Modern Equilibrium Line Altitude Research Articles

Controls on the altitude of Scandinavian cirques: What do they tell us about palaeoclimate?

Cirques are glacially eroded, bowl-shaped depressions, characterised by steep headwalls and flat or overdeepened floors. Given their association with past glaciers, cirques are sometimes used as proxies for palaeoclimate. However, cirques are shaped over multiple glacial cycles, and their usefulness as palaeoclimate indicators therefore remains open to question. In this paper, we map 3984 glacier-free cirques across the Scandinavian Peninsula and analyse variations in cirque floor altitude (CFA). We explore the relationships between CFAs and cirque aspect, latitude, longitude, and distance to the coast. We test the validity of using CFAs as indicators of palaeoclimate through comparison with the equilibrium-line altitudes (ELAs) of 513 modern cirque glaciers. Results indicate that both CFAs and modern cirque-glacier ELAs decrease with latitude and vary with aspect, being generally lowest on east-facing slopes. However, the clearest and strongest trend in both CFAs and modern cirque glacier ELAs is an increase in elevation with distance from the modern coast (i.e., distance ‘inland’). This likely indicates that similar climatic gradients, particularly an inland reduction in precipitation, acted to regulate former sites of glacier initiation (reflected by CFAs) and modern glacier ELAs. This would imply that CFAs are a useful proxy for palaeoclimate. However, we note that both CFAs and modern ELAs reflect the general topography of this region (with increasing elevations moving inland), and the glacial history of the area (indirectly linked to palaeoclimate) may have played a role in regulating where cirques have formed. For these reasons, we suggest that palaeoclimatic interpretations derived from CFAs should be treated with caution.

Controls on the lifespans of Icelandic ice caps

The increasingly common disappearance of glaciers is striking, and it is often used to highlight the effects of anthropogenic climate change to the public. This approach is more effective when modern glacier and climate change is placed within a frame of reference. Here we present a new, efficient method to assess the timing of glacier birth and death. We model equilibrium line altitudes (ELAs) of 22 Icelandic glaciers through time, and in doing so produce the first regional estimates of glacier lifespans. We force the model with three Holocene temperature reconstructions and five future-climate projections, while also exploring the effects of glacial-isostatic adjustment and variable precipitation. Mass balance parameters are tuned for each ice cap using modern data. Geologically-constrained inception ages are used to validate individual simulations. The timing of Icelandic glacier inception likely ranged from before the Holocene to as late as the Little Ice Age. Anthropogenic warming is expected to cut short the lifespans of glaciers that have existed for thousands of years. By quantifying the controlling factors we show that topography and the rate of summer temperature change are the primary controls of glacier lifespans in Iceland. Topography, represented by the difference between the modern ELA and the highest local topography, is ten-times as important as ELA sensitivity to temperature. This easy-to-measure topographic metric exerts a first-order control on the timing of inception and the future loss of accumulation area and it can be used to contextualize ongoing glacier disappearance.

Reconstruction of the Little Ice Age glaciers and equilibrium line altitudes in the Kodar Range, southeast Siberia

Little Ice Age (LIA), a time of global cooling widely manifested in the 14th-19th centuries, was characterized by worldwide expansion of mountain glaciers. Unfortunately, little is known about the LIA glacier changes in remote areas of continental Siberia. In this work we studied the evolution of the glaciers of the Kodar Range, southeast Siberia. These glaciers are mainly of small size (<0.5 km2), cirque and cirque-valley, with predominant northern and north-eastern aspects, located at elevations of 1900–2800 m a.s.l. Using field and satellite imagery data over the 2001–2013 period (Landsat ETM+, Cartosat-1, WorldView-2) we mapped the glaciers, equilibrium line altitudes (ELAs) and the LIA moraines. Modern ELAs vary from 1945 to 2615 m a.s.l. (mean value is 2286 ± 162 m) and demonstrate NNW to SSE increasing regional trend (from 2060 to 2430 m a.s.l.) with local ELA variability due to orography of ±114 m. The extents of the LIA glaciers were reconstructed from terminal moraines, located in front of the glacier termini. These moraines are of fresh morphology, non-vegetated, and locally dated by lichenometry at the mid-18th century. Since the LIA the total glacier area decreased from 19 km2 to 9 km2 (by 53%) and glacier termini retreated, on average, by 440 m. South and south-west facing glaciers shrunk 1.5–2 times stronger than those with northern and north-eastern aspects. Using the toe-to-headwall altitude ratio (THAR) and toe-to-summit altitude method (TSAM), the LIA ELAs were calculated. The LIA ELAs lowered, on average, by 40–60 m, while the ELA changes on south and south-west facing glaciers (mean value of 80–100 m) was 2–2.5 times greater than of the northern. ELA depression during the LIA corresponded to decrease in mean summer temperature of 0.9–1.1 °C (if precipitation was equal to the present). Temperature increase was mainly responsible for ELA rising since the LIA, however, the response of small glaciers could be modulated by other factors (e.g. local insolation or precipitation). Stronger reduction of the south and south-west facing Kodar glaciers can be explained by their greater sensitivity to both solar radiation and snow nourishment with southern cyclones and East Asian monsoon.

