribution of last glaciation moraine limits in the Cordillera del Paine area at the southeast margin of the Hielo Patagonico Sur (South Patagonian Icefield), according to Marden (1994). Mapa esquematico mostrando la distribuci6n de Ios Ifmites de las morrenas de la ultima glaciaci6n en al area de Cordillera del Paine en el margen sureste del Hielo Patag6nico Sur (de acuerdo a Marden 1994). THE QUATERNARY GLACIATION OF CHILE 379
the range of conventional radiocarbon dating, but is relatively fresh in appearance, Mercer (1983) considered it could have been deposited during marine isotope stage 4 (MIS4); Porter (1981) was more cautious and did not r-ule out a penultimate glaciation age for this unit. Llanquihue II drift is better constrained with radiocarbon dates. Outwash gravels overlain by Llanquihue II till contains peat clasts as young as 20,100 +1-500 yr BP (Mercer 1976) and this indicates the advance culminated after ca.20.000 yr ago. Recession of the Llanquihue II glacier is Fig.
Porter (1981) estimated that a snowline lo Patagonico N orte (HPN) and Hielo depression of ca.1 OOOm must have occurred Patagonico Sur (HPS). Moreover, a for an outlet glacier to reach the western multitude of smaller icecaps and glacier side of the Llanquihue lake basin. Recent systems are scattered throughout this zone modelling experiments have suggested that wherever the topography is high enough, the conterminous icecap over this part of as on Is la Santa Ines and Cordillera Darwin, the Andes could not be generated by where glaciers descend to sea level. The temperature depression alone and that a Equilibrium Line Altitudes of glaciers in this substantial increase in mean annual region range between 500m in the west and precipitation must also have occurred 1350m in the east (Kerr and Sugden in press). (Hulton et al. 1994). Very little is known about the extent of iv. The Chilean Channels and Fuego Quaternary glaciation in the Chilean Patagonia: South of latitude 46°S the Channels region and it has commonly been climate is dominated by wet maritime assumed that the expanded Patagonian westerly air chilled in its passage across icefields terminated on the continental shelf the Humboldt Current. Although there is west of the land masses (Caldenius 1932, seasonal movement north and south of the Denton and Hughes 1981). However, recent zone of intense precipitation, much of the palaeoecological studies in the area suggest west coast of Chile south of latitude 46°S is that land refugia for some plants and wet and windy all year round. This explains flightless beetles possibly existed on Isla the presence of two substantial icefields Wellington during the last glaciation centred on relatively low (ca.2000m) (Ashworth et a1.1991). This has important topography in temperate latitudes, the Hie- implications for reconstructions of glacial- age climate in the region as it suggests a rather restricted ice extent when the global ice volume was at its highest. Farther south, large moraine systems extend eastwards from the southern margin of the HPS and from the former icecap centred on Isla Desolacion, Isla Santa Ines and Cordillera Darwin. The moraines were mapped by Caldenius ( 1932) who believed that most of them represent late stages of the last glaciation. Studies of glacial deposits in Argentina by Mercer (1976) demonstrated that the most extensive Patagonian glaciation occurred about one million years ago. Meanwhile, paleo- magnetic data from glacial sediments around Lago Buenos Aires (Lago General Carrera) have shown that the system of five complex moraine belts represent Pliocene and Quaternary glaciations (Clapperton 1983; Morner and Sylwan 1989). Similarly, the stacked drifts composing the peninsulas in Magellan Strait, viz. Segunda Angostura, Primera Angostura and Punta Dungeness Fig. I 0: Schematic diagram showing the present and (Fig.1), may represent a long history of last glaciation snowline altitudes throughout various late Cenozoic glaciation. latitudes of the Andes. The sequence of glacial advances during Diagrama esquematico mostrando !as alturas de la linea de the last glaciation in this region of Chile nieves en la actualidad y durante la ultima glaciaci6n a traves has now been mapped in detail in two de Ios Andes a varias latitudes. places, around Torres del Paine and in THE QUATERNARY GLACIATION OF CHILE 381
Magellan Strait. In the Lago Sarmiento Preliminary data on the late Quaternary basin adjacent to the Paine massif (Fig.8) glacial sequence in Magellan Stait suggests four major moraines may be associated that Magellan glacier advanced along the with advances of the last glaciation (Marden strait at least five times during the last 1994). The easternmost (Stage P, Fig.8) is glaciation, the last occasion occuring after composed of deformed lacustrine sediments 12.000 yr BP. Following the last glaciation and could represent the initial 'clear-out' maximum, Magellan glacier probably of the lake basin during an early stage of withdrew rapidly southwards due to rapid the last glaciation. The next two are similar ablation through calving in a proglacial in form and weathering characteristics and lake 200-SOOm deep (Porter et a1.1992). may mark limits reached during a later The glacier possibly remained at stage of the last glaciation, prior to the topographic pinning points in the southern global glaciation maximum, but there is no part of Magellan Strait (as at Paso del dating control. The fourth moraine is older Hambre) between ca.14.000-12.000 yr BP than a tephra dated to 12.000 yr BP and is before advancing into deposits containing morphological very fresh. While it would Reclus tephra (Fig.9) dated to ca.l2.000 yr 'fit' with the Llanquihue Ill stage of the BP (R.McCulloch, personal communication Lakes Region in terms of relative position, 1993). This advance may have produced a its precise age is not known. renewed rise in proglacial lake level that Younger moraines and drift