Stephen C. Porterpeter/Resources/Seminar...Porter, 1975 , 1977; Rodbell, 1992) . Studies have shown...

*Tel.: #1-206-543-1904; fax: #1-206-543-3836.E-mail address: [email protected] (S.C. Porter).

Quaternary Science Reviews 20 (2001) 1067}1091

Snowline depression in the tropics during the Last Glaciation

Stephen C. Porter*Department of Geological Sciences and Quaternary Research Center, University of Washington, Seattle, WA 98195-1360, USA

Abstract

Five primary methods have been used to reconstruct Pleistocene snowlines or equilibrium-line altitudes (ELAs) in the tropics(23.53N}23.53S) during the last glaciation, but each has inherent errors that limit the accuracy of the results. Additional potentialerrors in determining ELA depression involve estimates of modern snowline altitude, dating resolution, topographic reconstruction offormer glaciers, orographic e!ects, the presence of rockfall debris on glaciers, and calculation of regional ELA gradients. Eustaticsea-level lowering during the last glaciation is an additional factor in#uencing estimates of ELA depression (!ELA). In cases wheremodern snowline lies above a mountain summit, only a minimum value for !ELA can be obtained. At 12 tropical sites in Africa, theAmericas (to 103S latitude), and Paci"c islands, estimates of average !ELA range from 440 to 1400m, but most fall in the range of800}1000m (mean $1""900$135m). In a regional study of ELA depression in the southern tropical Andes (8}223S), an average!ELA of ca. 920$250m has been reported. Based on the assumption that glacier mass balance was controlled solely byablation-season temperature, and assuming a full-glacial temperature lapse rate of !63C/km, depression of mean annual temper-ature in glaciated alpine areas was ca. 5.4$0.83C. If adjusted for a sea-level fall of !120m at the glacial maximum, this value isreduced to 4.7$0.83C. The "gure is based on the (unlikely) assumption that accumulation on alpine glaciers has been invariant;nevertheless, it is similar to values of temperature depresson (5}6.43C) for the last glaciation obtained from various terrestrial sites, butcontrasts with tropical sea-surface temperature estimates that are only 1}33C cooler than present. ! 2001 Elsevier Science Ltd. Allrights reserved.

1. Introduction

Recurring questions regarding the magnitude of tropi-cal climate change during the last glacial age haveemerged since publication of the CLIMAP Project Mem-bers (1976, 1981) reconstruction of ice-age sea-surfacetemperatures (SSTs). The CLIMAP reconstruction,which focused on the last glacial maximum (LGM) re-vealed large areas in the tropics to have had SSTs aswarm as, or even slightly warmer than, those ofthe present. The CLIMAP project considered theLGM to date to 18,000 !"Cyr BP [21,648 (21,484)21,313 cal yr BP; equivalent calibrated ages ($1") havebeen obtained using CALIB 3.03 (Stuiver and Reimer,1993) for ages (18,000 yr, and using Stuiver et al. (1998)for ages '18,000 yr]. Rind and Peteet (1985) sub-sequently noted con#icts between ice-age paleotempera-tures generated by a general circulation model

experiment, which used the CLIMAP SSTs as boundaryconditions, and low-latitude terrestrial paleoclimateproxy data. Their analysis employed pollen evidence andestimates of snowline depression from four tropical sites(Hawaii, the Colombian Andes, equatorial Africa, andNew Guinea), and led them to conclude that theCLIMAP reconstruction underestimated the amount oftropical temperature depression, which likely amountedto 5}63C.

Renewed interest in this topic has been generatedby evidence and modeling that point to colder tropicaltemperatures than those implied by the CLIMAPreconstruction (e.g., Guilderson et al., 1994; Stute et al.,1995; Thompson et al., 1995; Bush and Philander, 1998;Farerra et al., 1999). Basic to much of the discus-sion about colder glacial-age tropics has been thequestion of snowline depression (e.g., Broecker, 1995;Hostetler and Mix, 1999; Lee and Slowey, 1999), yetmost of the snowline data used in the arguments has notbeen rigorously evaluated. Since Rind and Peteet (1985)questioned the CLIMAP conclusions more than a dec-ade ago, additional information has emerged that nowpermits a more thorough assessment. This paper focuses

0277-3791/01/$ - see front matter ! 2001 Elsevier Science Ltd. All rights reserved.PII: S 0 2 7 7 - 3 7 9 1 ( 0 0 ) 0 0 1 7 8 - 5

on the set of paleoclimatic data bearing on snowlinedepression in the tropics during the last glaciation. Inassessing these data, potential sources of error are alsoconsidered in arriving at site-speci"c and globally aver-aged values.

1.1. Glacier equilibrium-line altitudes

The snowline, de"ned as the lower limit of perennialsnow on the landscape, is equivalent to the "rn limiton temperate alpine glaciers, which is the lower limitof snow at the end of the ablation season. On suchglaciers, the "rn limit approximates the equilibrium line,the locus of points along which the annual mass balanceis zero. In most recent paleosnowline studies, the equilib-rium line is regarded as synonymous with the snowline,and its altitude, following Meier and Post (1962), isdesignated the equilibrium-line altitude (ELA). The dif-ference between the modern ELA (ELA

!) and that of

some earlier time (e.g., the last glaciation, ELA") is

a measure of equilibrium-line (i.e., snowline) depression(!ELA).

The mass balance of a glacier, and #uctuations of theglacier's equilibrium line, are controlled by a numberof climate-related processes. For most low-latitudetemperate glaciers, the most important controls are accu-mulation-season precipitation and ablation-season tem-perature. Together these parameters encompass a rangeof possible conditions controlling the ELA. Therefore,a unique value for past precipitation or temperaturecannot be derived from the !ELA alone (Porter, 1977;Seltzer, 1994). In most published paleosnowline studies,no di!erence in precipitation is assumed (in most casesprobably erroneously) between the present and theLGM, and a change in temperature is obtained by as-suming a "xed atmospheric lapse rate. In cases whereindependent evidence for one parameter (i.e., LGM pre-cipitation or temperature) is available from another cli-mate proxy, then !ELA can provide an estimate of theother parameter.

2. Methods

In studies of glaciated low-latitude mountains,"ve common methods have been used to reconstructformer ELAs. Because the methods di!er in theirapproach, the results they produce are not strictlycomparable.

2.1. Cirque-yoor altitude

When a glacier just "lls a cirque, its steady-state ELAtypically lies not far above the average altitude of thecirque #oor (Fig. 1a). Therefore, cirque-#oor altitude has

sometimes been used as a convenient proxy for formerELAs (e.g., PeH weH and Reger, 1972; Nogami, 1972, 1976;Fox and Bloom, 1994). While this approach is reasonablein situations where Pleistocene glaciers terminated atcirque thresholds, in such cases the cirque glaciers disap-peared when snowlines rose above cirque levels at theend of the Pleistocene, meaning that site-speci"c ELAdepression cannot be calculated directly. Furthermore, inmany glaciated tropical mountain ranges and on largevolcanoes, glaciers expanded beyond cirques to formvalley glaciers, and in these circumstances ELAs laybelow (often well below) the altitudes of cirque #oors. Insuch cases, snowline reconstructions based on cirque-#oor altitudes may substantially underestimate actualsnowline depression.

2.2. Upvalley limits of lateral moraines

For a glacier in a balanced (steady-state) condition,the upvalley limit of its contemporary lateral moraineslies at the equilibrium line, below which ice-#ow pathsare diverging and ascending. If lateral moraines of aformer glacier are well preserved, then the altitudeof their upvalley limits may closely approximate theformer ELA (Fig. 1b). Whereas this method has beenused with success in some areas (e.g., Andrews, 1975;Mahaney, 1990), in many alpine regions lateral morainesare absent or poorly preserved and at best provide onlylower limiting estimates for contemporaneous ELAs.Meierding (1982) considered the highest lateral morainealtitude to be the least reliable of several methods fordetermining Pleistocene ELAs in the Front Range ofColorado.

