Past glaciation in the tropics

Quaternary Science Reviews 28 (2009) 790–798

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Past glaciation in the tropics Stefan Hastenrath* Department of Atmospheric and Oceanic Sciences, University of Wisconsin-Madison, 1225 West Dayton Street, Madison, WI 53706, USA

a r t i c l e i n f o

a b s t r a c t

Article history: Received 18 August 2008 Received in revised form 1 December 2008 Accepted 3 December 2008

Tropical glaciers are considered along three meridional profiles, for the Australasian sector, Africa and the Americas. Evaluated are the annual mean freezing level 0 C, modern equilibrium line altitude MEL, and past equilibrium line altitude PEL. The calculation of 0 C is based on a 1958–1997 global data set; MEL refers to estimates concerning the first half of the 20th century; and the timing of the PEL is not generally known. 0 C stands around 4000–5000 m, with lower levels in the outer tropics. MEL is reached in the Australasian sector on four mountains, and in Africa on three mountains, near the Equator, near or above 0 C. In the American cordilleras many peaks are still glaciated, above 0 C, but in the arid southern tropical Andes even summits above 6000 m do not reach MEL. The PEL stands between 3000 m and 5000 m, high in the equatorial zone, but highest in the arid southern tropical Andes. The height difference between MEL and PEL is of order 1000 m, with regional differences. Deglaciation dates range between 15,000 and 8000 years BP, with later timing towards the higher elevations. This synopsis suggests priority targets for further research: morphological mapping and age determination in the High Atlas of Morocco; timing of deglaciation in the High Semyen of Ethiopia and the Altos de Cuchumatanes of Guatemala; exploration in the highlands of Lesotho and of the Dominican Republic; and glaciation in the arid southern tropical Andes. Ó 2008 Elsevier Ltd. All rights reserved.

1. Introduction The glaciers in the high mountains of the tropics merit particular attention in relation to the long-term variations of climate. The present review focuses on events since the era of largest ice extent in the midlatitudes, commonly referred to as Last Glacial Maximum (LGM), around 21,000 years BP (Bush and Philander, 1998; Porter, 2001; Otto-Bliesner et al., 2006). Past and recent glaciation and freezing level in the tropics are appraised along three broad meridional profiles, namely the Australasian sector, Africa, and the Americas (Fig. 1). Dates of deglaciation are ascertained in accordance with elevation. This synopsis serves to identify the most prominent pending challenges for future field research.

2. Climatic controls The existence and maintenance of perennial ice, glaciers, require particular conditions in the climatic environment. In part of the terrain the annual mean temperature must be below freezing. The mass gain in solid precipitation must balance the loss through

* Tel.:þ1 608 262 3659; fax: þ1 608 262 0166. E-mail address: [email protected]. 0277-3791/$ – see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.quascirev.2008.12.004

ablation and ice outflow. The ice equilibrium line is defined as the region where solid precipitation equals the ablation through melting and/or sublimation, and is accordingly determined by specific conditions of the mass and heat budgets (Kuhn, 1981, 1989; Hastenrath, 1995, pp. 399–412; Kaser, 1999; Benn et al., 2005; Mark et al., 2005; Smith et al., 2008). Of interest in the following is a modern equilibrium line (MEL) as estimated from conditions in the first half of the 20th century, and a past equilibrium line (PEL) deduced from fossil glacial morphology. MEL differs from the recent progressively changing conditions (Hastenrath, 2005, 2008), which are not well established. Furthermore, the timing of past larger ice extent corresponding to PEL is not generally known. Appropriately Klein et al. (1999) posit the concept of last local glacial maximum LLGM. Similarly, attention has been called to the asynchroneity of glacier variations between different regions (Gillespie and Molnar, 1995). Essential for reference is the 1958–1997 annual mean freezing level constructed from the pertinent database (Kalnay et al., 1996) for the three meridional profiles indicated in Fig. 1 (details in Figs. 3, 6 and 9 of Sections 3–5). Not detailed here, the freezing level in the tropics rose from the first to the last decade of the 40-year data set. As illustrated in the profiles, the mean freezing level stands low in the midlatitudes and rises to the inner tropics. In the midlatitudes with their lower temperature and water vapor content, the mean freezing level may provide the major control for MEL and PEL. By

S. Hastenrath / Quaternary Science Reviews 28 (2009) 790–798

AMERICAS 30°N

HAWAII

AFRICA

791

90°E

AUSTRALASIA

90°E

150°E

DOMINICANA

0°

30°S 90°W

0°

Fig. 1. Orientation map showing the domains of the maps in Figs. 2, 5 and 8, and of the profiles in Figs. 3, 6 and 9, for the Australasian sector, Africa and the Americas, respectively. Dashed-line rectangles contain the domains for which upper-air temperature was evaluated.

