Rock glacier distribution and paleoclimate in Italy

Permafrost, Phillips, Springman & Arenson (eds)
© 2003 Swets & Zeitlinger, Lisse, ISBN 90 5809 582 7
Rock glacier distribution and paleoclimate in Italy
F. Dramis
Dept. Geological Sciences, “Roma Tre” University, Largo S. Leomardo M.1, Rome, Italy
C. Giraudi
ENEA- Casaccia, via Anguillarese, S. Maria di Galeria (RM), Italy
M. Guglielmin
ARPAlombardia, via Restelli 1a, Milano, Italy
ABSTRACT: On the basis of radiometric datings of rock glaciers and other proxy data, the Authors reconstruct the
evolution of permafrost in the Italian mountains since the Last Glacial Maximum. The minimum ages of active
rock glaciers in the Alps range from 1100 to 2200 yr BP, even though younger ages are not excluded; instead, those
of inactive rock glaciers range from 2300 to 5000 yr BP, but older ages are not excluded. In the Apennines, inactive
rock glaciers indicate three different phases that started earlier than in the Alps (ca 8000 yr BP) and fit relatively
well with the two younger phases of the Alps (3200–3700 yr BP and ca 1200 yr BP). Only one rock glacier in the
Apennines is considered to be still active. Lake-level fluctuations and palinological records show that the minimum
ages of rock glaciers are well correlated with dry climate stages. This suggests that changes in precipitation may
play a fundamental role in the evolution of permafrost and related landforms.
1 INTRODUCTION
2 ROCK GLACIER DISTRIBUTION IN THE
ITALIAN MOUNTAINS
The term rock glacier relates to a tongue- or lobateshaped body of blocks with surface features such as
furrows, trenches and ridges, indicating creep movement (Wahrhaftig & Cox, 1959; Haeberli, 1985; Evin,
1987; Barsch, 1992). The internal structure of rock
glaciers is still poorly understood, even though some
boreholes, drilled in the Alps and in North America
(Barsch et al., 1979; Johnson & Nickling, 1979;
Haeberli et al., 1988; Vonder Mühll, 1993; Clark et al.,
1996; Guglielmin et al., 2001), have emphasized that
the upper layer is always composed of open-work bouldery material, and that underneath frozen ground or
massive ice can be found.
Despite the ongoing debate on the origin of rock
glaciers and the included ground ice, only few studies
have taken into consideration the morphochronologicalpaleoclimatic history of permafrost in mountain areas
(Kerschner, 1985; Calkin et al., 1988; Morris, 1988;
Calderoni et al., 1998; Konrad et al., 1999; Haeberli
et al., 1999; Guglielmin et al., 2001). However, such
investigation could provide a source of climatic information at the millennial-scale, enabling the different
geomorphologic roles of glacial and periglacial processes to be identified.
In this paper, on the basis of radiometric ages of
rock glaciers and other available proxy data, we try to
point out some major paleoclimatic stages in the Italian
Alps and Apennines since the Last Glacial Maximum
(LGM).
The rock glacier inventory of the Italian Alps, compiled
by Guglielmin & Smiraglia (1997) mainly from airphoto interpretation, shows the distribution of more
than 1600 rock glaciers over a total area of 220 km2,
with a density of 0.059 rg/km2. Only 20% of these rock
glaciers are considered dynamically active, while over
80% consist of fragments of metamorphic rocks. This
high percentage is obviously influenced by the prevailing rock types in the Italian Alps even though,
as demonstrated in several Italian Alpine sectors
(Guglielmin, 1997), the density of rock glaciers with
respect to the outcropping areas of metamorphic rocks
is at least twice that for carbonate rocks. Slope rock
glaciers clearly prevail in the Graian, Pennine and
Lepontine Alps, whereas cirque rock glaciers predominate in the Maritime, Cottian, Rhaetian, Atesine, and
Carnic Alps. The mean minimum altitudes reached by
the fronts of inactive and active rock glaciers in the
various alpine sectors are shown in Table 1. Both, the
active and inactive rock-glacier altitudes show vast
differences in the various Alpine sectors. Also, the difference in altitude between active and inactive rock
glaciers varies significantly, reaching a minimum value
in the Maritime Alps (103 m), even though this value,
as well as those found for the Lepontine and Dolomite
Alps (where active rock glaciers are rare), has a low
level of statistical significance.