2.3. Glaciation threshold

The glaciation threshold (GT) for a speci"ed area (nor-mally a 7.5# topographic quadrangle or its equivalent:e.g., ca 60 km# at 453 latitude) is the mean altitude be-tween the lowest mountain with a glacier on it andthe highest without (Fig. 1c). Although this method isnot applicable to isolated peaks, such as volcanoes,it has proved useful for assessing regional snowlinetrends across mountain ranges (e.g., "strem, 1966;Porter, 1975, 1977; Rodbell, 1992). Studies have shownthat the GT essentially parallels the regional ELA trend,but commonly lies 100}200m higher (Meierding, 1982;S. C. Porter, unpublished data). A limiting problemwhen using the GT to determine Pleistocene snowlinedepression is the need to identify and map former gla-cierized and nonglacierized peaks for a speci"c time (e.g.,the LGM) throughout a rather broad region. Such exten-sive "eldwork normally is impractical, and so subjectiveassessments of the extent and age of past glaciation

1068 S.C. Porter / Quaternary Science Reviews 20 (2001) 1067}1091

Fig. 1. Common methods used to derive past equilibrium-line altitudes in the tropics. See text for details. Cirque-yoor method: The ELA of a cirqueglacier is inferred to lie above, but not far above, the cirque #oor (CF). However, if a glacier expands beyond the cirque threshold, the ELA will be lowerthan the cirque #oor. Lateral-moraine method: The upglacier limit of a lateral moraine approximates the ELA of the glacier that constructed themoraine. Glaciation-threshold method: The average altitude between the highest nonglacierized summit (Sn) and lowest glacierized summit (Sg) de"nesthe glaciation threshold (GT) in a restricted area. Altitude-ratio method: In the median-altitude variant of this method, the ELA lies midway in altitudebetween the head of the glacier (A

#) and the terminus (A

$). In the terminus-head altitude ratio (THAR) approach, the THAR equals the ratio of the

altitude di!erence between the terminus and the ELA divided by the total altitude range of the glacier. The ELA can be estimated by adding thealtitude of the terminus to the product of the total altitude range and an assumed THAR. Accumulation-area ratio method: In using this method, anaccumulation-area ratio (AAR) is used, based on the ratio of the accumulation area (Sc) to the total area of the glacier (where Sa is the ablation area).Empirical studies suggest that a steady-state (SS, when the mass balance"0) AAR of 0.65$0.5 is appropriate for most temperate, relativelydebris-free glaciers. The surface topography of the former glacier is reconstructed based on glacial-geologic data. From the glacier's area}altitudedistribution (here depicted as a cumulative curve) and an assumed AAR, an ELA value is obtained.

are usually based on analysis of topographic maps oraerial/satellite imagery. Despite the inherent uncertain-ties and subjectivity involved (Meierding, 1982), this

method has proved useful in assessing snowline depress-ion in some areas (e.g., the Cascade Range: Porter et al.,1983, Fig. 4-15).

S.C. Porter / Quaternary Science Reviews 20 (2001) 1067}1091 1069

2.4. Altitude ratios

Use of the median altitude of a former glacier asa proxy for past snowline altitude is based on theempirical observation that the "rn limit on temperateglaciers at the end of the ablation season often lies abouthalfway between the head of a glacier and its terminus(Fig. 1d). HoK fer (1879) used a variation of this approachby computing the arithmetic mean of the altitude ofa glacier's terminus and the average altitude of themountain crest at the glacier's head. The median altitudemethod, in theory, is easy to apply if adequate altitudedata are available (i.e., topographic maps with a resolu-tion of ca 30m or less, "eld data based on altimetrymeasurements, or digital-elevation data), and if a formeralpine glacier had a normally distributed area vs. altitudecurve. Nevertheless, whereas determination of the lowerlimit of a glacier based on end moraines or outwashheads may be relatively straightforward, assigning anupper altitudinal limit to the former glacier in the cirqueregion is generally subjective and arbitrary. High, steepcirque headwalls can lead to a potential range of esti-mates di!ering by tens to hundreds of meters.

The median altitude method assumes that the ratio ofa glacier's range in altitude above the equilibrium line tothe total altitudinal range of the glacier is 0.5. A variationof this method has also been used in which the ratio[termed the toe-to-head (i.e., terminus-to-head) altituderatio, or THAR] is some lower value (Fig. 1c). Forexample, Meierding (1982) reported that THARs of0.35}0.40 generated the most accurate results in the Col-orado Front Range. The resulting ELAs were ca100}150m lower than those derived using the medianaltitude (THAR"0.5).

2.5. Accumulation-area ratio

The accumulation-area ratio (AAR) of a glacier is theratio of the glacier's accumulation area to the sum of itsaccumulation and ablation areas (Fig. 1e). Empiricalstudies of modern glaciers have shown that understeady-state conditions the AAR typically falls between0.5 and 0.8 (i.e., 0.65$0.15) (Meier and Post, 1962),meaning that the accumulation area occupies approxim-ately two-thirds of the glacier's total area. In calculatingpast ELAs using the AAR method, a steady-state condi-tion is assumed and the glacier's extent and topographyare determined using glacial-geologic data such as lateralmoraines, erratics, and trimlines (Porter, 1981). An initial(estimated) ELA is selected using the altitude ratiomethod. Contours of the glacier surface are then drawn,consistent with principles of glacier #ow (contours ofa glacier in a balanced state typically are concave up-glacier in the accumulation area and convex in the abla-tion area, with the degree of concavity or convexityincreasing with increasing distance from the equilibrium

line). The area between each pair of successive contours isthen measured and used to generate a cumulative curvethat graphically displays the glacier's area/altitude distri-bution. Assuming a steady-state AAR of 0.65, the ELAcan be determined from the graphical plot. Error limitsare derived by assuming a range of AAR values(e.g.,$0.05 or$0.10).

2.6. Comparison of methods

Meierding (1982) assessed the relative reliability ofvarious paleo-ELA methods based on data from theFront Range of Colorado. Using "rst-order trend-surfaceanalyses, he found that the cirque-#oor, median-altitude,and lateral-moraine methods had the greatest root meansquare error (RSME"97}148m), whereas the THAR("0.40) and AAR ("0.65) methods produced the mostconsistent results (RSME"ca 80m). A similar study inNorway by Torsnes et al. (1993) also concluded that theAAR method produced the most reliable results. Theonly similar comparative study in low latitudes was madeby Osmaston (1989a) in his study of glaciated equatorialAfrican mountains. He concluded that a modi"ed versionof the altitude-ratio method gave the best results. Over-all, the general lack of reliable topographic informationand detailed "eld mapping for many tropical glaciatedareas, as well as limited radiometric age control, meansthat errors inherent in most of these methods may bemagni"ed at low-latitude sites.

2.7. Additional potential sources of error

In addition to di!ering results obtained from the sev-eral ELA methods outlined above, as well as the errorspeculiar to each method, several other sources of errorenter into the calculation of LGM snowline depression.

2.7.1. Altitude of the modern snowlineOn many tropical mountains, glaciers are absent or the

modern snowline altitude is known only approximately.Furthermore, in a time of generally warming climate, thetransient nature of the snowline means that values ob-tained from direct observations a decade or more ago, orfrom topographic maps based on them, may underesti-mate the present snowline altitude. Where direct obser-vational data are unavailable, modern ELAs obtainedfrom recent glacier maps or aerial photography andemploying the median altitude or AAR methods likelyo!er the best estimates. Nevertheless, errors of tens ofmeters or more may result.

2.7.2. Age of the LGM glacial limitIn few cases has the limit of the last glaciation been

radiometrically dated in tropical mountains, and in noinstance has it been closely bracketed by dates. There-fore, synchrony of moraines likely built during the LGM


commonly is inferred, based mainly on relative-age cri-teria and the pattern of moraine sequences, and on infer-red regional or global correlations.

Recent studies in several (nontropical) mountainranges report that glaciers advanced repeatedly duringthe last glaciation (marine isotope stage 2), and that theresulting moraines often are nested or closely spaced.These ice advances typically date between ca 27,000and 16,000 !"CyrBP [31,300 and 18,972 (18,876)18,784 cal yr BP] (e.g., Phillips et al., 1990; Gosse et al.,1995; Lowell et al., 1995; Swanson and Porter, 1999).Although comparable moraine sequences may exist inthe tropics (e.g., New Guinea: Blake and LoK %er (1971);Andes: Clapperton (1987) and Thouret et al. (1996)), nonehas yet been closely dated. The actual age of an `LGMamoraine may lie anywhere within this ca 12,500 yr range.Nevertheless, the juxtaposition of such moraines impliescomparable snowline depressions ()50m di!erence)during these successive advances.

2.7.3. Paleoglacier reconstructionsA potential source of error in the AAR method is

associated with the topographic reconstruction of a for-mer glacier, which is necessarily subjective. Below theequilibrium line, terminal and lateral moraines and trim-lines provide altitudinal constraints along a glacier's for-mer margin, whereas above the former ELA little controlusually exists. Errors in circumscribing the accumulationarea tend to be minimal because of steep valley wallsupglacier from the equilibrium line; thus, an erroneousaltitude estimate for the glacier margin in this zone onlyminimally a!ects the lateral extent of the accumulationarea.

Errors related to topographic reconstruction are mini-mized in the case of small glaciers with normally distrib-uted area}altitude curves. Large, complex glaciers, andthose having a trend perpendicular to the regional ELAgradient, may generate unreliable results.

2.7.4. Orographic ewectsSmall glaciers con"ned to deep cirques on leeward#anks of mountains, or shaded by steep mountain walls,may persist at altitudes well below those of glaciers onfully exposed slopes. In general, ELAs based on geomet-rically simple glaciers in exposed sites are likely to pro-vide the most regionally consistent ELA values. Inaddition, orographic e!ects (e.g., unequal exposure tosun, unequal accumulation) may lead to a range of tens ofmeters in the altitude of the "rn limit on a given glacier.

2.7.5. Anomalies resulting from a cover of rockfall debrisAn extensive cover of rockfall debris can insulate an

alpine glacier and greatly reduce ablation (Clark et al.,1994). Such glaciers tend to be relatively insensitive toa warming climate and typically advance to lower alti-tudes than do nearby debris-free glaciers. Clark et al.