contrast, within the tropics there is a greater diversity of climatic environments at the ice equilibrium line, MEL and PEL. Thus, in the humid inner tropics the equilibrium line is relatively low, the environment warm, and ablation is effected predominantly through the energetically inexpensive melting. In this situation the ice equilibrium line is sensitive to temperature possibly more than to precipitation variations. As glaciers reach down to relatively low elevations and vegetation limits are high, there are good prospects for radiocarbon dating of moraines. In the more arid outer tropics the climatic environment at the equilibrium line is drastically different. For example, in the arid southern tropical Andes in particular, there are mountains of more than 6000 m without perennial snow. The ice equilibrium region is cold, ablation is exclusively through the energetically expensive sublimation, and the ice extent is sensitive to variations in precipitation but not in temperature. The vertical separation between glaciers and vegetation is large, so that radiocarbon dating holds little promise. Not detailed in the profiles (Figs. 3, 6, and 9, Sections 3–5) are azimuth asymmetries of ice distribution on individual mountains. Most readily understood are the North–South differences. Planetary radiation geometry results in larger insolation on the equatorward facing slopes, thus favoring stronger glaciation on the poleward faces. This effect increases away from the Equator. Such meridional asymmetries are, for example, found on Kilimanjaro in East Africa, on the Mexican volcanoes, and in the Ecuadorian Andes (Hastenrath, 1981, pp. 19–24; Hastenrath, 1984, pp. 63–142, 293–296). More remarkable are zonal asymmetries. In the more humid regions there is a tendency for lower-reaching glaciation in the azimuths of major moisture supply. For example, in the Ecuadorian Andes (Hastenrath, 1981, pp. 52–60) lower glacier limits rise from the eastern to the western Cordillera, and in both mountain chains the eastward slopes of individual volcanoes are more heavily glaciated than those facing West. In the drier mountain regions, the direction of atmospheric moisture transports gives way to local circulations as major controlling factor of zonal asymmetries in ice extent. The mountain wind system leads to enhanced cloudiness in the afternoon, which reduces insolation on the westward facing slopes, and thus favors glaciation there. The observed tendency for lower-reaching glaciation at westerly azimuths in the more arid mountain regions appears plausible from this mechanism. On the perennially clouded mountains of the humid tropics, such processes would be less effective in controlling the azimuth distribution of ice extent. Lower glacier limits in the West than to the East are, for example, found for the modern and past glaciations of Kilimanjaro, for the past glaciation of Ethiopia, for the Mexican volcanoes, and the southern tropical Andes (Hastenrath, 1984, pp. 50–142, 293–296). Apart from certain topographic factors, the spatial distribution of ice thus strongly reflects the predominant climatic patterns.

3. Australasia In the Australasian sector (Fig. 1) the recent glaciation is limited to vanishing glaciers on the highest mountains near the Equator, but there is evidence of past glaciation at various latitudes. Figs. 1 and 2 provide map orientation, Fig. 3 a meridional– vertical profile showing mountain summits, freezing level, MEL and PEL, and Fig. 4 exhibits the timing of deglaciation versus elevation. For the Shesan Mountain on Taiwan PEL has been estimated at 3400 m, with glaciation ages of 44,000 and 19,000, and a deglaciation date of 11,000 years BP at 3700 m (Cui et al., 2000, 2002; Porter, 2001). For Kinabalu on Borneo, PEL has been reported of 3660 m and deglaciation at 9200 years BP at 4000 m (Koopmans and Stauffer, 1968; Stauffer, 1968; Flenley and Morley, 1978).

AUSTRALASIA 30 °N

TAIWAN

KINABALU NEW GUINEA

°S 30

KOSCIUSZKO

TASMANIA

150°E

Fig. 2. Map for the Australasian sector. Dashed-line rectangles contain the domains for which upper-air temperature was evaluated. Square dots indicate the location of mountains with recent glaciation and round dots the location of mountains with past glaciation. The mountains plotted are on Taiwan Shesan; on Borneo Kinabalu; on New Guinea the four mountains with recent glaciation, Ngga Pilimsit, Jaya, Takora, Mandala, and further Scorpio, Giluwe, Wilhelm, Saruwaged Range and Albert-Edward; on the Snowy Mountains of Australia Kosciuszko; and in Tasmania the West Coast Range.