Holocene glacier and climate variations in Vestfirðir, Iceland, from the modeling of Drangajökull ice cap

Drangajökull is a maritime ice cap located in northwest (Vestfirðir) Iceland. Drangajökull's evolution is therefore closely linked to atmospheric and ocean variability. In order to better constrain the Holocene climate and glacier history of Vestfirðir we model the past evolution of Drangajökull ice cap. Simulations from 10 ka to present are forced by general circulation model output, ice-core-based temperature reconstructions, and sea-surface temperature reconstructions. Based on these 10-thousand year simulations, Drangajökull did not persist through the Holocene. We estimate that air temperatures were 2.5–3.0 °C higher during the Holocene Thermal Maximum than the local 1960–1990 average. Simulations support Drangajökull's late Holocene inception between 2 and 1 ka, though intermittent ice likely occupied cirques as early as 2.6 ka. Drangajökull is primarily a Little Ice Age ice cap: it expanded between 1300 and 1750 CE, with the most rapid growth occurring between 1600 and 1750 CE. The maximum Holocene extent of Drangajökull occurred between 1700 and 1925 CE, despite the lowest late Holocene temperatures, occurring between 1650 and 1720 CE. Between 1700 and 1925 CE temperatures were likely 0.6–0.8 °C lower than the 1950–2015 reference temperature. The modern equilibrium line altitude (ELA) is bracketed by topographic thresholds: a 1 °C temperature increase from the modern ELA would eliminate the ice cap's accumulation area, while a reduction of 0.5 °C would lead to the rapid expansion of the ice cap across Vestfirðir. The proximity of Drangajökull to topographic thresholds may explain its late inception and rapid expansion during the Little Ice Age.

Glacier change in the Gangdise Mountains, southern Tibet, since the Little Ice Age

Delineating glacier change during the Little Ice Age (LIA) is of great importance when attempting to understand regional climatic changes and can also help to improve the understanding of any predictions of future glacial changes. However, such knowledge is still lacking for some critical regions of the Tibetan Plateau (TP). In this study, we mapped 4188 contemporary glaciers and reconstructed 1216 LIA areas of glacial coverage in the Gangdise Mountains to the north of the Himalaya using Google Earth satellite imagery. We estimated their paleoglacial areas and equilibrium line altitudes (ELAs) based on the toe-to-headwall altitude ratio (THAR) method. Results show that most glaciers are small (<1 km2), with slope/hanging glaciers the most common (2844 out of 4188 glaciers), while valley glaciers have the greatest coverage (1009.0 km2 out of a total area of 1723.7 km2). Contemporary glaciers have retreated significantly since the LIA, with reductions in length of between 5.5% and 94.7% (mean glacier length 696 m; mean reduction in length 41.7%) and reductions in glacier area of between 4.1% and 94.9% (mean glacier area 0.42 km2; mean reduction in area 44.8%). These reductions have occurred under different local climatic and topographic conditions. The contemporary ELA ranges from 5516 to 6337 m asl; the LIA ELA ranged from 5476 to 6329 m asl. Contemporary and LIA ELA values rise from southeast to northwest. As a general rule, the rise in the ELA value decreases from the eastern to the central Gangdise Mountains and then increases westward, with a mean ELA rise of 45 m. Multiple regression models suggest that 46.8% of the glacier area loss can be explained by glacier elevation, area, and slope. However, only 15.5% of the rise in ELA values can be explained by glacial geometric, topographic, or locational parameters. The spatial pattern of modern ELA values in this region appears inversely related to precipitation, which decreases from southeast to northwest, implying that precipitation is one of the key controls of ELAs. This is also consistent with results from elsewhere in High Asia. In contrast to the Gangdise Mountains' eastern and western sectors, glaciers in the central sector have undergone less change, i.e., in terms of reductions in length, area loss, and rises in ELA. Topography can of course also influence glacial change by creating shielding and/or rainshadow effects and by affecting local temperatures.

