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 DOI: 10.1177/0959683607082438

2007 17: 1033The HoloceneBarbara Stenni, Laura Genoni, Onelio Flora and Mauro Guglielmin

An oxygen isotope record from the Foscagno rock-glacier ice core, Upper Valtellina, Italian Central Alps  

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1996). New high-resolution isotopic records (δ18O, δD and deu-terium excess) from an ice core drilled in the frontal part of theFoscagno rock glacier in the Italian Central Alps are presented in thispaper. Moreover, two other drilling sites (S1 and S3) on the samerock glacier are considered. The empirical relationship existingbetween either D/H or 18O /16O and condensation temperature(Dansgaard, 1964) has long been used as a tool in ice corepalaeotemperature reconstruction studies. The main aim of the pres-ent study was to investigate further the origin of the massive icefound inside the Foscagno rock glacier, and if possible to obtainpalaeoclimate information from the ice.

Study site

The map (Figure 1A) shows the geographical and geomorphologiclocation of the Foscagno rock glacier, located in the Upper Valtellina,

Introduction

The very few available geochemical records retrieved by rock gla-ciers may provide information about past climate conditions (Clarket al., 1996; Steig et al., 1998; Humlun, 1999; Haeberli et al., 1999;Konrad et al., 1999). In fact, three important characteristics of rockglaciers allow them to be used as a palaeoclimate proxy: (i) per-mafrost occurrence within them, (ii) their slow velocities and (iii) theinsulation effect of their debris cover. The presence of permafrostconditions and the slow velocities of the rock glaciers provide veryold ages, while the third factor permits the preservation of glacio-chemical signatures of palaeoclimatic significance. The GalenaCreek ice core in the Absaroka Mountains in northwesternWyoming, which probably dates back to 1100–2000 yr BP, suggestsa preservation of the original geochemical stratigraphy (Clark et al.,

Abstract: New high-resolution isotopic records (δ18O, δD and deuterium excess), from an ice core drilled in theFoscagno rock glacier (Italian Central Alps), are presented. The δ18O data suggest a clear division between anupper part (2.5 and 4 m), showing relatively homogeneous values, and a middle part (4–7.65 m), showing seasonalvariations of this parameter. The isotopic analyses confirm previous results (crystallographic and chemical analy-ses) suggesting a division of this relict glacier ice body into an upper part, between 2.5 and 4 m, where meltingand refreezing processes occur, and a middle part, between 4 and 7.65 m, where the isotopic signal is preserved.Larger deuterium excess variations (d = δD−8*δ18O) are found in the massive ice (below 4 m depth) rather thanin the overlying ice. These are in antiphase with the δ18O but without any clear correspondence with the presenceof the debris layers. Postdepositional processes could have affected, at least partially, the isotopic content of theoriginal precipitation. The radiocarbon dating of a leaf (Salix spp.) found in the massive ice from another nearbyborehole in the same rock glacier gave a calendar age ranging between AD 765 and 1260. The expected δ18O val-ues of the present-day precipitation in the Foscagno valley are of the same order as those found in the massive ice(−12.4‰). This similarity would suggest climate conditions not very different from present day, in good agree-ment with other available palaeoclimate reconstructions for this period. However, only more abundant precipita-tion would make the existence of a glacier possible in a climate not very different from that of the present.

Key words: Rock glacier, δ18O, δD isotope, relict glacier ice, permafrost, Holocene, Italian Alps.

The Holocene 17,7 (2007) pp. 1033–1039

© 2007 SAGE Publications 10.1177/0959683607082438

*Author for correspondence (e-mail: mauro.guglielmin@uninsubria.it)

An oxygen isotope record fromthe Foscagno rock-glacier ice core,Upper Valtellina, Italian Central AlpsBarbara Stenni,1 Laura Genoni,1 Onelio Flora1 andMauro Guglielmin2*

( 1Department of Geological, Environmental and Marine Sciences, University of Trieste, Via E. Weiss 2,34127 Trieste, Italy; 2Department of Functional and Structural Biology, University of Insubria,Via J.H. Dunant 3, 21100 Varese, Italy)

Received 11 September 2006; revised manuscript accepted 10 April 2007

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one of the longest and most studied rock glaciers of the Italian Alps(Figure 1B). Within it, previous geophysical investigations revealeda patchy permafrost distribution, with a thickness ranging between 8and more than 50 m (Guglielmin et al., 1994). On the bases of geo-physical investigations and of geomorphological evidence, thePACE (Permafrost and Climate in Europe) borehole was cored inJune 1998 at an elevation of 2510 m, in the right lobe of the activepart of the rock glacier, reaching a depth of 24 m (see location inFigure 1A and B). The S1 and S3 boreholes (Figure 1A) were drilledin September 2002 in the active part of the rock glacier, in the samelobe as the main PACE borehole, and in the left lobe, respectively.Another borehole (S2) was drilled just outside the rock glacier at2650 m a.s.l. in an area reached by the Foscagno glacier during the‘Little Ice Age’ until AD 1932 (Guglielmin et al., 2001).