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0°

TASMANIA

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NEW GUINEA NAGA PILIMSIT JAYA TRIKORA MANDALA SCOPIO WILHELM GILUWE SARUWAGED ALBERT EDWARD

0

KINABALU

TAIWAN

m

30°S

Fig. 3. Meridional profile for the Australasian sector. Solid line shows elevation of 0 C isothermal surface, annual mean over the years 1958–1997 (source: Kalnay et al., 1996); dots denote elevation of the highest summits; short horizontal lines indicate past equilibrium line PEL, and modern equilibrium line MEL characteristic of the first half of the 20th century. Orientation map is given in Fig. 2.

For New Guinea field research has been particularly productive, concerning both the past and recent glaciations (Galloway et al., 1972; Peterson and Hope, 1972; Loeffler, 1972, 1976, 1977, 1982; Hope et al., 1976; Hastenrath, 1985; Prentice et al., 2005). MEL is reached on four mountains, although on one of them ice has completely disappeared in recent years (Klein and Kincaid, 2008). As overall representative values of 4600 m and 3600 m have been proposed for MEL and PEL, respectively, with glaciation ages of 27,000 and 23,000 years BP (Porter, 2001). Deglaciation dates have been reported of 13,000–8,000 years BP at 3400–4500 m (Hope et al., 1976; Flenley, 1979). For the Snowy Mountains of Southern Australia, PEL is reported of 2100 m, and glacial advances around 59,000, 32,000, 19,000, and 17,000 years BP (Barrows et al., 2001, 2002). For Tasmania, reports are of PEL at 800 m in the West to 1500 m in the Northeast, with glaciation from before 20,000, maximum around 19,000, and deglaciation before 10,000 years BP (Colhoun, 1985; Colhoun et al., 1996).

4500

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GILUWE ALBERT EDWARD

KINABALU

WILHELM JAYA 3500

TAIWAN

GILUWE SCORPIO

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AUSTRALASIA 15000

YEARS B.P.

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Fig. 4. Variation of minimum deglaciation age with elevation for Australasia. Sources are: Stauffer (1968), Peterson and Hope (1972), Hope et al. (1976), Flenley and Morley (1978), Flenley (1979), Cui et al. (2000, 2002). Locations are indicated in the map Fig. 2.

Fig. 3 shows PEL standing higher in the inner tropics than at the poleward extremities of the profile. MEL is reached only on the highest mountains in equatorial New Guinea. Remarkably, it is plotted below the freezing level. Indeed, the two informations must not be taken as synchronous: the recent glaciers are not in equilibrium, and the 40-year mean of upper-air temperature does not detail warming trends. Following the pioneering work of Flenley (1979), Fig. 4 exhibits deglaciation dates between 13,000 and 8000 years BP, with later timing towards the higher elevations. 4. Africa On the African continent, the recent glaciation is limited to three high mountains near the Equator and there is abundant evidence of past glaciation at other latitudes. Figs. 1 and 5 provide map orientation, Fig. 6 offers a meridional profile of mountain summits, freezing level, MEL and PEL, and Fig. 7 plots deglaciation dates by elevation. For the High Atlas of Morocco there have been since the 1940s recurrent reports of fossil glacial morphology (Dresch, 1941; Mensching, 1953; Heybrock, 1955; Messerli, 1967; Chardon and Riser, 1981; Messerli and Winiger, 1992). However, no mapping of moraine and cirque systems has as yet been published and nothing is known about the chronology. Field evidence of former glaciation on the high mountains within and around the Sahara desert has been the subject of thorough reviews (Messerli et al., 1980; Messerli and Winiger, 1992). Such evidence is lacking for the Sinai (28 N). For the Hoggar there are reports of a nivation zone on East-facing slopes at 2400–2600 m and of some forms possibly indicative of small glaciers on the summits. In Tibesti distinct nivational forms are apparent only above 3000 m, especially around the northern peaks, but are lacking on the southern higher summits. The nivation forms are not controlled by aspect and are less clear than in the Hoggar. For Jebel Marra (13 N) Williams et al. (1980) report no evidence of former glaciation. For Mount Cameroon (4 N) field observations (Messerli and Baumgartner, 1978) indicate the absence of fossil glacial forms; recent volcanic ash cover being suggested as a possible factor. On the mountains of Ethiopia there is abundant evidence of past glaciation (Hastenrath, 1974b, 1977, 1984, 1995; Potter, 1976; Gasse, 1978; Williams et al., 1978; Gasse and Descourtieux, 1979; Hurni, 1982, 1989; Porter, 2001; Mark and Osmaston, 2008). For High Semyen in Northern Ethiopia mapping of fossil morphology (Hastenrath, 1974b) yields a PEL estimate of about 4100 m. Despite


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M. KENYA KILIMANJARO 3000

RUWENZORI

°S

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LESOTHO

30

YEARS B.P.