(1994) suggested that the steady-state AAR on such gla-ciers might be reduced from ca 0.65 to as little as 0.10.Estimates of paleo-ELAs for debris-covered glaciersbased on the AAR or THAR methods therefore mayresult in anomalous ELA values and produce erroneousregional ELA gradients.

2.7.6. ELA gradientAn error can be introduced in calculating !ELA if the

regional snowline gradient is not considered. If there isno present or past ELA gradient and both the modernand LGM ELAs are determined for glaciers on the oppo-site #anks of a mountain range, the !ELA on each #ankwill be the same (Fig. 2, !ELA

!). However, consider the

common case where the modern and LGM ELAs aredetermined for opposite #anks of a mountain (e.g., Por-ter, 1979) or mountain range (e.g., Porter et al., 1983),across which there is a marked precipitation gradient. Ifregional trend surfaces of present and past ELAS aredetermined, then !ELA may be less than if no gradientexists (assuming uniform lowering on both #anks) (Fig. 2,!ELA

#), or the !ELA on one #ank may di!er from that

on the opposite #ank if the modern and paleo-ELAgradients converge or diverge (Fig. 2, !ELA

$%"). Where

possible, therefore, trend surfaces of present and formerELAs should be calculated and their di!erence deter-mined in order to obtain the most reliable estimates ofELA depression.

2.7.7. Adjustment for lowered sea levelIn most ELA reconstructions, sea-level lowering at the

LGM is not considered. However, the eustatic fall of sealevel had the e!ect of raising the altitude of mountainsummits by the amount of the sea-level drawdown. As-suming that sea level fell ca 120 m (e.g., Fairbanks, 1989;Bard et al., 1990; Rohling et al., 1998), this amount shouldbe subtracted from the calculated !ELA to obtain anadjusted ELA depression with respect to the changingworld sea-level datum (Broecker, 1997). For example, ifthe present snowline (ELA

!) on a glacier lies at 3900 m

(Fig. 3) and the reconstructed ELA"

at 3000m, then theapparent !ELA"900m. However, during full-glacialtime, the ELA

"lay at an altitude of 3120m, rather than

3000m. Therefore, the di!erence between the presentELA

!(3900m) and full-glacial ELA

", (3120m) is 780 m

("!ELA adjusted for sea-level fall, designated !ELA%

in Fig. 3). The sea-level factor becomes relevant if !ELAis used in conjunction with an atmospheric lapse rate toestimate temperature depression during full-glacial time,for it will reduce the estimate by ca 10}15% (see below).

3. Tropical mountain glaciers (23.53N}23.53S)

Data on full-glacial snowlines are available for 18mountain areas in tropical Africa, Central and South


Fig. 2. Where no modern (ELA!) or full-glacial (ELA

") snowline gradients exist, !ELA is the same on opposite sides of a mountain or mountain range

(!ELA!). Where ELA gradients exist, the !ELA may be less (!ELA

#) or greater (!ELA

$) than in the no-gradient case, or may di!er on opposite sides

of a mountain or mountain range (!ELA", !ELA

&).

Fig. 3. When adjusted for fall of sea level from its modern level (SL!) to its full-glacial level (SL

"), !ELA is reduced by an amount equivalent to the

sea-level fall (!ELA%). A fall in sea level of 120m had the e!ect of raising the altitude of the summit and the reconstructed !ELA

"by this amount.

America, and several glaciated Paci"c islands (Fig. 4).Some permit only minimum estimates of snowlinedepression because the highest summit lies belowthe modern snowline, but for more than half, the full-glacial snowline depression can be calculated. In addi-tion, regional data on snowline depression have beengenerated for the tropical Peruvian, Bolivian, andChilean Andes.

3.1. Africa

3.1.1. Ethiopian highlandsThe highlands of Ethiopia, which reach altitudes of

more than 4000m, are too low to intersect the modern

snowline, but the highest summits developed glaciersduring the last glaciation. The Simen Mountains (13314#N) culminate in Ras Dejen (4543m), the highest moun-tain in Ethiopia (Fig. 5a). Hurni (1989) mapped moraines,cirques, and periglacial features that presumably date tothe last glaciation (no radiometric dates are available). Of20 former glaciers, those that formed in NW- to NE-facing cirques terminated as low as 3760 m; those occupy-ing S-facing catchments reached only as low as 4400m.Former ELAs were estimated on the basis of the medianaltitude of the glaciers (using end moraines and the top ofcirque headwalls), as well as the altitude of the upper endof lateral moraines, giving an average value of ca 4250m.Minimum !ELA was therefore 290m (Table 1).


Fig. 4. Map showing location of glaciated areas in the tropics where ELA"

and !ELA estimates have been obtained.

Glacial landforms (cirques, U-shaped cross-valley pro-"les, moraines, striations) were used by Potter (1976) tomap the extent of a former ice cap ('140km#) atopMt. Badda (7352# N; published altitudes are inconsistentand range from 4350 to 4133m), ca 160km southeastof Addis Ababa (Fig. 5b). End moraines in W-trendingvalleys were noted as low as 3650m. Lateral moraines,inferred to be of last-glacial age, reach as high as 4000m.If a summit altitude of 4350m is adopted, then minimumELA depression was 350m. A comparable minimum!ELA results if the median altitude method is used.

3.1.2. Kilimanjaro, TanzaniaMt. Kilimanjaro (3305#S), the highest mountain in Afri-

ca, now supports (5 km# of glacier cover. This volcanicmassif includes two summits; the highest, Kibo (5895m),lies west of a lower peak, Mawenzi (5147m) (Fig. 5b).During a succession of glaciations, glaciers on the vol-cano expanded to cover ca 153km# (Osmaston, 1989a).End moraines of the last (`Maina) glaciation on Kiboand Mawenzi reach as low as ca 3250m.

To determine snowline depression on Kilimanjaro,Osmaston (1989a) used a modi"cation of Kurowski's(1891) method, which assumed that net accumulation isa linear function of altitude and that the ELA lies at themean altitude of the glacier area. Osmaston included anarbitrary weighting factor to take possible nonlinearity ofthe accumulation trend into account, and concluded thatthis approach, which he called the Altitude}Height}Ac-cumulation (Alt}Ht}Acc) method, was likely to givemore reliable results.

Osmaston's (1989a) analysis disclosed an asymmetricaldistribution of glaciers, and an ELA

!that slopes gently

eastward on Kibo (5455}5360m) and lies at ca 5030m onMawenzi. ELA

"gradients slope west}northwestward

across Kibo (4540}4575m) and eastward across Mawenzi(4300}4240m), leading to unequal values of !ELA ondi!erent sides of the mountain (Fig. 5c). These he at-tributed to complex meteorological in#uences. Osmastonconcluded that a !ELA of 770$60m between the Mainglaciation and a Recent ice advance (i.e., middle-tolate-Neoglaciation) explained his results on most ofMazwenzi and Kibo. The calculated rise in ELA

!since

the Neoglacial maximum (a minimum of 60m) increasesthe !ELA to at least 830$160m (Table 1).

3.1.3. Ruwenzori, UgandaThe Ruwenzori Mountains (0320-25#N) reach altitudes

of more than 5000m and contain many small glaciersthat collectively cover ca 4.5 km# (Fig. 5d). During thelatest (Lake Mahoma) of at least three Pleistoceneglaciations, ice covered ca 260 km# and terminated aslow as 2070m on the eastern slope (Osmaston, 1989b).A date of 14,750$290 !"C yrBP[17,981 (17,647)17,315 cal yr BP] from Mahoma Lake, provides a min-imum age for moraines that impound the lake (Living-stone, 1962, 1975).

Osmaston (1989b) calculated ELAs of present andformer glaciers using the Area}Height}Accumulationmethod described above. He derived a su$cient numberof measurements to de"ne the regional ELA gradient,which descends to the east}southeast. The ELA

!along

an approximately west}east transect across the rangedescends from ca 4720 to 4270 m (Osmaston, 1989b,Fig. 7b), the estimated average ELA

!being ca 4600m.

During the Lake Mahoma glaciation the ELA"

sloped


Fig. 5. Sites in Africa where ELA"

and !ELA estimates have been obtained. (a) Ras Dejen, Simien Mountain, Ethiopia; (b) Mt. Badda, Ethiopia; (c)Kilimanjaro, Tanzania; (d) Ruwenzori Mountains, Uganda; (e) Mt. Kenya, Kenya; (f) Mt. Elgon, Kenya}Uganda, and Aberdare Mountains, Kenya.See text for details.


eastward from ca 4100 to 3600m (Osmaston, 1989b,Table 10). Along this W}E transect, the !ELA increasedfrom ca 620 to 670m.