Factors responsible for driving the glaciation in the Sarchu Plain, eastern Zanskar Himalaya, during the late Quaternary

ABSTRACTDetailed geomorphological investigation supported by field stratigraphy and optical dating enabled us to identify three major events of glaciation in the eastern Zanskar Himalaya (Sarchu Plain). The oldest and most extensive glaciation is the Sarchu Glaciation Stage‐1 (SGS‐1) that pre‐dates the Last Glacial Maximum (LGM) and is assigned to the cold and wet Marine Isotope Stage‐4 (MIS‐4). The second glacial advance (SGS‐2) is optically dated to ∼20 ka and the youngest SGS‐3 is assigned to the 8.2 ka cooling event. Evidence of deglaciation is preserved as outwash terrace gravels and is assigned to the pluvial MIS‐3 and the early Holocene (∼9 ka). Increased strength of westerlies (winter precipitation) and associated temperature decline were responsible for driving the glaciation in the study area, a conclusion that accords well with modern meteorological data showing a reasonable correlation between the modern equilibrium line altitude, enhanced mid‐latitude westerlies (winter precipitation) and winter temperatures. The phases of deglaciation occurred during periods of warmer climatic excursions. Overall a broad correspondence between the Sarchu Plain glaciation phases and northern latitude ice sheet dynamics is suggested, implying that the glaciers in the trajectory of the mid‐latitude westerlies are likely to respond in phase with northern latitude glaciation.

Synchronous Last Glacial Maximum across the Anatolian peninsula

AbstractUludağ is a prominent mountain in northwestern Turkey where glacial deposits have been documented in the Kovuk Valley and the glacial history has been reconstructed based on 31 cosmogenic 10Be exposure ages from glacially transported boulders and bedrock. The results suggest that the Kovuk Glacier began advancing before 26.5±1.6 ka. It reached its maximum extent at 20.3±1.3 ka, followed by a re-advance at 19.3±1.2 ka, both during the global Last Glacial Maximum (LGM) within Marine Isotope Stage 2. The timing of the LGM glaciations in the Kovuk Valley is consistent with the investigated LGM glaciations in other mountains of Anatolia, the Mediterranean and the Alps. Based on the geomorphological ice margin reconstruction and using the accumulation/ablation area ratio (AAR) approach, the equilibrium line altitude (ELA) of the Kovuk LGM glacier was c. 2000 m above sea level for an estimated AAR of 0.67. This indicates a c. 1000 m lowering of the ELA for the LGM compared with the modern ELA estimate. These lines of evidence are consistent with the LGM glaciers that have been documented elsewhere in the Anatolian Mountains.Supplementary material: An input file for exposure age calculation in the CRONUS-Earth online calculator and an KML file for sample locations are available at http://www.geolsoc.org.uk/SUP18878

Cirques, peaks, and precipitation patterns in the Swiss Alps: Connections among climate, glacial erosion, and topography

Glacial erosion, a process influenced by climate, has been implicated in limiting the relief of mountain ranges. However, climate itself is sensitive to large-scale topography, suggesting that climate, topography, and glacial processes form a coupled system. Large spatial gradients in precipitation exist in the southern Swiss Alps, allowing us to study this coupling in a region of climate variability. More than 500 cirques were identified, and neighboring peaks were found to co-vary in elevation with cirque floors ( R 2 0.64). Cirque headwall relief does not vary with precipitation or cirque floor altitude. These relationships confirm the hypothesis that cirque formation restricts peak altitudes via slope processes that limit the relief of cirque headwalls. We compared the position of the regional equilibrium line altitude (ELA) estimated from modern climate to a surface defined by the cirque floors. The modern ELA and cirque floor surfaces are similar in shape, illustrating the impact of spatial variability in precipitation on glacial processes and topography. Precipitation variability in this region is partially dictated by large-scale topography. Therefore, precipitation patterns, glacial process domains, and topography must evolve together.