15000

0°

10000

Fig. 7. Variation of minimum deglaciation age with elevation for Africa. Sources are: Livingstone (1962), Hamilton and Perrott (1978), Gasse and Descourtieux (1979), Perrott (1982). Locations are indicated in the map Fig. 5.

Fig. 5. Map for Africa. Symbols are as for Figs. 2 and 8. The mountains plotted are, in and around the Saharan uplands the High Atlas of Morocco, Sinai S, Hoggar H, Tibesti T, Marra M, Cameroon C; in Ethiopia High Semyen, Badda B, Cilalo, Cacca, Enguelo; in East Africa the three mountains with recent glaciation, Ruwenzori, Mount Kenya, Kilimanjaro, and the two mountains with past glaciation Elgon E and Aberdares A; in Lesotho the Tabana Ntlenyana.

Hurni’s (1982, 1989) attempt at radiocarbon dating, an absolute chronology of fossil glaciation on High Semyen is still lacking. Evaluation of air photographs reveals traces of fossil glaciation on Mounts Badda, Cilalo, Cacca, and Enguolo in Southern Ethiopia (Hastenrath, 1977). Gasse (1978) and Gasse and Descourtieux (1979) obtained a radiocarbon date for the bottom of a small bog in a cirque at 4040 m on the East side of Mount Badda. From this it is inferred that the ice had completely disappeared before 11,500 years BP. Equatorial East Africa still has glaciers on its three highest mountains, Ruwenzori, Mount Kenya, and Kilimanjaro, and there is evidence of past glaciation on Elgon and Aberdares (Livingstone, 1962; Hamilton and Perrott, 1978; Perrott, 1982; Hastenrath, 1984, 1985, 1995, pp. 399–412; Mahaney, 1988, 1989; Rosquist, 1990;

5000

Shanahan and Zreda, 2000; Porter, 2001; Kaser and Osmaston, 2002; Mark and Osmaston, 2008). Shanahan and Zreda (2000) using cosmogenic dating report glaciation dates of more than 250 ka on both Mount Kenya and Kilimanjaro; for Mount Kenya moraine ages of 28, 14.6, 10.2 and 8.6 ka and around 200 years; for Kilimanjaro moraine ages of 20, 17.3, 15.8 and 13.8 ka. Correspondence to the moraine stages in Table 1 part (A) remains to be ascertained. The MEL is estimated on Ruwenzori at about 4600 m, with higher values in the West than in the East; on Mount Kenya at about 4700 m, with higher values in the Northwest than in the Southeast; and on Kilimanjaro at about 5400 m, with higher values in the East than West (Hastenrath, 1984; Porter, 2001; Kaser and Osmaston, 2002). Thus, the MEL representative of the early 20th century stands somewhat above the 1958–1997 mean freezing level (Fig. 6). Referring back to a remark in Section 2, here is a unique opportunity to compare MEL with recent mass budget measurements on Mount Kenya and Kilimanjaro. The 1958–1997 mean freezing level at the Equator stands at 4170 m. On Mount Kenya, the l8-year monitoring of Lewis Glacier (Hastenrath, 2005)

AFRICA

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ELGON RUWENZORI KENYA ABERDARES KILIMANJARO

BADDA

SEMYEN

HOGGAR

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m

30°S

Fig. 6. Meridional profile for Africa. Symbols are as for Figs. 3 and 9. Orientation map is given in Fig. 5.

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Table 1 Past glaciation in (A) East Africa and (B) the Ecuadorian Andes. For moraine stages III, II, I, and R (recent, 1970s) information is presented on ice extent and elevation range of moraine systems. Synchroneity between moraine systems in (A) and (B) is not implied (sources: Hastenrath, 1981, p. 56, 57, 60; 1984, p. 51, 52, 54; 2005, p. 184). III (A) East Africa Ice extent in 106 m2 Ruwenzori Mount Kenya Kilimanjaro Aberdares Elgon

(200) 240 200 23 95

II

I

40 81 66

Elevation range of moraine systems in hundreds of m Ruwenzori 23 43–35 Mount Kenya 34 43–40 Kilimanjaro 34 (50–45) Aberdares 38–32 Elgon 36–33 (B) Ecuador Ice extent in 106 m2 Western cordillera Eastern cordillera

987 1063

R

12 2 17

4 1 5

46–33 43–40 52–46

38 332

47 208

Elevation range of moraine systems in hundreds of m Western cordillera 46–32 50–40 Eastern cordillera 46–30 47–42

