UNIVERSITY OF CINCINNATI
Date:______
I, ______, hereby submit this work as part of the requirements for the degree of: in:
It is entitled:
This work and its defense approved by:
Chair: ______
A Comparison of Glacial Chronologies between the Eastern and Western Cordilleras, Bolivia
By Colby A. Smith M.S. University of Maine, 2003 B.S. University of Maine, 2001
A Dissertation Submitted in Partial Fulfillment of the Requirements of the Degree of Doctor of Philosophy (in Geology)
The Graduate School The University of Cincinnati May 2008
Advisory Committee:
Thomas V. Lowell, Professor of Geology, University of Cincinnati (primary advisor) Bryan G. Mark, Assistant Professor of Geology, Ohio State University David B. Nash Professor of Geology, University of Cincinnati Lewis A. Owen, Professor of Geology, University of Cincinnati Donald T. Rodbell, Professor of Geology, Union College
i Abstract
The timing and forcing of glacial advance in the tropical Andes remains uncertain with regard to both higher latitude and intra-tropical glacial fluctuations over the last glacial cycle and through the Holocene. Surface exposure age dating of glacial deposits in both the Eastern and
Western Cordilleras of Bolivia coupled with glacial mass balance modeling provides a step towards understanding these relationships.
36Cl Surface exposure age dating at Nevado Sajama in the Western Cordillera, Bolivia suggests that glaciers retreated from moraines at ~16.9 ka -10.2 ka, 7.0 - 4.4 ka, and 4.7 - 3.3 ka.
In the Eastern Cordillera of Bolivia at Nevado Illimani glaciers retreated from moraines at 15 ka
-13.0 ka, 10.5 ka - 8.5 ka, and 3.0 ka -1.5 ka based on 10Be surface exposure ages. The new data from the Eastern Cordillera agree well with other Late Glacial and late Holocene aged moraines in the Eastern Cordillera while the data from the Western Cordillera agree well with the one previous study. However, this study provides the first dates on Holocene moraines in the
Western Cordillera. The major chronological contribution from the Western Cordillera is that the Late Glacial aged advance appears to be the most extensive of the last glacial cycle and approximately synchronous with the Late Glacial advance in the Eastern Cordillera. If indeed the two advances occurred at the same time, reconstruction of past equilibrium line altitudes
(ELA) suggests that that the regional ELA was nearly flat between the Eastern (4870 m a.s.l.) and Western Cordilleras (>4690 m a.s.l.) as opposed to today where it rises 360 m from the
Eastern to the Western Cordillera. It is hypothesized that the change in the slope of the regional
ELA results form increased winter precipitation from the west.
A positive degree-day glacier mass balance model was applied to the paleo-ELAs associated with the moraines dated at Nevado Illimani. Unique solutions were obtained for
ii periods where independent estimates of either precipitation or temperature are available.
According to the model only a ~2.3 oC cooling is required to advance glaciers to their Late
Glacial extents if precipitation was 50-75% higher. The model was also applied to the Western
Cordillera. However, it did not produce the correct modern ELA. This suggests that PDD models are not applicable to regions where sublimation is the dominant ablation mechanism.
iii
iv Acknowledgments
I would like to thank the members of my committee for their help over the past four years. All committee members have read and re-read drafts of these manuscripts and added useful comments for which I am grateful. However, many committee members have gone beyond the usual editing duties. I would like to thank Don Rodbell and Bryan Mark for introducing me to fieldwork in South America in 2005. I would like to thank Lewis Owen for the use of his lab and the help and insight he has given me regarding cosmogenic exposure age dating. Finally, I would like to thank Tom Lowell for his leadership and friendship over the past decade.
Beyond my committee, I would like to thank the Parque Nacional de Sajama for permission to work in the park. Also, the Universidad de San Andres and Jose Escobar and
Monica Escobar for organizing the logistics in Bolivia.
Preface
My dissertation is a compilation of three separate manuscripts. Chapter 2 has been submitted to the Journal of Quaternary Science with Thomas V. Lowell and Marc W. Caffee of the Department of Physics, Purdue University as co-authors. Chapter 3 is in preparation for submission to Quaternary Research with Thomas V. Lowell, Lewis A. Owen, and Marc W.
Caffee as co-aruthors. Chapter 4 will likely be submitted to the Journal of Glaciology with
Thomas V. Lowell as a co-author.
v Table of Contents
Chapter 1. Introductory Remarks………………….…………………………………………..1
Chapter 2. Late Glacial and Holocene Glacial Chronology and Geomorphologic Evidence for the Presence of Cold Based Glaciers at Nevado Sajama, Bolivia………...... 9 Abstract………………………………………………………………………………9 Introduction………………………………………………………………………..…9 Geologic and Climatic Setting…………………………...…………………………10 Previous Work…………………………...…………………………………….…....11 Methods…………………………...………………………………………….....…..12 Results…………………………...………………………………………….....…....14 Discussion……………………...…………………………………………...…....…19 Conclusion……..………………...…………………………………………...…..…28 Acknowledgements…………………………...…………………………………….29 References……………………….…………………………………………...…..…29 Tables…………………………...…………………………………………...…..….34 Figures…………………………...…………………………………………...…..…38
Chapter 3. Late Quaternary glacial chronology Nevado Illimani, Cordillera Real, Bolivia: Implications for paleoclimatic reconstructions across the Andes………………48 Abstract……………………………………………………………………………..48 Introduction.………………………………………………………………………...48 Geologic Setting…………………………………………………………….………50 Methods……………………………………………………………………………..51 Results……………………………………………………………………………....52 Discussion…………………………………………………………………………..56 Conclusions………………………………………………………………………....60 Acknowledgements………………………………………………………………....61 References…………………………………………………………………………..61 Tables…………………………………………………………………………….....67 Figures………………………………………………………………………………70
Chapter 4. Present, future, and past tropical Andean ELAs: Assessing the relative importance of precipitation and temperature on glacier mass balance using a positive degree-day model…………………………………………………..…….80 Abstract……………………………………………………………………………..80 Introduction…………………………………………………………………………80 Glaciological Setting……………………………………………………………..…82 Data…………………………………………………………………………………83 Methods…………………………………………………………………………..…84 Selection of Model Variables……………………………………………………….83 Results……………………………………………………………………………....87 Discussion…………………………………………………………………………..88 Conclusions……………………………………………………………………...….94 References……………………………………………………………………….….95
vi Figures……………………………………………………………………………..101
Chapter 5. Concluding Remarks……………………………………………………………...107
List of Tables and Figures
Chapter 1. Introductory Remarks……………….….…………………………………………..1 Figure 1……………...………………………………………………………………..6 Figure 2……………...………………………………………………………………..7 Figure 3……………...………………………………………………………………..8
Chapter 2. Late Glacial and Holocene Glacial Chronology and Geomorphologic Evidence for the Presence of Cold Based Glaciers at Nevado Sajama, Bolivia………...... 9 Table 1………………………………………………………………………………34 Table 2……………………………………………………………………………....35 Table 3…………………………………………………………………………...….36 Table 4………………………………………………………………………………37 Figure 1…………………………………………………………………………...…38 Figure 2……………………………………………………………….……………..39 Figure 3……………………………………………………………………………...40 Figure 4………………………………………………………………………….41-42 Figure 5………………………………………………………………………….43-44 Figure 6……………………………………………………………………………...45 Figure 7……………………………………………………………………………...46 Figure 8…………………………………………………………………………...…47
Chapter 3. Late Quaternary glacial chronology Nevado Illimani, Cordillera Real, Bolivia: Implications for paleoclimatic reconstructions across the Andes………………48 Table 1 ………..……………………………………………………………….……67 Table 2…………………………………………………………………………...68-69 Table 3…………………………………………………………………………...….70 Figure 1…………………………………………………………………………...…70 Figure 2……………………………………………………………….……………..71 Figure 3……………………………………………………………………………...72 Figure 4………………………………………………………………..…………….73 Figure 5……………………………………………………………………..……….74 Figure 6…………………………………………………………………..……...75-76 Figure 7……………………………………………………………………………...77 Figure 8…………………………………………………………………………...…78 Figure 9…………………………………………………………………………...…79
vii Chapter 4. Present, future, and past tropical Andean ELAs: Assessing the relative importance of precipitation and temperature on glacier mass balance using a positive degree-day model…………………………………………………..….….80 Figure 1………………………………………………………………………….....101 Figure 2……………………………………………………………….……………102 Figure 3…………………………………………………………………………….103 Figure 4………………………………………………………………..…………...104 Figure 5……………………………………………………………………..……...104 Figure 6…………………………………………………………………..………...105 Figure 7…………………………………………………………………………….105 Figure 8………………………………………………………………………….…106
Chapter 5. Concluding Remarks……………………………………………………………...107
viii Chapter 1. Introductory Remarks
The oxygen isotopic records from high latitude ice-cores broadly indicate climatic correlation between the two polar hemispheres over the past 25 ka. For example, the GISP II core from Greenland (Blunier and Brooks, 2001) and the Dome C core from Antarctica (Lorius et al., 1979) both show glacial conditions prior to 17 ka followed by warming, a cold reversal, and then moderately stable Holocene climate starting at about 11.5 ka (Fig. 1). Although broad synchronicity between the glacial and interglacial periods can be established, the precise timing and structure of the climate change is quite different. In Greenland, there is abrupt warming at the start of the Bølling /Alerød followed by abrupt cooling and abrupt warming bracketing the
Younger Dryas cold period (Blunier and Brooks, 2001). In Antarctica, warming starts earlier than in Greenland and it is more gradual with only a slight cold reversal that also starts earlier than the Younger Dryas.
It has been proposed that changes in ocean circulation explain the asynchronicity of climate between the two polar hemispheres (Broecker, 1998; Blunier and Brooks, 2001).
However, more recent ice-core data suggest a more complex relationship (Jouzel et al., 2001). In order to understand better the relationship between the paleo-climate of the Northern and
Southern Hemispheres it is necessary to examine how high latitude climatic signals propagate to lower latitudes, specifically the tropics.
In tropical regions, the oxygen isotopic record is ambiguous for two reasons. First, the isotopic record is not a simple temperature relationship as it is at higher latitudes (Dansgaard,
1964). It has been interpreted to respond to changes in precipitation (Ramirez et al., 2003) or changes in temperature (Thompson et al., 1998), and it may respond to both. Second, the oxygen
1 isotopic records from the topical Andes have similar structures and chronologies to those from higher latitudes but in no discernable geographic pattern. At Nevado Sajama (Thompson et al.,
1998) the isotopic record shares a similar chronology and abrupt changes with the GISP II core
(Blunier and Brooks, 2001) while at Nevado Illimani (Ramirez et al., 2003) the record shows early warming and gradual change similar to the Antarctic record (Fig. 2). Sajama is located at
18.0 oS in the Western Cordillera of Bolivia, and Illimani is located at 16.6 oS in the Eastern
Cordillera of Bolivia. Thus, isotopic records similar to those from both polar regions can be found within 150 km of each other in the tropics of the Southern Hemisphere. Although this suggests different climatic conditions in the Eastern and Western Cordilleras it does not indicate whether the climatic differences are temperature or precipitation since it is unclear what tropical isotopic records track.
Examining the glacial geologic record at both Nevado Sajama in the Western Cordillera and Nevado Illimani in the Eastern Cordillera can provide new information regarding the past temperature and precipitation conditions that is not available from the isotopic record. At the latitudes of the mountains moisture originates from the Atlantic in the east and is advected across the continent. This results in high precipitation in the Amazon Basin and in the foothills of the
Eastern Cordillera. Due to the rain shadow produced by the Eastern Cordillera, the Western
Cordillera is semi-arid, and the Atacama Desert on the western slope of the Western Cordillera is hyper-arid. A result of this precipitation gradient is that although the temperature is fairly constant between the Western and Eastern Cordilleras, the glacial ELAs and glacial margins are much higher in the Western Cordillera (Fig. 3).
In important difference between the glaciers of the Western and Eastern Cordilleras is that the glaciers in the west are hundreds of meters above the mean annual 0 oC isotherm (Klein
2 et al., 1999) while glaciers in the east intersect 0 oC isotherm (Francou et al., 1995). Since glaciers on Sajama only rarely experience melting conditions they are believed to be relatively insensitive to temperature changes while still responding to precipitation changes. Glaciers on
Illimani are believed to respond readily to temperature changes. Since the glaciers respond to different forcings it has been hypothesized that they may have different chronologies
(Hastenrath, 1971). Thus, mapping and dating of glacial deposits was undertaken at Nevado
Sajama and Nevado Illimani to test the hypothesis regarding chronologic asymmetry between the two cordilleras and to compare the glacial geologic record to oxygen isotopic record.
In this dissertation, chapter one discusses new 36Cl surface exposure ages from glacial deposits on Nevado Sajama in the Western Cordillera of Bolivia, as well as providing the first geomorphic report on the presence of cold-based glaciers in the past. Cold-based glacier deposits are subtle features that may be easily misinterpreted in the field. Misidentification of cold-based glacier deposits has implications for the interpretation the glacial geologic record of the high Andes. Since only a handful of dates exist for glacial deposits in the Western Cordillera the data provided here more than doubles the existing data set.
Chapter two provides a 10Be surface exposure age chronology from Nevado Illimani in the Eastern Cordillera of Bolivia. The chronological and equilibrium line altitude (ELA) data from Illimani are compared to data from north and south along the range as well as with the new data from the Western Cordillera. Comparison of dated paleo-ELA reconstructions yields an interpretation different than that of regional ELA studies of undated deposits.
Chapter three applies a positive degree-day model to glacial mass balance in the tropical
Andes. The model is calibrated using modern data from Zongo Glacier, Bolivia and the sensitivity of the mass balance to changes in temperature and precipitation are examined. Once
3 calibrated the model inputs are pertibated in order to suggest 1) future changes in ELA with predicted warming, and 2) past climatic conditions associated with known, dated, ELAs derived from the pervious two chapters.
The dissertation ends with some concluding remarks. The major contributions of the work are presented in the context of how the glacial geologic record compares to the oxygen isotopic record in the tropical Andes. Additionally, there are some significant geomorphological results from the Western Cordillera. Because scientific understanding of the glacial and climatic history of the Western Cordillera is limited some comments are made about future research directions in this region.
References
Blunier and Brooks, 2001. Timing of millennial-scale climate change in Antarctica and
Greenland during the last glacial period. Science, 291, 109-112.
Broecker, W.S., 1998. Paleocean circulation during the last deglaciation: A
bipolar seasaw? Paleoceanography, 13, 119-121.
Dansgaard, W., 1964. Stable isotopes in precipitation. Tellus, 16, 4.
Francou, B., Ribstein, P., Saravia, R., Tiriau, E., 1995. Monthly balance and water discharge of
an inter-tropical glacier: Zongo Glacier, Cordillera Real, Bolivia, 16oS. Journal of
Glaciology, 42, 61-67.
Hastenrath, S.L., 1971. On the Pleistocene snow-line depression in the arid regions of the South
American Andes. Journal of Glaciology, 10, 255-267.
Jouzel, J., Masson, V., Cattani, O., Falourd, S., Stievenard, M., Stenni, B., Longinelli, A.,
Johnsen, S.J., Steffenssen, J.P., Petit, J.R., Schwander, J., Souchez, R., Barkov, N.I.,
4 2001. A new 27 ky high resolution East Antarctic climate record. Geophysical Research
Letters, 28, 3199-3202.
Klein, A.G., Seltzer, G.O., Isacks, B.L., 1999. Modern and last local glacial maximum
snowlines in the Central Andes of Peru, Bolivia, and Northern Chile. Quaternary Science
Reviews, 18, 63-84.
Lorius, C., Merlivat, L., Jouzel, J., Pourchet, M., 1979. A 30,000 yr isotope climatic record from
Antarctic ice. Nature, 280, 644-648.