The sites of two 14C datings of palaeosoils, buried under the leftlobe of the active part and under the front of the inactive part ofthe rock glacier (Calderoni et al., 1998), are reported in Figure 1A.The former gives an uncalibrated age (14C age) of 2200 ± 60 yr BP(AD 1950), corresponding to calendar ages ranging between 2010 and2345 yr BP, while the latter gives an uncalibrated age of 2700 ± 70yr BP, corresponding to calendar ages ranging between 2740 and2950 yr BP, according to Stuiver et al. (1998).

Methods

Coring was performed with a diamond drill head and a triple sam-pler (T121) using compressed and refrigerated air as well as liquid.

The cutting of the ice cores was carried out in a cold room(−20°C) at the University of Milano Bicocca. The PACE ice corewas cut with a sampling resolution of about 20 cm in the massive,highly fractured ice between 2.5 and 4 m and of about 4 cm in themassive ice between 4 and 7.6 m. For a complete description ofthe stratigraphy of the PACE borehole see Guglielmin et al. (2001and 2004). The S1 and S3 drillings are characterized by a discon-tinuous presence of ice with a lower sampling resolution than thatapplied to the PACE borehole.

The ice core samples were measured for their oxygen andhydrogen isotope composition using the well established tech-nique of CO2/H2 water equilibration by means of an automaticequilibration device on line with the mass spectrometer (Epsteinand Mayeda, 1953; Horita et al., 1989). The results are reported asdelta units (δ) per mil (‰) against the V-SMOW isotopic stan-dard. The analytical precision of δ18O and δD measurements(where δ18O or δD = {[(18O /16O)

sample/(18O/16O)

V-SMOW]−1} × 1000)

are better than ± 0.05‰ and ± 0.7‰ respectively. The δ18O and δDmeasurements were performed on the same water aliquot.

Results and discussion

The massive ice, found between 2.5 and 7.65 m depth of the PACEdrilling, can be divided into two parts according to its macroscopiccharacteristics: the upper core between the depths of 2.5 and 4 m(Figure 2A), and the middle core between those of 4 and 7.65 m(Figure 2B). The upper part was almost completely crushed becauseof a high fracture frequency (Figure 2A) while the middle partshowed layers of almost transparent ice, abundant rock inclusions,and subhorizontal debris bands and layers of foliated ice (Figure2B). The debris bands are mainly coarse grained (gravel, composedof angular clasts) although there are some debris bands composedonly of sand and finer material (Figure 2B). Within the lower partbetween 7.6 and 14.5 m, the ground ice occurs as thin ice layers andlenses alternating with ice-bonded sediment layers (Figure 2C). Theextent of the massive ice in the other two boreholes is lower than inthe PACE one, with a maximum thickness of 5 m, and limited todepths of 5.1–7 m and 7.5–7.9 m in S1, and of 2.6–5 m in the caseof S3, respectively. Foliated ice layers occur in the massive ice ofS3, while only between 6–6.1 m and 7.5–7.9 m in S1.

The δ18O measurements, performed in the massive ice of thePACE ice core, are reported against depth in Figure 3A. The lowresolution δ18O records obtained from the S1 and S3 drillings arereported in Figure 3B. The distance between the three boreholesand the heterogeneity in the massive ice distribution prevent usfrom making a direct comparison at the same depth. However, themain trends are in fairly good agreement below 4 m depth. In thispaper, we focus mainly on the high-resolution isotopic recordobtained from PACE. The oxygen isotope data suggest a cleardivision between the upper and the middle core, in agreement withthe stratigraphic pattern. The upper core (between 2.5 and 4 m)shows quite homogeneous data gathering around a mean δ18Ovalue of −12.2‰, while the values obtained from the middle core(4–7.65 m) range between −17.0 and −10.3‰, with a mean valueof −12.4‰.