Last Glacial Maximum climate inferences from integrated reconstruction of glacier equilibrium-line altitude for the head of the Urumqi River, Tianshan Mountains

The relationship between climate and glacial equilibrium-line altitude (ELA) during the Last Glacial Maximum (LGM) has recently received considerable interest. However, in the headwaters of Urumqi River in northwest China, a systematic reconstruction for the paleo-ELAs had not been completed to permit regional and large-scale paleoclimate inferences. Recent improvements in dating and understanding of glacial extents provide the possibility to use glacial reconstruction to infer the paleoclimate. To improve the ability to obtain paleoclimate information, the ELAs during the local LGM (MIS2–3) and modern time (1998–2004) were constructed by using an integrated method. The regional tectonic uplift is considered when calculating the LGM ELAs in order to improve their precision and thus obtain a better estimate of the paleoclimate. The estimated ELA values were input into a robust equation to derive the forcing paleoclimatic conditions. During the local LGM in the study area, the ELA was between 3420 and 3560 m a.s.l., 493–633 m lower than modern ELA. Considering energy and mass balance in the equation to account for the ELA decrease, during the LGM, the temperature decreased by 5.42–6.30 °C and the precipitation was ∼30% compared to the present.

Climatic implications of reconstructed early–mid Pliocene equilibrium-line altitudes in the McMurdo Dry Valleys, Antarctica

AbstractEarly–mid Pliocene moraines in the McMurdo Dry Valleys, Antarctica, are more extensive than the present alpine glaciers in this region, indicating substantial climatic differences between the early–mid Pliocene and the present. To quantify this difference in the glacier–climate regime, we estimated the equilibrium-line altitude (ELA) change since the early–mid Pliocene by calculating the modern ELA and reconstructing the ELAs of four alpine glaciers in Wright and Taylor Valleys at their early–mid Pliocene maxima. The area–altitude balance ratio method was used on modern and reconstructed early–mid Pliocene hypsometry. In Wright and Victoria Valleys, mass-balance data identify present-day ELAs of 800–1600ma.s.l. and an average balance ratio of 1.1. The estimated ELAs of the much larger early–mid Pliocene glaciers in Wright and Taylor Valleys range from 600 to 950±170ma.s.l., and thus are 250–600±170m lower than modern ELAs in these valleys. The depressed ELAs during the early–mid-Pliocene most likely indicate a wetter and therefore warmer climate in the Dry Valleys during this period than previous studies have recognized.

Late Pleistocene glacial chronology of the Pietrele Valley, Retezat Mountains, Southern Carpathians constrained by 10Be exposure ages and pedological investigations

The Southern Carpathians are located in the transition zone between Mediterranean and continental climates and constituted the major areas of ice accumulation in the Carpathians during Pleistocene glaciations. An absolute glacial chronology had not yet been established. In this study we constrain the glacial history of the Pietrele Valley in the Retezat Mountains located in the Southern Carpathians and find that the timing of the late Würmian glacial advance is asynchronous to global climate records, likely reflecting changes in moisture availability in the area. The former ice extent of two major glacial advances (M1 or Lolaia glacial advance and M2 or Capra-Judele glacial advance) in the valley was reconstructed based on mapping and surveying of glacial features. Field relations and different pedological weathering indicators of till imply that the M1 glacial advance occurred well before the global Last Glacial Maximum (LGM). Its deposition during the penultimate glaciation is unlikely; no boulders suitable for exposure dating were found on M1-moraines. Seventeen exposure ages show that the moraines of the maximum late Würmian M2-glacial advance were abandoned around 16.1±1.6 ka. Another late glacial advance (M3 or Stevia advance) occurred during the Younger Dryas chron as constrained by two exposure ages. We suggest that the glacial advance that occurred during the time of the global LGM was less extensive than the M2 glacial advance and the deposits were overrun by the lateglacial M2 glacier, as we found no field evidence of glacial deposits between the M1 and the M2 moraines. We explain this obvious asynchronity of the local late Würmian ice extent with that of global records of ice accumulation with increased aridity in the Carpathians during the global LGM due to a lower sea-level. The reconstructed Late Pleistocene equilibrium line altitude (ELA) shows a distinct increase in the trend from the Alps and the Bavarian Forest to the Carpathians in comparison to the modern ELA. This finding supports the high aridity in the study area during the Late Pleistocene. Two distinct lateglacial moraines were deposited during times of reported colder and wetter climatic conditions around the North Atlantic (Heinrich-events H0 and H1).