Ramirez, E., Hoffmann, G., Taupin, J.D., Francou, B., Ribstein, P., Caillon, N., Ferron, F.A.,
Landais, A., Petit., J.R., Pouyaud, B., Schotterer, U., Simoes, J.C., Stievenard, M., 2003.
A new Andean deep ice core from Nevado Illimani (6350 m), Bolivia. Earth and
Planetary Science Letters, 212, 337-350.
Thompson, L.G., Davis, M.E., Mosley-Thompson, E. Sowers, T.A., Henderson, K.A.,
Zagorodnov, V.S., Lin, P.-N., Mikhalenko, V.N., Campen, R.K., Bolzan, J.F., Cole-Dai,
J., Francou, B., 1998. A 25,00-year tropical climate history from Bolivian ice cores.
Science, 282, 1858-1864.
5 Figure 1. The oxygen isotopic records from the GISPII (Blunier and Brooks, 2001) and Dome C (Lorius et al., 1979) ice-cores in Greenland and Antarctica respectively.
6
Figure 2. A. The oxygen isotopic records from Nevado Sajama (Thompson et al., 1998) plotted with the similar isotopic data from the GISPII ice-core in Greenland (Blunier and Brooks, 2001). B. The oxygen isotopic record from Nevado Illimani (Ramirez et al., 2003) plotted with the similar isotopic data from the Dome C ice-core in Antarctica (Lorius, et al., 1979).
7
Figure 3. Glaciers in the Western Cordillera are situated well above the 0 oC isotherm while glaciers in the Eastern Cordillera intersect it. Therefore, changing temperature (ie the elevation of the 0 oC isotherm will have little effect on glaciers in the Western Cordillera while affecting Eastern Cordilleran glaciers.
8 Chapter 2. Late Glacial and Holocene Glacial Chronology and Geomorphologic Evidence for the Presence of Cold Based Glaciers at Nevado Sajama, Bolivia
Abstract
Past studies of regional paleo-equilibrium line altitude in the central Andes are based on the assumption that large outermost moraines visible in remotely sensed images of the Western
Cordillera date the last glacial maximum (LGM). However, field investigation and geomorphic mapping at Nevado Sajama, Bolivia indicates the large moraines are relic features with smaller moraines overlying and extending beyond them. 36Cl exposure ages from the smaller moraines suggest that they date to late glacial times ~16.9 ka -10.2 ka. Although late glacial deposits have been found throughout the Central Andes, the extent of these deposits relative to LGM deposits varies both between the Western and Eastern Cordilleras and north-to-south along the Western
Cordillera. In the Western Cordillera in the zone of easterly winds, the late glacial appears to be the most extensive glacial advance of the last glacial cycle. Geomorphic evidence also suggests that some late glacial moraines were deposited by cold based ice, a previously unreported finding in the tropical Andes. Retreat from other glacial features occurred at about 7.0 - 4.4 ka and 4.7 -
3.3 ka. These are the first dated Holocene glacial deposits in the Western Cordillera of Bolivia.
Introduction
The primary records for reconstructing paleo-climate in the tropical Andes are ice-cores, lake-levels, and glacial fluctuations. In modern systems, both precipitation and temperature affect all three proxies to differing and often poorly constrained extents. Although it is known that during the last glacial cycle and into the Holocene rapid changes in temperature and
9 precipitation occurred, disentangling the magnitude of these changes has proven difficult due to the fact that proxy records track multiple variables and the chronology of glacial fluctuations is not well constrained. Here we provide the first report on the presence of cold-based ice deposits in the tropical Andes, and suggest how these findings may be responsible for the poorly constrained glacial chronologies of the central Andes. We examine the glacial geomorphology and glacial chronology on Nevado Sajama in the Western Cordillera of Bolivia and report on 36 new 36Cl exposure age dates on glacial deposits from Sajama.
Geologic and Climatic Setting
Nevado Sajama (18.11o S, 68.88o W) is a stratovolcano reaching an elevation of 6542 m on the eastern side of the Western Cordillera (Fig. 1). Although the age of Sajama is unknown, other volcanos in the area range in age from early Miocene (Worner et al., 2000) to Pleistocene
(Avilia-Salinas, 1991). Sajama was selected because it is among the southernmost mountains in the Western Cordillera with modern glaciers (Ammann, et al., 2001). Although mountains to the south rise above the 0 oC isotherm, the area is too arid to support glaciers. Having a modern glacial datum facilitates comparison of past glacial and climatic conditions to those of the present. Also, there are existing ice-core (Thompson et al., 1998), meteorological (Hardy et al.,
1998), ELA (Klein et al., 1999), and snowline (Arnaud et al., 2001) data from Sajama.
Fieldwork was conducted in Huaqui Jihuata and Patokho Valleys on the east side of the mountain (fig. 2, fig. 3). As discussed below, these valleys are representative of others on the mountain and contain multiple glacial drifts and moraines.
Meteorlogical data from 1990-2001 from the village of Cosapa (3922 m) at the base of
Sajama indicate a mean annual temperature of 7.3 oC. If a lapse rate of 6.85 oC/ km, based on
10 monthly radiosonde data from El Alto in 1988 (SENAMHI, unpublished data), is applied to this temperature, then the elevation of the annual 0oC isotherm is 4990 m, which is below the termini of the glaciers, extending to ~5240 m.
The mean annual precipitation at Cosapa is 351 mm. Approximately 85% of precipitation falls during the austral summer monsoon from December through March
(SENAMHI, unpublished data). Since the field area is located in the zone of easterly winds, precipitation increases towards the Amazon Basin in the east and decreases towards the Atacama
Desert in the west.
Previous Work
Very little is known about the glacial chronology of the Western Cordillera of the Central
Andes. Two data sets, one from 19o S (Clayton and Clapperton, 1997) in the zone of easterly trade winds and one from 29o S (Zech et al., 2006) across the arid diagonal (Ammann, et al.,
2001) in the zone of westerlies are all that constrain the timing of glaciation in the range making up the western boundary of the Altiplano. Clayton and Clapperton (1997) dated peat from an ice-contact fan that grades into a delta of paleo-Lake Tauca on the western Altiplano. The four ages range between 16.7 ± 0.4 and 12.7 ± 0.3 ka before present when converted to calendar years using Calib 5.1 (Stuiver and Reimer, 1993). From these data, Clayton and Clapperton (1997) concluded that glacial advance on the Altiplano is roughly synchronous with paleo-Lake Tauca during a period of cool, wet conditions, and that precipitation came from the east. Zech et al.
(2006) used eleven 10Be exposure ages to date five landforms in northern Chile. It was concluded that a minor advance occurred at 11.6 ± 1.2 ka, and major advances occurred at 14.0 ±
1.4 ka, and 24.1 ± 2.4 ka. Zech et al. pointed out the overlapping ages between their data set and
11 that of Clayton and Clapperton (1997) and suggested synchronicity in the Western Cordillera, between the zone of easterlies and the zone of westerly circulation during a period of cold, wet, conditions. However, the source of the precipitation could not be determined.
Methods
A geomorphic map of Nevado Sajama was constructed based on field investigation and aerial photograph interpretation Fig. 2, Fig. 3. Google Earth digital globe imagery from 2007 was used as the base for the final map. Aside from snow and ice, and bedrock, which are self explanatory, the map is divided into 8 units including 4 drift units, moraines, ice-contact slopes, rock glaciers and rock glacier deposits, and scree and fan deposits.
To complement the geomorphic mapping, 36 samples for 36Cl exposure dating were collected. 36Cl was selected because the andesitic rocks lack the quartz necessary for 10Be exposure age dating. Field sampling involved removing at least 600 g of rock from boulders or bedrock with a chisel and hammer. Samples were collected from geomorphically significant locations such as drift sheets (ground moraine), moraine ridges, or striated bedrock. During sampling, care was taken to select large boulders of no less than 0.75 m in height that had no apparent signs of post depositional movement or exhumation and few signs of surface degradation. Preference was given to larger, faceted, boulders on the crests of moraines.
Location, elevation, shielding, and slope data were recorded for each sample.
Upon return to the laboratory, samples were crushed, and a 10 g aliquot was collected for elemental analysis at SGS laboratory, Toronto, Canada in order to determine the chemistry of the rocks. Elemental analyses were conducted on four samples, and the mean values of the four
12 samples were used as input to calculate the ages of the remaining thirty-two samples. The results of the four elemental analyses and the mean values are presented in Table 1.
The remaining crushed samples were then sieved to < 1 mm. About 30 g of each sample was cleaned of any organics or carbonate in HNO3 prior to dissolution in a HF and HNO3 solution along with ∼1g of 35Cl carrier. Following dissolution the samples were centrifuged, and the supernatant was reserved. AgCl was precipitated from each sample by the addition of silver nitrate. The samples were then vacuum filtered to separate the AgCl. Next, the AgCl was dissolved by the addition of ammonium hydroxide, reserved, and re-precipitated with silver nitrate. Following, another round of centrifuging the fluid was discarded, and the AgCl was dissolved prior to chlorine purification by ion exchange chromatograph columns. The chlorine was precipitated a final time prior to centrifuging, drying, and targeting. Chlorine ratios were measured by AMS at PRIME LAB, Purdue University, West Lafayette, Indiana. A more complete description of the laboratory methods are described by Ayarbe (2000).
Kober et al. (2007) measured bedrock erosion rates along a transect from the Atacama
Desert across the Western Cordillera. Andesite in the Western Cordillera was found to erode at a rate of 0.835 cm/ka. We adopt this value for all age calculations except the striated bedrock which is assumed to have 0 cm/ka erosion in order to preserve the striations.
Age calculations were made using the PRIME LAB WebCN online calculator that involves the shielding values of Dunne (1999) and the latitude and altitude scaling values of
Stone (2000) and the production rates of Phillips et al., (2001). The sample ages, locations, and
Cl ratios are listed in Table 4. The errors presented in Table 4 account for the instrument measurement error only.
13 Results
Geomorphology
The most prominent glacial features in the field area consist of distinct drift units which are easily identifiable both in the field and on aerial and satellite images (Table 2, fig. 5). Four drift units are mapped on Nevado Sajama. Although not all units are found in all valleys, a pattern emerges when the geomorphology of the entire mountain is examined. The oldest unit,
Drift 1, is present in all moraine bounded valleys that radiate out from the mountain. The minimum elevation of Drift 1 ranges between 4350 m and 4635 m. Drift 1 is hummocky and composed of bouldery sediments that form numerous ridges ~3 m high that trend both along and across the axis of the valley. In many places the streams do not flow down the center of the valley, rather they flow between the red hummocky Drift 1 and the valley wall. Lakes are often found in depressions in this drift unit. Drift 1 is believed to be a ground moraine composed of previously englacial and supraglacial debris. This interpretation accounts for the lower elevation of the unit changing with aspect discussed below as well as the hummocky nature of the deposit.
Up valley from Drift 1 is Drift 2: a generally light tan colored unit. It is composed of sediments varying in size from silt to boulders. In Patokho Valley, a small lake is within this unit. The light colored drift unit is at the upper extent of the gently sloping valley. There is abundant grass and some small trees on this unit. The color change from dark to light tan is apparent on black and white aerial photographs and is even more apparent in the field. The difference in color between Drift 1 and Drift 2 is a result of differing lithologies sourced from different areas the mountain. Small ~2-3 m high lateral and frontal moraines bound this drift in many valleys. Drift 2 is found in 12 of 15 valleys. The minimum elevation of the unit is between 4585 m and 4925 m. Drift 2 is believed to be a ground moraine composed of previously
14 englacial and supraglacial debris bounded by moraines related to the same glacial advance similar to the origin of Drift 1.
Drift 3 is located up valley from Drift 2. Drift 3 generally has a steep down valley slope composed of largely light colored gravel-sized sediment although boulders are also common.
Lithologically, Drift 3 is similar to Drift 2, and the color change is believed to be a result of reduced chemical weathering of the younger Drift 3. The boulders are often fractured or shattered. A hummocky surface lies above the steep slope. Vegetation is virtually absent at this elevation. The slopes of this unit display well developed stone stripes indicative of prolonged periglacial activity. Most of the stripes are composed of gravel to cobble sized clasts.
Periglacial processes may be actively exhuming the larger boulders that were sampled for exposure age dates. Drift 3 is present in 8 of 15 valleys, and its minimum elevation is between
4745 m and 5000 m. In the two valleys from which exposure age samples were collected, Drift 3 is not a simple ground moraine as the previously mentioned drift units. The flat top and steep downvalley slope are suggestive of a rock glacier deposit as is the lack of lateral moraines.
However, the modern rock glaciers above Drift 3 do not grade into Drift 3. Because rock glaciers depend upon sub-freezing temperatures, this indicates either a rapid change in temperature or a different origin for the deposits. In order to explain, the origin of these deposits we take a modern analog approach. In Phajokhoni Valley, south of Huaqui Jihuata Valley, a glacier exists within the inner most drift unit. The surface of the glacial ice is debris rich as result of rock fall from the cliffs. Down slope from the active glacier, bounded by lateral moraines, are hummocky deposits formed by the retreat of debris ladened ice. Although, the distal slopes of the lateral moraines are ∼60 m high there are essentially no proximal slopes because the hummocky debris has infilled this area. The mapped drift units in Huaqui Jihuata
15 and Patokho Valleys are believed to have been deposited in a similar manner by debris rich ice.
However, the formation of large distal slopes is hindered in the confined valleys.
The final drift unit, Plateau Drift, is not found in the valleys radiating from the mountain.
Rather, it is found on high elevation, low angle, plateaus that are separated from Sajama by secondary peaks or large exposures of bedrock. The Plateau Drift is generally light in color and hummocky. No moraines are associated with thePlateau Drift. It is believed that the Plateau
Drift was deposited by small cold-based ice-caps.
Moraines fall into two distinct groups. There are large ∼60 m- 80 m high lateral moraines that enclose the lower portions of the valleys, and there are the small moraines ∼1 m - 4 m high that are inset, crosscut, or overlie the larger moraines. The large lateral moraines are mapped as having ice-contact slopes although the moraines are likely much older than these planer, over- steepened slopes. Depending on location, moraines are composed of boulder to silt sized sediments and were deposited in ice-marginal positions. Often moraines are associated with the down valley extents of Drifts 1 and 2. The lowest elevation moraine mapped is at 4245 m and the highest is at 5445 m in both cases these are small moraines.
The large lateral-terminal moraines that radiate from Nevado Sajama comprise the most visually obvious evidence of past glacial expansion on the mountain. These moraines and others like them in the Western Cordillera have been used in reconstructions of paleo-ELAs during the local last glacial maximum (Klein et al., 1998). However, at least on Sajama, the large lateral- terminal moraines that bound the lower reaches of the valleys predate this time. Smaller moraines only a few meters high overlie or crosscut the larger moraines. Huaqui Jihuata Valley serves as an example. First, there are small moraines that crosscut the large left lateral moraine
(fig. 3). Also, a small left lateral moraine overlies the large valley forming left lateral moraine
16 (fig. 4). In the upper reaches of the valley the crest of the smaller moraine is deposited on the ice-proximal slope of the large valley-forming moraine. This overlapping relationship demonstrates that the smaller moraines are younger than the larger moraines.
Active rock glaciers and rock glacier deposits are found on some south facing slopes below bedrock exposures. The secondary peaks surrounding Sajama are high enough to support rock glaciers while providing ample debris. Active rock glaciers are unvegetated, bouldery deposits, with steep crescentic hummocks. The up valley side of the rock glaciers grade into scree slopes or fans while down slope margin is an over steepened slope supported by the interstitial ice. The minimum elevation of the rock glaciers is about 4800 m. By definition, rock glaciers form only areas of perennially frozen ground. The lowest extent of rock glaciers has been used as a maximum limiting altitude for the 0oC isotherm (Kerschner, 1978).
Scree slopes are found down slope from many bedrock exposures and are mapped together with fan deposits. Fan deposits are found not only against the mountain but also on and in between ice-distal slopes of the large lateral moraines described above. Thus, the scree and fan unit encompasses a broad spectrum of deposits from high angle slope deposits (screes) to the low angle water lain deposits (fans) outside of the large valley forming moraines.
