Latest Pleistocene Glacial Maximum

地学雑誌 Joumal of Geography 103(7)867-885 1994 Snowline Altitude and Climate in the Peruvian Andes (5-17•‹ S) at Present and during the Latest Pleistocene Glacial Maximum Andrew N. FOX * and Arthur L. BLOOM * Abstract The present snowline in the Peruvian Andes (5-17°S), rises from as low as 4.7•}0.1km on the eastern (windward) to more than 5.3 •} 0.1km on the western (leeward) side of the central Andes. The effect of temperature on snowline altitude is isolated from the effect of precipition by subtracting the altitude of the mean annual 0•Ž isotherm from the altitude of the snowline. This difference, defined as the normalized snowline altitude, increases with decreasing precipitation. The lowest late Pleistocene snowline rose from east to west and ranged in altitude from 3.2 to 4.9 (•}0.1) km. Both the present and lowest late Pleistocene snowlines indicate that moisture at both times was derived principally from tropical easterly winds. An east-west precipitation gradient steeper than present is inferred for the eastern slopes of the centralAndes from the steeper late Pleistocene snowline gradient. Mean annual temperatures were 10•}1.9•Ž cooler that today at 3.52 km, as calculated from a late Pleistocene snowline as much as 1.4•}0.2 km lower than today. Mean annual precipitation was 25 to 50% less than today along the eastern side, and more than 75% less on the western side of the central Andes. These estimates of lower temperature and decreased precipitation are more extreme than previous estimates. They imply that the amount of glacial-age cooling elsewhere, such as in western North America, may also have been underestimated by previous researchers because they did not adequately consider the effect of reduced ice-age precipitation on snowline lowering. America (Fig. 1). The single region chosen I. Introduction for discussion here is the portion of Peruvian This paper is a review of portions of the Andes between latitude 5 •‹S, and about 17 •‹S, Ph. D. dissertation of the first author (Fox, and westward of 69.5 •‹W longitude (Fig. 1). 1993), summarized and edited by the second The limits of the region were defined by the author, to illustrate some new techniques for availability of good quality topographic maps estimating and interpreting present and former at a scale of 1 : 100,000 with a contour interval snowlirie altitude in mountain regions. Fox of 50 m, compiled from stereographic aerial (1993) analyzed snowline altitudes in three photographs. The methods illustrated here regions of the Andes Cordillera of South are applicable to any other mountainous region * Department of Geological Sciences , Cornell University. 2122 Snee Hall, Ithaca, NY 14853, USA. 867 Fig. 1 Map of the central Andes showing the location of the three regions discussed in Fox (1993) Area A, the Peruvian Andes, is discussed here. The thin solid line is the 3 km topographic contour for geographic reference in subsequent figures. The thin dashed lines are international borders. with maps of at least comparable quality, Thematic Mapper images were extensively climatic records, and aerial photographs or used to compile the extent of modern snowfields other documentation of snowlines, both past and to estimate the extent of snow cover during and present. In the original study, Landsat the latest Pleistocene glacial maximum. The 868 age of the latest glacial maximum in the production and is shown by blue contour lines Peruvian Andes is not known, but was probably with a contour interval of 50 m. about 18,000 to 20,000 years ago. Snow cover is unevenly distributed along Many authors have discussed the probable the axis of the central Peruvian Andes and is effect of variable precipitation on estimates of notably concentrated on mountain peaks higher snowline lowering and cooling during the last than 5 km elevation from 8.5 •‹S to 13.5 •‹S glacial maximum. In particular, if the glacial latitude, and along the eastern and western climate was generally drier than at present, margins of the Peruvian Altiplano (Fig. 2). the snowline on mountains would not have Clapperton (1991) gave a good general review lowered as much as it would have if precipita- of the tectonic and volcanic controls on the tion had remained the same and only a colder spatial distribution of high topography and temperature had determined the amount of glaciation in the Andes. snowline lowering. Conversely, estimates of Polygons were digitized around the perimeter tempera ture decrease during the latest glacial of all snow-covered areas shown on the topo- maximum that are based on an assumption of graphic maps using the ESRI ARC/INFO ge- no change in precipitation will be systematically ographic information system (GIS). Total snow too small. This creates special problems for cover in the Peruvian Andes, calculated by interpreing