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Snowline Depression in the Tropics During the Last Glaciation Stephen C

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Snowline Depression in the Tropics During the Last Glaciation Stephen C 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 potential errors in determining ELA depression involve estimates of modern snowline altitude, dating resolution, topographic reconstruction of former glaciers, orographic e!ects, the presence of rockfall debris on glaciers, and calculation of regional ELA gradients. Eustatic sea-level lowering during the last glaciation is an additional factor in#uencing estimates of ELA depression (!ELA). In cases where modern snowline lies above a mountain summit, only a minimum value for !ELA can be obtained. At 12 tropical sites in Africa, the Americas (to 103S latitude), and Paci"c islands, estimates of average !ELA range from 440 to 1400 m, but most fall in the range of 800}1000 m (mean $1""900$135 m). In a regional study of ELA depression in the southern tropical Andes (8}223S), an average !ELA of ca. 920$250 m has been reported. Based on the assumption that glacier mass balance was controlled solely by ablation-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 !120 m at the glacial maximum, this value is reduced 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, but contrasts with tropical sea-surface temperature estimates that are only 1}33C cooler than present. ! 2001 Elsevier Science Ltd. All rights reserved. 1. Introduction experiment, which used the CLIMAP SSTs as boundary conditions, and low-latitude terrestrial paleoclimate Recurring questions regarding the magnitude of tropi- proxy data. Their analysis employed pollen evidence and cal climate change during the last glacial age have estimates of snowline depression from four tropical sites emerged since publication of the CLIMAP Project Mem- (Hawaii, the Colombian Andes, equatorial Africa, and bers (1976, 1981) reconstruction of ice-age sea-surface New Guinea), and led them to conclude that the temperatures (SSTs). The CLIMAP reconstruction, CLIMAP reconstruction underestimated the amount of which focused on the last glacial maximum (LGM) re- tropical temperature depression, which likely amounted vealed large areas in the tropics to have had SSTs as to 5}63C. warm as, or even slightly warmer than, those of Renewed interest in this topic has been generated the present. The CLIMAP project considered the by evidence and modeling that point to colder tropical LGM to date to 18,000 C yr BP [21,648 (21,484) temperatures than those implied by the CLIMAP 21,313 cal yr BP; equivalent calibrated ages ($1") have reconstruction (e.g., Guilderson et al., 1994; Stute et al., been obtained using CALIB 3.03 (Stuiver and Reimer, 1995; Thompson et al., 1995; Bush and Philander, 1998; 1993) for ages (18,000 yr, and using Stuiver et al. (1998) Farerra et al., 1999). Basic to much of the discus- for ages '18,000 yr]. Rind and Peteet (1985) sub- sion about colder glacial-age tropics has been the sequently noted con#icts between ice-age paleotempera- question of snowline depression (e.g., Broecker, 1995; tures generated by a general circulation model Hostetler and Mix, 1999; Lee and Slowey, 1999), yet most of the snowline data used in the arguments has not been rigorously evaluated. Since Rind and Peteet (1985) questioned the CLIMAP conclusions more than a dec- * Tel.: #1-206-543-1904; fax: #1-206-543-3836. ade ago, additional information has emerged that now E-mail address: [email protected] (S.C. Porter). permits a more thorough assessment. This paper focuses 0277-3791/01/$- 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 1068 S.C. Porter / Quaternary Science Reviews 20 (2001) 1067}1091 on the set of paleoclimatic data bearing on snowline sometimes been used as a convenient proxy for former depression in the tropics during the last glaciation. In ELAs (e.g., PeH weH and Reger, 1972; Nogami, 1972, 1976; assessing these data, potential sources of error are also Fox and Bloom, 1994). While this approach is reasonable considered in arriving at site-speci"c and globally aver- in situations where Pleistocene glaciers terminated at aged values. cirque thresholds, in such cases the cirque glaciers disap- peared when snowlines rose above cirque levels at the 1.1. Glacier equilibrium-line altitudes end of the Pleistocene, meaning that site-speci"c ELA depression cannot be calculated directly. Furthermore, in The