The upper and lower elevations of rock glaciers, valley forming moraines, and the four drift units are plotted in figure 5 along with the estimated modern snowline. The elevations of the large lateral-terminal moraines and the drift units within them are remarkably consistent around the mountain. The lowest extents are concentrated on the south side of the mountain as expected in the Southern Hemisphere. These relationships lend credence to the belief that the deposits are glacial drifts. Also of note, the drift units terminate higher on the mountain than the large valley forming moraines. The inset positions of the drifts indicate a younger age for these
17 deposits. Areas of Plateau Drift are distinctive because they are not bounded by large valley forming moraines. On the north-east and south-west aspects of the mountain this can be explained by the presence of bedrock ridges and secondary peaks blocking the areas of Plateau
Drift from high elevation accumulation areas on Sajama. However, the two areas of Plateau
Drift on the north side of the mountain drain high elevation accumulation areas, but lack the large valley forming moraines.
In figure 5, the aspect of rock glaciers are plotted relative to the summit. However, they are often on the south facing slopes of local ridges. According to Kerschner (1978), the termini of active rock glaciers must lie above the annual 0o C isotherm. Rock glaciers on Sajama extend to ~4800 m. This provides a slightly lower elevation 0o C isotherm than the 4990 m estimate calculated using meteorological data from Cosapa village and a lapse rate of 6.85 oC/km
(SENAMHI, unpublished data). However, the estimates are within 200 m of each other and in general agreement. Since the lowest glacier margin on Sajama is currently at ~5240 m these findings are consistent with the belief that the annual 0o C isotherm is below the glacier termini.
However at this location the vertical distance between the 0o C isotherm and the glacier termini is less pronounced than the maximum value of 1000 m suggested by Klein et al. (1999).
36Cl Exposure Ages
As is usually the case with exposure age dates, there is scatter within the individual dates for any single landform. This is due primarily to two processes, exhumation and inheritance, that affect the number of cosmogenic nuclides present in a sample (Briner et al., 2005). Absolute ages of samples can be altered by erosion of the sampled rock surface. Removal of the outer cosmogenic nuclide rich material makes the measured age younger than the actual age of a
18 landform. However, assuming that erosion rates are the same for all rocks, erosion of rock surfaces does not account for scatter in the data set. Sample locations are plotted on the geomorphic map (Fig. 3) with between three and eight samples collected from each landform.
The exposure ages range from 0.2 ka within a couple hundred meters of active glacial ice to 27.8 ka on striated bedrock (Table 3). The outermost small moraines date to late glacial times.
Drifts 2 and 3 date to the middle and late Holocene respectively. These are among the first
Holocene ages from glacial deposits in the Western Cordillera.
Discussion
Geomorphic Interpretation
Further evidence for the relic nature of the large lateral-terminal moraines comes from
Patokho Valley. Here, a thin bouldery drift sheet bounded by a small bouldery moraine blankets both the ice-proximal and ice-distal slopes of the valley-forming moraine. The bouldery drift is not mapped in figures 2 and 3 because it is not clearly visible in remotely sensed images and thus difficult to interpret around the mountain. However, the bouldery moraine that bounding the margin of the boulder drift is visible on these remotely sensed images. Both the drift sheet and the moraines are clearly visible in the field as can be seen in figure 6. The drift is only a thin veneer of boulders deposited atop the unaltered large right lateral-terminal moraine. Several small, bouldery, concentric, recessional moraines exist within the bouldery drift unit. These deposits are unique from others that we examined in the field. The bouldery drift sheet and bouldery moraine have all of the characteristics of being deposited by cold based ice.
19 Since cold based ice deposits have not been reported in the tropics, we present a brief discussion here. The deposits of cold-based ice are distinct from those of wet based ice. Staiger et al. (2005) indicate that cold based ice: (1) deposits bouldery moraines or drift; (2) deposits drift atop undisturbed landscapes; and (3) does not form glaciofluvial features while wet based ice does just the opposite. The outermost moraines in Patokho Valley meet all of these requirements. They are composed entirely of boulders and do not have fine matrices. The bouldery moraines are draped over the large valley-forming moraine without altering it. In fact, the outermost bouldery moraine overlies an undisturbed fluvial terrace where it descends onto the floor of Patokho Valley. There is a distinct lack of glacial fluvial landforms associated with this moraine set figure 6. Thus, we infer that cold-based ice deposited the bouldery outermost moraines in Patokho Valley.
Cold-based ice deposits subtle landforms; the bouldery drift and bouldery moraine are barely discernable on aerial photographs. This problem is compounded by the fact that the large lateral moraine is undisturbed by the overlaying cold-based ice deposit. Although the cold-based ice deposits are more apparent in the field, they could still be misinterpreted by workers not accustomed to the subtle deposits of cold based ice. For example, if the bouldery drift had not been recognized as a separate deposit, then samples collected from the boulders of the relatively young drift sheet may have been believed to date the much older valley forming moraine leading to a far different long term geomorphic history of the mountain. With the increase in the use of exposure age methods, recognizing cold-based ice deposits overlying older moraines is critical to the correct interpretation of the glacial geologic history of the high Andes.
20 Timing of Glaciation in the Western Cordillera
Briner et al. (2005) examined exposure ages on independently dated moraines in Alaska and discussed possible ways of assigning ages as arithmetic mean, weighted mean, or oldest age.
For moraines far from cirque headwalls, Briner et al. (2005) determined that exhumation was the dominant source of error and inheritance was rare. So the older or oldest ages on the moraine more closely approximate the age of the moraine. Other workers (Zreda et al., 1994; Phillips et al., 1997) have found similar results. Thus, we favor ages on the older sides of the distributions for any single landform.
Given the many unknowns and the lack of independent dating methods in the area, we take a conservative approach to assigning ages. Rather than choosing a single age, a range from the arithmetic mean to the maximum age is used. For example, ignoring errors, the dates on
Drift 2 are 2.2 ka, 3.3 ka, 5.0 ka, and 7.0 ka. The mean age is 4.4 ka. Thus, we assign an age for the deposit between 7.0 ka and 4.4 ka. Although a number of metrics is listed in Table 4, the ages of key geomorphic features are as follows: the outermost moraine in Patokho Valley ( beyond Drift 1) is 16.9 -11.8 ka; the outermost moraine in Huaqui Jihuata Valley (beyond Drift
1) dates to 14.0 -10.2 ka; the Plateau Drift in Patokho Valley is 7.0 - 6.5 ka; Drift 2 is 7.0 - 4.4 ka; and Drift 3 is 4.7 - 3.3 ka, and the samples collected near the modern ice margin are ∼0.2 ka
(Table 4). Although there is some overlap between ages of different deposits, the ages are in the correct morphostratigraphic order with older deposits in ice-distal positions and younger deposits in ice-proximal positions (fig. 7).
With the addition of the 36Cl dates from Nevado Sajama, the glacial chronological data set from the Western Cordillera begins to approach a size where preliminary comparisons can be made north and south along the range and between the Eastern and Western Cordilleras.
21 As mentioned previously, the only glacial chronology in the Western Cordillera north of the arid diagonal is that of Clayton and Clapperton (1997) who dated organic material in a proglacial fan that grades into a delta of paleo-lake Tauca. These radiocarbon ages were interpreted to date the maximum advance of the last glacial cycle to shortly after 13,300 ± 90 14C
(15,717 ± 211 calendar, Stuiver and Long,1993). The outermost moraines at Sajama date to16.9
-11.8 ka and 14.0 -10.2 ka. Thus, our findings are partially consistent with those of Clayton and
Clapperton (1997) in that there is a large amount of overlap between the ages of the bouldery moraine in Patokho Valley at Sajama and the proglacial fan deposit of Clayton and Clapperton
(1997). However, the outermost moraine in Huaqui Jihuata Valley at Sajama postdates the age from further south on the Altiplano. This is likely the result of the fact that the date from the proglacial fan is a maximum limiting age and the exposure age dates from Sajama are minimum limiting ages.
Zech et al. (2006), working at 29o S in the zone of westerlies, found that a minor glacial advance occurred at 11.6 ± 1.2 ka, and major advances occurred at 14.0 ± 1.4 ka, and 24.1 ± 2.4 ka. Correlation of the 14.0 ± 1.4 ka advance to the 15.7 ± 0.2 ka advance of Clayton and
Clapperton (1997) led Zech et al. (2006) to conclude that glacial activity was synchronous north and south of the arid diagonal. While this may be true during late glacial times, nothing as old as
24 ka has been found in the zone of easterlies. Thus, there is some ambiguity regarding synchronicity of glaciation north to south along the Western Cordillera. Although data are sparse in the region, this may suggest that the climate controls on glaciation in the Western
Cordillera differ through time and space.
In the Western Cordillera, the maximum advance of the last glacial cycle appears to have occurred during late glacial times. This contrasts with the Eastern Cordillera where recent work
22 suggests an early local Last Glacial Maximum at ∼32 ka (Smith et al., 2005) and an early warming at ∼ 22 ka (Seltzer et al., 2002). 10Be exposure ages from end moraines in the Eastern
Cordillera of Peru and Bolivia suggest that glaciers reached their maximum extents at ~34 ka well before the global LGM (Smith et al., 2005). Sediment cores from both Lake Junin, Peru and Lake Titicaca, Peru-Bolivia show an abrupt change from clastic rich to organic rich sedimentation between 22 ka and 19.5 ka suggesting the retreat of glaciers prior to the Bølling /
Allerød warming in the Northern Hemisphere (Seltzer et al., 2002). These data suggest that during the last glacial cycle the maximum glacial advance occurred thousands of years earlier in the Eastern Cordillera than in the Western Cordillera only a couple hundred kilometers away.
However, as Schaefer et al (2006) point out the data set of Smith et al. (2005) indicates a termination at 17.3 ± 1.9 ka that overlaps the dates indicative of retreat from the late glacial moraines at Sajama. Regardless, the assumption, used to reconstruct regional paleo-ELAs, that the local LGM occurred synchronously in the Eastern and Western Cordilleras may not be valid.
Our Holocene record of glacial activity with advances at 7.0 - 4.4 ka; 4.7 - 3.3 ka represents the first published absolute ages of moraines in the Western Cordillera, Bolivia. The only published date of Holocene glacial activity in the Western Cordillera comes from Enciero
Valley in Northern Chile. Here, Grosjean et al. (1998) radiocarbon dated peat from within a moraine to 2600 14C years before present. Due to the dearth of Holocene data in the Western
Cordillera, we are forced to compare our record to that of the Eastern Cordillera. Rodbell (1992) reports Holocene glacial moraines in the Cordillera Blanca, Peru at 7-6 ka; 3.4-1.8 ka; 1.3-0.4 ka, and within the past several hundred years. Three out of the four periods of glacial advance correlate between cordilleras indicating regional advances at ~7.0-6.0 ka, ~3.3 ka, and within the
23 past couple hundred years. These data suggest synchronicity between the two cordilleras during the Holocene.
The relative timing of glacial advance and retreat in the Eastern and Western Cordilleras is an important paleoclimatic proxy because different forcing mechanisms are believed to dominate the different cordilleras. In the Eastern Cordillera, the 0o C isotherm intersects glaciers near the equilibrium line altitude (ELA) (Francou, 1995). In the Western Cordillera, the 0o C isotherm is up to 1000 m below the termini of the glaciers (Klein et al., 1999). Thus, it has long been hypothesized that glaciers in the Eastern Cordillera are more sensitive to temperature changes while glaciers in the Western Cordillera are more sensitive to precipitation changes
(Hastenrath, 1971). Accordingly, if the glaciers in the Eastern and Western Cordilleras respond to different forcings, then it is possible if not likely that the two areas have different glacial chronologies.
Given the current dating uncertainties, the late glacial moraines in the Western Cordillera are indistinguishable from the late glacial moraines in the Eastern Cordillera. However, the lack of more extensive local last glacial maximum moraines in the Western Cordillera stands in contrast to results from the Eastern Cordillera. Three possible scenarios may explain these findings. 1) There was a more extensive local last glacial maximum advance in the Western
Cordillera that has yet to be mapped and dated. 2) There was no advance in the Western
Cordillera that coincided with the local last glacial maximum advance in the Eastern Cordillera.
3) The local last glacial maximum advance was less extensive than the late glacial advance in the
Western Cordillera.
With dated moraines in only three valleys in the Western Cordillera it is possible that in other locations in the cordillera the local last glacial maximum was more extensive than the late
24 glacial advance. Although this cannot be ruled out, the existing data set suggests a late glacial maximum. The complete absence of a local last glacial maximum advance in the Western
Cordillera seems unlikely. Ohmura et al., (1992), demonstrated that the dominant forcing on the elevation of a glacial ELA is temperature. Since paleo-temperature is unlikely to vary across the
Altiplano there was probably an advance in the Western Cordillera coincident with the local last glacial maximum in the Eastern Cordillera. The similar Holocene chronologies seem to support correlation between the cordilleras. Therefore, the most likely interpretation is that the local last glacial maximum was less extensive (not a maximum) than the late glacial advance in the
Western Cordillera.
While local ELAs are dominantly controlled by temperature (Ohmura et al., 1992), regional ELAs reflect precipitation gradients. This is well demonstrated by the modern rise in
ELA and glacial extents between the Eastern and Western Cordilleras (Klein et al., 1998). With these relationships in mind we propose a thought experiment to explain apparent asymmetry between the glacial chronologies of the Eastern and Western Cordilleras. Given modern conditions, a reduction in temperature with no change in precipitation would cause advances in both cordilleras, but the rise of the ELA and glacial extents from East to West would not be significantly affected. Thus, the new glacial margins in the Western Cordillera would still be at a higher elevation than the new glacial margins in the Eastern Cordillera. However, if the same cooling were coupled with a change in the precipitation gradient such that both cordilleras received nearly the same amount of precipitation then the ELAs and glacial extents would be similar in both ranges. As a working hypothesis to explain the differences between the Eastern and Western Cordilleras we suggest the above scenarios are indicative of the local last glacial maximum and the late glacial respectively.
25
Paleo-ELAs
Arnaud et al (2001) used 14 images from 12 years of satellite imagery from Sajama and an estimated sublimation rate to calculate annual snowlines normalized to the end of the dry season (August 15). Averaging these data provides a modern mean annual snowline of approximately 5630 m. Based on the uppermost elevation of the lateral moraines we suggest minimum paleo-ELAs of 4690 m and 5000 m for the outermost moraines and Drift 2 respectively. No lateral moraines are apparent for Drift 3. Changes in ELA were 940 m and 630 m during deposition of Drifts 1 and 2. We stress that the ∆ELA values are maximums since the paleo-ELAs derived from the elevation of lateral moraines are always minimum values.
Since the glaciers of the Western Cordillera are believed to be sensitive to changes in precipitation (Hastenrath, 1971) the standard method of determining paleotemperature by multiplying the change in ELA by the lapse rate is an insufficient measure of paleoclimatic conditions. Ohmura et al., (1992) cataloged the environmental conditions at the ELAs of 70 glaciers worldwide. A strong correlation between summer temperature and precipitation at the
ELA was found. Although tropical glaciers are underrepresented in this data set, Clayton and
Clapperton (1997) have shown that the tropical glaciers for which there are data correlate well with the global relationship of Ohmura et al. (1992). Thus, we use this relationship to suggest past environmental conditions during deposition of the outermost small moraines and the moraines associated with Drift 2 on Nevado Sajama.