3.1.4. Mt. Kenya, KenyaThe glaciers of Mt. Kenya (5202m; 0309#S) have been

shrinking in area and now cover (1 km# (Young andHastenrath, 1991). Modern ELAs are estimated to lie atca 4700}4725m (Mahaney, 1990, Fig. 11.8) and glaciertermini at 4650$100m. Moraines of the Liki II gla-ciation (Osmaston, 1989b; Mahaney, 1990, Table 11)extend as low as 3200 m, and are older than 15,000!"Cyr BP [18,003 (17,916) 17,830 cal yrBP]. Osmaston(1989b, Table 11), using the median altitude method,calculated an ELA

"of 4200m for the glaciers originating

on the highest summit (Batian), and a !ELA of 600 m(Osmaston, 1989b, Table 12). Mahaney (1990, Fig. 11.8),on the other hand, estimated that the full-glacial ELA,based on lateral moraine altitudes, lay between ca 3680(SW slope) and 4000m (NW slope) (Fig. 5e); most likely itaveraged close to ca 3700m. Based on his data, the!ELA ranged between ca 725 and 1020m. The discrep-ancy between Mahaney's results (Fig. 5e, Table 1) andOsmaston's may largely re#ect the di!erent methodolo-gies used.

3.1.5. Other glaciated African summitsTwo additional low-latitude mountains, each lying be-

low the modern snowline, developed large glaciers duringthe last glaciation (Osmaston, 1989b, Table 11). OnMt. Elgon (4320m; 1330#N), which had 75 km# of ice atthe glacial maximum [prior to ca 11,000 !"CyrBP;12,966(12,917)12,865cal yrBP], moraines extend as lowas 3350m. Osmaston (1989b) calculated that the ELA

"lay at 3600}3900m, which means a minimum !ELA of420}720m (Fig. 5f). In the Aberdare Mountains (4001m;0315}45#S), which had 23 km# of ice cover at the glacialmaximum, LGM moraines reach as low as 3200m.The calculated ELA

"is 3700 m and !ELA was '300 m

(Fig. 5f).

3.2. Mexico and Central America

3.2.1. Mexican volcanoesThe high stratovolcanoes of Mexico's Cordillera

NeovolcaH nica display evidence of repeated Pleistoceneglaciations. Two of the best-documented records arefrom IztaccmHhuatl (5286m; 19305}15#N) and Ajusco(3937m; 19312.5#N) (White, 1962; Heine, 1976, 1978,1984; White and Valastro, 1984) (Fig. 10). Initially, chro-nologies of glaciation were based on relative-dating cri-teria, limiting radiocarbon ages (primarily of paleosols,wood fragments, peat, and calcrete), and correlation withglacial sequences elsewhere (Heine, 1984). Heine (1984)reported evidence of glacier advances at ca 35,000

and 12,000 yrBP [ca 39,800 and 14,111 (13,992)13,880 cal yr BP], but none that correlated with the mar-ine isotope stage 2 maximum. White (1981), using rela-tive-age criteria, correlated the Hueyatlaco moraines(Diamantes Substage, Second Advance) on IztaccmHhuatland Santo TomaH s Substage moraines on Ajusco withPinedale moraines of the western United States that aregenerally regarded as correlatives of the isotope stage2 maximum (e.g., Richmond, 1986). Subsequently, Whiteand Valastro (1984) inferred that the Santo TomaH s drift ismore than 25,000 !"C yr (ca 29,000 cal yr) old, based ona single radiocarbon date of a bulk sample from theB horizon of a buried soil. The soil is developed on SantoTomaH s till and lies stratigraphically below tephra onwhich another buried soil is developed that dates to15,090$150 !"CyrBP [18,186 (18,009) 17,832 cal yrBP].Recent $'Cl surface-exposure ages for the outer Hueyat-laco moraines on ItaccmHhuatl indicate that the main LatePleistocene advance probably culminated 19,000}18,000$'Cl yr ago, and that the inner moraines are 15,000}14,000 $'Cl yr old (VaH squez-Selem, 1998).

White (1981) calculated modern and past ELAs forglaciers on the western slope of IztaccmHhuatl and thenorthern and eastern slopes of Ajusco. Based on meanaltitudes and an AAR of 0.65, he determined an averageELA

!of ca 4880m for glaciers on IztaccmHhuatl and

3970m for the Diamantes Second Advance, indicating anLGM snowline depression of 910 m (Fig. 6a). For Ajusco,which lies 65 km west of IztaccmHhuatl, White calculatedthat the ELA

"for the Santo TomaH s advance was 3270m.

This is ca 155}170m below ELAs calculated for twoNeoglacial ice advances and ca 665 m lower than thepresent ice-free summit. Absence of glacier ice on Ajusco(White and Valastro, 1984, Fig. 2) implies an ELA

!of

'3937 m, and a full-glacial !ELA of '665m (Fig. 6b).

3.2.2. Altos de Cuchumatanes, GuatemalaHastenrath (1974) reported evidence of glaciation in

the Altos de Cuchumatanes (15330#N), a carbonate karstupland that reaches altitudes of nearly 3800m (Fig. 6c).An end-moraine complex, with up to 20m of relief,descends to 3470}3600m. Hastenrath's published recon-naissance maps do not permit detailed topographic re-construction of the former glaciers. He estimated that theassociated ELA lay at ca 3650m, which is close to themedian altitude of the glaciated terrain. The correspond-ing minimum !ELA is 150m. Although no dates areavailable, Hastenrath considers the moraine character-istics comparable to those of presumed LGM age in themountains of Venezuela, Costa Rica, and Mexico.

3.2.3. Sierra de Talamanca, Costa RicaIn the Cordillera de Talamanca, culminating in Cerro

ChirripoH (9329#N; 3819m) and now below the regionalsnowline, two groups of moraines delimit former small


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1999

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1999

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uvia

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100

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Yes

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1999

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ian

And

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(0.4

5)51

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Yes

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inet

al.(

1999

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Chi

lean}B

oliv

ian

And

esTH

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(0.4

5)54

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1100

Yes

Kle

inet

al.(

1999

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oliv

ian

And

esTH

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(0.4

5)54

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900}

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Yes

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al.(

1999

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ics

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imum

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Fig. 6. Sites in Mexico and Central America where ELA"and !ELA estimates have been obtained. (a, b) IztaccmHhuatl and Ajusco, Mexico; (c) Altos de

Cuchumatanes, Guatemala; (d) Cerro ChirripoH , Costa Rica. See text for details.

valley glaciers (Weyl, 1956; Hastenrath, 1973) (Fig. 6d).The upper basins of the main glaciated valleys lie at3450}3550m and contain several small moraine- androck-dammed lakes. A !"C date of basal sedimentsfrom moraine-impounded Lago de Morrenas (3480m)indicates that the basin was deglaciated priorto 10,140$120 !"C yrBP [12,112 (11,808) 11,123cal yrBP] (Horn, 1993). At a site near El Empalme(2400m), ca 60 km northwest, the paH ramo (alpine) pollenzone dates to 20,750 !"Cyr BP (ca 24,200 cal yr BP) andrepresents a treeline depression of at least 650m (Martin,1964); possibly, this date is close to the time of maximumsnowline depression as well. Assuming that glaciersheaded close to 3700 m (i.e., just below most crest alti-tudes) and terminated as low as ca 3300}3400m (Hasten-rath, 1973; Orvis and Horn, 2000), the full-glacial ELA(median-altitude method) lay at ca 3500}3550m.This value is comparable to that recently derived byOrvis and Horn (2000) of 3506}3523m, and representsa minimum ELA depression of ca 295}315m (Fig. 6d).They reported that the annual 03C isotherm lies at4900m, or ca 1400m above the late Pleistocene ELA,

which therefore is a possible minimum value for ELAdepression.

3.3. Andes of South America

3.3.1. Sierra Nevada de MeH rida, VenezuelaSchubert (1974, 1984) and Schubert and Clapperton

(1990) reported evidence of multiple glacier advances inthe Sierra Nevada de MeH rida (altitudes to 5000m) be-tween 8315# and 9300#N latitude in the central VenezuelanAndes. He assigned the mapped drifts to several stades ofthe MeH rida Glaciation, of presumed Late Pleistocene age.The oldest recognized drift (Early Stade), which lacksdating control and has no clear morainal morphology,may date to marine isotope stage 4 or 6 (Clapperton,1993, Table 14.1a). Well-developed moraines of the lastglaciation (Late Stade) form a nested and overlappingsuccession between 3000 and 3500m altitude. A min-imum age of 12,650$130 !"CyrBP [15,128 (14,874)14,621 cal yr BP] was obtained for peat upvalley from theyoungest of these moraines. Peat beneath and abovethick outwash just beyond the outer moraine limit is


dated 19,080$820 and 16,500$290 !"C yrBP [22,700and 19,736 (19,423) 19,153 cal yrBP], respectively, point-ing to an isotope stage 2 age for the Late Stade. A datefor basal peat on the #oor of a cirque demonstratesdeglaciation near the range crest by 11,470 !"CyrBP[13,463 (13,384) 13,317 cal yrBP] (Schubert and Clapper-ton, 1990).

Schubert and Valastro (1974) mapped the glacial geol-ogy in the PaH ramo de la Culata (northern VenezuelanAndes), where end moraines of the last glaciation aretraceable as low as ca 3150m and cirque headwallsupvalley average 4450m. Based on the median altitudemethod, the ELA

"for Pico BolmHvar (&5000 m) would

have been at ca 3800m (Fig. 7a). This is about 900 mbelow the modern snowline [ca 4700m according toSchubert (1974), but more recently above 4700 m(Schubert, 1998)]. However, this !ELA value should beconsidered only approximate, for the ELA gradient hasnot been considered and the modern snowline value isonly an estimate.

