state and dynamics of a tropical rock glacier on the …...caquella meteorological station between...
TRANSCRIPT
References
Fabre, D., Francou, B., Jomelli, V., Kaiser, B., Arnaud, Y., Pouyaud, B., Smiraglia, C., Valla. F. - 2001.Analyse de la structure d'un glacier
rocheux du domaine tropical (Caquella, Sud Lipez, Bolivie). La Houiile Blanche. Vol. 3-4, pp. 124-132.
Francou B., Denis Fabre D., Pouyaud B., Jomelli V. & Arnaud. Y. - 1999. Symptoms of degradation in a tropical rock glacier, Bolivian
Andes. Permafrost and Periglacial Processes. Vol.10 (1), pp. 91-100.
Ikeda, A., Matsuoka, N. & Kääb, A. - 2008. Fast deformation of perennially frozen debris in a warm rock-glacier in the Swiss Alps: an
effect of liquid water. J. Geophys. Res., 113, F01021, doi:10.1029/2007JF000859
Kääb, A., R. Frauenfelder and I. Roer. - 2007. On the response of rockglacier creep to surface temperature increases.
Global and Planetary Change. 56(1-2), 172-187.
Payne D. - 1998. Climatic implications of rock glaciers in the arid Western Cordillera of the Central Andes. Glacial Geology and
Geomorphology, 1998, rp03/1998.
Bodin X.1*, Francou B.2,3, Arnaud Y.4, Fabre D.5 & Wagnon P.4
State and dynamics of a tropical rock glacier on the Altiplano (Bolivia, 21.5°S)State and dynamics of a tropical rock glacier on the Altiplano (Bolivia, 21.5°S)
(1) Instituto de Geografía, Pontificia Universidad Católica de Chile, Santiago, Chile
(2) Laboratoire d'étude des Transferts en Hydrologie et Environnement, CNRS-UJF, Grenoble, France
(3) Institut de Recherche pour le Développement, Quito, Ecuador
(4) Laboratoire de Glaciologie et de Géophysique de l'Environnement, CNRS-UJF, Grenoble, France
(5) Conservatoire National des Arts et Métiers, Paris, France
5 - ConclusionThis study was the first of this kind to provide a detailled panorama of the state and the dynamics of a tropical rock glacier thanks to
various complementary datasets. A partial confirmation of the degrading state of the rock glacier arises from the high surface velocity
recorded between 1998 and 2002 and from the interpretation of geoelectrical soundings. Meteorological measurements reveal a
complex surface energy balance, strongly influenced by latent heat fluxes and rise the question of the hydric functioning of the
permafrost.
4 - Discussion
Results of others studies on temperate mountains (Kääb et al., 2006; Ikeda et al., 2009) suggest that the
combination of low resistivity and relatively high surface velocity recorded on the Caquella rock glacier may
be explained by the presence of warm permafrost (temperature close to 0°C and presence of liquid water).
According to the same authors, the relatively high annual and seasonal variability of surface kinematics might
also result from that specific thermal state of the ground.
As already pointed out by Francou et al (1999), this tropical rock glacier, although still located above the 0°C
isotherm may therefore be experiencing degradation of its permafrost. Additional geophysical data (like
electrical resistivity tomography) and thermal monitoring are nevertheless required to determine more
accurately the internal structure of the rock glacier and its physical and thermal state.
The asynchronous combination of the air temperature and snow thickness seasonal cycles lets us thinking that
the thermal response of the ground is probably not as highly influenced by the snow cover here as it is under
temperate climate. Furthermore, in spite of their short duration, the high sublimation rates of snow measured
on the rock glacier suggest that meltwater from the snowpack would be a small contributor to the hydric state
of the ground. It is possible that the water alimentation of the permafrost body results from the burying of snow
under debris, as well as is probable that past nivo-meterological conditions, as attested by the (probably) LIA
moraine, were significantly different from those of today.
The long-term continuation of the monitoring is therefore required, as well as the acquisition of new datasets
on other intertropical Andean rock glaciers (Trombotto et al., submitted).
4.1 - Present state of a tropical rock glacier 4.2 - Permafrost / climate relationship at intertropical latitude
3 - Dynamics of the rock glacier
Jul-98 Jan-99 Jul-99 Jan-00
-15
-10
-5
0
5
10
15
Air
te
mp
era
ture
[°C
]
0
20
40
60
80
100 Rela
tive
hum
idity
[%]
3.2 - Seasonal and annual surface kinematics between 1998 and 2002
Between 1998 and 2002, the measured horizontal velocities
(campaigns on 07/98, 06/99, 05/00, 04/02) range from 124 to 17
cm/yr, and the vertical velocities from -61 to 5 cm/yr .