For the outermost late glacial moraines four calculations are made based on the assumptions that the glacial advance was caused by 1. only an increase in precipitation; 2. only a decrease in temperature; 3. an increase in precipitation of 200 mm and some cooling Hastenrath
26 and Kutzbach, 1985) and; 4. an increase in precipitation of 500 mm and some cooling (Grosjean,
1994) (fig. 8). If precipitation alone were to account for the ELA lowering then there would need to be a massive increase of ~1700 mm of precipitation annually. This is better than a 500% increase in precipitation, and we do not propose that it happened. Rather, we provide it as a limiting value. According to scenario 2, a 5.3 ± 1.5 oC cooling could have caused the ELA depression in the absence of any increased precipitation. While this magnitude of temperature depression is reasonable (Stute et al., 1995), it does not account for the increased precipitation recorded in other proxy data. If precipitation increased by 200 mm as suggested by Hastenrath and Kutzbach (1985), then the required cooling would be 4.6 ± 1.5 oC. Finally, if precipitation increased by 500 mm as estimated by Grosjean (1994) then the required cooling would be only
3.6 ± 1.5 oC . Scenarios 2, 3, and 4 suggest temperature depressions that agree with other records. Stute et al., (1995) and Guilderson et al. (2001) suggest tropical South American late glacial temperature depressions of 5.4 ± 0.6 oC and ~4.5 oC based on dissolved noble gas concentrations in ground water, and the oxygen isotopic record from corals respectively. Given the other paleo-climatic records scenario 3 seems most appropriate.
For Drift 2 deposited between 7.0 ka and 4.4 ka there is neither a corresponding paleo- lake nor an estimate of paleo-precipitation. Thus, we provide only the limiting values of scenarios 1 and 2 predicting maximum increases in precipitation and decreases in temperature respectively. In order to lower the ELA to 5000 m without changing the temperature precipitation would have to be increased by nearly 1 m. The same ELA lowering could be achieved without changing precipitation if the temperature were 3.3 oC cooler.
The rise of modern ELAs from east to west across the central Andes, and the fact that the termini of modern glaciers in the Western Cordillera are above the 0 oC isotherm, suggest that
27 Western Cordilleran glaciers are sensitive to changes in precipitation. This is true relative to glaciers in more humid regions such as the Eastern Cordillera. However, considering the large magnitude of precipitation change required to lower ELAs to their late glacial elevations in the absence of a temperature depression, these findings suggest that even in the semi-arid Western
Cordillera temperature is still an important control on glaciers.
Conclusions
The geomorphology of Nevado Sajama suggests that the large lateral-terminal moraines that bound the lower reaches of the valleys are not of LGM origin as previously suggested.
Rather they are relic landforms with younger moraines overlying or crosscutting them. The presence of multiple drift units of both warm and cold-based ice dating to the late glacial and
Holocene times suggests an active and complex glacial geologic history of the mountain that is likely found in other locations in the high tropical Andes.
Moraines that pre-date the global LGM have been reported in the Eastern Cordillera
(Smith et al., 2005). Since the oldest dated moraines on Nevado Sajama appear to be late glacial in age an apparent asymmetry exists between the Eastern and Western Cordilleras. We hypothesize that this is due to changes in the east-west precipitation gradient over time.
Specifically, if the gradient remained unaltered from its present state during full glacial times but become nearly flat during late glacial times then the local last glacial maximum advance in the
Western Cordillera would be less extensive than the late glacial advance.
28 During the Holocene, ages of Eastern (Rodbell, 1992) and Western Cordilleran moraines overlap each other again with some uncertainty. Therefore, at varying times our data both support and refute an out of phase relationship between the Eastern and Western Cordilleras.
Acknowledgements
Thanks to the Parque Nacional Sajama for permission to work in the national park.
Thanks also to J.M. Escobar and M. Escobar for liaison and logistics within Bolivia. Thanks to
M. Bourgeois and S. Ma at PRIME LAB for assistance preparing samples for dating and calculating ages. Fieldwork was partially funded by GSA Grant Number 8405-06.
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33 Table 1. Results of elemental analysis, and mean values used for calculating other ages. composition Unit BV -06-33 BV-06-34 BV-06-35 BV-06-36 Mean Al2O3 % 14.1 15.4 14.8 14.3 14.65 B ppm 41 11 22 24 24.5 Ca % 1.92 2 1.45 1.94 1.8275 CaO % 2.86 3.51 2.27 3.32 2.99 Fe2O3 % 3.99 4.79 4.53 4.36 4.4175 Gd ppm 4 3 3 4 3.5 K % 3.71 3.28 3.94 3.7 3.6575 K2O % 4.23 3.99 4.51 4.33 4.265 MgO % 1.65 1.7 1.1 1.33 1.445 MnO % 0.06 0.06 0.04 0.06 0.055 Na2O % 3.56 4.15 3.69 3.76 3.79 P2O5 % 0.26 0.43 0.65 1.12 0.615 SiO2 % 64.8 62.5 65.3 64.6 64.3 TiO2 % 0.71 0.83 0.79 0.79 0.78
34 Table 2. Descriptions of key deposits shown on the geomorphic maps or mentioned in the text.
Deposit Dimensions/ Description Vegetation Cover Boulder Density LIA ∼3 m high ridge bounded None High proximally by depression recently vacated by glacier. Deposit is ice-cored. Drift 3 High amount of gravel and None Moderate cobble sized sediments. Steep down valley slope. Active stone stripes. Drift 2 Light tan unit of mixed Low Moderate sediment sizes bounded by moraines of ∼4 m. Plateau Drift Hummocky drift often Low Moderate without associated moraines. Drift 1 Hummocky deposit with Moderate Moderate linear ridges. Composed of distinctive red boulders and finer sediments. Bouldery Moraine ∼2 m high composed of Low High exclusively boulders. Outermost Moraine ∼3 m high composed of Moderate Low Huaqui Jihuata sediments of mixed sizes. Valley
35
Table 3. Sample ages, locations, and Cl ratios. Age ka (0 Age ka (0.835 Altitude 36Cl/Cl E- Sample erosion) mm/ka) Latitude Longitude (m) 15 Drift BV-06-01 9.37 ± 0.19 9.95 ± 0.23 18.1088 68.8077 4428 591 ± 12 1 BV-06-02 7.46 ± 0.18 7.81 ± 0.20 18.1088 68.8078 4429 551 ± 12 1 BV-06-03 14.13 ± 0.32 15.67 ± 0.40 18.1088 68.8079 4425 1034 ± 23 1 BV-06-08 15.07 ± 0.44 16.86 ± 0.55 18.1087 68.8103 4498 1175 ± 33 1 BV-06-09 9.01 ± 0.25 9.56 ± 0.29 18.1087 68.8105 4501 685 ± 18 1 BV-06-13 11.26 ± 0.28 12.18 ± 0.33 18.1092 68.8116 4499 897 ± 21 1 BV-06-14 9.7 ± 0.24 10.34 ± 0.28 18.1091 68.8119 4492 674 ± 16 1 BV-06-34 8.43 ± 0.23 8.9 ± 0.26 18.1201 68.7931 4267 629 ± 16 1 BV-06-35 7.25 ± 0.21 7.74 ± 0.24 18.1199 68.7942 4279 526 ±14 1 BV-06-36 12.78 ± 0.39 14.02 ± 0.48 18.1199 68.7940 4287 1119 ± 33 1 BV-06-28 4.93 ± 0.14 5.05 ± 0.15 18.0966 68.8319 4731 451 ± 12 2 BV-06-29 2.24 ± 0.10 2.2 ± 0.10 18.0963 68.8322 4747 176 ± 7 2 BV-06-30 3.32 ± 0.11 3.34 ± 0.11 18.0967 68.8354 4737 299 ± 9 2 BV-06-31 6.74 ± 0.18 7.02 ± 0.20 18.0951 68.8354 4732 620 ±16 2 BV-06-25 6.15 ± 0.19 6.36 ± 0.21 18.0882 68.8213 4639 463 ± 13 Plateau BV-06-26 5.91 ± 0.16 6.1 ± 0.18 18.0883 68.8213 4625 463 ± 12 Plateau BV-06-27 6.76 ± 0.20 7.03 ± 0.23 18.0881 68.8213 4629 545 ± 16 Plateau BV-06-10 3.24 ± 0.10 3.25 ± 0.11 18.1077 68.8477 5060 280 ± 8 3 BV-06-11 4.57 ± 0.17 4.67 ± 0.18 18.1068 68.8480 5060 463 ± 16 3 BV-06-12 2.51 ± 0.11 2.5 ± 0.12 18.1074 68.8478 5061 226 ± 9 3 BV-06-17 4.29 ± 0.16 4.37 ± 0.17 18.1052 68.8491 5081 410 ± 14 3 BV-06-18 2.67 ± 0.09 2.67 ± 0.10 18.1050 68.8494 5085 252 ± 8 3 BV-06-19 2.72 ± 0.12 2.71 ± 0.13 18.1052 68.8497 5083 258 ± 11 3 BV-06-32 3.95 ± 0.13 4.01 ± 0.14 18.1062 68.8476 4957 384 ± 12 3 BV-06-33 2.28 ± 0.08 2.25 ± 0.09 18.1055 68.8483 5071 177 ± 5 3 BV-06-23 0.25 ± 0.03 0.21 ± 0.03 18.1212 68.8607 5169 48 ± 3 LIA BV-06-24 0.16 ± 0.04 0.11 ± 0.04 18.1208 68.8604 5229 35 ± 3 LIA BV-06-04 8.49 ± 0.24 8.49 ± 0.24 18.1167 68.8214 4584 755 ± 20 Bedrock BV-06-05 8.05 ± 0.37 8.05 ± 0.37 18.1168 68.8214 4589 729 ± 32 Bedrock BV-06-06 27.8 ± 0.75 27.8 ± 0.75 18.1207 68.8109 4480 1869 ± 48 Bedrock BV-06-07 20.3 ± 0.46 20.3 ± 0.46 18.1207 68.8109 4471 1408 ± 31 Bedrock BV-06-15 8.36 ± 0.19 8.36 ± 0.19 18.1036 68.8258 4675 759 ± 16 Bedrock BV-06-16 7.94 ± 0.23 7.94 ± 0.23 18.1036 68.8259 4687 670 ± 18 Bedrock BV-06-20 13.37 ± 0.25 13.37 ± 0.25 18.0996 68.8322 4712 1243 ± 23 Bedrock BV-06-21 10.18 ± 0.30 10.18 ± 0.30 18.0998 68.8324 4695 932 ± 26 Bedrock BV-06-22 11.3 ± 0.38 11.3 ± 0.38 18.0996 68.8324 4692 1148 ± 38 Bedroc
36
Table 4. Landform age estimates.
Number of Minimum Maximum Mean Age Standard Unit samples Age (ka) Age (ka) (ka) Deviation 1 10 7.74 16.86 11.30 3.24 2 4 2.20 7.02 4.40 2.10 Plateau 3 6.10 7.03 6.50 0.48 3 8 2.25 4.67 3.30 0.93 LIA 2 0.11 0.21 0.16 0.07 Bedrock 9 7.94 27.80 12.87 6.85
37
Figure 1. Digital elevation map of the Altiplano with key locations indicated. The box in the inset South American map approximates the enlarged map.
38 Figure 2. Geomorphic map of Nevado Sajama. The box approximates the area enlarged in figure 3.
39 Figure 3. Geomorphic map of the field area on the east side of Nevado Sajama. The numbers indicate the locations of samples which are described in Table 4.
40 Figure 4. A. Oblique Google Earth image of Huaqui Jihuata Valley, Nevado Sajama, Bolivia.
View is towards the east-north-east. 1 = Drift Unit 1. 2 = Drift Unit 2. 3 = Drift Unit 3. P =
Plateau Drift. RG = Active Rock Glaciers. The box refers to the area of figure 4 B. B. Google
Earth image of left lateral moraine expanded from A. The dotted line labeled indicates the crest of the older valley forming moraine. The younger moraine is overlying the older valley forming moraine on the ice-proximal slope.
41
42
Figure 5. Photographs of selected deposits mentioned on the geomorphic maps or in the text.
A. The apparent Little Ice-Age Deposits. Note the meter scale in the foreground and the proximity to the glacier in the background. B. Drift 3. C. Drift 2. D. Plateau Drift on the bench. For scale, the boulders are about 1 m in height. E. The hummocky unit in the bottom of the valley is Drift 1. For scale, the valley is about 400 m wide. F. Photograph of the bouldery moraine interpreted to have been deposited by cold-based ice near the mouth of Patokho Valley.
For scale, the boulders are about 1-1.5 m in diameter. For reference, samples 1, 2, and 3 were collected from this portion of this moraine. G. The outermost moraine below Huaqui Jihuata
Valley.
43
44
Figure 6. Aspects and elevations of rock glaciers, valley forming moraines, and associated drifts on Nevado Sajama.
45 Figure 7. Plot of ages of deposits vs. their respective distances from the summit of the mountain.
Hollow points represent samples collected from drifts, solid points represent samples collected from moraines, and “X”s represent samples collected from striated bedrock. Note the general trend of younger deposits closer to the summit of the mountain.
46 Figure 8. Using the data set of Ohmura (1992) paleoclimatic conditions are estimated for the paleo-ELAs of the outermost moraines and drift unit 2. A. The star indicates the current environmental conditions at 4690 m, the paleo-ELA of the outermost moraines. Ascending the graph are points assuming no increase in precipitation, a 200 mm, a 500 mm increase in precipitation, and no change in temperature. B. The star indicates the current environmental conditions at 5000 m the paleo-ELA associated with drift unit 2. Ascending the graph are points assuming no change in precipitation, and no change in temperature.
47 Chapter 3. Late Quaternary glacial chronology Nevado Illimani, Cordillera Real, Bolivia: Implications for paleoclimatic reconstructions across the Andes
Abstract
Regional glacial geologic studies in the in the central Andes suggest an equilibrium line altitude (ELA) lowering of ~1000 m during the late Pleistocene. Detailed, localized, work on dated glacial features, however, rarely shows this magnitude of ∆ELA. New 10Be surface exposure ages from moraines on Nevado Illimani, Cordillera Real, Bolivia suggest that glaciers retreated from moraines during the periods 15.0 ka -13.0 ka; 10.5 ka - 8.5 ka, and 3.0 ka - 1.5 ka.
Late Glacial moraines at Illimani are associated with a ∆ELA of ~400 m, which is consistent with other local reconstructions of Late Glacial ELAs in the Eastern Cordillera of the central
Andes. Reconstructed ELA gradients across the Andes based on dated Late Glacial moraines suggest a change in slope of the regional ELA that is believed to be a result of increased precipitation from the west.
Introduction
Lake level data from Lake Titicaca provide solid evidence that the Altiplano experienced periods of increased precipitation during glacial (26 ka -18 ka) and Late Glacial (13 ka -11.5 ka) times (Baker et al., 2001a). These data do not provide, however, the source of this precipitation.
Haug et al. (2001) and Baker et al. (2001b) suggest that the increased precipitation may have been advected to the Altiplano from the Atlantic as a result of the southward displacement of the intertropical convergence zone and the strengthening of the South American summer monsoon.
48 Alternatively, Heusser (1989) suggests the increased precipitation may have originated in the
Pacific and been transported to the Altiplano by winter storms associated with the northward displacement of the westerly wind system.
The glacial geologic record, however, can help in reconstructing regional precipitation gradients and help elucidate the moisture source. Reconstructions of former glacial ELAs are used to derive paleo-climatic conditions, which are a combination of temperature and precipitation. Ohmura et al. (1992), however, showed that temperature correlates strongly to the elevation of the ELA of a glacier. The regional ELA's gradient, however, reflects the regional precipitation gradient, and can be used to extract precipitation source (Harrison, 2005; Benn et al., 2005 ). Glacial ELAs and extents in the Eastern Cordillera, for example, are hundreds of meters lower than glaciers in the Western Cordillera (Francou et al., 1995; Klein et al., 1999;
Jordan, 1999) despite having essentially the same temperatures at both locations. The rise in
ELA to the west is a result of decreasing precipitation to the west. Thus, examining the ELAs in both the Eastern and Western Cordilleras over time can provide hints as to the regional precipitation gradient and the source of the precipitation.