199
(Pre Boreal). After the LGM, the Apennine climate
underwent a warming trend (Giraudi & Frezzotti,
1997) with peaks of aridity and wetter periods. The
oscillations of Fucino Lake show a negative hydrologic
balance between 20,000 and 17,000 yr BP and between
15,000 yr BP and the early Holocene (Giraudi, 1998).
The Holocene evolution of climate in the Mediterranean area does not show marked changes in temperature. Much more important were changes in the
hydrologic balance (Orombelli & Ravazzi, 1996; Alley
et al., 1997). Several proxy data, such as lake level
records or the sequences of aggradation/degradation
phases of travertine dams and soils, indicate widespread
fluctuations in the precipitation regime (Vinken, 1968;
Cilla et al., 1996; Giraudi, 1998; Ricci Lucchi et al.,
2000).
In the Apennines, the early Holocene after 8900 yr
BP was generally warmer than the present according
to pollen data (Brugiapaglia, 1995), but the hydrologic
balance of the lakes was not positive before about
7000 yr BP (Giraudi, 1996). The sedimentary sequences
of travertine-dammed swampy-lacustrine deposits in
the Apennine valleys, as well as the flora record from
peaty sediments in the Alps, indicate warm and wet conditions until 5000–4600 yr BP, when a strong decrease
in precipitation was recorded at some localities of the
Alps (Burga, 1987; Stumbock, 1996) and Apennines
(Giraudi, 1998). The greatest peaks of aridity, coupled
with a drop in air temperature (Röthlisberger, 1986),
were recorded between 3700 and 3200 yr BP (Calderoni
et al., 1998; Giraudi, 1998; Ricci Lucchi et al., 2000),
while a cold and dry episode occurred in the Alps
between 1100 and 1200 yr BP (Denton & Karlen,
1973).
At least some of the main dry episodes recorded in
the Mediterranean area seem to have occurred at a
more global scale as well. Karlen (1998) found four
aridity peaks at 9500, 8200, 3800, and 1200 yr BP
from lake level changes in Sweden, while Dramis &
Umer (2000) reported four peaks of aridity at about
8000, 5000, 3500, and 1200 yr BP from travertinedammed swampy sequences of northern Ethiopia. The
latter dates are well correlated with the Holocene fluctuations of lake levels in the Main Ethiopian Rift
(Street, 1979). Comparable stratigraphic sequences of
climatic events have also been observed in Egypt
(Kröpelin, 1987) and Morocco (Alley et al., 1997).
Table 1. Summary of rock glacier characteristics in the
Italian Alps and Apennines.
Mountain sector
Number
of RG
1
2
3
Maritime Alps
Cottian Alps
Graian Alps
Pennine Alps
Lepontine Alps
Rhaetian Alps
Atesine
Dolomites
Carnic Alps
Northern Apennines
Central Apennines
Southern Apennines
66
48
126
199
79
581
370
105
20
0
40
1
2230
2631
2679
2573
2228
2510
2594
2366
–
–
2540
–
2127
2285
2288
2340
2107
2128
2281
2106
1744
–
1570
1750
103
346
391
233
181
382
313
260
–
–
970
–
Legend: 1 – minimum mean altitudes of the active rock glacier
fronts; 2 – minimum mean altitudes of the inactive rock glacier
fronts; 3 – difference between 1 and 2.
About 40 rock glaciers could be identified in the
Apennines (Giraudi, 2002), almost all located along the
eastern side of the Central Appenines, and in particular
on the Gran Sasso Massif (2912 m a.s.l.), the Maiella
Massif (2793 m a.s.l.), and Mt. Velino (2486 m a.s.l.).
These rock glaciers are all inactive except for a small
talus-derived feature located on the north-facing slope of
Mt. Amaro (2793 m a.s.l.) with its front at an altitude
of 2540 m a.s.l. This rock glacier is considered to be
active on the basis of its geomorphological appearance
(Dramis & Kotarba, 1992) and of BTS measurements
(Dramis & Guglielmin, in preparation). The difference
in altitude between active and inactive rock glaciers
is very great compared to the Alps, and seems to be
determined by the older age of the relict rock glaciers
found in the Apennines.