3.3.2. Cordillera Central, ColombiaGlaciers and perennial snow"elds on the high Andean

volcanoes Nevado del RumH z (5200m), Nevado de SantaIsabel (4950m), and Nevado del Tolima (5200m) coverca 36 km# between latitudes 4335# and 5310#N (Herd,1974, 1982). According to Hoyos-Patin$ o (1998), thesnowline on Nevado del RuH iz lies at 4900 m on thewestern #ank and 4800m on the eastern #ank (Fig. 7b).On Nevado de Santa Isabel, the snowline lies at 4800 mon the western #ank and 4700 m on the eastern #ank(Hoyos-Patin$ o, 1998) (Fig. 7c). These values are60}170m higher than those measured by Herd (1974,1982) in 1972}73. Herd's values were based on averagetransient summer snowline, which was estimated to lie ca100m below the actual snowline.

Herd (1974) mapped the approximate extent of latePleistocene glaciers on the massif and obtained a date of13,760$150 !"C yrBP [16,705(16,498)16,286cal yrBP]for peat directly beneath a tephra that overlies the outer-most drift of the last glaciation. His mapped glacier limitapproximates the closely nested limits of the early andlate Murillo drifts of Thouret et al. (1996). The lateMurillo advance predates basal peat overlying the mo-raines having an age of 16,220$80 yrBP [19,247(19,100) 18,970 cal yrBP], while the early Murillo mo-raines likely date between 28,000 and 21,000 !"CyrBP[28,000 and 23,100 cal yrBP] based mainly on regionaltephrochronology. In the Eastern Cordillera of Colom-bia, the greatest advance of the last glaciation is believedto have culminated between ca 23,500 and 19,500!"Cyr BP (ca 28,000 and 23,100 cal yrBP) (Helmenset al., 1996).

Herd (1974) determined the late Pleistocene ELA usingthe median-altitude method, assuming that ice divides ofthe full-glacial ice caps on the volcanoes represented the

headward limits of individual ice tongues. The recon-structed ELA on Nevado del RumHz, along a section ob-lique to the modern ELA gradient, rose from ca 3550 mon the eastern slope to ca 4000m on the western slope.On Nevado de Santa Isabel, the Late Pleistocene ELAalong the same transect as the present ELA gradient rosefrom ca 3500 to 4000m (Herd, 1974). In both cases, Herdcalculated the !ELA as ca 950$50m. However, if themodern snowline values of Hoyos-Patin$ o (1998) are used,the mean !ELA increases to 1075 for Nevado del RumH zand 1000m for Nevado de Santa Isabel (Figs. 7b, c).

3.3.3. Ecuadorian AndesThe Ecuadorian Andes comprise two parallel ranges,

the Cordillera Occidental (western range; 0322#N}1329#S)and the Cordillera Oriental (eastern range; 0301#N}2320#S), separated by intermontane basins and formingdissected plateaulike surfaces at 3500}4000m altitude(Clapperton, 1987) (Fig. 7d). The ranges are surmountedby 14 glacierized stratovolcanoes more than 4600 m high(Jordan and Hastenrath, 1998). Clapperton (1987, 1993)summarized the glacial geology of the mountains andnoted that deposits of `full-glaciala age typically include3}4 lateral and/or terminal moraines that date broadly towithin the interval 34,000}12,000 !"C yrBP [38,700 and14,111 (13,992) 13,880 cal yr BP). In discussing his ap-proach for determining ELA depression, he noted thatsome earlier studies used cirque-#oor altitudes to calcu-late !ELAs. He cautioned that during the glacial max-imum, most cirques were buried under continuousice"elds, implying that cirques likely originated duringearlier intervals of reduced glacier cover. Therefore, suchcirques cannot be used for constructing former snow-lines. Exceptions were mountain ridges in southern Ecua-dor that are too narrow to have supported ice"elds or icecaps.

Clapperton (1987) estimated the ELA!

based on "eldobservations and aerial photographs, deriving values of4800}4000m for Chimborazo (6310m) in the CordilleraOccidental and 4600}4830m for Antisana (5790m) in theCordillera Oriental (Fig. 7d). The values for ELA

!on

Chimborazo volcano compared closely with those basedon the upper limits of lateral moraines. He used themedian-altitude method to calculate ELA depressionalong transects across the two cordilleras, noting, how-ever, that the results were `crude approximationsa be-cause the estimated modern ELAs were imprecise andthe reconstructions were based on topographic mapswith a large (40m) contour interval. In both cordilleras,eastward-sloping ELA

!and ELA

"gradients are present,

and !ELA is greatest on the eastern #ank of each range(Fig. 7d). Mean !ELA values increase from west to eastacross the Andes, from 810}920m in the Cordillera Occi-dental to 970}1120 in the Cordillera Oriental (based onvalues in Clapperton's Fig. 7; values in Clapperton'sTable 2 are somewhat greater).


Fig. 7. Sites in tropical South America where ELA"

and !ELA estimates have been obtained. (a) Pico BolmHvar, Venezuela; (b, c) Nevado del RumH z andNevado de Santa Isabel, Colombia; (d) Chimborazo and Antisana, Ecuadorian Andes; (e) Cordillera Blanca and Cordillera Oriental, Peruvian Andes.See text for details.

3.3.4. Peruvian AndesStudies of snowline depression in Peru have centered

mainly in the Cordillera Blanca (7340#S) and Cordillera

Oriental (8308#}9358#S) (Fig. 7e). Rodbell (1992) used theTHAR method and derived measurements for ca 20glaciers in each mountain range using 1 : 100,000-scale


topographic maps with 50m contours, supplemented bymeasuring on 1 : 25,000-scale aerial photographs the up-per limits of preserved lateral moraines. Results obtainedusing THAR values of 0.2 and 0.4 typically di!ered by ca150}350m. The accuracy of resulting ELAs was esti-mated to be $50m. The age of full-glacial moraines isunknown, but radiocarbon dates from moraine-dammedlakes and bogs provide minimum ages for deglaciationin the Cordillera Oriental of 12,100$190 !"CyrBP[14,382 (14,114) 13,870 cal yrBP] and in the CordilleraBlanca of 13,280$190 !"CyrBP [16,140 (15,859)15,559 cal yr BP].

3.3.4.1. Cordillera Oriental. Rodbell (1992) obtainedELA

"values for former glaciers lying east and west of the

main divide, as well as west of a local divide lying ca20km west of the main divide (Fig. 7e). No glaciers arepresent in the study area, but ELA

!is based on a re-

gional glaciation threshold estimate of 4620m (Seltzer,1987). Paleoglaciers on the western side of the localdivide had an average ELA

"of 3850-3900m, represent-

ing a !ELA of 750}950m. Corresponding ELA"

valueswest and east of the main divide are 3540}3640 and3150}3300m, respectively. The !ELAs are 900}1150 and1100}1350m, respectively, with estimated mean values ofca 850}1000 and 1200m for the two respective sides ofthe range (Rodbell, 1992). These results demonstratea W}E snowline gradient, with snowline depression in-creasing toward the Amazon Basin, which is, and appar-ently was, the primary source of precipitation.

3.3.4.2. Cordillera Blanca Rodbell's (1992) estimates ofELA

!in the Cordillera Blanca range from 4985$120 m

west of the divide to 5030$110 m east of the divide,indicating no discernable gradient (Fig. 7e). The cal-culated ELA

"is 4400$100 to 4250$110 m

(THAR"0.2 and 0.4, respectively) west of the divide and4200$170 to 4350$150m east of the divide, giving anaverage of ca 4300 m for the range as a whole. Thecalculated !ELA is 440}900 and 530}970m, respectively.Rodbell (1992) suggested that, based on comparison withELA

!and glaciation threshold values, the average

!ELA for the range is ca 700m.

3.3.4.3. Regional Andean reconstructions. Nogami(1976) compared the modern snowline along the entireAndean cordillera (103N}553S), based on the altitude ofexisting glaciers shown on aerial photographs (pre-1976),with a Pleistocene snowline reconstructed using the cir-que-#oor method. The regional pattern disclosed thatpast and recent snowline surfaces rise westward betweenthe northern equatorial Andes and northern Chile(ca 303S), at which latitude a shift occurs to northeast-ward-rising snowlines. The di!erence between Nogami'stwo surfaces (!ELA) is less than 1000m.

Fox and Bloom (1994) made a regional study of LateQuaternary snowlines in the Peruvian Andes (5}173S).They based their estimate of modern snowline altitude onthe lower limit of snow depicted on 1 : 100,000-scale aer-ial photographs (1955}63). The data were transferred totopographic maps (50-m contour interval), and an errorof $100m was assumed. They showed that the snowlinerises from ca 4700 m in the northern and eastern Andesto more than 5300m in the south and west, and hasa westward-rising gradient, especially in the north. Theoverall pattern is similar to that derived by Nogami(1976). Full-glacial snowline was reconstructed based onthe cirque-#oor method, assuming that glaciers occupiedthe lowest cirques contemporaneously. The time of thisoccupation, however, has not been dated. The recon-struction indicates that snowline depression reacheda maximum of 1400$200m on the eastern side of thePeruvian Andes but decreased westward to a minimumof 600m in the western ranges and on most of thePeruvian Altiplano (Fig. 8). An average value was notgiven, but based on their Fig. 7 it appears to be in therange of 800}1000m in the north, decreasing to600}800m in the south.