Regional reconstructions of past ELAs in the tropical Andes of Peru and Bolivia consistently find a ~1000 m lowering of the modern ELA during the late Pleistocene (Nogami,
1976; Satoh, 1979; Fox and Bloom, 1994; Klein et al., 1999; Porter, 2001). These studies use a variety of methods to determine both the paleo-ELA and the modern ELA. Due to the regional nature of the studies, however, the data are collected from remotely sensed images or topographic maps compiled from remotely sensed data. These regional data sets do not have absolute chronologies associated with them. Smaller scale field studies, with associated chronologies, rarely show this magnitude of ∆ELA. Seltzer (1992 and 1994) calculates ∆ELAs
49 of 300-500 m in the Cordillera Real, Mark et al. (2002) estimated ELA depressions of 170-230 m in the Cordillera Vilcanota and at the Quelccaya Ice-cap, Smith et al. (2005) found ∆ELAs of
300-600 m on the Junin Plain and the west side of the Cordillera Real. Smith et al. (2005) report a 800-1000 m lowering of the ELA in one location on the east side of the Cordillera Real.
Part of this difference in ∆ELA is explained by the chronologies associated with the moraines used to reconstruct paleo-ELAs. The ∆ELAs of Seltzer (1992 and 1994) and Mark et al. (2002) are Late Glacial. Although Smith et al. (2005) mapped and dated Late Glacial moraines, they only provide ∆ELAs from moraines that date to ~32 ka. Therefore, the Late
Glacial moraines that are inset to the 32 ka moraines would have smaller ∆ELAs. Studies based on remotely sensed data do not have chronologies attached to them. Rather, the outermost moraine is assumed to date to the Last Local Glacial Maximum a period of time that is poorly defined in the South American tropics.
To define paleo-ELAs and determine paleo-precipitation sources we develop a chronology based on geomorphic mapping and 10Be surface exposure ages for Nevado Illimani.
The Late Glacial chronology at Illimani and the ∆ELA compare favorably to those set forth by
Seltzer (1990; 1992; 1994) and Smith et al. (2005). These findings from the Eastern Cordillera are then compared to paleo-ELA reconstructions from the Western Cordillera for the same Late
Glacial period (Clayton and Clapperton, 1997; Smith et al., in review) in order to determine change in the regional slope of the ELA.
Geologic Setting
Fieldwork for this study was carried out in Pasto Grande Valley between 4000 m and
5200 m above sea level (a.s.l.) on the north side of Nevado Illimani in the Cordillera Real,
50 Bolivia (Fig. 1). The Cordillera Real is a range of 6000 m peaks in the Eastern Cordillera that bounds the east side of the Altiplano, a high elevation plateau at about 3800 m a.s.l. Currently, precipitation in the region originates from the tropical Atlantic and is transported by northeast trade winds over the continent. While some cities in the Beni region east of the Eastern
Cordillera receive about over 2000 mm of rain per year (SENAHAMI, website), the Altiplano is semi-arid. The intensively studied Zongo Glacier, about 60 km northwest of Illimani along the axis of the Cordillera Real provides a good source of meteorological data. There the mean annual precipitation for hydrological years 1992-1996 was 794 mm, and the mean annual temperature was 7.2 oC at 3922 m (Francou et al., 1995; Wagnon, et al., 1999). The mean annual
0 oC isotherm is at about 4895 m a.s.l using a lapse rate of 7.4 oC / km (Francou et al., 1995), in conjunction with the measured temperature at 3922 m. Modern glaciers in Pasto Grande Valley extend to about 4900 m apparently terminating at the mean annual 0 oC isotherm.
Methods
Geomorphic mapping of the field area allowed for a better understanding of the morphostratigraphic relationships between glacial geomorphic features. The map, constructed from Bolivian aerial photograph number 1052 taken in 1963, also serves to plot the thirty-five samples of grano-diorite collected from moraines and dated using 10Be exposure age methods.
When selecting rocks to sample care was taken to choose large intact boulders that were unlikely to have been transported down slope or exhumed subsequent to deposition of the moraine. A minimum of 600 g of rock was collected using a hammer and chisel from nearly horizontal surfaces. Latitude, longitude, and elevation data were collected for each sample site using a handheld GPS. Shielding values were measured with a pocket transit at 30o intervals.
51 Processing of the rock samples to isolate Be was carried out at the University of
Cincinnati. Samples were crushed and sieved to isolate the 250-500 µm fraction. The samples were then leached in aqua regia overnight prior to two 24 hour leachings in 5% HF/HNO3. Next the mafic minerals were removed from the sample by heavy liquid (lithium heteropolytungstate) separation. Then the samples were leached a final time in 1% HF/HNO3 for 24 hours to obtain pure quartz.
Approximately 20 g of pure quartz from each sample was mixed with 0.4935 mg/g or
1.26 mg/g or Be carrier and dissolved in concentrated HF. Following perchloric acid fuming to remove F atoms, the samples were passed through anion and cation exchange columns to remove
Fe and Ti. The Be fraction was collected and precipitated with ammonium hydroxide. The resultant beryllium hydroxide gel was rinsed before being oxidized in quartz crucibles at 750 oC for 5 minutes to produce beryllium oxide. Nb powder was mixed with beryllium oxide and the combination was pounded into stainless steel targets for measurement of 10Be/9Be ratios by accelerator mass spectrometry at PRIME Laboratory, Purdue University. A full description of laboratory methods can be found in Dortch (2008).
Results
Geomorphology of Pasto Grande Valley
Currently, three small separate glaciers feed into Pasto Grande Valley. Although the aerial photograph used as the base for the map (Fig. 2) had enough snow cover to make the two glaciers in the south west tributary valley appear to be joined, they are in fact separate glaciers.
These two glaciers would have been joined during deposition of the youngest moraine set. Here called Group C, (Fig. 3, Table 1) which is composed of well developed left and right lateral
52 moraines that join terminal moraines and enclose a small pond at an elevation of 4785 m a.s.l.
Hummocky debris, the upper portions of which are likely ice-cored, is Inset to the Group C lateral moraines. A series of relic ice-contact fans grade to the Group C end moraines.
About midway down valley at an elevation of 4380 m a.s.l., two sets of end moraines, cut by a small stream, perch atop a 120 m bedrock step in the valley floor. These end moraines, termed Group B, can be traced up into laterals that have been removed by slope processes.
Down valley from the Group B end moraines the valley drops steeply over the bedrock step.
There are some traces of end moraines below the bedrock step, but they are poorly preserved due to stream incision. The associated lateral moraines are nearly buried in active scree.
A left lateral moraine terminates between Groups B and C at the junction with the southwest tributary valley. This moraine, part of Group A2, is much higher than the Group C moraines and represents an older more extensive glacier configuration when all three glaciers coalesced. The single Group A2 left lateral can be traced down valley where it becomes a series of smaller lateral moraines on the slope above the Group B end moraines before projecting over the bedrock step. The valley floor levels out down valley from the bedrock step. Small fragments, not more than 10 m in length, of both left and right lateral moraines of Group A remain along the valley walls. The laterals are neither continuous nor well preserved due to steep bedrock walls and active scree. Two end moraines, Group A1, cross the valley floor at an elevation of 4000 m a.s.l. The Group A1 end moraines are located just up valley from a bedrock gorge. They are the outermost preserved moraines observed in Pasto Grande Valley. Older fragments of left lateral moraines, however, are present down valley from the Group A1 end moraines. The laterals and end moraines of Group A are subdivided because the lateral moraines cannot be traced continuously to the end moraines. The longitudinal profiles for the left lateral
53 moraine and the floor of the valley (Fig. 4) indicate that elevations of the lateral moraines project in a straight line to the terminal moraines. However, would not be expected with the pronounced bedrock step. Rather, the laterals should more closely mimic the topography of the valley floor.
Although either scenario is possible, the dearth of data points immediately below the bedrock step prevents a reliable correlation between the Group A1 and A2 moraines. Thus for the purpose of dating, Groups A1 and A2 are treated separately in our study.
10Be AMS measurements were made in two groups at PRIME Lab, Purdue University.
The first group, highlighted in gray, contains 12 samples and 2 blanks, and uses a carrier with a concentration of 0.4935 mg/g. The second group contains 23 samples and 3 blanks, and uses a carrier with a concentration of 1.26 mg/g.
Glacial Chronology
The values used to calculate 10Be ratios are presented in Table 2. Since the blanks returned high values in the first group they were not used in calculating ages; rather the mean value from the prior column run (3.59 * 10-14) was used. However, for the second group the mean of the three blank values (1.815 * 10-15) was subtracted from the 10Be/9Be ratios prior to calculating ages.
Ages are calculated using the CRONUS online calculator using the scaling values of Lal
(1991) and Stone (2000). We understand that there are a number of different geomagnetic scaling schemes and that depending on which one is used ages may vary by upwards of 20%
(Fig. 5) Thus, we present our ages without accounting for geomagnetic changes through time.
However, the data needed to re-calculate ages using different scaling values are present in Tables
54 2. Ages, assuming 0 erosion, and without scaling for changes in geomagnetic variability, are shown in Table 2.
Paleo-climate Reconstructions
To reconstruct climatic conditions based on past glacial margins we use the terminus to headwall altitude ratio (THAR) described by Mierding (1982) and Porter (2001) with a THAR value of 0.37 as suggested by Seltzer (1992) while working in the Cordillera Real. Also, we use the summit elevation (6350 m a.s.l.) as that of the “headwall” because the glaciers extend to the summit of the mountain. Thus, former ELAs are calculated according to:
ELA = (elevation of end moraine) + [0.37* (6350 m - elevation of end moraine)]
Changes in ELA are calculated by subtracting the estimates of paleo-ELA from 5270 m a.s.l., which is the mean ELA of Zongo Glacier for hydrological years 1992-1997 (Francou et al.,
1995; Wagnon, et al., 1999). Temperature changes are calculated by using a lapse rate of 7.4 oC
/ 1000 m as measured by Francou et al. (1995) at Zongo Glacier.
To verify the use of the THAR method we also reconstructed the Late Glacial paleo-ELA using the accumulation area ratio (AAR) method (Mierding, 1982; Porter, 2001; Benn et al.,
2005). Seltzer (1992) suggested an AAR of 77% in the Cordillera Real. However, use of this value in Pasto Grande Valley produces a Late Glacial ELA below the uppermost extent of the
Late Glacial lateral moraine. Therefore, a lower AAR is required to produce a higher paleo-
ELA. Using an AAR of 66% yields a Late Glacial ELA of 4865 m, which is only 5 meters lower than that predicted by the THAR method. An AAR of 66% closely approximates the AAR of
55 temperate latitude glaciers (Mierding, 1982), but is on the low side for tropical glaciers (Benn et al., 2005). Considering the scarcity of continuous lateral moraines, and the sensitivity of the
AAR method to what ratio is used, we adopt the paleo-ELAs determined by the THAR method.
However, all variables and values are shown in Table 4.
Reconstructions of former ELAs using the THAR method are slightly higher than the maximum elevation of lateral moraines of the same ages for group C and A (assuming that A1 correlates with A2). In both cases the lateral moraines extend to steep bedrock faces where preservation is unlikely. The associated temperature depressions assuming a lapse rate of 7.4 oC/km (Francou et al., 1995) and no change in precipitation are 3.0 oC, 1.2 oC, and 0.2 oC based on ∆ELAs of 400 m, 160 m, and 20 m for groups A1, B, and C respectively.
Discussion
Ages of Moraine Groups
The 10Be ages are presented as probability plots for each moraine group (Fig. 6). At a distance of several kilometers from the cirque headwall, exhumation is a far more common source of error than inheritance (Briner et al., 2005). This has led some workers (Zreda and
Phillips, 1994; Phillips et al., 1997; Briner et al., 2005) to assign ages based on the single oldest date. While we do not select the oldest single sample we recognize that the age of a landform is likely to be on the older side of the distribution. Thus, we present minimum ages for stabilization of the moraines follow the retreat of ice from the landforms.
Age range assignments of the four groups of moraines are taken to be from the peak of the probability distribution to the older end of the distribution (Fig. 6). Although the Group C moraines are not believed to be the result of a single advance as the lateral moraine stratigraphy
56 is complex, the final retreat from these compound moraines occurred after 3.0 ka, at about 1.5 ka. Ice retreated from Group B moraines between 10.5 ka and 8.5 ka. Ice retreated from Group
A1 end moraines between 15.0 ka and 13.0 ka. Ages from Group A2 lateral moraines appear to straddle the ages of Group A1 and B. Thus, our current interpretation is that they were deposited during a time when ice extended beyond the bedrock step, but were not vacated and fully stabilized until ice retreated from moraine Group B.
Paleo-climatic Implications
The glacial record from Illimani can be compared with the Altiplano lake record. Baker et al. (2001a) used several proxies to determine the paleo-salinity of Lake Titicaca. These data were used to determine whether the lake was overflowing and fresh (as it is today) or a closed basin and more saline (as it has been in the past). Thus, a paleo-precipitation record of wet and dry periods exists (Fig.. 7). The data of Baker et al. (2001a) correlate well with the paleo-lake record of the southern Altiplano (Baker et al., 2001b; Placzek et al., 2006).
Lake levels were rising or overflowing 3.5 ka - 3.2 ka and 2.8 ka - 2.0 ka. The retreat from moraine Group C approximately coincides with the drying experienced at 2.0 ka. Lake
Titicaca was fresh and overflowing 10.0 ka - 8.5 ka (Baker et al., 2001a) coincident with the formation of moraine Groups B. However, retreat from moraine Group A between 15.0 ka and
13.0 ka coincides with a dry period according to Baker et al. (2001a) followed by a wet period between 13.0 and 11.5 ka. Thus, there is some ambiguity regarding the correlation of glacier fluctuations with changes in precipiation.
While glaciers in the Western Cordillera do not intersect the 0 oC isotherm and are hypothesized to be sensitive to changes in precipitation (Hastenrath, 1971; Klein et al., 1999) the
57 glaciers in the Eastern Cordillera intersect the 0 oC isotherm and are believed to be most sensitive to changes in temperature (Klein et al., 1999). This hypothesis is supported both in modern mass balance studies and paleo-climate studies. At Zongo Glacier, Francou et al. (1995) showed a strong positive correlation between temperature and runoff. Seltzer et al. (2002) suggest that the drastic reduction in magnetic susceptibility values in Lake Titicaca at ~22 ka indicate glacier retreat during a time of increased precipitation. Thus, the glaciers must have retreated due to warming.
Changes in temperature control ELA over time, however, spatial changes in ELA at a given time are controlled by precipitation. The modern ELA in the central Andes rises to the south and to the west (Seltzer, 1990; Klein et al., 1999) as a result of decreasing precipitation in these directions. Estimates of the modern ELA in the Cordillera Vilcanota and Quelccaya Ice-
Cap region of southeastern Peru range between 5100 m and 5300 m a.s.l. (Mercier and Palacios,
1977; Thompson, 1979; Mark et al., 2002). Mass balance measurements at Zongo Glacier in the
Cordillera Real on the eastern margin of the Altiplano put the mean ELA at 5270 m a.s.l. in northeastern Bolivia (Francou et al., 1995; Wagnon, et al., 1999). At Nevado Sajama in the
Western Cordillera of northern Bolivia, however, the modern ELA is 5630 m a.s.l. based on more than a decade of remotely sensed images (Arnaud et al., 2001). South of Sajama (18o S) the ELA rises above the 6000 m peaks. The modern ELA is a reflection of the precipitation gradient. In the central Andes, precipitation comes from the Atlantic. Thus, the Eastern
Cordillera receives more precipitation and has lower ELAs than the Western Cordillera. The lack of glaciers between 18o S and 27o S results from neither tropical (i.e., easterly) nor mid- latitude (i.e., westerly) precipitation penetrating to these latitudes in sufficient quantities to
58 maintain glaciers (Ammann et al., 2001). However, the slope of the regional ELA has changed over time.