The 1998-1999 year shows a mean vertical velocity / 3D velocity ratio
of -0.49 whereas it is only of -0.17 and -0.15 during 1999-2000 and
2000-2002 years, indicating a higher vertical component in the total
movement during the first year than during the 3 next ones.
Jan-99 Jan-00 Jan-01 Jan-0220
40
60
80
100
Hori
zo
nta
l ve
locity [cm
/yr] -50
-40
-30
-20
-10
0
10
Vertic
al v
elo
city
[cm
/yr]
Jan-99 Jan-00 Jan-01 Jan-02
down
middle
up
Annual vertical and horizontal velocities between 1998 and 2002 of the three main morphologic parts of the Caquella rock glacier.
Fluctuations of vertical and horizontal displacements along the longitunal profil of the Caquella rock glacier during the dry 1998 season, the wet 1998-1999 and the whole 1998-1999 year.
Daily air temperature and relative humidity recorded at the Caquella meteorological station between March 1998 and March 2000 and dates of seasonal geodetic measurements.
nov98-jun99jul98-nov98
jul98-jun99 0 400 800 1200 1600Distance [m]
-80
-60
-40
-20
0
Vertical displacement [cm/yr]
0
40
80
120
160
Horizontal displacement [cm/yr]
5400
5450
5500
5550
5600
5650
Altitud
e [m
. asl]
jul98-jun99nov98-jun99jul98-nov98
On a seasonal scale, it appears that, on average for the whole rock glacier, both vertical and horizontal velocities (absolute
values) measured during the dry 1998 season (jul.98-nov.98) where lower than those during the wet 1998-1999 season
(nov.98-jun.99). The amount of movement during the wet season reaches about 52 to 61% of the total annual movement, except
for the lowest points on the front where the wet part is only of 38%.
25-Apr 26-Apr 27-Apr 28-Apr-12
-8
-4
0
4
Te
mp
era
ture
[°C
]
0
10
20
30
40
50
Rela
tive
hum
idity
[%]
Relative humidity
Air temperature
Vertical profile of ground temperature: mean, minimum, maximum and +/- 1SD envelop (April 2001 - March 2002).
3.1 - Characteristics of the surface energy balance and the ground thermal regime
The computation of the different energy fluxes above a 40-cm thick snow cover, during a 5-day period entering the
austral dry winter of 2001, indicates a negative balance (approx. -85 W/m²) and a cooling of the ground.
As it typically occurs on high mountain at very high altitude, the energy balance measurements show high
sublimation rates (2-3 mm we/day) of the snow, which would generally lead to the disappearance of the snow cover
in 2 to 3 months.
The ground thermal monitoring between April 2001 and March 2002 shows that:
- the mean annual temperatures of that upper part of the active layer were comprised between -0.6 (-50 cm) and
-0.2°C (-170 cm);
- the quasi absence of snow cover during the winter allows a strong coupling with the atmosphere and a deep
freezing of the ground;
- the timelag of the minimum temperature is shifted by quasi 2 months at -170 cm compared to the surface
- the near-0°C curtain effect observed at -170 cm attests the progressive freezing of ground moisture.
May-01 Jul-01 Sep-01 Nov-01 Jan-02 Mar-02-8
-6
-4
-2
0
2
4
6
8
Te
mp
era
ture
[°C
]
-8
-6
-4
-2
0
2
4
6
8
T -50cm
T -70cm
T -100cm
T -130cm
T -170cm
-8 -4 0 4 8Temperature [°C]
-200
-160
-120
-80
-40
0
Dep
th [
m]
25-Apr 26-Apr 27-Apr 28-Apr0
200
400
600
800
1000
1200
Ra
dia
tive
flu
x [
W/m
²]
LWup
LWdown
SWup
SWdown
Data from meteorological station between 24th and 28th of April 2001. Up: air temperature and relative humidity; down: radiative fluxes.
Daily ground temperatures at 5 depths between April 2001 and March 2002.