The δ18O variations observed in the massive ice below 4 m couldbe related to seasonal variations of temperature. In fact, the δ18O ofprecipitation is essentially related to the condensation temperatures(Dansgaard, 1964). On the contrary, the upper part, characterized bymore homogeneous values, could be affected by postdepositionalprocesses. The amplitude of the supposed seasonal variations, of

1034 The Holocene 17,7 (2007)

Figure 1 Foscagno rock glacier. (A) Location. Legend: 1, boreholesites; 2, buried soils sites; 3, limit of active rock glacier; 4, limit ofinactive rock glacier; 5, sharp ridge. (B) View of the rock glacier withthe location of the ice core (white arrow) and the surroundings

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about 5–6‰, is however lower than the ones observed nowadaysin the precipitation of the area (Longinelli and Selmo, 2003) withvalues around 10‰. This decrease in amplitude could be relatedto a smoothing effect occurring during the firnification process inthe snow column of the accumulation area.

The isotopic variations observed in the Galena Creek ice coreby Steig et al. (1998) were also interpreted as seasonal variations

even if melting in association with debris layers accounts for partof the observed isotopic stratigraphy. In that case δ18O amplitudesof 1–2‰ were observed.

Crystallographic and chemical analyses (Guglielmin et al., 2004)have already suggested that the ice characteristics between 2.5 and4 m are consistent with superimposed ice that formed as a conse-quence of melting and refreezing processes. Instead, the ice between4 and 7.65 m can be considered also as a glacier ice body but withthe seasonal signal preserved. This interpretation was mainly basedon arguments reported below. The upper part of the core was com-posed of white ice, very rich in elongated bubbles (0.5–2 mm),while the middle part showed layers of almost transparent ice, withsome undeformed small bubbles (0.3–0.5 mm), abundant rockinclusions and subhorizontal debris bands and layers of foliated icewith large tabular crystals alternating with thin bubbly layers.Crystal size ranged between 5 and 30.5 mm, while C-axes had amean dip ranging from 9° to 36° generally randomly orientated oralong subhorizontal bands in the middle core. The sediment layers(mainly silty sand, sometimes with gravels) were always subhori-zontal and did not show any evidence of postdepositional deforma-tion. All these crystallographic characteristics suggest that the iceshould be considered as a relict glacier ice body. The chemicalanalyses performed showed a much higher ionic content in theupper than in the middle part of the core and similar to the lowerone. In particular, the correlation between sodium and chloride inthe middle core was very high, while it was poor (R2 < 0.40) in theupper and lower core, where the high Na/Cl ratios (respectively, 3.1and 6.5) indicate a strong sodium contribution from the soil. Thechemical data suggest that the upper part suffered melting andrefreezing processes that did not affect the middle one (Guglielminet al., 2004). Importantly, the average concentrations of sulphateand nitrate are comparable with those typical of a pre-industrial era(Guglielmin et al., 2004).

The δ18O and δD measurements allow us to calculate the deu-terium excess, which is defined as follows: d = δD−8*δ18O(Dansgaard, 1964). This second order parameter is mainlydependent on the climatic conditions in the precipitation sourceregions (Merlivat and Jouzel, 1979). Co-isotopic (δ18O and δD)studies are necessary and widely used in order to determinewhether melting–refreezing processes have affected a ground icebody (Jouzel and Souchez, 1982; Souchez and Jouzel, 1984).

The δ18O and deuterium excess values, obtained from the PACEborehole, are reported in Figure 4. As shown before, in the case ofthe δ18O, larger d variations are found in the massive ice ratherthan in the ice below 4 m depth. An anti-phase behaviour isobserved between the two records in the massive ice. The uppercore (2.5–4 m) shows a mean d value of 8.8‰, while the valuesobtained from the middle core (4–7.65 m) range between 7.3 and16.4‰, with a mean value of 9.9‰.

Steig et al. (1998) pointed out large deviations in d in theGalena Creek ice core, immediately below debris layers, enrichedwith respect to the background and showing an anti-phase withδ18O values. They interpreted that behaviour as reflecting meltwa-ter infiltration in correspondence with the debris layer represent-ing the summer ablation season. However, this infiltrationaccounts for only part of the observed isotope stratigraphy, withthe isotopic changes reflecting both meltwater infiltration and sea-sonal variations. In the Foscagno ice core we have different dspikes in anti-phase with the δ18O, but there does not seem to be aclear correspondence with the presence of the debris layers.