During the Late Glacial, the regional ELA may have been much flatter from east to west and rising from south to north on the southern Altiplano (Fig.. 8). If the calculated Late Glacial
∆ELA values are subtracted from what the authors used as the modern ELA, then the absolute elevation of the Late Glacial ELA can be determined. Thus, the Late Glacial ELA was 4935 m in the Cordillera Vilcanota (Mark et al., 2002), 5045 m at the Quelccaya Ice-Cap (Mark et al.,
2002), 4640-4840 m in the Cordillera Real (Seltzer,1992; 1994), 4870 m a.s.l. in Pasto Grande
Valley, Cordillera Real (this study), 4690 m at Nevado Sajama (Smith et al., in review), and
4424 m at Cerro Azanaques on the south central Altiplano (Clayton and Clapperton, 1997).
The difference between the modern ELAs in the Cordillera Real and at Nevado Sajama is
360 m (Francou et al., 1995; Wagnon, et al., 1999; Arnaud et al., 2001), but during Late Glacial times the difference was between ~0 and 180 m (Seltzer, 1992 and 1994; Smith et al., in review).
Since precipitation controls the slope of the modern regional ELA, it is likely that precipitation also controlled the slope of the Late Glacial ELA. The flattening of the east to west slope of the
ELA suggests a change in relative amounts of precipitation with the Western Cordillera becoming nearly as wet as the Eastern Cordillera. A suggested by Heusser (1989), a plausible explanation for this would be an increase in winter precipitation from the west as described by
Vuille and Ammann (1997). Although often overlooked, Vuille and Ammann (1997) indicate that austral winter snowfall is a regular occurrence between 18 oS and 28 oS in the Western
Cordillera. An intensification of these "cut-off" storms in the past may explain the change in the slope of the regional ELA across the Andes. This hypothesis contrasts with the glacial modeling
59 results of Kull and Grosjean (2000) who found that less than 15% of total snowfall was derived from the west during the Late Glacial.
Additional evidence for the increased precipitation being sourced from the west comes from the north to south gradient of the ELA. On the currently unglaciated 5140 m Cerro
Azanaques (19o S), Clayton and Clapperton (1997) suggest a mean Late Glacial ELA of 4424 m a.s.l. Although it is impossible to calculate modern ELAs at this latitude due to the lack of glaciers (Ammann et al., 2001) it is likely that the ELA is higher than that of Sajama, 5630 m a.s.l. (Arnaud et al., 2001). However, during Late Glacial times this relationship was apparently reversed with lower ELAs being reported further south at both Cerros Azanaques and Tunupa
(Clayton and Clapperton, 1997).
The above interpretation hinges on the Late Glacial moraines from southern Peru and north and central Bolivia all being formed at the same time. Although the absolute chronology is ambiguous (Fig. 9), it is suggestive of a synchronous Late Glacial advance/stillstand. Thus, the interpretation of an altered slope to the regional ELA forced by a change in precipitation source during the Late Glacial is put forth as a tentative hypothesis.
Conclusions
Glaciers retreated from moraines at 15 ka -13.0 ka, 10.5 ka - 8.5 ka, and 3.0 ka -1.5 ka in
Pasto Grande Valley on the north slope of Nevado Illimani. Although each of these periods exhibited a decrease in precipitation, we suggest that the glacial advances were primarily the result of cooling. Our findings are in broad agreement with those of others since Late Glacial and
Neoglacial moraines have been found in the Eastern Cordillera. Interestingly, early Holocene moraines are not common (Seltzer, 1990).
60 If the Late Glacial advance at Nevado Illimani is synchronous with other Late Glacial moraines in the central Andes, then a marked change in the slope of the regional ELA occurred during the Late Glacial. East to west the regional ELA was nearly level suggesting a change in the distribution of precipitation that may indicate increased winter precipitation from the west.
These findings contrast with previous undated reconstructions of regional ELAs based on remotely sensed data that suggest increased precipitation from the east.
Acknowledgments
Thanks to J.M. Escobar and M. Escobar for logistics in Bolivia. Thanks to J. Dortch for instruction and assistance in the laboratory. The fieldwork was funded in part by GSA Grant
Number 8405-06.
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66
Table 1. Descriptions of key deposits shown on the geomorphic maps or mentioned in the text.
Deposit Dimensions/ Description Vegetation Cover Boulder Density End Moraine C ∼20 m high sharp crested None High moraine. Ice-proximal slope is steep, slumping and bounded by ice-debris. The ice-distal slopes abut bedrock cliffs. End Moraine B ∼2-3 m high moraine set. Low High Both the left and right laterals can be traced to an end moraine that rests atop the bedrock step in the valley floor. End Moraine A1 The moraine set is Low Moderate composed of two ∼3-4 m end moraines with rounded tops and shallow slopes. Lateral Moraine A2 A series of several left High Moderate lateral moraines ∼2-3 m high that extend over the bedrock step in the valley.
67 Table 2. Data used for calculating10Be surface exposure ages, and the ages and errors.
Be Carrier North East Thick- Shielding Internal Exposure External Sample Quartz Carrier Conc. Lat. Long. Elev. ness 10Be/9Be Error factor uncertainty age uncertainty (g) (g) (mg/g) (DD) (DD) (m) (cm) (year) (year) (year) - - 3.61E- BV40 30.1358 0.9259 0.4935 16.59902 67.79279 4394 2.5 4.89E-14 15 0.9327 76 930 0.114 - - 1.37E- BV41 31.9126 0.9383 0.4935 16.59896 67.79281 4383 4.0 4.81E-13 14 0.9503 277 9647 0.923 - - 3.25E- BV42 30.4032 0.9654 0.4935 16.59993 67.79547 4425 6.5 2.40E-14 15 0.9602 70 412 0.08 - - 8.22E- BV46 16.3478 1.0075 0.4935 16.61245 67.80310 4870 4.0 1.38E-13 15 0.9322 287 4632 0.511 - - 8.75E- BV53 23.2192 0.9622 0.4935 16.59828 67.79323 4379 5.0 4.00E-13 15 0.9491 253 11415 1.073 - - 1.18E- BV57 26.9777 0.9629 0.4935 16.59865 67.79555 4490 1.0 4.81E-13 14 0.9541 270 10833 1.025 - - 5.74E- BV59 26.6397 0.9198 0.4935 16.61230 67.80326 4869 3.5 1.21E-13 15 0.8901 117 2361 0.245 - - 2.23E- BV64 14.5790 0.9368 0.4935 16.60195 67.79699 4524 7.5 4.17E-13 14 0.9549 953 17569 1.868 - - 4.41E- BV65 30.1787 0.9635 0.4935 16.61293 67.80455 4940 6.0 9.55E-14 15 0.9094 81 1653 0.171 - - 5.02E- BV67 33.5864 0.9355 0.4935 16.61309 67.80493 4929 4.5 6.29E-14 15 0.8831 82 943 0.118 - - 1.07E- BV69 31.7906 0.9793 0.4935 16.57599 67.79489 4021 2.5 3.78E-13 14 0.9685 261 9095 0.87 - - 1.07E- BV77 25.4012 0.9257 0.4935 16.57535 67.79407 3983 3.0 3.78E-13 14 0.8798 399 5782 0.661 1.40E- Cblk1 0.0000 0.9825 0.4935 - - - - 4.61E-13 14 - - - - 1.02E- Cblk2 0.0000 0.9163 0.4935 - - - - 3.34E-13 14 ------1.55E- BV37 23.0731 0.4284 1.2600 16.59951 67.79437 4405 4.0 2.87E-13 14 0.9491 502 8706 0.94 - - 1.69E- BV38 19.1548 0.3744 1.2600 16.59961 67.79448 4405 1.5 2.89E-13 14 0.9324 575 9199 1.018 - - 8.04E- BV39 20.8132 0.3946 1.2600 16.59991 67.79470 4427 2.0 2.74E-13 15 0.9503 259 8235 0.795 - - 1.50E- BV44 20.4788 0.3929 1.2600 16.60178 67.79663 4524 1.5 2.88E-13 14 0.9679 456 8217 0.878 - - 1.83E- BV45 19.4510 0.393 1.2600 16.60239 67.79694 4525 1.5 2.92E-13 14 0.9671 589 8787 0.995 - - 1.68E- BV47 19.4456 0.3807 1.2600 16.61266 67.80356 4880 2.5 9.20E-14 14 0.9228 473 2073 0.509
68 - - 1.09E- BV49 20.1372 0.3654 1.2600 16.61244 67.80297 4853 1.5 8.23E-14 14 0.9430 278 1637 0.315 - - 1.44E- BV50 19.2408 0.4148 1.2600 16.58698 67.79637 4203 2.5 2.62E-13 14 0.8960 623 10508 1.144 - - 3.41E- BV52 20.3838 0.3825 1.2600 16.59823 67.79330 4375 1.0 4.16E-13 14 0.9032 1158 13455 1.689 - - 1.23E- BV54 20.3154 0.4157 1.2600 16.59830 67.79326 4381 4.0 2.38E-13 14 0.9603 439 7825 0.838 - - 2.16E- BV55 23.5409 0.393 1.2600 16.59875 67.79559 4472 4.0 4.27E-13 14 0.9450 615 11592 1.225 - - 1.49E- BV56 22.8624 0.391 1.2600 16.59871 67.79558 4468 3.0 3.24E-13 14 0.9555 423 8694 0.899 - - 4.40E- BV58 22.5338 0.4091 1.2600 16.59860 67.79554 4483 3.5 4.26E-13 14 0.9554 1336 12354 1.749 - - 1.64E- BV60 19.6124 0.4099 1.2600 16.61256 67.80349 4816 2.5 8.57E-14 14 0.8732 534 2197 0.571 - - 2.58E- BV61 21.0824 0.4075 1.2600 16.61245 67.80371 4865 3.0 1.05E-13 14 0.8995 742 2482 0.776 - - 1.28E- BV68 22.6460 0.4018 1.2600 16.57668 67.79620 4052 0.5 3.63E-13 14 0.9309 464 12473 1.23 - - 1.37E- BV70 20.9487 0.3979 1.2600 16.57603 67.79482 4017 4.0 3.45E-13 14 0.9694 534 12739 1.281 - - 1.72E- BV71 20.0066 0.4327 1.2600 16.57597 67.79477 3999 2.5 3.10E-13 14 0.9196 803 13564 1.477 - - 2.52E- BV72 21.9802 0.3649 1.2600 16.57590 67.79425 3991 5.0 4.10E-13 14 0.9709 876 13607 1.521 - - 5.85E- BV73 19.5496 0.3922 1.2600 16.57546 67.79453 3994 3.0 3.84E-13 14 0.9590 2447 15260 2.817 - - 1.89E- BV74 22.1256 0.4046 1.2600 16.57542 67.79443 3998 2.5 3.11E-13 14 0.9101 754 11646 1.304 - - 1.38E- BV75 21.2485 0.4195 1.2600 16.57537 67.79455 4001 1.5 3.47E-13 14 0.9710 554 13119 1.321 - - 1.53E- BV76 21.9093 0.4417 1.2600 16.57537 67.79405 3989 0.5 2.77E-13 14 0.9721 620 10518 1.143 7.09E- Cblk3 0.0000 0.4091 1.2600 - - - - 1.80E-14 15 - - - - 5.50E- Cblk4 0.0000 0.3772 1.2600 - - - - 2.25E-14 15 - - - - 4.05E- Cblk5 0.0000 0.4022 1.2600 - - - - 1.40E-14 15 - - - -
69 Table 3. Former ELAs relative to the modern ELA of 5270 m a.s.l.
Moraine Elevation Extent of AAR AAR THAR THAR THAR Group of End Lateral 77% 66% Method Method Method Moraines Moraines Paleo- Paleo- Paleo- Change in Temperature (m) (m) ELA (m) ELA (m) ELA (m) ELA (m) Change (C) A 4000 4677 4640 4865 4870 400 -3.0 B 4378 - 4840 5040 5110 160 -1.2 C 4600 5187 5055 5230 5250 20 -0.2
Figure 1. Digital elevation model of the Altiplano showing the locations of key sites mentioned in the text. The box on the inset map of South America approximates the enlarged area of the
DEM.
70 Figure 2. Aerial photograph and geomorphic map of Pasto Grande Valley. The moraines are sorted into four boxed groups A1, A2, B, and C. The locations of samples are indicated.
71
Figure 3. Views of the moraine sets described in this study. A. View from atop the bedrock step looking down valley towards the Group A1 end moraines. B. Looking up valley at the left lateral Group A2 Moraine. C. The rocky deposit in the foreground extending to the right side of the photograph is the end moraine of Group B. Note the steep drop in the valley profile just down valley from this moraine. D. Looking up the tributary valley at the laterals and end moraines of Group C.
72 Figure 4. The longitudinal profile of the floor of Pasto Grande Valley (solid line). Note the pronounced bedrock step in the profile. The locations of samples collected from moraine Groups
A1 and A2 are shown as squares. The dashed line approximates the former glacier profile if A1 and A2 deposits were formed at the same time.
73 Figure 5. Plots for different scaling models compared Lal (1991) and Stone (2000) over the last
40 ka.
74 Figure 6. Probability plots showing the age distributions of moraine Groups A1, A2, B, C.
Numbers refer to ages at the peaks of the distributions as well as the highs and lows surrounding the peaks.
75
76
Figure 7. Plot of multiple climate proxies from the Central Andes. The dashed line is the δ18O record from the Sajama Ice-Core. The solid line is the δ18O record from the Huascaran Ice-core.
The gray shaded regions represent periods during which Lake Titicaca was either rising or overflowing. The probability plots at the bottom give the age ranges for the end moraines
Groups A1, B, and C.
77 Figure 8. A schematic cross section of the Western Cordillera, Altiplano, and Eastern Cordillera shows the present ELA (solid line) and the Late Glacial ELA (dashed line). Note how the modern ELA rises from east to west due to decreasing precipitation in this direction. During the
Late Glacial the ELA is nearly level across the Altiplano.
78 Figure 9. Age comparison of moraines from the Eastern Cordillera of Southern Peru and Bolivia, and the Western Cordillera of Bolivia. For comparison, radiocarbon dates have been calibrated using Calib 5.1 (Stuiver and Reimer, 1993). A. Calibrated ages of moraines from the Cordillera
Real, Bolivia (Seltzer, 1992). B. Calibrated ages of the HII moraines at Quellcaya Ice Cap, Peru
(Rodbell and Seltzer, 1993). C. 10Be surface exposure ages of moraines in the Junin region, Peru and the Cordillera Real, Bolivia (Smith et al., 2005). D. Calibrated ages of the HII moraines at
Quellcaya Ice Cap, Peru (Kelly et al., in review). E. Calibrated maximum age of moraine at Cerro
Azanaques, Western Altiplano, Bolivia (Clayton and Clapperton, 1997). F. 36Cl surface exposure ages of moraines on Nevado Sajama, Western Cordillera, Bolivia (Smith et al., in review). G. 10Be surface exposure ages of moraines on Nevado Illimani, Cordillera Real,
Bolivia (this study).
79 Chapter 4. Present, future, and past tropical Andean ELAs: Assessing the relative importance of precipitation and temperature on glacier mass balance using a positive degree-day model
Abstract
A positive degree-day glacier mass balance model is calibrated using modern meteorological and mass balance data from Zongo Glacier, Cordillera Real, Bolivia. Once, calibrated the model is applied in three ways. First, the effect of predicted future rising temperatures on glacial ELAs is examined. Although the forecast warming of the next century will cause retreat of all glaciers and elimination of some lower elevation glaciers, the model suggests that glaciers will remain on the high peaks of the range with future ELAs of about 5640 m a.s.l. Second, the model is used to suggest possible precipitation and temperature scenarios responsible for known ELA lowerings during the Late Glacial, and early and late Holocene. If paleo-precipitation was as high as estimated, then the model suggests only a 2.3 oC cooling during Late Glacial times. When the model is applied to the Western Cordillera, it indicates an
ELA that is ~500 m below the actual ELA. This is believed to result from the fact that sublimation dominates the ablation regime in the arid Western Cordillera. Since sublimation is relative insensitive to changes in temperature, use of a PDD model is ineffective in this high, arid, tropical environment.