50–45 47–42

38 132

yields negative net balance still at 4900–5000 m, compared to the aforementioned MEL of 4700 m. For the summit plateau of Kilimanjaro negative net balance has been reported (Mo¨lg and Hardy, 2004) from monitoring at 5794 m, compared to the aforementioned MEL of 5400 m. The estimates of PEL in equatorial East Africa are for Ruwenzori 3850 m, for Mount Kenya 3900 m, and for Kilimanjaro 4600 m (Fig. 6), with differences between the various mountain sectors similar to those of MEL. The PEL has further been estimated at about 3950 m on Mount Elgon (Hamilton and Perrott, 1978) and at about 3900 m on the Aberdares (Perrott, 1982). Table 1 part (A) presents evaluations of ice extent for three moraine stages, showing a progressive decrease to the recent conditions. Turning to the timing of deglaciation, Kaser and Osmaston (2002, pp. 160–162) offer no such information for the two highest mountains, Kilimanjaro and Mount Kenya, and further express reservations on the interpretation of field data. For Ruwenzori Livingstone (1962) reports a deglaciation date of >14,700 years BP at 2970 m. For Elgon Hamilton and Perrott (1978) found a date of 11,500 years BP at 4150 m, and for the Aberdares Perrott (1982) reports 12,200 years BP at 3790 m. Fig. 7 shows from these four sources a later timing of deglaciation towards the higher elevations. Proceeding further southward to the southern extremity of the continent there are the high mountains of Lesotho. Based on geomorphic evidence, Harper (1969) suggested that the highest peaks of Lesotho were above the snowline during the Pleistocene with a broad periglacial zone extending some 900 m lower. Tabana Ntlenyana, the highest summit of Lesotho and Southern Africa as a whole, reaches 3485 m. During field work in 1971 (Hastenrath, 1972; Hastenrath and Wilkinson, 1973) landforms suggestive of glacial relief, cirques, were indeed observed in the highlands of Lesotho, but a clear moraine morphology could not be detected. Thus, geomorphic evidence of past glaciation in Southern Africa merits further exploration. This review for Africa invites further exploration of specific targets. For the High Atlas of Morocco a systematic mapping of fossil glacial morphology is called for, based on topographic maps and air photography and field exploration. Building on that an effort

should be undertaken to assess the age of deglaciation. For High Semyen of Ethiopia, mapping of fossil glacial morphology has been accomplished more than three decades ago, but dating of the deglaciation still remains to be done. In Southern Africa, the mountains of Lesotho remain a challenge concerning the extent and age of possible past glaciation. 5. Americas With the unique extent of the cordilleras, the meridional patterns of glaciation are particularly well evidenced in the Americas. Figs. 1 and 8 provide map orientation, Fig. 9 a meridional profile of freezing level, MEL and PEL, and Fig. 10 the timing of deglaciation with elevation. At the northern extremity of the profile, in the greater Caribbean–Central American region, glaciation has been extensively studied on the Mexican volcanoes (White, 1962, 1981; White and Valastro, 1984; Heine, 1984, 1988, 1994; Vazquez-Selem, 1997; Vazquez-Selem and Heine, 2004; Lachniet and Vazquez-Selem, 2005). MEL is estimated at around 4900 m and PEL at around 4000 m and 3300 m (Porter, 2001). Glaciations are reported at 36,000–32,000 and 10,000–8500, and an advance around 12,000 years BP (Heine, 1988, 1994). Proceeding southward on the Central American land bridge, fossil glacial morphology has been mapped in the Altos de Cuchumatanes of Guatemala (Hastenrath, 1974a; Weyl, 1980; Lachniet, 2007), with PEL at 3650 m. On the nearby Tajumulco, with 4220 m the highest mountain of Central America, no fossil glacial

AMERICAS 30

A

I P

DOMINICANA

CUCHUMATANES TALAMANCA

°S 30

Hu JH Q M Sa CQ

A Ch

Bo Mu Mi Co RT SI

CR SV L TC Ac

90°W

Fig. 8. Map for the Americas. Symbols are as for Figs. 2 and 5. The mountains plotted are in the Central American–Caribbean region the Mexican volcanoes Iztaccihuatl I and Popocatepetl P (with recent glaciation), and Ajusco A, the Altos de Cuchumatanes of Guatemala, the Cordillera de Talamanca of Costa Rica, and the highlands of the Dominican Republic; in the Northern Andes of Colombia and Venezuela the volcanoes Pico Bolivar Bo, Ruiz-Tolima RT, Cocuy C and Santa Isabel SI (with recent glaciation), as well as Mucubaji Mu and Paramo de Miranda Mi; of the numerous glaciated mountains in the Ecuadorian Andes only two are plotted, Chimborazo Ch and Antisana A; in the Andes of Peru and Bolivia the locations are shown of Huascaran H, Junin–Huaytapallana JH, Quelccaya Q, Misti M, Cordillera Real CR, Sajama Sa; in the Andes of Chile and Argentina the mountains plotted are Choquelimpie CQ, Cordillera Santa Victoria SV, Llullaillaco, L, Nevado Tres Cruces and Cerro Potro TC, and Aconcagua Ac.