Ice samples subjected to melting and refreezing normally alignon a straight line with a lower slope (called ‘freezing slope’) thanthat of the Meteoric Water Line (MWL; δD = 8δ18O + 10;Dansgaard, 1964) on which glacier ice samples, originating fromprecipitation, align (Jouzel and Souchez, 1982; Souchez andJouzel, 1984). The δ18O/δD diagram shown in Figure 5, obtained

Mauro Guglielmin et al.: Oxygen isotopes from a rock-glacier ice core 1035

Figure 2 Foscagno PACE ice core. (A) Cuttings of the upper part(2.5–4 m). (B) Massive ice of the middle part (4–7.65 m). (C) Frozendebris with lenses of segregated ice of the lower part (7.65–14.5 m).Note that the top of the core corresponds to the top of the page

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from all the results, does not show any strong evidence of melt-ing–refreezing, and all the data are plus or minus aligned on theMWL. Also the upper part of the core does not show any evidenceof this type. The regression line obtained from the samples of themassive ice shows a slope of 7.26, just a little lower than the localmeteoric water line, whose value is 7.68 (Longinelli and Selmo,2003). On the other hand, the freezing slope calculated by the theo-retical equation for a closed system (Jouzel and Souchez, 1982),using an initial meltwater δ18O value of −12.4‰, is 6.53.Consequently, postdepositional processes (meltwater infiltration,refreezing and so on) could, at least partially, have affected the iso-topic content of the original precipitation.

If we take into consideration the mean value obtained from themassive ice (−12.4‰) as being representative of the mean isotopiccomposition of the precipitation, we could, in principle, derivesome information about past climate conditions. The expectedδ18O values of the present day precipitation in the Foscagno valleyrange between −11.8 and −12.4‰ at elevations of 2500 and 2800m, respectively. We calculated these values on the basis of theδ18O elevation gradient observed in the precipitation for the area(Longinelli and Selmo, 2003) equal to −0.19‰/100 m. Theexpected values are of the same order as those found in the mas-sive ice of the frontal part of the rock glacier. This similaritywould suggest climate conditions not so far from the present day.

1036 The Holocene 17,7 (2007)

−18

−17

−16

−15

−14

−13

−12

−11

−10

−9

200 300 400 500 600 700 800

Depth (cm)

δ

δ 18

O (

V-S

MO

W)

A

−18

−17

−16

−15

−14

−13

−12

−11

−10

−9

200 300 400 500 600 700 800 900 1000 1100

Depth (cm)

S1S3

δ18O

(V

-SM

OW

)

B

Figure 3 δ18O records from the PACE borehole (A) and S1 and S3 boreholes (B) reported against depth (cm). The depths of the three boreholesare not directly comparable (see text)

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Age of the ice and palaeoclimaticimplications

Regarding the age of the ice, the radiocarbon data of two buriedpalaeosoils, both under the active and the inactive part of the rockglacier (Calderoni et al., 1998), suggest that the rock glacier hasbeen active since 2200 yr BP; however, they do not give us infor-mation about the age of the ice preserved within the rock glacier.Moreover, these ages are referred to bulk palaeosoils and, as

pointed out by different authors (eg, Hornes et al., 2004; Humlunet al., 2005), the contamination of this type of sample could givesignificantly different ages. It is also true that, in this particularcase, no evidence of cryoturbation and/or bioturbation were visi-ble in the soil and therefore we can accept the age of 2200 yr BPas a minimum age of the rock-glacier activity. Further hints comefrom a radiocarbon dating of a leaf (Salix spp.) found inside themassive ice at the S1 drilling site (7.7 m depth). This leaf wasfound embedded within an ice layer parallel to the foliations,suggesting an original primary sedimentation. Therefore the AMS

Mauro Guglielmin et al.: Oxygen isotopes from a rock-glacier ice core 1037

Figure 4 δ18O and deuterium excess records from the PACE borehole reported against depth (cm). The debris bands (vertical bars) are composedof coarse (grey) and fine (light grey) material

−130

−120

−110

−100

−90

−80

−70

−60

−50

−40

−16 −15 −14 −13 −12 −11 −10 −9 −8

230-400400-750 Meteoric Water Line

δD = 8δ18O+10

δD (

V-S

MO

W)

δ18O (V-SMOW)

δD = 7.26 δ18O + 0.71R2 = 0.96

Figure 5 δ18O/δD diagram showing all the results obtained from the PACE ice core above (open triangle) and below (solid dots) 4 m depth. Theregression line obtained from the samples of the massive ice (solid line) and the Global Meteoric Water Line (dashed line) are reported

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uncalibrated radiocarbon age of 1020 ± 20 BP, that corresponds toa calendar age ranging between AD 765 and 1260, is considered theage of the ice layer where the leaf was collected. However, we arenot sure whether the massive ice found inside S1 is the same ageas the PACE drilling, even if the two boreholes were cored in thesame morphological feature (right lobe of the active rock glacier)and at a very short distance from each other (less than 50 m).Moreover, the mean δ18O value of the massive ice between 4 and7.65 m in the PACE borehole is similar to that between 7.5 and 7.9m in S1 and equal to −12.4 and −12.6‰, respectively. In addition,the massive ice found inside the two boreholes shows similar crys-tallographic characteristics (ie, foliations, etc.).