Introduction
Glaciers respond dynamically to changes in climate. The margins of small mountain glaciers react to changes in mass balance on timescales of only a few years (Nesje, 1995).
However, climatic interpretations of glaciers are complicated by the fact that two variables
80 control glacial mass balance, ablation season temperature and accumulation season precipitation
(Nesje, 2005). Investigators in the tropical Andes have suggested that glacial retreat resulted from decreased precipitation (Rodbell and Seltzer, 2000), or increased temperature (Seltzer et al.,
2002). Although empirical data, predominantly from mid-to-high latitude glaciers suggest that temperature correlates strongly with glacial equilibrium line altitudes (ELA) (Ohmura et al.,
1992), precipitation cannot be ignored especially in arid regions.
Often, glacial geologists simply multiply the change in ELA by a reasonable lapse rate to estimate the temperature depression during a past glacial advance. Implicitly, this method assumes no change in precipitation. This approach becomes problematic when other proxy data suggest a marked change in precipitation. For example in the Central Andes, the presence of paleo-lakes on the Altiplano during Late Glacial times suggests that this period was between
50% and 75% wetter than today (Hastenrath and Kutzbach, 1985). A precipitation change of this magnitude cannot be ignored when interpreting past glacial ELAs (Seltzer, 1992).
An alternative method of determining paleo-climatic conditions derived from changes in glacial ELA is to model the relationship between temperature and precipitation. The basic premise of positive degree-day (PDD) models is that a correlation exists between temperature and ablation (Braithwaite and Olesen, 1989). The presumed correlation results from the fact that the primary energy sources associated with ablation, longwave radiation and sensible heat fluxes, are closely tied to temperature (Hock, 2003). While modeling energy balances on and around glaciers provides detailed physical controls on mass balance, PDD models provide a means to efficiently model mass balance with a minimum of measured data. Generally, only temperature and precipitation values are needed.
81 This paper uses a positive degree-day model (Braithwaite and Zhang, 2000) calibrated with modern mass balance and meteorological measurements to construct a vertical mass balance profile of Zongo Glacier, Cordillera Real, Bolivia. Once calibrated the model used to examine the relative influence of precipitation and temperature on ELA by pertibating the model inputs.
The model is first applied to predict potential future changes in ELA that may occur as a result of recent warming (Solomon et al, 2007). Next, the model is applied to dated changes in ELA in the Cordillera Real to determine potential precipitation/temperature solutions that may have been responsible for past glacial expansions (Smith et al, in review b). Finally, the model calibrated in the Eastern Cordillera is applied to paleo-ELA data from Nevado Sajama, Western Cordillera,
Bolivia (Smith et al., in review a). The differing controls on mass balance in the Eastern and
Western Cordilleras is examined in the context of the model.
Glaciological Setting
Zongo Glacier (16.28o S, 68.14o W) is located in the Cordillera Real, Bolivia. Its elevation is between about 6000 m and 4900 m, and it is adjacent to the 6088 m peak of Huayna
Potosi (Francou, et al., 1995). Described as being in the outer tropics by Favier et al. (2004), the region has a single wet season during the austral summer. Nearly 70% of the 886 mm of average annual precipitation falls between the beginning of October and the end of March (Ribstein et al.,
1995). The moisture for precipitation originates in the Atlantic and is transported across the
South American continent by easterly winds. Although, there is an orographic effect as warm moist air is forced up over the Andes, locally at elevations above the passes there is believed to be no change in precipitation with elevation (Francou et al., 1995). At high elevation in the tropics, diurnal temperature fluctuations far exceed seasonal temperature fluctuations. While
82 mean monthly temperatures only change ∼5 oC from January to July (Ribstein et al., 1995), diurnal temperatures routinely change by ∼20 oC. Thus, an annual mass balance cycle contains a wet season and a dry season with ablation throughout the year (Kaser and Osmaston, 2002).
This contrasts with higher latitude glaciers where the cycle is a cold accumulation season and a warm ablation season. The mean annual temperatures at Zongo station (4770 m) during hydrological years (September-August) 1991-1992, and 1992-1993 were 1.6 oC and 1.0 oC respectively, and the lapse rate has been measured at 0.0074 oC m-1 (Francou et al., 1995).
Data
Zongo Glacier has arguably the best mass balance record in the tropics. A fifteen year record (1992-2006) of annual mass balance is available from the French GLACIOCLIM website.
Mean vertical mass balance data for Zongo Glacier for hydrological years 1992-1993 through
2005-2006 are presented in Table 1 and plotted in Figure 1. While the mass balance data are easily accessible, long term continuous records of temperature and precipitation are not. Some information can be obtained from prior publications regarding Zongo Glacier (Francou et al.,
1995; Ribstein et al., 1995; Wagnon et al., 1999A; Wagnon et al., 1999B), but a continuous record is available for only three hydrological years (September 2003-August 2006). These data, also available from the GLACIOCLIM website, include half hourly temperature, precipitation, and radiation measurements for nearly this entire period. Due to the availability, continuity, and high resolution of these data, they are used to calibrate our model. Mean values and other relevant information are provided in Table 2.
83 Methods
Annual glacial mass balance is simply accumulation minus ablation expressed in millimeters of water equivalent (mm w.e.). The specific mass balance of a given altitudinal band is determined by multiplying the measured mass balance by the ratio of the area of the altitudinal band to the area of the entire glacier. The net specific mass balance of the entire glacier is then the summation of the specific mass balances of all altitudinal bands.
Annual mass balance at elevation j can be expressed as: bj = cj - aj (1) where bj is the annual mass balance (mm w.e.) while cj and aj are annual accumulation (mm w.e.) and annual ablation (mm w.e.) at elevation j respectively. PDD models suggest that a set amount of snow, ks, melts during one positive degree-day while a different set amount of ice, ki, melts during one positive degree-day. Thus ablation may be expressed as: aj = ksnow*PDDsnow + kice *PDDice (2)
-1 -1 The k values are in mm deg d and PDDs and PDDi are the number of positive degree-days devoted to melting each medium in a year (oC day). Since temperature and the number of
PDD/year change with elevation, k is considered constant over the elevation of the glacier.
Common values of k in the mid-to-high latitudes tend to be around 8 mm d-1 deg-1 (Braithwaite and Zhang, 1999). However, higher values have been recorded at lower latitudes and higher elevations. Zhang et al. (2006) report k values as high as 13.8 mm oC-1 d-1 in Western China.
Snow and ice have different values of k due to their differing albedos. Here we follow the methodology of Braithwaite and Zhang (2000) and set ksnow = 0.6 kice. We ignore mass balance changes resulting from refreezing of melted snow or ice (Braithwaite and Zhang, 1999).
Combining equations 1 and 2 yields:
84 bi = ci - (ksnow*PDDsnow + kice *PDDice) (3) which is the basis of the model when coupled with a temperature lapse rate. Conversions between positive degree-days and mean annual temperature followed the statistical approach of
Braithwaite (1984) in which predicted daily mean temperatures are distributed about an annual
(or monthly) mean temperature based on a standard deviation. By summing the positive values the number of positive degree-days for any mean annual temperature or vise versa can be determined. Although the measured mean standard deviation of daily mean temperatures at
Zongo is only 1.5 oC, this value produces too few positive degree-days. We found that a standard deviation of 2.2 most closely predicted the actual number of positive degree-days and adopted it here (Table 2). Thus, with the GLACIOCLIM temperature data recorded at 5050 m
(Table 2), and a lapse rate we can convert the vertical temperature profile of Zongo Glacier to a
PDD vertical profile using the method of Braithwaite (1984). Finally, appropriate values for accumulation and k can be determined while tuning the model. A more complete description of the methods used here can be found in Braithwaite and Zhang (2000).
Selection of Model Variables
The initial input required to select the other variables used in the model is temperature.
We used an annual temperature of 0.4 oC at 5050 m a.s.l. which is derived from the mean daily temperature of the three hydrological years 2003/2004 through 2005/2006 (GLACIOCLIM, website). The values for annual precipitation, temperature lapse rate, and the positive degree-day factor for ice (kice) were determined through model calibration. To calibrate the model it was run
99 times with differing values for precipitation, lapse rate, and degree day factor (kice), as done by Braithwaite and Zhang (2000). Values used for calibration include: 1) precipitation 500-1500
85 mm a-1 in 100 mm steps; 2) lapse rates 0.006, 0.007 and 0.008 oC m-1; and 3) degree day factors
14, 15, and 16 (mm oC-1 d-1). The values for precipitation and lapse rate bracket local field measurements (Francou et al., 1995), and the values for kice bracket the number required to calibrate the model output to the empirical mass balance data. The measured mass balance results (GLACIOCLIM, website) were subtracted from the modeled output for each model run.
This yields an estimate of model error in mm w.e. a-1 (Fig. 2). As reported by Braithwaite and
Zhang (2000) there is not a unique set of values that yields an error at or near 0 mm w.e. a-1, rather several different solutions are possible. Therefore we select values for precipitation, lapse rate, and Kice that are similar to those measured in the field. Chosen values are discussed below relative to empirical values.
We use the value 950 mm w.e. a-1 as the baseline precipitation in our model. Ribstein et al. (1995) report the mean precipitation between 1970 and 1992 at 4770 m near Zongo Glacier to be 886 mm a-1. Despite the 0 mm m-1 precipitation lapse rate (Francou et al., 1995), the fifteen year mean mass balances in the upper reaches of the glacier record a mean annual accumulation exceeding 900 mm a-1 (GLACIOCLIM, website). Thus, the sum of precipitation and wind blown snow must be even larger than this.
A lapse rate of 0.007 oC m-1 is the value used in the model. It compares favorable with the measured lapse rate over Zongo Glacier of 0.0074 oC m-1 (Francou et al., 1995) and radiosonde data from El Alto 0.0069 oC m-1 (SENAHAMI, unpublished data).
We select a high, but not unreasonable, positive degree-day factor for ice of 14.4 mm w.e. oC-1 d-1. Ribstein et al. (1995) report monthly mean temperatures at Zongo Glacier for the period September 1991-August 1993, and Francou et al. (1995) report monthly ablation measurements from two groups of stakes, at two elevations, over the same two-year period (n =
86 48). Coupling the statistical method of calculating positive degree-days (Braithwaite, 1984) with the temperature data of Ribstein et al. (1995) provides the number of positive degree-days for each month during the two-year period. The monthly ablation measured by Francou et al. (1995) divided by the number of positive degree-days per month yields quasi-empirical data for positive degree-day factors at Zongo Glacier. Although the values cover a large range (-52 to 71 mm w.e. oC-1 d-1) and have a high standard deviation (21 mm w.e. oC-1 d-1), when plotted in a histogram (Fig. 3) the peak lies between 15 and 20 mm w.e. oC-1 d-1. If all values are used, then the mean is 12 mm w.e. oC-1 d-1. However, if negative values (the result of net monthly accumulation) are ignored the mean k-value is 18 mm w.e. oC-1 d-1. These values are similar to those reported by Zhang et al. (2006) in western China. Thus, it seems reasonable that high elevation, low latitude, glaciers may have higher degree-day factors than glaciers at lower elevations and higher latitudes. For the model as it relates specifically to Zongo Glacier, we use
o -1 -1 o -1 -1 kice=14.4 mm w.e. C d and ksnow= 0.6 kice= 8.6 mm w.e. C d .
Results
The modeled results using the above-mentioned values for precipitation, lapse rate, and positive degree-day factor are plotted in Figure 1. The abrupt kink in the modeled vertical mass balance line is a result of the hypsometry of Zongo Glacier and it does not represent any problem with the model.. The surface area of the 100 vertical meter band centered on 5050 m a.s.l. makes up nearly 15% of the surface area of the entire glacier. Because this low elevation band is relatively large, a disproportionate amount of ablation occurs here when net specific mass balance is calculated.
87 Although this kink does not appear in the average vertical mass balance profile, it is present in the data of many individual hydrological years. In order to make this model more generally applicable throughout the region, we try to minimize the hypsometric effect specific to Zongo
Glacier by disregarding the area data. Rather than using the specific net mass balance of Zongo
Glacier, we use the generic net mass balance in the absence of hypsometric data. In affect, we assume that every elevation band has the same area. The error analysis for this general model is plotted in Figure 2 along with the Zongo specific errors. Again there is no unique solution, but we choose values close to empirical values. For the general model we maintain the lapse rate of
o -1 -1 o -1 -1 0.007 C m , and the precipitation of 950 mm a , but change kice to 15.1 mm w.e. C d and
o -1 -1 ksnow to 9.1 mm w.e. C d . When plotted along with the empirical mean net vertical mass balance data from Zongo (Fig. 4), the modeled results agree closely. This close correlation is our strongest defense for using a PDD model in the tropics.
Discussion
Braithwaite and Zhang (2000) originally developed the model used in this paper to test mass balance sensitivities of glaciers around the world to climate change. Due to the dearth of tropical mass balance data, tropical glaciers are understandably underrepresented even in more complete subsequent publications (Braithwaite et al., 2002). Therefore, we would be remiss to proceed to a discussion of past glacier extents and paleo-climate without first discussing the sensitivity of Zongo Glacier to changes in climate. For this exercise we use the original model of net specific mass balance of Zongo Glacier.
Temperature mass balance sensitivity is defined as the change in net specific mass balance due to a +1 oC change in temperature (Braithwaite et al., 2002). The modeled sensitivity
88 data of Braithwaite et al. (2002) are presented in figure 5 versus mass balance amplitude; the modeled sensitivity of Zongo Glacier (-1194 mm w.e.) has been added. Although Zongo does not plot along the trend line with the data from temperate and sub-polar regions, it does plot in the same region as other tropical glaciers, specifically, Lewis Glacier on Mt. Kenya suggesting that tropical glaciers are especially sensitive to temperature changes.
Future Changes
Now that the model has been verified for modern conditions, we begin to pertibate the inputs in order to predict changes in glacier ELA with changing climate. Health of Andean glaciers is of social importance because glaciers buffer the timing of runoff from the mountains.
That is, glacial runoff provides water during the dry season. For this section and the remainder of the paper we will be using the generic model that does not include the hypsometry of Zongo
Glacier. Thus, we will be applying the model to the glaciers of the Cordillera Real.
The technical summary of the Intergovernmental Panel on Climate Change (IPCC) report
(Solomon et al, 2007) uses a number of future scenarios to predict climate change during this century. The conservative-to-moderate scenarios A1T and B2 both estimate a likely warming of
2.4 oC by the decade 2090-2099. Thus, we examine the effects of a warming of this magnitude on the glaciers of the Cordillera Real. Using the generic model we change only the temperature in 1 oC increments and determine the new ELAs by noting the Y-intercept of a line drawn between the lowest elevation positive balance and highest elevation negative balance (Fig. 6).
Likewise, if we change only precipitation while maintaining the modern temperature a plot of
ELA with precipitation can be constructed (Fig. 7). With the predicted warming of 2.4 oC
(Solomon et al, 2007) local ELAs would rise to ∼5642 m a change of nearly 340 m. A greater
89 than 200 % increase in precipitation is required to off set completely this dramatic rise in ELA.
According to the IPCC report, the tropical Andes are likely to experience little change in annual precipitation (Solomon et al, 2007). Thus according to the model, glaciers are unlikely to disappear completely from the Cordillera Real since many of the peaks exceed 6000 m.
However, lower elevation glaciers may disappear.
Past Changes
Rather than using predicted future climate conditions to model a future ELA, we now use the model to examine the potential past climate conditions associated with known paleo-ELAs.