limits (IV cut impressive shorelines near Porvenir and and V on Fig.8) are late-glacial in age. The on the northern tip oflsla Dawson (Fig.lc). easternmost, situated some 20km from the In conjunction with evidence emerging modern icefront, indicates a stillstand from the Chilean Lakes region for repeated during deglaciation and must have occurred advances of these climatically-sensitive just before ca.12.000 yr BP. This is known glacier systems during the last glaciation, from the reworked pumice clasts forming the Paine and Magellan sequences suggest uppermost layers of the outwash, the pumice responses to the global climatic fluctuations having travelled through the glacier system during MIS3 and MIS2 implied by the ice from the accumulation zone close to Reclus core records volcano (Fig.l c). Geochemical analyses and radiocarbon dating have established that this volcano had a large eruption at DISCUSSION ca.l2.000 yr BP (R.McCulloch, personal communication 1993). Recession from this Know ledge of the succession of Quaternary stillstand limit was followed by a glacier fluctuations in Chile is important readvances to form a moraine complex for understanding the nature of climatic sometime before 8.700 yr BP, the minimal changes throughout almost 40 degrees of radiocarbon age of basal peat in a kettle latitude in the Southern Hemisphere. This basin immediately inside the end moraine. review of existing data about such The palaeoclimatic significance of these fluctuations ratses the following data is that deglaciation after the last gla- conclusions cial maximum in this area was not rapid. 1. The overall pattern of climatic belts Moreover, moraine complex V (Fig.8) may affecting Chile remained essentially simi- well have been deposited by an readvance lar to the modern one, as indicated by the of Grey glacier responding to a climatic palaeo-snow line gradients (Fig.l 0). These perturbation induced by theY ounger Dry as imply that precipitation decreased from east stadia! event of the North Atlantic to west in northern Chile and from west to (Clapperton 1993c) particularly if the east in central and southern Chile, as now. icefield was already in a relatively expanded 2. Although the climatic pattern was si- position. Marden ( 1994) has explained these milar, there were substantial changes in differences between Paine and sites farther temperature and effective precipitation, and north in terms of migrations of the belt of in the location of westerly rain belts. For high precipitation. example, in the semi-arid northern Andes, 382 CLAPPERTON
the presumed lowering of mean annual Wider Implications temperature during Quaternary glacial intervals must have been accompanied by Evidence emerging from radiocarbon-dated an increase in effective mean annual glacial sequences in the Llanquihue and moisture, otherwise glaciers clould not have Magellan areas suggests that icefield outlet formed. Similarly, extensive icefields could glaciers advanced to similar limits several not have built up in the Chilean lakes region times during the last glaciation. As new ice without a significant rise in precipitation. core data from Greenland and marine core Moreover, Quaternary icefields would data from the North Atlantic imply probably have been more extensive in atmospheric fluctuations of a quasi-cyclical southern Chile if precipitation had been as nature throughout the last glaciation (Bond high as the modern total. et al., 1993 ), the Chilean icefields may also 3. Increased effective precipitation in have responded in a similar fashion, northern Chile during glacial intervals may assuming high sensitivity to variations in have resulted from greater cloudiness and temperature and precipitation. The validity lower evaporation induced by a cooler of such correlation awaits detailed glacial atmosphere. Higher precipitation in the chronologies from the various glaciated lakes region may have resulted from a regions of Chile. northerly migration of the zone of greatest The nature and age of glacier fluctuations westerly humidity, due to a weaker Pacific in northern and north-central parts of Chile anticyclonic system and an expanded sub- remain poorly known and require detailed Antarctic cover of sea ice. investigation. This is an important task to 4. The estimated snowline depression of undertake since it is vital to compare 800-1 OOOm for late Quaternary glacial palaeoclimatic records between subtropical intervals in Chile may not indicate that the arid and temperate humid regions to atmosphere cooled by as much as 5-6.5°C understand the precise nature of global (assuming a 0.65°Cil OOm lapse rate), since climatic changes during the late Quaternary. higher precipitation in some regions would have had a significant effect in lowering the snow line. An overall cooling by at least ACKNOWLEDGMENTS 4 °C seems likely, but could have been more or less at different sites, where local I am most grateful to Dra.Carolina Villagran influences such as exposure to katabatic and colleagues of the University of Chile winds were probably important. for inviting me to participate in the 5. The apparent rise in the glacial snow line symposium in Santiago and to CONICYT towards latitude 28-30°S from both for financial support. Thanks are due also northern and southern directions suggests to the Natural Environment Research that this currently hyper-arid region Council of Britain for supporting fieldwork remained as dry during glacial intervals, in Chile over the past three years with despite the (assumed) lowering of Research Grant GR3/7637. atmospheric temperature. This implies that the seasonal westerlies failed to penetrate or migrate more than about 2 degrees of latitude farther north than their modern limit, and that the humid (tropical) easterlies did not expand much farther south than their current (seasonal) extent. THE QUATERNARY GLACIATION OF CHILE 383
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