795

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ACONCAGUA

C. S.VICTORIA LLULLAILLACO TRES CRUCES, CP

HUSCARAN JH QUELCCAYA MISTI C.REAL CHOQUELIMPIE, SA

ANTISANA CHIMBORAZO

0

TALAMANCA P.BOLIVAR, MU, MI COCUY RUIZ-TOLIMA, S.ISABEL

AJUSCO IZTACCIHUATL POPOCATEPETL CUCHUMATANES

m

30°S

Fig. 9. Meridional profile for the Americas. Symbols are as for Figs. 3 and 6. Orientation map is given in Fig. 8.

forms were found (Hastenrath, 1963), conceivably due to later volcanic activity, similar to the conjecture by Messerli and Baumgartner (1978) for Mount Cameroon in Africa. In Central America, fossil glacial morphology has further been mapped in the Cordillera de Talamanca of Costa Rica, with PEL estimated at 3500 m (Hastenrath, 1973; Weyl, 1980; Orvis and Horn, 2000; Lachniet and Seltzer, 2002; Lachniet, 2004, 2007; Lachniet and Vazquez-Selem, 2005). Horn (1990) reported a deglaciation date of >10,000 years BP at 3480 m, as plotted in Fig. 10, and expressed the need for further comparative investigations on the altitude dependence of deglaciation dates. Intriguing and enigmatic are reports from the highlands of the Dominican Republic on the Caribbean island of Hispaniola, around 19 N with a summit of 3087 m (Schubert and Medina, 1982; Orvis et al., 1997). Landforms suggestive of fossil glacial morphology are

4500

JUNIN HUAYTAPALLANA

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C.REAL

4000 MIRANDA COCUY MUCUBAJI 3500 RUIZ-TOLIMA

3000

TALAMANCA

AMERICAS

15000

YEARS B.P.

10000

Fig. 10. Variation of minimum deglaciation age with elevation for the Americas. Sources are: Gonzalez et al. (1965), Herd and Naeser (1974), Salgado-Labouriau et al. (1977, 1988), Wright (1983), Seltzer and Wright (1989), Horn (1990), Orvis and Horn (2000), Seltzer (1992). Locations are indicated in the map Fig. 8.

reported from elevations above 2200 m. Mappings and dates are not available. In context, the possibility of past mountain glaciation remains inconclusive. The Northern Andes have been the object of progressive field investigations (Herd and Naeser, 1974; Schubert, 1974, 1984; Schubert and Clapperton, 1990; Thouret et al., 1996; Porter, 2001). On Pico Bolivar in the Venezuelan Andes PEL is estimated at 3800 m and MEL around 4700 m (Porter, 2001). On Nevado del Ruiz in the Colombian Andes PEL is around 3800 m, with higher elevations in the West than East; and MEL is around 4850 m, again higher in the West than East (Porter, 2001). On Nevado de Santa Isabel in the Colombian Andes, PEL is around 3750 m, higher in the Northwest than Southeast; and MEL is around 4750 m, higher in the Northwest than Southeast. Deglaciation dates have been reported in the Venezuelan Andes for Mucubaji (Salgado-Labouriau et al., 1977) and Paramo de Miranda (Salgado-Labouriau et al., 1988; Schubert and Clapperton, 1990), and in the Colombian Andes for Cocuy (Gonzalez et al., 1965) and Ruiz-Tolima (Herd and Naeser, 1974). These are plotted in Fig. 10. In the Ecuadorian Andes numerous mountains in the western and eastern cordilleras are still glaciated (Hastenrath, 1981; Clapperton and McEwan, 1985; Clapperton and Vera, 1986; Clapperton, 1987a,b; Porter, 2001). Of these Chimborazo and Antisana are plotted in Figs. 8 and 9. As detailed elsewhere (Hastenrath, 1981, pp. 19–24, 52–60) and indicated in Section 2, there are marked azimuth asymmetries; glacier limits rise from the eastern to the western cordillera, and even on individual mountains ice is more extensive on the eastern than the western slopes, all reflecting the direction of the major moisture supply. MEL is estimated at around 4850 m on Chimborazo and 4700 m on Antisana, and PEL around 4000 m and 3650 m, respectively (Hastenrath, 1981; Porter, 2001). Table 1 part (B) presents evaluations of ice extent for three moraine stages, showing a progressive decrease to the recent conditions. The lowest moraines III are covered by a mantle of volcanic ash and carry vegetation; the complex consists of multiple moraine ridges (see Hastenrath, 1981, photo 22), as also noted by Clapperton and McEwan (1985). Moraines II have no ash cover and mostly no vegetation. Still higher up is moraine complex I near the ice margin of the early 20th century. Pictorial evidence from 1872 to 1903 (El Altar and Antisana, Hastenrath, 1981, photos 22–38; Hastenrath, 2008, photos 2.1.1–2.3.10) shows the glaciers inside the moraine system II. Clapperton and Vera (1986) and Clapperton (1987b) list