Although debate continues over the timing of the so-called "Local Last Glacial Maximum" in the tropical Andes (Seltzer, 1992; Seltzer et al., 2002; Smith et al., 2005), strong evidence exists for a glacial re-advance (or still-stand) during Late Glacial times (Seltzer, 1990; Smith et al., 2005;
Smith et al., in review). Again, the exact timing of deposition of the Late Glacial moraines is uncertain specifically with regard to the Younger Dryas period (Rodbell and Seltzer, 2000; Kelly et al., in review). However, examining the suite of environmental conditions that may have caused the Late Glacial re-advance does not require knowing the exact date. For our purposes we put it roughly between 15 ka and 12 ka as suggested by Smith et al. (2005). Less extensive,
Holocene, moraines have also been reported in the Cordillera Real (Gauze et al., 1986; Smith et al., in review). Here we estimate paleo-environmental conditions for paleo-ELAs of 4870 m,
5110 m, and 5220 m corresponding to Late Glacial, early Holocene, and late Holocene moraines respectively at Nevado Illimani in the Cordillera Real about 60 km southwest of Zongo Glacier
(Smith et al., in review). The model is assumed to apply to the entire Cordillera Real for the following discussion.
90 To calculate potential paleo-environmental conditions associated with lowered ELAs, the input temperature was lowered in 0.5 oC increments and the precipitation was adjusted iteratively for each new temperature until the model predicted an ELA within 5 m of the paleo-ELAs. This produced a temperature vs. precipitation curve for each of the paleo-ELAs (4870 m, 5110 m, and
5250 m) (Fig. 8). According to the model, any temperature, precipitation point along the line is a possible climatic solution responsible for the lowered ELA.
Since there is not a unique temperature and precipitation solution for each paleo-ELA, we must rely on available proxy data to provide either one variable or the other. If precipitation was indeed 50-75% higher during Late Glacial times as suggested by Hastenrath and Kutzbach
(1985) then the model suggests a relatively small magnitude of cooling, only 2.2-2.4 oC, is required to extend glaciers to their Late Glacial margins. This is within the error of the Late
Glacial cooling (3.5 ± 1.6 oC) calculated by Seltzer (1992) based on ELA depressions in the
Cordillera Real.
However, greater magnitude cooling has been suggested in tropical South America during Late Glacial times. For example, Stute et al. (1995) suggested a cooling of 5.4 oC based on dissolved noble gases in groundwater, and Guilderson et al. (2001) suggested a cooling of 4.5 oC based on the oxygen isotopic composition of tropical corals. If either one of these cooling scenarios is correct, then the model predicts a decrease in precipitation a conclusion that is in agreement neither with the elevated lake levels found in Lake Titicaca (Baker et al., 2001A) nor the presence of paleo-lakes on the Altiplano at this time (Baker et al., 2001B).
Quantitative temperature and precipitation data for the Holocene are scarce. However,
Baker et al. (2001A) provide a 25 ka continuous record of when Lake Titicaca was overflowing
(as it does today) or when lake level was rising or falling. The record is interpreted to track
91 changes in precipitation relative to the modern since a drop in lake level of only ∼25 m would create a closed basin lake and result in an altered geochemical signature. Baker et al. (2001A) report Lake Titicaca was overflowing between 10.0 ka and 8.5 ka, and it was either rising or overflowing between 2.8 ka and 2.0 ka. Thus, glacier retreat from the early (∼8.5 ka) and late
(∼1.5 ka) Holocene moraines in the Cordillera Real coincides with the end of periods that were at least as wet today. Therefore, we can estimate maximum temperature depressions of 1.4 oC and
0.3 oC for the early and late Holocene respectively.
Model Application to the Western Cordillera
Glaciers in the Western Cordillera of Bolivia lie above the annual 0 oC isotherm while glaciers in the Eastern Cordillera intersect the 0 oC isotherm (Klein et al., 1999). Since Western
Cordilleran Glaciers are precipitation limited and Eastern Cordilleran glaciers are temperature limited, it has been hypothesized that the glaciers of the Western Cordillera are more sensitive to precipitation changes while the glaciers of the Eastern Cordillera are more sensitive to temperature changes (Hastenrath, 1971). Thus, it seems appropriate to test that hypothesis by applying the model to the Western Cordillera. However there is no mass balance data from the
Western Cordillera, thus we must apply the generic mass balance model based on Eastern
Cordilleran conditions. Positive degree-day models based on the mass balance record of Zongo
Glacier have been applied to sites as far away as Hawaii (Blard et al., 2008). Thus, in principle applying the methodology to the Western Cordillera should not present a problem.
We run the model with a modern temperature of -0.6 oC at 5050 m a.s.l. (one degree
-1 o -1 lower than at Zongo Glacier), a precipitation value of 351 mm a , and a kice value of 4.5 mm C d-1 (Vuille, 1996; Kull and Grosjean, 2000) (Fig. 9). The new temperature value is derived by
92 extrapolating meteorological data from the village of Cosapa, near the base of Sajama, upwards to the elevation at which temperature was measured at Zongo Glacier using a lapse rate 0.007 oC m-1 (SENAHAMI, unpublished data). The temperature derived from the Cosapa village data yields a temperature of -0.6 oC at 5050 m a.s.l. Thus, it is 1.0 oC colder at 5050 m a.s.l. on
Sajama compared to 5050 m a.s.l. at Zongo Glacier. Precipitation is also measured at Cosapa over the same time period as the temperature data (1990-2001) (SENAHAMI, unpublished data) and the precipitation lapse rate is assumed to be 0 mm m-1. The PDD of 4.5 mm oC-1 d-1 is the same value used by Kull and Grosjean, (2000) at the same latitude in northern Chile, and based on empirical measurements made by Vuille (1996). Also, this lower kice value is similar to those reported by Zhang et al. (2006) for the high, arid, regions in western China.
Running the model with these modern conditions produces an ELA of 5120 m a.s.l. The modern ELA on Sajama is 5630 m a.s.l based on the elevation of snow on remotely sensed data normalized to the end of the dry season (Arnaud et al., 2001). Changing one variable at a time the values required to model the correct modern ELA are an increase in temperature of 3.5 oC, a decrease in precipitation of 95%, or a PDD factor for ice of 85 mm oC-1 d-1. The fact that the
PDD model is ineffective at reproducing a vertical mass balance profile in the Western
Cordillera after effectively doing so in the Eastern Cordillera highlights the different ablation mechanisms between the Western and Eastern Cordilleras.
At Zongo Glacier, Wagnon et al. (1999) measured mass loss due to melting and sublimation from September 1996 through August 1997. During this period, 994 mm w.e. were lost to melting while 202 mm w.e. were lost to sublimation. Of the mass lost to sublimation, nearly 60% occurred during the four-month dry period May-August and only 13% occurred during the four-month wet period November-February. The predominance of melting (85% of
93 mass lost) in the Eastern Cordillera provides the explanation why the PDD model works.
Temperature, in the form of PDD, is closely linked to longwave radiation and sensible heat fluxes, which are the primary energy sources responsible for melt (Hock, 2003).
However in the Western Cordillera, sublimation is the dominant ablation mechanism.
Arnaud et al. (2001) used a Penman model to calculate an annual sublimation of 379 mm at the summit of Sajama during the period October 1996 through September 1997. During the same period, upwards of 540 mm of precipitation fell at Cosapa. We use this as a limiting value because the meteorological record of November is missing. Regardless, the calculated sublimation at the summit of Sajama is over 50% of the accumulation. Although sublimation rates depend on temperature, other factors such as wind speed, and humidity can mask changes in temperature. Thus, glaciers in arid, high elevation, tropical environments where sublimation is the dominant mode of ablation have been shown do be relatively insensitive to temperature
(Mote and Kaser, 2007). In this regard it is not surprising that a temperature (PDD) model is ineffective at predicting ELA in the Western Cordillera. However, more complex glacial models that account for sublimation have been applied in to the Western Cordillera (Kull and Grosjean,
2000)
Conclusions
A positive degree-day model can be tuned to closely replicate the vertical mass balance profile of Zongo Glacier in the Eastern Cordillear, Bolivia. Once calibrated, the inputs of temperature, precipitation, and positive degree-day factor can be adjusted to determine the response of glacier ELAs to predicted future rises in temperature, or inversely to determine the likely paleo-environmental conditions responsible for past ELA depressions. Model results
94 suggest the following. 1) A warming of 2.4 oC would raise ELAs about 340 m and drastically reduce the glaciated area of the Cordillera Real. 2) If precipitation was 50-70 % higher, then
Late Glacial temperatures were depressed only about 2.3 oC. While the model is confidently extended to the Eastern Cordillera, it fails to account for sublimation and is thus ineffective in the arid Western Cordillera where sublimation dominates the ablation regime.
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100 Figure 1. Specific vertical mass balance profile of Zongo Glacier. The triangles are the empirical mean balance values multiplied by the percent of total area represented by each altitudinal band (ie specific mass balance) for hydrological years 1991/1992 through 2005/2006.
The solid line is the model derived vertical specific mass balance given modern conditions of
o o -1 -1 950 mm accumulation, 0.4 C at 5050 m, and kice = 14.4 mm C d . The dashed line is the modeled vertical specific mass balance given an increase in mean annual temperature of 1 oC .
101 Figure 2. Model errors associated with different model inputs. The solid dots represent model runs of the specific mass balance model that accounts for the hypsometry of Zongo Glacier and should be read on the left vertical axis. The hollow dots represent model runs of the generic model that assumes equal area in each altitudinal band. It should be read on the right vertical axis. The lower part of the figure describes the different variables used in each model run. Since many different solutions provide low errors, we chose precipitation, lapse rate, and positive degree-day factors that approximate local conditions.
102 Figure 3. Histogram of quasi-empirical k-values. Ribstein et al. (1995) measured ablation at two elevations (4900 m and 5200 m) for two hydrological years (1991/1992, and 1992/1993).
The number of degee-days for each month was calculated using the mean monthly temperatures
(Ribstein et al., 1995) and the statistical method of Braithwaite (1984). It should be noted that these values are not specific to either snow or ice as only surface lowering was reported.
103 Figure 4. Vertical mass balance profile. The triangles are the empirical mean balance values for hydrological years 1991/1992 through 2005/2006. The dashed is the modeled vertical mass
o balance given the modern conditions of 950 mm accumulation, 0.4 C at 5050 m, and kice = 15.1 mm oC-1 d-1. This model does not use the hypsometric data specific to Zongo Glacier.
Figure 5. Model specific mass balance amplitude (average of accumulation and ablation) is plotted against temperature sensitivity (change in net specific mass balance given a 1oC temperature increase). The Zongo Glacier data point from this study (1.18 mm w.e. a-1, -1.19 mm w.e. oC-1) is plotted along with the data of Braithwaite et al. (2002).
104
Figure 6. The generic model (independent of Zongo's hypsometry) is used to estimate the elevation of the ELA with changes in temperature.
Figure 7. The generic model (independent of Zongo's hypsometry) is used to estimate the elevation of the ELA with changes in precipitation.
105 Figure 8. Modeled climatic conditions for the Late Glacial, early Holocene, and late Holocene
ELAs using the generic model (independent of Zongo's hypsometry).
106 Chapter 5. Concluding Remarks
Examination of the tropical glacial geologic record provides paleo-climatic information that is unavailable from the oxygen isotopic record preserved in tropical ice-cores. This is most evident during the Holocene since glacial advances occurred in both the early and late Holocene in both the Western and Eastern Cordilleras during periods of relatively stable isotopic composition. This may result from either the glaciers being more sensitive to the same climatic changes that affect the ice-cores, or the glaciers responding to different climatic conditions than those that affect the ice-cores. Prior to the Holocene, the ice-core record may be affected by changes in the source and seasonality of precipitation. The hypothesized changes in precipitation sources may be reflected in the glacial chronology.
There appears to be some chronologic asymmetry between the Western and Eastern
Cordilleras during full glacial conditions. In the Eastern Cordillera, the Late Glacial moraines are inset to older late Quaternary moraines. However in the Western Cordillera, the most extensive glacial deposits on Nevado Sajama date to the Late Glacial. Since temperature is unlikely to change significantly across the Altiplano, this asymmetry suggests that glaciers in the
Western and Eastern Cordilleras respond differently to changes in precipitation or more likely there were different precipitation regimes in the two cordilleras.
Aside from the paleo-climatic implications of this study, there are two notable geomorphic findings from Nevado Sajama. First, the large valley forming moraines, previously interpreted to have been deposited during the Last Glacial Maximum, are relic features with subtle Late Glacial moraines deposited on top of them. Second, the presence of small bouldery moraines deposited atop an unaltered landscape in the absence of glaciofluvial features indicates the past presence of extensive cold-based ice on the mountain.
107 Previously, regional reconstructions of paleo-ELA have depended on the assumption that the large outermost moraines in both cordilleras were deposited synchronously during the Last
Glacial Maximum. Due to the chronologic asymmetry between the Western and Eastern
Cordilleras and the presence relatively small late Quaternary moraines, this assumption needs to be reexamined. The discovery of cold-based ice deposits overlying undisturbed moraines highlights the need for careful geomorphic mapping so that ages from the younger cold-based deposits are not interpreted as ages for the older, underlying, undisturbed moraines. The presence of extensive cold-based ice deposits in close temporal and geographic proximity to deposits of wet-based ice may provide important clues to both the paleo-climatic and glacial geologic history of Nevado Sajama.
In general the moraine chronology is difficult to compare to the ice-core record both because it is discontinuous and lacking resolution. Therefore, future work to refine the comparison of oxygen isotopic and glacial geologic records should focus on continuous high- resolution glacial records. Such archives can be found in the sedimentary sequences of proglacial lakes. Examining the sedimentation rates in radiocarbon dated, proglacial, lacustrine deposits has been used successfully as a proxy for up-valley glacial extents with periods of high sedimentation rate correlating to periods of expanded glacier margins.
A number of small lakes exist inset to the large valley forming moraines at Nevado
Sajama. Sediment cores from a number of these lakes should show temporal correlation in sedimentation rates. Additionally, the age of the bottom of the lacustrine sequence will provide a minimum age for the final retreat of glaciers from the lake. While these lakes will provide useful high-resolution records, they will limited to the Holocene. In order to obtain a longer record a
108 lake beyond the Late Glacial moraines must be cored. Such lakes exist across the border in Chile at the same latitude as Sajama, and should also be targeted.
Once the sediment cores are collected, dated, and analyzed, it will be necessary to determine whether the glacial sedimentological record tracks precipitation, temperature, or a combination of the two. This can be determined by comparing the Western Cordilleran glacial lacustrine record with the Eastern Cordilleran glacial lacustrine record from Lake Titicaca (Baker et al., 2001a) as well as the data from the Salar de Uyuni (Baker et al., 2001b). The abrupt change from glacial/clastic to organic sedimentation in Lake Titicaca (Seltzer et al., 2002) has been used to suggest warming and glacial retreat in the Eastern Cordillera. While the interbeded lacustrine silts and salts of the Salar de Uyuni have been used as a precipitation proxy for the
Altiplano. Thus, the controls on glacial sedimentation in the Western Cordillera may be determined by comparison to existing paleo-temperature and paleo-precipitation proxies that may then be compared to the ice-core data to help establish the role of the tropics in the climate system.
References
Baker, P.A., Rigsby, C.A., Seltzer, G.O., Fritz, S.C., Lowenstein, T.K., Pacher, N.P., Veliz, C.,
2001a. Tropical climate changes at millennial and orbital timescales on the Bolivian
Altiplano. Nature, 409: 698-701.
Baker, P.A., Seltzer, G.O., Fritz, S.C., Dunbar, R.B., Grove, M.J., Tapia, P.M., Cross, S.L.,
Rowe, H.D., Broda, J.P., 2001b. The history of South American tropical precipitation for
the past 25,000 years. Science, 291: 640-643.
109 Seltzer, G.O., Rodbell, D.T., Baker, P.A., Fritz, S.C., Tapia, P.M., Rowe, H.D., Dunbar, R.B.,
2002. Early warming of tropical South America at the last glacial-interglacial transition.
Science, 296: 1685-1686.
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