2 - State of the Caquella rock glacier
Re
sult
s o
f v
ert
ica
l ele
ctri
cal s
ou
nd
ing
s (J
uly
19
97
)
2.2 - Internal structure of the rock glacier
As already described by Francou et al. (1999) and Fabre et al. (2001), the
vertical electrical soundings (VES) reveal relatively low resistivities of
the 2nd layer (between 4300 and 44000 Ohm.m), which would
generally correspond to mainly sandy material with low interstitial ice
content and/or temperature close to 0°C. Those results are in good
agreement with observations made in a crevasse in 1996.
Strong spatial variations of the internal structure of the rock glacier
appear, and three zones can be distinguished:
1) the root, with a thick active layer above a thin frozen level;
2) the median part, with a 40-m thick frozen layer below an active
layer of 4m;
3) the terminal part, with very few or no ice.
1x1021x10
31x10
41x10
5
Resistivity (Ω.m)
-40
-30
-20
-10
0
Depth (m)
1 10 100 1000AB/2 (m)
1x102
1x103
1x104
1x105
Resistivity (Ω.m)
Standard error = 0.071
CAQ_96_4 (5580 m)
18m - 11100 Ohm.m
8m - 44000 Ohm.m
?m - 1000 Ohm.m
1x102
1x103
1x104
Resistivity (Ω.m)
-60
-50
-40
-30
-20
-10
0
Depth (m)
1 10 100 1000AB/2 (m)
1x102
1x103
1x104
Resistivity (Ω.m)
Standard error = 0.093
CAQ_96_5 (5470 m)
2m - 1400 Ohm.m
55m - 4350 Ohm.m
?m - 350 Ohm.m
1x103
1x104
1x105
Resistivity (Ω.m)
-50
-40
-30
-20
-10
0
Depth (m)
1 10 100 1000AB/2 (m)
1x103
1x104
1x105
Resistivity (Ω.m)
Standard error = 0.100
CAQ_96_1 (5415 m)
4m - 4200 Ohm.m
39m - 38000 Ohm.m
?m - 2000 Ohm.m
1x103
1x104
1x105
Resistivity (Ω.m)
-30
-20
-10
0
Depth (m)
1 10 100AB/2 (m)
1x103
1x104
1x105
Resistivity (Ω.m)
Standard error = 0.128
CAQ_96_3 (5360 m)
3m - 2300 Ohm.m
25m - 5300 Ohm.m
?m - 15000 Ohm.m
1x102
1x103
1x104
Resistivity (Ω.m)
-30
-20
-10
0
Depth (m)
1 10 100AB/2 (m)
1x102
1x103
1x104
Resistivity (Ω.m)
Standard error = 0.098
CAQ_96_2 (5300 m)
?
4m - 900 Ohm.m
?m - 1800 Ohm.m
2.1 - Geomorphological and topoclimatic contexts
Glacial deposits on the Eastern flank of the Cerro Caquella are marked by
widespread Quaternary moraines, until 4500 m asl., and a very reduced
LIA moraine at 5675 m asl.
Below this latter, the 20-ha rock glacier develops until 5250 m asl. (approx.
altitude of the present 0°C isotherm) in a moderately sunny context and
displays a typical morphology of transverse ridges and furrow and a
80-m high front.
Ge
om
orp
ho
log
ica
l an
d t
op
ocl
ima
tic
con
tex
t
Morphologic characteristics of the rock glacier and location of the datasets (VES = vertical electrical sounding)
Potential global solar radiation received at the rock glacier surface (computed in Solar Analyst, ArcGIS 9.2)
1 - IntroductionOn the Southern Bolivian Altiplano, active rock glaciers are found on the flanks of volcanoes above 4800-5000 m asl. (Payne et al., 1998). Due to aridity,
glaciers are quasi absent in the region, and rock glaciers hence represent a climatic indicator as well as a water reservoir. In this context, the study of the
Caquella rock glacier, initiated in 1996 by IRD, intends to bring insights on the present state and the recent dynamics of a rock glacier at low (21°S)
latitude.
Results from geomorphological analysis and geophysical investigations are first presented. Then we used climatic datasets, including a 5-days surface
energy balance assessment, ground temperature measurements during one year and a 2-year topoclimatic monitoring, to precise the characteristics and
controls of the permafrost thermal regime in this context. Repeated geodetic measurements of blocks at the surface of the rock glacier finally allow to
discuss the relationship between climate and kinematics of the rock glacier under present warming conditions.
Overview of the Cerro Caquella Eastern face and the (circled) rock glacier (photo X. Bodin, avril 2009).
The Caquella rock glacier at the end of the wet season (photo IRD, avril 2008).
Cerro Caquella (5940 m asl)