Klein et al. (1999) subsequently expanded on the workof Fox and Bloom (1994) to include not only the Andes ofsouthern Peru, but also Bolivia and northern Chile(Fig. 8). They adopted Fox and Bloom's estimate of themodern snowline altitude in Peru (see above), and usedLANDSAT Thematic Mapper imagery to map the lowerlimit of snow cover in the southern part of their studyregion. Maps with scales of 1 : 50,000 (20m contours) and1 : 250,000 (250m contours) were used. They noted thelack of adequate dating control for the last glaciation, butassumed that moraines mapped as (local) LGM wereconstructed contemporaneously. In Peru, cirque-#ooraltitudes were used to determine LGM snowlines; how-ever, the age of the last cirque glaciation is unknown, andso their assumed age equivalence is unproven. Further-more, many cirques lay at the heads of valley glaciers. Inthe southern region, an adjusted THAR of 0.45 was usedto calculate speci"c snowlines, as well as a regional snow-line pattern for the last glaciation. In the area where datafrom the two methods overlap, the di!erence in estimatedELA between the methods is ca 175m. The con"gurationof the reconstructed glacial snowline is similar to that ofFox and Bloom's (1994) modern snowline. Their cal-culated snowline depression over the tropical Andesaveraged 920$250m. However, consistent with the con-clusions of Fox and Bloom (1994), the calculated !ELAis ca 1200 m in the eastern cordillera of Peru and Bolivia,and 500}800m to the west [see also Seltzer (1992, 1994),who calculated an ELA depression of only 300$100 mon the western slope of the Cordillera Real]. In the aridranges along the border of Bolivia and Chile, snowlinedepression reached 1000}1200m (Klein et al., 1999,Fig. 7).


Fig. 8. Map of the Peruvian, Bolivian, and northern Chilean Andesshowing pattern of regional ELAg surface (after Klein et al., 1999). Boldlines perpendicular to trend of isolines show arbitrary transects depic-ted in Fig. 10 and included in Table 1.

3.4. Pacixc Islands

3.4.1. Taiwan Shan, TaiwanAlong the Taiwan Shan, which forms the high crest of

Taiwan, 62 peaks exceed 3000m altitude; the highest, YuShan, reaches 3997m. Kano (1934}35) identi"ed 35 cir-ques in the northern sector of these mountains, lyingmainly on the eastern side of the range. Moraines oncirque #oors and beyond cirque thresholds de"ne thelimits of former cirque and valley glaciers, as well asa small ice cap. Although no dates were available, Kanoassigned the landforms to the last glaciation and notedthat cirque altitudes range from ca 3500 to 3730m. Hesuggested that glaciers extended down to 3300m on thenorthern and eastern slopes of the highest peak, but wereca 300m higher on the southern and western sides of therange.

Ono (1988, Fig. 1; Y. Ono, pers. comm. 2000) estimatedthe full-glacial snowline on Taiwan to lie at ca 3400m,midway between an ELA

"of 3350m on Xue Shan and

3450m on Yu Shan (to the south), derived using thecirque-#oor and glaciation threshold methods (Fig. 9a).Based on this "gure, and an inferred average crest alti-tude of 3800m, the !ELA was '400m.

3.4.2. Mauna Kea, HawaiiThe summit of Mauna Kea (4206m; 19350#N) on the

island of Hawaii lacks perennial glacier ice. However, at

the LGM, an ice cap covered 70 km# of the upper slopesabove ca 3200m altitude (Porter, 1979) (Fig. 9b). Twosurface-exposure ages have been obtained for boulders atthe surface of the youngest (Makanaka) drift and one forglacially abraded rock near the summit (20,300$2300,18,900$800 yr, and 14,700$500 $'Cl yr, respectively;Dorn et al., 1991). These imply that the LGM occurredduring marine oxygen-isotope stage 2 and that the sum-mit was deglaciated by ca 15,000 yr ago.

Based on the AAR method, the full-glacial ELA, cor-rected for ca 35m of postglacial isostatic subsidence(2.5m/10$ yr; Porter, 1979), averaged ca 3780m and hadan eastward-sloping gradient. The minimum !ELA wasca 425m. The July freezing isotherm now lies close to4715m, about 500 m above the summit, and likely ap-proximates the ELA

!. Assuming a comparable relation-

ship between July temperature and ELA"

during theLGM, and using an AAR of 0.60$0.05, !ELA was935$190m.

3.4.3. Mt. Kinabalu, BorneoGlacial-erosional features below the summit of Mt.

Kinabalu (4101m; 6305#N) on the island of Borneo werereported by Koopmans and Stau!er (1967) and Stau!er(1968). They estimated a glacial-age snowline of3735$75m (12,000}12,500 ft) for the mountain basedon the median-altitude method (Fig. 9c). The downvalleyextent of ice was inferred from possible moraines at ca2835 and 3230 identi"ed on aerial photographs. Theglacial deposits have not been dated. If the landforms arecorrectly identi"ed as moraines and the higher one datesto the LGM, as inferred here, then based on the median-altitude method, the average ELA

"lay at ca 3665m and

!ELA was at least 435m. Koopmans and Stau!er esti-mated that the ELA

!lies at ca 4570$150m, which

would imply a !ELA of 905$150m during the lastglaciation.

3.4.4. Papua New GuineaNew Guinea is the only equatorial Paci"c island

(5}93N) with numerous highlands (ca 3800}4500m) thatgenerated a Late Pleistocene glacier cover. Most glacial-geologic studies have focused on the eastern half of theisland (Papua New Guinea), which is more accessiblethan Irian Jaya to the west. Although no glaciers exist inthe eastern highlands, LoK %er (1972) inferred that thesnowline lies at ca 4600m, the reported altitude of thesnowline in the glaciated areas of Irian Jaya (Verstappen,1964; Allison, 1976). The LGM snowline was determinedby LoK %er (1972) using the arithmetic mean of the alti-tudes of terminal moraines and the mean altitude of thecatchment area, as well as the altitudes of the lowestcirque #oors (Fig. 9d), and is similar to estimates basedon the glaciation threshold (LoK %er, 1971). Bowler et al.(1976) estimated the age of the LGM to be ca18,000}16,000 !"CyrBP [21,648 (21,484) 21,313}18,972


(18,876) 18,784 cal yr BP] based on pollen-derived esti-mates and limiting !"C ages from two highland sites.Hope and Peterson (1976) reported that maximum de-pression of vegetation zones occurred 18,500}16,000!"Cyr [22,000}18,972 (18,876) 18,784 cal yr] ago, andthat in several areas substantial ice retreat had occurredby 14,500}14,000 yr [17,453 (17,369) 17,287}16,879(16,792) 16,704 cal yr] ago.

Mt. Giluwe (4368m), a dome-shaped stratovolcano at63S latitude, was mantled by a large ice cap (188 km#)during the last glaciation. Blake and LoK %er (1971) de-scribed end moraines that form concentric belts aroundthe mountain as low as 2750}3000m, and alluvium, in-terpreted as outwash, that overlies peat having an age of23,600$1100 !"C yr (ca 27,400 cal yr) old. LoK %er (1972)calculated an ELA

"of 3500}3550m (Fig. 9d), which

represents a minimum !ELA of ca 820}870m.For other areas of less-extensive glaciation lying be-

tween ca 53 and 93S latitude, LoK %er (1972) estimated theELA

"to lie between 3500 and 3700m (Fig. 8d). However,

recent studies in the Sarawaged Range, using moderntopographic maps, suggest an ELA

!as low as 3400 m

(M. Prentice, pers. comm., 2000). If a snowline gradientexisted across New Guinea, it was very gentle and cannotbe de"ned on the basis of available data. Adopting Ver-stappen's (1964) value of 4600m for the modern snowline,LoK %er (1972) concluded that snowline depression inthese areas during the last glaciation was ca 900}1100m.

4. Discussion

As summarized above, data bearing on snowline de-pression in tropical latitudes are restricted to easternAfrica, Central and South America, and several Paci"cislands. Di!erent methods have been used in derivingELA

!, ELA

", and !ELA, but the results are not strictly

comparable. The AAR method, often regarded as themost reliable and consistent, has been used infrequentlyin tropical snowline studies. Altitude ratios (median alti-tude, THAR, Alt}Ht}Acc) have been employed in mostcases. In the few instances where lateral moraines wereused to de"ne former ELAs, the results were similar tothose obtained using altitude ratios. Cirque-#oor alti-tudes likely are the least-reliable method, especially with-out associated "eld studies and adequate dating control.Although cirques may record some average regional levelof glacial conditions (e.g., Porter, 1989), they do notnecessarily record a synchronous glacial event, such asthe LGM. Di!ering methodologies, therefore, introducepotential variance to !ELA estimates, the magnitude ofwhich is di$cult to assess. In some temperate-latitudestudies (e.g., Meierding, 1982), methodological di!erencesamounted to 100m or more in calculated ELA

"; compa-

rable di!erences of 100}200m probably should be ex-pected in tropical snowline studies.