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ages of 2000 years B.P., for moraine complexes around 3500 m, 3900 m, and 4250 m, respectively. One may conjecture correspondence to the aforementioned systems III, II and I in Ecuador. Correspondence between moraine systems in Ecuador versus East Africa is not known, pending absolute dating. For Peru’s Cordillera Blanca (Rodbell, 1992; Kaser, 1999; Porter, 2001; Smith et al., 2008), with the highest summit Huascaran of 6768 m, MEL is estimated at around 5000 m and PEL at 4300 m, with somewhat lower elevation in the East than West, again reflecting the direction of the major moisture supply. MEL and PEL rise from North to South in the Peruvian– Bolivian Andes (Hastenrath, 1967, 1971; Seltzer, 1990; Fox and Bloom, 1994). For the region of Junin and Huaytapallana, at around 11–12 S and 75–76 W, and at 4200–4600 m, deglaciation dates have been reported ranging between 7000 and 14,000 years BP (Wright, 1983; Seltzer and Wright, 1989), as plotted in Fig. 10. The large temporal spread may be appreciated in the context of topography; thus Klein et al. (1999) remark that the Altiplano is characterized by high altitude and low relief, which they consider a factor for the relatively small elevation difference between MEL and PEL in this region. Seltzer et al. (2002) report deglaciation dates without indication of elevation. For the Quelccaya Ice Cap in Southern Peru, 5645 m, MEL is estimated at around 5300 m and PEL around 4900 m (Mercer and Palacios, 1977; Seltzer, 1990; Fox and Bloom, 1994). Mercer and Palacios (1977) and Mercer (1984) report radiocarbon age determinations of peat deposits, of 12,240 years BP at 4750 m, and of 9980 years BP at 5070 m. In spatial context and in accordance with the reservations expressed by Kaser and Osmaston (2002, pp. 160–162, 168) and Clapperton (2000) these may not represent deglaciation. On the Peruvian–Bolivian Altiplano high lake stands at 12,500 – 11,000 years BP, the paleolake Tauca episode, coincide with more extensive glaciation (Hastenrath and Kutzbach, 1985; Seltzer, 1990; Clayton and Clapperton, 1997), indicative of the important moisture control. For the Cordillera Real of Bolivia (Seltzer, 1992), with highest summit Ilimani 6438 m, MEL is estimated at about 5100 m and PEL around 4800 m, and deglaciation dates are reported of 10,400 years BP at 4300 m and of 10,020 years BP at 4330 m, also plotted in Fig. 10. For the Andes of Southern Peru, Bolivia and Northern Chile, Klein et al. (1999) mapped a rise of MEL and PEL towards the South and West. The arid region of the Andes extending from the Altiplano into Northern Chile and Argentina has received increased attention in recent decades (Messerli et al., 1992; Espizua, 1993; Clapperton, 1995, 2000; Zipprich et al., 2000; Amman et al., 2001; Zech et al., 2008). In the realm of the tropical easterlies the aridity increases westward, in the latitudes of the westerlies the greatest aridity is found in the East, hence the notion of ‘‘the aridity diagonal’’ (Zipprich et al., 2000). From observations on Choquelimpie, 5327 m, and nearby Parinacota, 6348 m, Amman et al. (2001) report MEL around 5600 m and PEL around 4800 m; for Sajama, 6542 m, they estimate MEL between 5400 m and 5700 m. For Llullaillaco, 6739 m, perennial snow and firn has been reported but no MEL (Hastenrath, 1971; Messerli et al., 1992; Amman et al., 2001). Three moraine stages have been mapped at 4900 m, 5100 m and 5500 m, but no dates are available, understandably a challenge in this extremely arid region without vegetation. For Cordillera de Santa Victoria in Northwestern Argentina, with maximum elevation >5000 m, Zipprich et al. (2000) report PEL around 4500 m, higher in the Southwest and lower in the Northeast, thus reflecting the moisture flow from the Northeast. They also note that there is no indication of a glaciation maximum at the LGM. For Tres Cruces and Cerro Potro Amman et al. (2001) report MEL around 5300 m and

