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| Titel |
Reconstruction of Equilibrium Line Altitudes of Nevado Coropuna Glaciers (Southern Peru) from the Late Pleistocene to the present |
| VerfasserIn |
J. Úbeda, D. Palacios, L. Vázquez |
| Konferenz |
EGU General Assembly 2009
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 11 (2009) |
| Datensatznummer |
250022092
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| Zusammenfassung |
| The Nevado Coropuna (15º31’S-72º39’W) is a volcanic complex located 200 km NE of the
city of Arequipa, in the Southern Peruvian Andes. The summit area in the complex is covered
with a glacier system formed by dozens of branches descending in all directions totaling
many km2 in surface area. The study of the volcanic complex and its glaciers is of
great interest because it is the main water reserve for tens of thousands of people,
because of the risk scenario created by the presence of ice masses on a volcano with
relatively recent activity, and because it constitutes an excellent geoindicator of the
effects of climate change on ice masses in the western mountain chain of the Central
Andes.
This research aims to analyze glacier evolution using as geoindicators variations in
glacier surface and equilibrium line altitudes (ELAs), defining deglaciation rates based on
those variations and preparing forecasts with them on when the ice masses might disappear if
the same rates were to occur in the future. In addition, a first estimation is attempted of the
chronologies of the last phase of volcanic activity and the last phase of maximum
glacier advance that can be attributed to the Late Glacial or Last Glacial Maximum
periods.
To achieve these aims, digital topography with 50m contour interval, two orthophotos of
the central section of the Coropuna complex (15-6-1955 and 21-10-1986), an ASTER
satellite image (12-11-2007) and geomorphological mapping of the volcanic complex created
in a previous phase of the research (Ubeda, 2007) were integrated into a Geographical
Information System (GIS). The GIS was used to determine the global extent of the
glacier system, and in more detail, that of two groups (NE and SE) in 1955, 1986
and 2007. Using the geomorphological cartography as a basis, the extent of the
glaciers during their last advance in the Little Ice Age (LIA) and their last maximum
advance were calculated. Next, surface areas for all phases were calculated using
automatic functions within the GIS operating environment. To reconstruct the ELAs of
the glaciers, the Area x Altitude Balance Ratio (AABR) method was used. This
method is extensively described in Osmaston (2005). To determine the rates of
deglaciation, variations observed for 2007 in surface areas and ELAs against their values in
1986, 1955 and the Little Ice Age (LIA) were used as geoindicators. Establishing
deglaciation rates has allowed forecasts to be made as to when the complete disappearance
of ice mass could occur for three future scenarios, considering the hypothetical
reproduction in each scenario of the rates of deglaciation observed since 1986 (Scenario
1), 1955 (Scenario 2) and the LIA (Scenario 3). To determine the chronology of
the last maximum advance of the glaciers and the last volcanic manifestations,
samples were taken from moraine blocks and glaciated rocky thresholds, and also
from lava ejected during the last eruption, in the eastern sector of the complex.
Due to their recent external appearance, since they have been channeled by glacial
valleys and have been affected by ice masses only at the head, these lavas had been
dated as Holocene. Absolute dating was performed using cosmogenic methods
(Cl36).
As a result of applying the proposed method, glacial system surface areas have
been estimated for 2007 (47 km2), 1986 (54 km2) and 1955 (56 km2), implying a
reduction of ~18% in 52 years. The process appears to have speeded up in the
last decades (~13% in only 21 years). Surfaces were also estimated and ELAs
reconstructed for the NE and SE groups in 2007, 1986, 1955, the Little Ice Age and during
the last maximum advance. Glaciers from the NE group show an area during all
periods (2.3, 2.7, 2.9, 3.3 and 30 km2) smaller than SE group glaciers (8.1, 9.9, 10.3,
11.9 and 66.5 km2). An individual analysis of glaciers in the NE and SE groups in
2007 shows a reduction in surface area two to four times greater than that observed
between 1955 and 1986. ELAs are also higher for all periods in the Northern section
(5968, 5930, 5923, 5886 and 5186 m) than in the Southern section (5862, 5806,
5787 and 4951 m). The depression in ELAs during the LIA was similar in the NE
(~82 m) and in the SE (~86 m). However, the 2007 ELA shows a depression of
106 m in the Southern direction. The magnitude of this depression has shown a
marked tendency towards reduction in recent decades (136 m in 1955 and 124 m in
1986). Furthermore, the decrease in ELA depression seems to occur faster, with -Z
~12 m between 1955 and 1986 and -Z ~18 m between 1986 and 2007. However,
during the Little Ice Age (~110 m) that value was closer to the current value (106
m).