4.1. Estimates of modern ELAs

Estimates of the modern snowline altitude probablyconstitute the least-reliable component of the !ELA cal-culations. In many cases, summit altitude is used asa minimum altitude for the modern snowline, recogniz-ing that the ELA

!must lie some unknown distance

above a nonglacierized summit. In other cases, the alti-tude of the modern snowline is estimated, either based onlimited "eld work (usually by noting the level of thetransient snowline during some part of the ablation sea-son), by assuming a relationship between the snowlineand the summer (July Northern Hemisphere) or annualmean freezing isotherm based on radiosonde data, or byusing the mapped limits of snow and glacier cover depic-ted on published topographic maps. In the latter ap-proach, generally it is assumed that snow/ice conditionsshown on an array of maps that cover an area or regionare essentially contemporaneous and represent a steadystate, and that the contour interval is suitable for relative-ly high-resolution interpolation between contours (e.g.,)30 m).

The lack of a consistent and rigorous method of deter-mining the modern snowline in the tropics could intro-duce an error of up to several hundred meters in some ofthe reported !ELA results. Furthermore, in a time whenglobal climate is warming, snowline values measuredseveral decades apart may di!er by tens of meters. Poten-tial errors may also occur when the ELA

!is inferred to

lie at the level of the summer freezing isotherm, if thisassumed relationship is not always valid.

4.2. Reconstructed Pleistocene ELAs

Data from 26 sites in the tropics permit reconstruc-tions of full-glacial snowline (ELA

") (Table 1; Fig. 10).

However, for only 11 of these has the ELA"gradient been

determined; nevertheless, in only two cases (IztaccmHhuatland Cordillera de MeH rida) may the lack of a calculatedsnowline gradient have introduced a signi"cant errorinto the !ELA calculation.

For most localities or regions, the age of the recon-structed ELA

"is unknown, but it is commonly assumed

to equate with the `globala LGM (i.e., ca 21,000}15,000!"Cyr BP; ca 24,400}17,453 (17,369) 17,287 cal yrBP).Available radiocarbon dates (Table 1) generally areinadequate to verify whether the reconstruction rep-resents full- or late-glacial (or even pre-LGM) condi-tions.

In South America, regional reconstructions for theAndes of Peru, Bolivia, and Chile (Nogami, 1976; Foxand Bloom, 1994; Klein et al., 1999), mainly using thecirque-#oor method, add additional data for the tropicsthat supplement information from speci"c areas. Despitethe acknowledged uncertainties and assumptions in-volved in these studies, including a potential error of


Fig. 9. Sites in tropical Paci"c islands where ELA"and !ELA estimates have been obtained. (a) Taiwan Shan, Taiwan; (b) Mauna Kea, Hawaii; (c) Mt.

Kinabalu, Borneo; (d) Papua New Guinea. See text for details.

$100}200m at any site, they serve to emphasize thepattern, signi"cance, and overall consistency of regionalELA gradients along and across mountain systems.

4.3. Snowline depression

For 12 of the tropical sites the modern ELA has beendetermined or estimated with su$cient con"dence that!ELA at the LGM can be calculated. For the others, thereported values are approximate, or only minimum esti-mates (Fig. 10; Table 1).

To assess snowline depression, the data are dividedinto two groups. The "rst group (mainly north of 103Slatitude) includes speci"c estimates at tropical sites inAfrica, the Americas, and Paci"c islands (Fig. 10). Thetwo summits of Kilimanjaro are considered a single lo-cality, and average values are used for the CordilleraBlanca and the Cordillera Oriental reported by Rodbell

(1992). The second group (mainly south of 103S latitude)includes regional reconstructions spanning segments ofthe tropical (central) Andes between 73 and 223S latitude.The mean !ELA for the "rst set of data (n"12) is900$135m. The Ruwenzori forms an outlier from theotherwise reasonably tight data set; if it is excluded, theresulting mean is not statistically di!erent at 1"(925$115m).

The second group of data (n"8) is represented by onecomposite data set (Fox and Bloom, 1994; not plotted inFig. 10) and 7 transects parallel to ELA gradients at 23latitude intervals, with values derived from Klein et al.(1999, Fig. 7) (Fig. 8; Table 1). These data illustrate a sub-stantial regional range of !ELA values, especially perpen-dicular to the trend of the Andes. Through this sector ofthe cordillera, as far south as 183S latitude, the ELA

"is

lowest, and !ELA values are greatest, on the Amazonianslope. Although the !ELA varies regionally, Klein et al.


Fig. 10. (a) Modern and full-glacial ELAs for areas in the tropics (23.53N}23.53S latitude) that supported Pleistocene mountain glaciers. Wheresummits now lie below the snowline, minimum ELAs are shown as the summit altitude. In some cases ELA

!and (or) ELA

"are shown with a range of

values, primarily resulting from ELA gradients across mountains or mountain ranges. See text and Table 1 for details. (b) Full-glacial snowlinedepression (!ELA) for tropical mountains and mountain ranges that supported Pleistocene glaciers. Minimum values represent summits that lie belowthe modern snowline. In areas where ELA gradients exist, the number shown is the median of a range of values. Areas north of about 103S latitude havea mean !ELA of 900$135m, whereas a regional study of Andes south of this latitude produced an estimated mean of 920$250m (Klein et al., 1999).

(1999) calculated an average value of 920$250m, themean being close to that of the "rst group of data dis-cussed above (Fig. 10b). In the following discussion, thevalue for the "rst data set will be used to represent globaltropical snowline depression during the LGM.

A snowline depression of 900$135m for tropical gla-ciers at the LGM is similar to estimates obtained formany temperate-latitude late Pleistocene mountain gla-ciers in both the Northern and Southern hemispheres(e.g., Porter, 1975; Porter et al., 1983; Furrer, 1991).Departures from the general average may be related toany of a number of factors, including local variations in

temperature depression, distance from precipitationsources, or nonuniform changes in accumulation (i.e.,precipitation) and radiation (as in#uenced by cloudiness,surface albedo, and topographic shading) in di!erentareas. The close similarity of derived !ELA values fortropical and temperate latitudes argues for a funda-mental temperature control of !ELA and implies a reas-onably consistent decline of air temperature during theglacial maximum in extra-polar alpine regions near mari-time sources of precipitation.

As discussed earlier, because sea level was ca 120 mlower than today at the LGM (Fig. 3), a !ELA of


900$135m is equivalent to a !ELA%of 780$135m as

a result of the changing ocean reference level.

5. Paleotemperature inferences from snowline data

Paleotemperature values based on snowline depress-ion are frequently cited, but often uncritically. As Seltzer(1994, p. 159) has emphasized, `climatic interpretations ofELA depression will always lack unique solutions be-cause of the complexity of the problema (see also Porter,1977). The most common approach has been to calculatelowering of temperature (usually annual, summer, orJuly, but not always clearly stated) based on an assumedLGM atmospheric temperature lapse rate. Inferred lapserates vary widely. For example, LoK %er (1970) applieda lapse-rate range of !5 to !63C/km in the highlandsof New Guinea; Porter (1979) used a lapse rate of!5.33C/km for Mauna Kea, Hawaii; Clapperton (1987)applied a lapse rate of !6.53C/km in Ecuador, a valuealso used by Rodbell (1992) and Seltzer (1987) for thePeruvian Andes; Wright (1983) and Osmaston (1989a)used a lapse rate of !73C/km in Peru and East Africa,respectively; and Fox and Bloom (1994) used a nonlinearlapse rate for the tropical Andes (!6.53C/km at!3.5 km altitude to nearly !103C/km at 6 km). Hos-tetler and Mix (1999) adopted a `nominal tropical lapseratea of !5.5 3C/km. This range in lapse-rate values(!5.3 to !103C/km) by itself translates into a 4.53Crange of values for temperature lowering, assuminga snowline depression of 1000m.

Assuming a mean tropical lapse rate of !6$13C/km,and no change in precipitation, an average snowlinedepression of 900$135m translates into a full-glacialmean temperature depression of 5.4$0.83C. Using thefull range (550}1400m) of reported tropical !ELA(Table 1), the lapse-rate approach produces temperaturedepressions ranging from 3.3 to 8.43C. Adjusted for sea-level lowering of 120 m, average temperature depressionis 4.7$0.83C.