Late Quaternary glacier advances in the upper catchment area of the Indus River (Ladakh and Western Tibet)

Geomorphological mapping in the Markha valley and the northern Nimaling Mountains (Northern Zanskar Range) has revealed eight prominent glacial stages. The two oldest glaciations are marked by glacial abrasion forms as ice-scour limits and glacial benches above the lower and middle Markha valley. These glaciations probably occurred prior to the last glacial cycle. Furthermore, terminal and lateral moraines, glacial tongue basins, basal moraines and tills provide evidence for three glacial stages in the Markha and Nimaling valleys, when the equilibrium-line altitude (ELA) depression was 670–350 m. The glaciations probably occurred during the LGM, the Lateglacial and the Lower Holocene as indicated by the topographic sequence, the ELA depressions and the tentative glacial stratigraphic divisions for the North Indian areas. Large moraine complexes in the vicinity of the present glaciers probably represent three glacial stages of Neoglacial and Historical glacial advances. The ELA depressions are 10–70 m. The present glaciation of the Nimaling Mountains, largely restricted to north flanks and the glacier basins below, covers an area of 45–55 km 2. The modern ELA is calculated at 5650–5750 m a.s.l. on north faces, rising to about 5900–6000 m a.s.l. on south faces. In the central and southern parts of the Ayila Ridge (Western Tibet) present ELAs range from 5700 to 5800 m a.s.l. on the north-facing slopes to 6000–6100 m a.s.l. on southern slopes. Eight former glacier stages are preserved on the southwest flank of these mountains by lateral and terminal moraine ramparts. The ELA depressions amount to 640–950 m.

Last Glacial Maximum equilibrium line altitudes in the circum-Caribbean (Mexico, Guatemala, Costa Rica, Colombia, and Venezuela)

Equilibrium line altitude (ELA) estimates for Last Glacial Maximum (LGM) paleoglaciers in Mexico, Guatemala, Costa Rica, Colombia, and Venezuela were determined using the accumulation area balance ratio (AABR), accumulation area ratio (AAR), toe-to-headwall altitude ratio (THAR), and the maximum altitude of lateral moraine (MALM) methods. LGM glacial expansions are chronologically constrained in Mexico, the Mérida Andes of Venezuela, the mountains around Bogotá, Colombia, and the Ruíz-Tolima massif, Colombia. Undated glacial sites are tentatively correlated to dated sites on the basis of similar moraine morphology and weathering characteristics. LGM ELAs are 3400–3950 m in central Mexico, 3544 m for Guatemala, 3477±13 m for Costa Rica, 4104±197 m for the Sierra Nevada de Santa Marta, Colombia, 3480 m for the Ruíz-Tolima region of Colombia, 3345±130 m for the mountains around Bogotá, Colombia, 4151±181 m for the Sierra Nevada de Cocuy, Colombia, and 3576±163 m for the Mérida Andes of Venezuela. As the modern ELA and/or 0 °C isotherm is found at 4900±200 m, LGM ELA depression in the circum-Caribbean region was between 500 and 1625 m.

Last Glacial Maximum equilibrium-line altitudes and paleoclimate, northern Uinta Mountains, Utah, U.S.A.

AbstractNineteen former valley glaciers were reconstructed for their Last Glacial Maximum (LGM) extents in the northern Uinta Mountains, Utah, U.S.A. Mean equilibrium-line altitudes (ELAs) calculated by four methods (accumulation–area ratio, toe–headwall altitude ratio, lateral moraines and cirque floors) range from 3050 to 3300 m a.s.l. Modern mean summer temperatures (Ts) at the ELAs range from 8.7° to 11.2°C, while modern winter precipitation (P) ranges from 354 to 590 mm snow water equivalent (SWE). Based on the difference in elevation of mean ELAs across the range, LGM P values must have ranged from 940 to 3040 mm SWE, assuming the modern summer lapse rate was the same during the LGM. A Ts depression of 5.5°C is required for these precipitation values to plot in the range of modern ELA values. The reconstructed increase in P at the western end of the range is 10 times the modern increase, reflecting the influence of pluvial Lake Bonneville. Assuming ELA depression (ΔELA) resulted from this P increase and a uniform 5.5°C Ts decrease, the regional LGM ΔELA was approximately 900 m.

