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

* xbodin@gmail.com

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)