In these simple, straightforward calculations, changesin the accumulation component of glacier mass balancehave been ignored. Intuitively, it would seem to be animportant factor in some alpine regions. However, Sel-tzer (1994) assessed its importance and concluded thatrelatively large changes in precipitation would be re-quired to a!ect ELA substantially. In tropical areas withhigh precipitation, the limiting control on glacier extentlikely is the altitude of the 03C isotherm (Hostetler andClark, 2000). Of equal or greater importance may be thelow seasonality of tropical climates, which leads to a rela-tively constant height of the freezing isotherm (Kleinet al., 1999). At these latitudes, ELAs that lie above thelevel of the 03C isotherm are sensitive to accumulationchanges, for above this level all precipitation falls assnow. At lower altitudes, as temperature rises above 53C,

precipitation falls as rain. In the Andes, for example, theprecipitation gradient is not uniform: rainfall reachesa maximum at ca 1000m altitude, above which it de-creases. A drop in freezing level, therefore, may converta larger percentage of the precipitation to snow andsigni"cantly increase accumulation, without a changein net precipitation. In this way, a relatively uniform dropin air temperature might produce regionally variableglacier mass balances leading to nonuniform ELAdepression.

It is apparent that estimating paleotemperatures usingsnowline data involves some substantial uncertainties.Not only are there pitfalls in the use of di!erent method-ologies, as well as signi"cant potential ranges of error,but the multiple factors that control glacier mass balanceand ELA do not permit an unequivocal and uniquepaleotemperature solution. Nevertheless, the regionaland global averages for tropical data suggest that thesimplistic lapse-rate approach may at least providea "rst-order approximation of regional and global tropi-cal temperature reduction at the LGM.

6. Other tropical LGM paleoclimate data and modelsimulations

The mean paleotemperature values based on snowlinedepression are in general accord with other paleotem-perature estimates from tropical land areas that suggestfull-glacial temperatures were substantially lower thantropical warm-season SSTs (Fig. 11).

The CLIMAP Project Members (1976, Fig. 3; 1981)derived a mean LGM tropical SST cooling of ca 1}33C,relative to modern (Fig. 11). More recent studies of thetropical oceans report LGM SSTs that were 1.7$0.7 to2.8$0.73C lower than present based on alkenone data(Lyle et al., 1992; Sikes and Keigwin, 1994; Bard et al.,1997) and Mg/Ca data (Lea et al., 2000), broadly consis-tent with mean CLIMAP estimates (Fig. 11). In contrast,oxygen-isotope and Sr/Ca data from corals at Barbadosimply LGM SSTs 5 to 63C colder than now (Guildersonet al., 1994). Crowley (2000), however, has questioned theinterpretation of the Sr/Ca data, noting that an SSTdepression of this amount would leave ice-age corals ator below their limit of habitability.

A variety of terrestrial climate proxy data suggests thatLGM temperature lowering was greater over land thanover the ocean. Representative estimates (Fig. 11 andTable 2), which are based on pollen data, noble-gasvalues in groundwater, and oxygen-isotope records inglacier ice, range from !5 to !123C (for additionaldata, see Farerra et al., 1999). Plotted with these temper-ature estimates in Fig. 11 are values based on !ELA and!ELA

%. These average snowline-based estimates, at 1

standard deviation, fall close to many of the other esti-mates of terrestrial temperature lowering, and therefore


Fig. 11. Estimates of temperature depression during the last glaciation (!Tg) at representative tropical sites, based on various climate proxies,compared with CLIMAP tropical sea-surface temperature (SST) di!erence between modern August and August 18,000 !"C yrBP (CLIMAP ProjectMembers, 1976, Fig. 3). Temperature depression based on average tropical !ELA is shown by bold line and 1" range by light shading; value adjustedfor sea-level lowering (!ELA

%) is shown by dashed bold line, and 1 " range by dark shading. Error estimates (1"), when reported, are shown by vertical

dashed lines. 1* eastern Atlantic Ocean alkenone data (Sikes and Keigwin, 1994); 2* Indian Ocean alkenone data (Bard et al., 1997); 3* centralPaci"c Ocean alkenone data (Lyle et al., 1992); 4* western and eastern Paci"c Ocean Mg/Ca data (Lea et al., 2000); 5*Hawaiian foram !!(O data(Lee and Slowey, 1999); 6* Barbados !!(O and Sr/Ca data (Guilderson et al., 1994); 7* noble gases in Oman groundwaters (Weyhenmeyer et al.,2000); 8* pollen in Guatemala lakes (Leyden et al., 1993); 9* noble gases in Nigerian groundwaters (Edmunds et al., 1999); 10, 11, and 12* pollendata from Panama, Brazil, and Ecuador, respectively (Colinvaux et al., 1996); 13* noble gases in Brazil groundwaters (Stute et al., 1995); 14* !!(O ofHuascaran ice cap, Peru (Thompson et al., 1995); 15* pollen data from Brazil (Colinvaux et al., 1996); 16* noble gases in Nambia groundwaters(Stute and Talma, 1998).

Table 2Representative tropical SST and terrestrial paleoclimatic data for the LGM

Data No. Latitude Location !SST (3C) Data Reference

SST data1 03 E Atlantic Ocean !1.8 Alkenone Sikes and Keigwin (1994)2 203N}203 S Indian Ocean !1.7$0.7 Alkenone Bard et al. (1997)3 0357#N Central Paci"c Ocean !1 Alkenone Lyle et al. (1992)4 0319'}2348#N W & E Paci"c Ocean !2.8$0.7 Mg/Ca Lea et al. (2000)5 21.36#N Hawaiian Islands !2 Foram !!(O Lee and Slowey (1999)6 13315#N Barbados !5 Sr/Ca Guilderson et al. (1994)

Terrestrial Data7 23330#N Oman !6.5$0.6 Noble gas Weyhenmeyer et al. (2000)8 16355#N Guatemala !6.5 to !8 Pollen Leyden et al. (1993)9 11330'}13330#N Nigeria !6 Noble gas Edmunds et al. (1999)

10 93N Panama *!5 Pollen Colinvaux et al. (1996)11 03 Brazil !6 Pollen Colinvaux et al. (1996)12 63S Ecuador !6 Pollen Colinvaux et al. (1996)13 73S Brazil !5.4$0.6 Noble gas Stute et al. (1995)14 93S Peru !8 to !12 !!(O of ice Thompson et al. (1995)15 21}223 S Brazil !6 to !9 Pollen Colinvaux et al. (1996)16 24330#S Namibia !5.3$0.5 Noble gas Stute and Talma (1998)

support the conclusion that LGM surface land temper-atures typically were depressed more than SSTs of adjac-ent oceans. Similar values have been reported ina modeling study of LGM climate that used a global

coupled ocean}atmosphere model of intermediate com-plexity (Ganopolski et al., 1998). The simulation showedthat tropical land areas cooled an average of 4.63C (com-parable to ELA

%, Fig. 11), whereas SSTs between 203N


and 203S cooled by 3.33C in the Atlantic, 2.43C in thePaci"c, and 1.33C in the Indian Ocean.

Farerra et al. (1999) used a variety of climate-proxydata to calculate average cold-season cooling at theLGM that ranged from !2.5 to !3.03 at present sealevel to ca !63 at 3000m altitude, suggesting nonlinearlapse rates. Such a relationship is apparent in a recent risein the tropical freezing level, which is closely linked to anincrease in SST (Diaz and Graham, 1996). Whereas theobserved tropical SST change was ca 0.2}0.33C, temper-ature change at the level of the freezing isotherm, basedon a standard lapse rate of !63/km, was ca 0.63C, ortwo to three times as great (Crowley, 2000). Modi"cationof tropical lapse rates at the LGM could well be relatedto a change in the mean altitude of the tropical inversion,in turn associated with a shift in the position and strengthof subtropical high-pressure cells (e.g., Hostetler andClark, 2000).

Betts and Ridgway (1992) evaluated several factorsthat may be related to the discrepancy between tropicalsnowline depression and SSTs. They computed that a de-crease in average tropical SST by 23 at the LGM and anincrease in tropical sea-surface pressure by 14mbar (theresult of a 120m fall in sea level) would account for an800m depression of the freezing isotherm. Signi"cantly,this result is consistent with recent (post-CLIMAP) esti-mates of SST lowering cited above (1.7}2.83C) and themean !ELA

%(780$135m) derived in the present study.

The mean !ELA%

value is also close to !ELAs (ca800}870m) reported by Hostetler and Clark (in press)based on mass-balance modeling of several tropical gla-ciers in New Guinea, Hawaii, Africa, and the Andes.

This review has focussed on the derivation and assess-ment of tropical paleosnowlines. The limited quantityand limitations of the existing information point to theneed for additional studies to enlarge and improve thedata set. However, even with adequate data, translatingsnowline depression into estimates of land-surface tem-perature depression is likely to persist as a challengingproblem. Among the important questions yet to be an-swered are: (1) what was the degree to which the massbalance of tropical glaciers at the LGM was in#uencedby a change in precipitation? and (2) were LGM lapserates di!erent than today, and were they linear or nonlin-ear? At present, suitable evidence to answer these ques-tions remains elusive.

Acknowledgements

The initial draft of this paper was written while I enjoy-ed hospitality of Nick Shackleton in the Godwin Labor-atory, Cambridge University, and Lloyd Keigwin in theMcLean Laboratory, Woods Hole Oceanographic Insti-tution. I thank Matsuo Tsukada for translation of a Ja-panese article used in this review, and Geo! Seltzer for

information about his snowline studies in SouthAmerica Helpful comments by reviewers Alan Gillespie,Michael Prentice, and Geo! Seltzer are greatly appreci-ated.

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