Late quaternary glacial stades in the Cordillera Central, Colombia, based on glacial geomorphology, tephra–soil stratigraphy, palynology, and radiocarbon dating

Using data from glacial geomorphology, tephra–soil stratigraphy and mineralogy, palynology, and radiocarbon dating, a sequence of glacial and bioclimatic stades and interstades has been identified for the last ca. 50000 yr in the Ruiz-Tolima massif, Cordillera Central, Colombia. Six Pleistocene cold stades separated by warmer interstades occurred: before 48000, between 48000 and 33000, between 28000 and 21000, from ≥16000 to ca. 14000, ca. 13000–12400, and ca. 11000–10000 yr BP. Although these radiocarbon ages are minimum-limiting ages obtained from tephra layers on top of tills, the tills are not significantly older because most are bracketed by dated tephra sets in measured stratigraphic sections. Two minor moraine stages likely reflect glacier standstill during cold intervals ca. 7400 yr BP and slightly earlier. Finally, glaciers readvanced between the seventeenth and nineteenth centuries. In contrast to the ice-clad volcanoes of the massif, ca. 34 km2 in area above an altitude of ca. 4800 m, the ice cover expanded to 1200 km2 during the Last Glacial Maximum (LGM) and was still 800 km2 during Late-glacial time (LGT). Glacier reconstructions based on the moraines suggest depression of the equilibrium line altitude (ELA) by ca. 1100 m during the LGM and 500–600 m during LGT relative to the modern ELA, which lies at ca. 5100 m in the Cordillera Central. Glaciers in this region apparently reached their greatest extent when the climate was cold and wet, e.g. during stades corresponding to Oxygen Isotope Stage 3; glaciers were still expanding during the LGM ca. 28000–21000 yr BP, but they shrank considerably after 21000 yr BP because of greatly reduced precipitation. © 1997 John Wiley & Sons, Ltd.

Paleoenvironmental Changes and Glacial Stades of the Last 50,000 Years in the Cordillera Central, Colombia

Using data from glacial geomorphology, tephra-soil stratigraphy and mineralogy, palynology, and radiocarbon dating, a sequence of glacial and bioclimatic stades and interstades has been identified for the past ca. 50,000 yr in the Ruı́z-Tolima massif, Cordillera Central. Six cold stades separated by warmer interstades occurred before 48,000, between 48,000 and 33,000, between 28,000 and 21,000, from ≥16,000 to ca. 14,000, ca. 13,000–12,400, and ca. 11,000–10,000 yr B.P. Although the radiocarbon ages are minimum-limiting ages obtained from tephra layers on top of tills, the tills are not significantly older because most are bracketed by dated tephra sets in measured stratigraphic sections. Two minor moraine stages likely reflect glacier pauses during cold intervals ca. 7400 yr B.P. and slightly earlier. Finally, glaciers readvanced between the 17th and 19th centuries. In contrast to the glacier cover (ca. 34 km2) on volcanoes of the massif during the last glacial maximum (LGM) the ice cover expanded to 1200 km2and was still 800 km2during late-glacial time (LGT). Glacier reconstructions based on the moraines suggest depression of the equilibrium line altitude (ELA) by ca. 1100 m during the LGM and 500–600 m during LGT relative to the modern ELA which lies at ca. 5100 m in the Cordillera Central. Glaciers in this region apparently reached their greatest extent when the climate was cold and moist, e.g., during stades corresponding to marine isotope stage 3; glaciers were still expanding during the LGM ca. 28,000–21,000 yr B.P., but they shrank considerably after 21,000 yr B.P. because of greatly reduced precipitation.