snowline, de"ned as the lower limit of perennial many glaciated tropical mountain ranges and on large snow on the landscape, is equivalent to the "rn limit volcanoes, glaciers expanded beyond cirques to form on temperate alpine glaciers, which is the lower limit valley glaciers, and in these circumstances ELAs lay of snow at the end of the ablation season. On such below (often well below) the altitudes of cirque #oors. In glaciers, the "rn limit approximates the equilibrium line, such cases, snowline reconstructions based on cirque- the locus of points along which the annual mass balance #oor altitudes may substantially underestimate actual is zero. In most recent paleosnowline studies, the equilib- snowline depression. rium line is regarded as synonymous with the snowline, and its altitude, following Meier and Post (1962), is designated the equilibrium-line altitude (ELA). The dif- 2.2. Upvalley limits of lateral moraines ference between the modern ELA (ELA ) and that of For a glacier in a balanced (steady-state) condition, some earlier time (e.g., the last glaciation, ELA") is a measure of equilibrium-line (i.e., snowline) depression the upvalley limit of its contemporary lateral moraines lies at the equilibrium line, below which ice-#ow paths (!ELA). The mass balance of a glacier, and #uctuations of the are diverging and ascending. If lateral moraines of a glacier's equilibrium line, are controlled by a number former glacier are well preserved, then the altitude of climate-related processes. For most low-latitude of their upvalley limits may closely approximate the temperate glaciers, the most important controls are accu- former ELA (Fig. 1b). Whereas this method has been mulation-season precipitation and ablation-season tem- used with success in some areas (e.g., Andrews, 1975; perature. Together these parameters encompass a range Mahaney, 1990), in many alpine regions lateral moraines of possible conditions controlling the ELA. Therefore, are absent or poorly preserved and at best provide only a unique value for past precipitation or temperature lower limiting estimates for contemporaneous ELAs. Meierding (1982) considered the highest lateral moraine cannot be derived from the !ELA alone (Porter, 1977; Seltzer, 1994). In most published paleosnowline studies, altitude to be the least reliable of several methods for no di!erence in precipitation is assumed (in most cases determining Pleistocene ELAs in the Front Range of probably erroneously) between the present and the Colorado. LGM, and a change in temperature is obtained by as- suming a "xed atmospheric lapse rate. In cases where independent evidence for one parameter (i.e., LGM pre- 2.3. Glaciation threshold cipitation or temperature) is available from another cli- The glaciation threshold (GT) for a speci"ed area (nor- mate proxy, then !ELA can provide an estimate of the other parameter. 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 and the highest without (Fig. 1c). Although this method is 2. Methods not applicable to isolated peaks, such as volcanoes, it has proved useful for assessing regional snowline In studies of glaciated low-latitude mountains, trends across mountain ranges (e.g., "strem, 1966; "ve common methods have been used to reconstruct Porter, 1975, 1977; Rodbell, 1992). Studies have shown former ELAs. Because the methods di!er in their that the GT essentially parallels the regional ELA trend, approach, the results they produce are not strictly but commonly lies 100}200 m higher (Meierding, 1982; comparable. S. C. Porter, unpublished data). A limiting problem when using the GT to determine Pleistocene snowline 2.1. Cirque-yoor altitude depression is the need to identify and map former gla- cierized and nonglacierized peaks for a speci"c time (e.g., When a glacier just "lls a cirque, its steady-state ELA the LGM) throughout a rather broad region. Such exten- typically lies not far above the average altitude of the sive "eldwork normally is impractical, and so subjective cirque #oor (Fig. 1a). Therefore, cirque-#oor altitude has assessments of the extent and age of past glaciation S.C. Porter / Quaternary Science Reviews 20 (2001) 1067}1091 1069 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 cirque glacier 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 lower than the cirque #oor. Lateral-moraine method: The upglacier limit of a lateral moraine approximates the ELA of the glacier that constructed the moraine.
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