3 PALEOCLIMATE EVOLUTION DURING
THE LATE PLEISTOCENE AND
HOLOCENE IN ITALY
The LGM was not a synchronous event in all the Italian
mountains, being dated between 21,000 and 23,000 yr
BP (Giraudi & Frezzotti, 1997). Subsequently, glacial
conditions persisted in the Alps and partially on the
Apennines until 11,000–11,500 yr BP, even though
interrupted by two recessional phases (Bölling &
Alleröd). The beginning of the Younger Dryas climatic
fluctuation was well reflected by the lowering of lake
levels, indicating above all an increase in summer
aridity rather than cooling of winter temperature
(Huntley et al., 1996; Wick, 1996). The Younger Dryas
was characterized by important changes in humidity
conditions until the sudden increase of temperature and
precipitations that marked the transition to the Holocene
4 ROCK GLACIER AGES
The relative dating of rock glaciers can be achieved with
indirect methods such as the analysis of stratigraphic
relationships with other dated deposits and landforms,
or a comparison between the minimum altitude of the
rock glacier fronts and the equilibrium line (ELA) of
200
Table 2.
14
C ages of active and inactive rock glaciers from the Alps and Apennines.
Rock glacier
14
Activity degree
Lab. code
Source
La Foppa 1
La Foppa 1
La Foppa 2
Foscagno
Foscagno
Monte Castelletto
Campo Valley
Cima Rossa
Pasquale Valley
Rhemé Valley
C. Imperatore
Val Maone
Val Maone
790 60
1120 60
5000 70
2200 60
2700 70
3430 70
1340 65
2710 70
2650 50
3965 140
8035 140
3180 40
780 40
Active
Active
Uncertain
Active
Inactive
Inactive
Active
Inactive
Inactive
Inactive
Inactive
Inactive
Inactive
Rome-200
Rome-375
Rome-204
Rome-208
Rome-209
Rome-206
Rome-207
Rome-376
BA-2335
GX 14742
UD-399
BA145529
BA145529
1
1
1
1
1
1
1
1
2
3
4
5
5
C age (yr BP)
Source: 1 – Calderoni et al., 1998; 2 – Guglielmin, unpublished data; 3 – Mortara et al., 1993/1992;
4 – Giraudi & Frezzotti, 1997; 5 – Giraudi, 2002.
All the available 14C ages (conventional ages, before
1950 AD) for the Italian rock glaciers are reported in
Table 2; their location is represented in Fig. 1.
All the ages of the Alpine rock glaciers were
obtained by dating the upper part (1–2 cm) of a paleosoil
buried by the rock glacier front. Instead, the Apennine
rock glacier ages were determined using the presence
of dated soils, lacustrine deposits, tephra layers and
loess deposits overlapping the rock glacier surface.
5 PALEOCLIMATIC SIGNIFICANCE OF
ROCK GLACIERS
Figure 1. Location of the studied rock glaciers. The black
stars indicate the location of the aged rock glaciers: 1 – La
Foppa-1, La Foppa-2, M. Castelletto, Foscagno; 2 – Campo
Valley; 3 – Cima Rossa; 4 – Pasquale Valley; 5 – Rhemé
Valley; 6 – Val Maone, Upper Val Maone; 7 – Campo
Imperatore; 8 – M. Amaro.
In literature, permafrost aggradation is related to cold
and dry climate conditions (e.g. Haeberli, 1985). On the
contrary, permafrost degradation is generally linked to
warm and wet periods, even though thick snow covers
can increase ground temperatures, and thus induce
permafrost degradation (Goodrich, 1982; Williams and
Smith, 1989). However, most investigators consider
rock glacier movement as being due only to permafrost creeping (e.g. Haeberli, 1985), and therefore during permafrost degradation periods the movement
should end.
The rock glacier ages of the Alps are “antequem”
ages and they indicate the minimum ages of the downward movement of the rock. Considering that permafrost aggradation can occur over a very short time
(Lunardini, 1993), we can reasonably assume that these
ages are also the indication of a permafrost aggradation
phase.
Instead, the rock glacier ages of the Apennines are
“postquem” ages as they indicate the minimum age of
the end of the rock glacier movement. Therefore they
should indicate the beginning of permafrost degradation
or a no-permafrost phase.
glaciers (Titkov, 1988; Kerschner, 1985). Absolute ages
of rock glaciers can be obtained through 14C dating of
soils or tephra layers buried by the rock glacier or
overlaying the rock glacier surface (Calderoni et al.,
1998; Johnson, 1998; Giraudi, 2002). They can be
achieved also by other methods such as lichenometry
(Calkin et al., 1988) or dendrochronology (Shroder &
Giardino, 1988), as well as by evaluating the weathering
degree of the surface blocks (Kirkbride & Brazier,
1995). Only two 14C ages of rock glacier ice (2250 yr
BP in both cases) are available: from the Murtel I rock
glacier, in Switzerland (Haeberli et al., 1999), and from
Galena Creek, in USA (Konrad et al., 1999). In order to
compare absolute ages with other proxy data (e.g. those
from pollen analysis, speleothems, travertine), we used
only the minimum ages for the selected rock glaciers.