The palaeotemperature reconstructions for the late Holocene (thelast 3000 years), based on pollen data (Davis et al., 2003), suggestrelevant differences within Europe, with greater temperature fluctu-ations in the southern than in the northern or central part. In partic-ular, for the period between AD 765 and 1260, the annual andsummer temperature values, in the southern regions, oscillatebetween −0.5 and +0.8°C. Temperatures similar to the present-dayvalues are reported by Mangini et al. (2005) from a stalagmiterecord in the Central Alps during the ‘Mediaeval Warm Period’(between AD 800 and 1300). Moreover, a stalagmite record from theSE Alps (Frisia et al., 2005) shows a complex structure for theMediaeval climate with slightly cool phases from AD 450 to 700 andfrom AD 900 to 1100. More recently, Vollweiler et al. (2006)recorded a warm period between AD 750 and 1250 analysing threestalagmites from the Alpine Spannagel cave (Austria) located in asite at the same altitude as our borehole (2500 m a.s.l.).

Considering that the age for the massive ice ranges between AD

765 and AD 1260, our δ18O data are not far from these palaeocli-mate reconstructions. In fact, the slight temperature variations(0.5–1°C) that seem to have occurred during this period probablyaffected the δ18O of the palaeoprecipitation (and so the ice) for atmost 0.7‰.

Conclusions

Although the possibility of a partial influence of meltwater infil-tration on the original isotopic signal cannot be excluded, an inter-pretation of the massive ice found in the Foscagno rock glacier asa relict of glacier ice, preserved within permafrost, seems to beplausible. In fact, a preserved chemical and isotope annual stratig-raphy in the massive ice suggests that it originated from snowfirnification and was not affected by strong postdepositionaleffects. The preservation of seasonal trends was possible becausethe Foscagno ice core was not subjected to strong percolationprocesses during the summer months.

The ice of the rock glacier has proved to be a potential source ofpalaeoclimatic information. The age of the massive ice likely falls inthe interval AD 765–1260, and its mean δ18O value is in good agree-ment with the other available palaeoclimate reconstructions.Moreover, the Foscagno glacier was a small cirque glacier, and it isreasonable to think that its response time to climatic changes was fast(Paterson, 1994) and therefore it should be assessed.

If it is true that the massive ice analysed is a relict glacier ice,as suggested by Guglielmin et al. (2004), and confirmed bythe present results, there is an apparent paradox to solve. In fact,the deposition of a Salix leaf within the snow suggests that also theFoscagno glacier occurred during the ‘Mediaeval Warm Period’,whereas now with similar air temperature it has completely disap-peared. Glacier advances have already been inferred for theMediaeval Period in other parts of the world, eg, Alaska (AD

400–700; Reyes et al., 2006), in East Greenland (AD 900–950;Geirsdóttir et al., 2000) or, closer to the examined area, in the Alps(ie, 1170 BP, Orombelli and Porter, 1982). As suggested also for

the ‘Little Ice Age’ in Norway by Nesje and Dahl (2003) and forAD 900–950 by Geirsdóttir et al. (2000) in Greenland, an increaseof precipitation especially in winter could be the explanation for apositive glacier mass balance. Unfortunately, no palaeoprecipita-tion data are available for the late Holocene in this area and, con-sidering the high present-day spatial variability of the snow coverin this sector of the Central Alps, our explanation still remainsspeculative. However, Holzhauser et al. (2005) reported glacieradvances in the Swiss Alps, from AD 800 to 900 and AD 1100 to1200 that are in the range of our Salix leaf determination. The gen-eral agreement found by these authors between glacier fluctua-tions and lake level variations, implies a strong influence of bothtemperature and moisture on the mass balance of glaciers.

Acknowledgements

We are very grateful to Marco Filipazzi and Giorgio Teruzzi(University of Milano Bicocca) for the sample cutting in the coldroom. Thanks also to Dr Nicoletta Cannone and Professor SergioSgorbati, who analysed the 14C-dated vegetation mat for this work.A special thanks to Professor Ole Humlun and an anonymousreviewer for the suggestions and comments that improve thepaper. Thanks also to Professor James Gilbert Burge for theEnglish revision. The research was funded by Istituto Nazionaleper la Montagna (IMONT) in the frame of the Cryoalp project.

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