Depression in ELAs during the last maximum glacier advance has been estimated at
~782 m (NE) and ~847 m (SE). During that period, the N-S depression reached a maximum
value of 235 m. These results agree with those obtained for the eastern range of the Central
Andes (Smith et al. 2005 a and b) and are also within the depression intervals and trends
proposed in regional-scale studies (Kelin et al. 1999). Analyses performed on a sample from
a block situated on a lateral moraine in the Queñua Ranra Quebrada (NE group of the
complex) suggest a chronology of ~17 Cl36 ky. for the last maximum ice mass advance.
This date is in agreement with the depression in SST temperature during the same
period, deduced from analyzing Mg/Ca ratios in marine foraminifera shells from the
Galapagos Islands (Lea, 2006). Using surfaces and ELAs as geoindicators, deglaciation
rates and the Horizon without glaciers (H0) have been calculated globally, for the
complete glacier system in scenarios 1 and 2, and for glaciers in the pilot group
in scenarios 1, 2 and 3. Results show that the deglaciation process is occurring
differentially. Whereas several masses could disappear in a few decades, others could be
preserved for centuries. Regarding the last phase of volcanic activity, a lava sample
has been dated at only ~2 Cl36 ky. Testing the proposed method has allowed the
modeling of glacier evolution using variations observed in surfaces and ELAs as
geoindicators.
Results from the global-scale analysis are only a preliminary approximation to the
problem. Detailed analysis of the glaciers in the NE and SE groups has yielded more precise
results. Forecasts about future glacier retreat, interest in finding out about their past evolution
and the absolute chronology of the last phase of volcanic activity, which confirms their recent
character, suggest the need to extend our understanding of the evolution of the Nevado
Coropuna’s volcanic complex and glacier system.
References
Klein, A.G., Seltzer, G.O. & Isacks, B.L., 1999. Modern and Local Last Glacial
Maximum snowlines in the Central Andes of Peru, Bolivia and Northern Chile. Quaternary
Research Reviews, 18: 3-84.
Lea, D.W., D.K. Pak, C.L. Belanger, H.J. Spero, M.A. Hall, and N.J. Shackleton, 2006.
Galapagos paleoclimate history of surface waters over the last 135,000 yr. Quaternary
Science Reviews, 25 (11-12): 1152-1167.
Osmaston, H., 2005. Estimates of glacier equilibrium line altitudes by the Area x
Altitude, the Area x Altitude Balance Ratio and the Area x Altitude Balance Index methods
and their validation. Quaternary International, 22–31: 138–139.
Smith, J., Seltzer, G.O., Rodbellb, D.T. & Klein, A.G., 2005 a. Regional synthesis of last
glacial maximum snowlines in the tropical Andes, South America. Quaternary International,
138-139:145-167.
Smith, JA, Seltzer, GO, Farber, DL, Rodbell, DT & Finkel, R.C., 2005 b. Early Local Last
Glacial Maximum in the Tropical Andes. Science, 308.
Ubeda, J., 2007. Geomorphological characterization of the northern sector of
the Central Volcanic Zone of the Central Andes. Approach to a case study: the
glacier of the volcano Nevado Coropuna complex. Research work to obtain the
Diploma of Advanced Studies, Universidad Complutense de Madrid, Madrid, 312 pp. |
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