201
The Alpine rock glaciers fit into the 5000, 3700–
3200 yr BP and 1200 yr BP dry periods, while only the
Apennine rock glaciers relate to the dry 8000, 3700–
3200, and possibly 1200 yr BP phases. The absence of
ages older than 5000 yr BP in the Alps does not exclude
possible phases of permafrost aggradation in the Early
Holocene. In fact, it is conceivable that the wider glacial extension in the Alps destroyed or buried previous
rock glaciers or other permafrost features. At the same
time, it must be emphasized that the main phases of
slope instability in the Italian Alps are related to the
wetter and warm periods of the Holocene. The abovementioned 3700–3200 yr BP phase of rock glacier
formation seems to have occurred at a global level, as
reported from different parts of the world, such as
Alaska (Calkin et al., 1988), Colorado (Morris, 1988),
and Tien Shan (Titkov, 1988). The 1200 yr BP phase
was reported also by Humlun (1998) from Antarctica.
It is important that all the minimum ages of rock glaciers follow warm and wet periods in which slope
instability was likely to have been high, at least in part
as a consequence of permafrost degradation (Dramis
et al., 1995; Davies et al., 2001; Harris et al., 2001).
The occurrence of landslides or an abrupt increase
in debris supply from the slope due to climate-induced
permafrost degradation may provide an explanation
for the bouldery top layer of rock glaciers.
It is also remarkable that, apparently in the same
morphologic and climatic conditions (e.g. altitude,
aspect), some rock glaciers can be affected by more than
one phase of permafrost degradation, such as in the
case of La Foppa. In fact, it is reasonable to hypothesize
that the La Foppa I rock glacier was interested by a
first permafrost aggradation phase before 5000 yr BP
and there are no reasons to think that the area could not
have undergone permafrost degradation between 5000
and 3700 yr and/or between 3200 and 1200 yr BP. Therefore we can hypothesize that, at least somewhere, the
rock glacier creep was discontinuous and followed a
step-like trend.
The relationships between precipitation regime and
rock glacier occurrence are also shown by the present
distribution of active rock glaciers, as compared with
the Mean Annual Air Temperature (MAAT) in several
Alpine and Central Apennine sectors. The MAAT, calculated with a 0.6°C/100 m adiabatic gradient for the
mean altitudes of active rock glaciers (MAF), is not
always the same at the altitude of the rock glacier fronts,
a datum which reveals a possibly significant role of
precipitation and, in particular, of snow cover. In fact,
the Maritime Alps, Lepontine Alps and Dolomites,
where rock glaciers reach altitudes lower than the 1°C
isotherm (Fig. 2), are the wettest sectors of the Italian
Alpine range (Guglielmin & Dramis, 1999) The apparent decrease of MAF in correspondence with areas
where there are greater precipitation can be explained
2800
0
-1
2600
Permafrost
-2
2200
-3
2000
-4
M
ar
iti
m
e
Co
tti
an
G
ra
ia
n
Pe
nn
i
Le ne
po
nt
in
e
Rh
ae
tia
n
A
te
sin
Ce
D
e
ol
nt
o
ra
l A mite
s
pe
nn
in
es
2400
MAF (m)
MAAT (ºC)
Figure 2. Relationships between MAAT and minimum
altitude of the active rock glacier front (MAF). Note that in
the Maritime Alps, Lepontine Alps, and Dolomites the
present MAAT is higher than 1°C.
as the effect of a prolonged permanence of insulating
snow cover in the summer and the resulting decrease
of ground temperatures (MGT).
6 CONCLUSION
The ages of rock glaciers from the Italian mountains
appear to correspond quite well with periods of aridity
both in the Apennines and in the Alps. The Alpine
rock glaciers fit into the 5000, 3700–3200 yr BP and
1200 yr BP dry periods, while only the Apennine rock
glaciers relate to the dry 8000, 3700–3200, and possibly
1200 yr BP periods. Moreover, especially during the
Holocene, rock glaciers seem to have developed following warm/wet periods, during which slope instability
was likely high.
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