Late Pleistocene equilibrium‐line reconstructions in the northern Peruvian Andes

Equilibrium‐line‐altitude (ELA) reconstructions using the toe‐to‐headwall‐altitude ratio method for paleoglaciers in the Cordilleras Blanca and Oriental, northern Peruvian Andes (7–10°S; 77°20′–77°35'W), indicate that ELAs during the last glacial maximum (LGM; marine isotope stage 2) were c. 4300 m in the Cordillera Blanca, c. 3900–3600 m on the west side of the Cordillera Oriental, and c. 3200 m on the east (Amazon Basin) side of the Cordillera Oriental. Comparison with estimated modern ELAs and glaciation thresholds indicate that ELA depression ranged from c. 700 m in the Cordillera Blanca to c. 1200 m on the east side of the Cordillera Oriental. This augments data from many mountain ranges in middle‐ and low‐latitude regions that indicate that ELAs during the LGM were depressed by c. 1000 m. Published palynological evidence for drier conditions during the LGM in the tropical Andes suggests that ELA depression of this amount involved a temperature reduction (&gt; 5–6°C) that greatly exceeded the tropical sea‐surface temperature depression estimates of CLIMAP (&lt; 2°C). The west to east increase in ELA depression during the LGM indicates that the steep modern precipitation gradients may have been even steeper during the LGM.

Nivation landforms in the western Great Basin and their paleoclimatic significance

More than 10,000 nivation landforms occur in the higher mountain ranges of the western Great Basin. They range from small, subtle hollows with head scarps a few meters high and a few tens of meters long to broad, clearly defined terraces as much as 220 m wide bounded by bold, steeply sloping head scarps as much as 30 m high and 1600 m long. Distribution of these nivation hollows is strongly influenced by elevation, slope orientation, local relief, and substrate lithology. About 95% occur between 2200 and 3000 m elevation, and nearly 80% are situated on north-northwest-to east-northeast-facing slopes. They occur mainly in areas of moderately sloping terrain and moderate local relief, and they are preferentially developed on relatively incompetent substrates including terrigenous sedimentary deposits, volcanic and metavolcanic rocks of intermediate composition, and deeply weathered granitoid rocks. Nearly all of these nivation hollows are relict. They are most abundant near areas of late Pleistocene glaciation but rarely occur within such areas. Most are veneered with colluvium and are well vegetated, and many hollows in the Mono Basin area are veneered with volcanic ash at least 700 yr old. Distribution of nivation hollows suggests that (1) the full-glacial nivation threshold altitude (NTA) rose from north to south at 190 m per degree of latitude, subparallel to, and approximately 740 m lower than, the full-glacial equilibrium-line altitude (ELA) and about 1370 m lower than the estimated modern ELA; (2) the difference between the full-glacial and modern ELAs indicates an approximate 7°C full-glacial mean-annual-temperature depression throughout the Great Basin; and (3) the full-glacial mean annual temperature at the NTA is estimated to have been approximately 0° to 1°C, assuming little change in accumulation-season precipitation.

Late Pleistocene Equilibrium-Line Altitudes and Modern Snow Accumulation Patterns, San Juan Mountains, Colorado, U.S.A.

Equilibrium-line altitudes (ELAs) are determined for 85 reconstructed late Pleistocene glaciers in the San Juan Mountains of Colorado. Paleo-ELA contours rise broadly from west to east across the range and wrap around the range's central core, where the highest ELA values are found. Detailed comparison of modern and paleosnowline variations within the range is not possible because there are too few modern snow fields. Snow survey data, however, allow comparison of modern snow accumulation patterns with paleo-ELAs. When the effect of elevation on accumulation is removed, modern accumulation patterns bear striking similarlity to paleo-ELA patterns, with lowest late Pleistocene ELAs in areas of highest modern snow accumulation and vice versa. Modern accumulation contours show the same west to east gradient and also wrap around the high, dry, central core of the range. Accumulation data suggest that theoretical modern ELAs would increase by approximately 600 m across the range, a figure closely comparable to cross-range Pleistocene ELA differences. These similarities suggest that accumulation-season general circulation patterns and moisture sources did not alter greatly between the time of the late Pleistocene glacial maximum and the present. They appear to rule out any strong increase in fall, winter, or spring southeasterly (Gulf of Mexico) flow as a mechanism for enhancement of precipitation in the range.