PEL around 4300 m. For Aconcagua, 6959 m, Espizua (1993) gives a MEL of 4500 m and PEL around 3400 m. Complementing this profile along the American cordilleras are reports (Porter, 1979, 2001; Blard et al., 2007; Pigati et al., 2008) of past glaciation on Hawaii (Fig. 1). Porter (1979, 2001) estimates for Mauna Kea, 4200 m, a PEL of 3780 m and a deglaciation date >15,000 years BP. This review for the Americas draws attention to some targets. For the Altos de Cuchumatanes of Guatemala the age of deglaciation is of interest in the context of the Cordillera de Talamanca of Costa Rica and the Northern Andes. The field evidence of fossil glacial morphology in the highlands of the Dominican Republic needs further exploration. Particular challenges arise in the arid southern Andes, with MEL above the highest summits, PEL not well defined, and little prospect for radiocarbon dating of glaciation and deglaciation in this extremely dry, vegetationless environment. 6. Conclusions With the global atmosphere progressively warming and the glaciers on the high mountains of the tropics out of equilibrium and shrinking, past glaciation cannot be compared to the present but at best referenced to recent evidence of ice extent and temperature conditions. Indeed, in situ monitoring on the two highest mountains of East Africa bears out negative net balance still at 5794 m and 4900–5000 m, respectively, well above the 4170 m freezing level. With these qualifications, tropical glaciers were here appraised along three broad meridional profiles, for the Australasian sector, Africa and the Americas, respectively. Evaluated along these profiles are the annual mean freezing level 0 C, reports of modern equilibrium line altitude MEL, and evidence of past equilibrium line altitude PEL. The calculation of the freezing level is based on a 1958–1997 global data set; MEL refers to estimates concerning the first half of the 20th century; and the timing of the PEL is not generally known. Thus one should appropriately think of last local glacial maximum LLGM instead of the conventional LGM. The freezing level 0 C stands around 4000–5000 m, with lower levels in the outer tropics. MEL is reached in the Australasian sector on four mountains of which one lost all ice in recent years, and in Africa on three mountains, all near the Equator and near or above the freezing level. In the American cordilleras many peaks are still glaciated, above 0 C, but in the arid southern tropical Andes even summits above 6000 m do not reach MEL. The PEL stands between 3000 m and 5000 m, high in the equatorial zone, but highest in the arid southern Andes. The height difference between MEL and PEL is of the order of 1000 m, with regional differences. Both MEL and PEL exhibit azimuth asymmetries: ice tends to be more extensive on the poleward flanks, in the sectors of major moisture supply, and in the drier mountain regions local circulations favor glaciation on the westward facing slopes. Deglaciation dates, over the altitude extent of about 3000 m to 4500 m, range between 15,000 and 8000 years BP, with later timing towards the higher elevations. This synopsis of field research spanning six decades calls attention to challenges for future work. Thus, in Africa, for the High Atlas of Morocco a systematic mapping of fossil glacial morphology and age determinations have still not been published. The wellmapped High Semyen of Ethiopia is still awaiting assessment of deglaciation date. For Tabana Ntlenyana in Lesotho further exploration is called for on extent and age of possible past glaciation. In the Americas, the age of deglaciation is to be determined for the Altos de Cuchumatanes in Guatemala. Fossil morphology in the highlands of the Dominican Republic merits further exploration.


Determination of past glaciation and chronology remains a challenge in the arid southern tropical Andes.

Acknowledgements This study was supported by the Variability of Tropical Climate Fund of the University of Wisconsin Foundation. I recall with gratitude all those who have helped me in my wanderings and field explorations over the decades: High Atlas of Morocco 1955; cordilleras of Central America and Mexico 1960–1963 and 1972–1973; Peruvian Andes 1964, 1969, 1976–1983; Lesotho 1971; Kilimanjaro 1971, 1973–1974; Mount Kenya 1971–1995; High Semyen 1973; Ruwenzori 1974; Ecuadorian Andes 1974–1979; Kosciuszko 1975; Papua 1975; Tasmania 1993; Taiwan 1997. At the University of Wisconsin Dierk Polzin assisted with the script processing and graphics. I thank the reviewers for encouragement and helpful feedback.

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