temperature change in the tropics summing the area of the snow cover polygons, during the latest glacial maximum. Many anal- is approximately 3,700 km2. yses of tropical mountains predict snowline Point elevations were recorded where the lowering on the order of 1,000 m during the perimeter of each snow-covered region crosses latest glacial maximum ; based on a moist adia- a topographic contour line to determine the batic lapse rate of 6 •Ž/1,000 m, tropical moun- local or orographic snowline altitude. Points tain cooling of at least 6 •Ž is commonly infer- were digitized at horizontal intervals of approx- red. If however, the climate was drier and imately 1 km to record local maxima and the snowline was therefore lowered less, the minima details of the orographic snowline. A cooling would have been even greater. The total of 5,730 points were digitized, with eleva- implicat:.ons of this effect are discussed in the tions ranging from 4,050 m to 6,000 m. final section of this paper. The position of the snowline determined from the topographic maps is assumed to be II. Present Snowline close to its perennial position. Temporal dif- Present perennial snow cover and the snow- ferences in the snowline altitude are generally line altitude in the Peruvian Andes was deter- small in tropical mountains because snowline mined from the snow cover shown on the is primarily controlled by the altitude of the topographic map series published at a scale of 0 •Ž isotherm, which has a smaller seasonal and 1 : 100,000 by the Instituto Geografico Militar, interannual range in the tropics than in moun- Lima, Peru. The maps were produced from tains at higher latitudes (Kuhn, 1981). stereographic aerial photographs that were The regional snowline altitude was determined taken between 1955 and 1963. Snow cover was by digitally contouring the orographic snowline photogrammetrically transferred from the aerial point observations (Fig. 2). Contours show the photographs to the topographic maps during calculated mean altitude of the point measure- 869 Fig. 2 Extent of present snow and ice digitized from the Peruvian 1 : 100, 000 scale topographic map series (Fox, 1993, Appendix A) Contours show the altitude of the present regional snowline in meters. Polygons located east of the available topographic map coverage represent the additional extent of snow cover observed on 1 : 250, 000 scale Landsat MSS images. ments within contiguous 20•~20 km regions. melting of snow on west-facing slopes (Has- The mean, rather than the maximum or mini- tenrath, 1967), Contours were interpolated bet- mum altitude, was selected because it removes ween snow-covered areas using the altitude of the bias introduced by preferential melting snow-free peaks and Seltzer's (1987) glaciation or preservation of snow due solely to local threshold estimates as controls. The error in slope aspect. Individual snowline observations determining the altitude of the snowline is est- generally vary from the mean by only 100 m imated to be •} 100 m, based on the local vari- or less, and are slightly higher on east- ability of the orographic snowline altitude ob- northeast than on west-southwest slopes beca- served on the topographic maps. use clouds, more common in the afternoon than Snowline contours generally parallel topog- in the morning, block insulation and reduce raphic contours, but rise from 4,700 m in the 870 northern and eastern Peruvian Andes to more the previous reports estimated the snowline at than 5,300 m in the south and west (Fig. 2). elevations greater than 5,500 m. Landsat Mul- Snowlin e gradients as steep as 11 m/km occur tispectral Scanner images acquired between perpendicular to the axis of the mountain belt October 1972 and May 1982 and Landsat TM on the east side of the Altiplano at 14 °S lati- images acquired between July 1984 and Septem- tude where regional topographic and precipi- ber 1986 support the lower snowline altitudes tation gradients are also steep, but an east- shown in Fig. 2. west snowline gradient of 5 m/km is more III. Climatic Controls on the Present representative of the northern region. Snowline Altitude The snowline contours shown in Fig. 2 are similar in trend and magnitude to previous Many previous studies have sought to identify estimates of the present snowline altitude that and understand the factors controlling the were calculated from far fewer orographic present snowline altitude so that paleoclimatic snowline measurements. On the basis of inferences can be made from measurements of of about 30 observations, Nogami (1976) showed lower snowlines in the past (Charlesworth, the present snowline increasing from 4,800 m 1957 ; Ostrem, 1966, 1972 ; Bradley, 1975 ; Ch- in the northern Peruvian Andes to 6,000 m in inn, 1975 ; Miller et al., 1975 ; Porter, 1975, the southwest.

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