Rock Glaciers in the Patagonian Andes: an Inventory for the Monte San Lorenzo (Cerro Cochrane) Massif, 47° S

ROCK GLACIERS IN THE PATAGONIAN ANDES: AN INVENTORY FOR THE MONTE SAN LORENZO (CERRO COCHRANE) MASSIF, 47° S

Daniel Falaschi1, Takeo Tadono2 and Mariano Masiokas1

1Instituto Argentino de Nivolog´ıa, Glaciolog´ıa y Ciencias Ambientales (IANIGLA), CCT-CONICET, Mendoza, Argentina 2Earth Observation Research Center (EORC), Japan Aerospace Exploration Agency (JAXA), Ibaraki, Japan

Falaschi, D., Tadono, T., and Masiokas, M., 2015. Rock (high mountain) environments around the world. glaciers in the Patagonian Andes: An inventory for the Monte They represent the geomorphologic, visible ex- San Lorenzo (Cerro Cochrane) Massif, 47° S. Geografiska pression of ice-supersaturated mountain permafrost Annaler: Series A, Physical Geography, xx, 1–9. DOI: creep in unconsolidated material, and are composed 10.1111/geoa.12113 of frozen but unconsolidated debris, which in ABSTRACT. Although rock glaciers in the Central and turn can be talus or till-derived (Barsch 1996). Desert Andes of Argentina and Chile have been previously Downslope movement of active rock glaciers is a studied in detail, much less attention has been paid to consequence of internal ice deformation, and thus the occurrence of these permafrost forms in Patagonia. rock glaciers are examples of cohesive flow features Recently, however, the establishment of the Argentinean (Barsch 1992). Glacier Inventory program, which intends to inventory and monitor all ice masses along the Argentinean Andes, has Rock glacier inventories are helpful in providing started a large amount of new geocryological research. basic information on the distribution of mountain The project is designed to provide reliable and worldwide (alpine) permafrost, and can assist in validating and comparable results, supported by well established technical adjusting permafrost distribution models (Boeckli procedures and background information. et al. 2012). Contrary to the rock glaciers in the Presented here is the first rock glacier inventory of the Central and Desert Andes (Lliboutry 1956), where Monte San Lorenzo (Cerro Cochrane) region in the southern Patagonian Andes. A total of 130 intact (9.86 km2) and 47 they have been extensively studied and in some fossil (1.45 km2) landforms were inventoried using two 2.5 m cases inventoried (e.g. Perucca and Esper Angillieri resolution ALOS Panchromatic Remote-sensing Instruments 2008; Azocar´ and Brenning 2010; Trombotto et al. for Stereo Mapping images. 2012; Falaschi et al. 2014), these landforms have Since the Argentinean federal initiative described above received much less attention in the Patagonian legally protects all rock glaciers in the country as water Andes. The relatively smaller size and lower reserves, and due to the little scientific knowledge concerning rock glaciers in the vast majority of the Patagonian Andes, abundance of Patagonian rock glaciers compared this inventory provides an important basis for political with the rock glaciers in the Central Andes, decision-making and opens further geocryological research and hence, their minor importance as freshwater avenues for the Patagonian region in general. suppliers, has resulted in a considerable lower amount of scientific efforts aiming to understand Key words: rock glacier inventory, Monte San Lorenzo, the interaction between climate and the cryosphere. Patagonian Andes, ALOS

Study site and previous studies Introduction Monte San Lorenzo (or Cerro Cochrane in Chile) Mountain permafrost in South America is com- (47° 35 S, 72° 18 W, 3706 m a.s.l.; Fig. 1) monly found at high elevations in the Andes, lies in the heart of the Patagonian Andes, at the and appears mostly as groups of rock glaciers. international border between Chile and Argentina, Active rock glaciers are striking tongue or forming a N–S stretching mountain range with lobate-shaped features present in many alpine an elevation above 3000 m a.s.l. Together with

Fig. 1. Study site location and geocryological map. the surrounding Cerro Penitentes, Cerro Hermoso assessed around 18% glacier retreat over 1985– and Cerro W, they represent the largest glaciated 2008. area beyond the Northern and Southern Patagonian It was probably Garleff and Stingl (Garleff 1977; Icefields at this latitude. Recently, Falaschi et al. Garleff and Stingl 1983, 1988) who first identified (2013) inventoried 213 glaciers (206 km2)and a series of present day periglacial landforms and

2 © 2015 Swedish Society for Anthropology and Geography ROCK GLACIERS IN THE PATAGONIAN ANDES processes in several W–E transects, from the would indicate that the 0°C isotherm altitude of Andes to the Patagonian steppe, and recognized present-day mean air annual temperature (MAAT) temperature as the key factor influencing the is located at 1725 m. a.s.l. Andean permafrost and periglacial phenomena. More recently, Trombotto (2000, 2002; Trombotto Liaudat 2008) did a comprehensive review of Materials and methods cryogenic processes, periglacial landforms and the We used two 2.5 m spatial resolution ALOS permafrost state in South America, inventorying Panchromatic Remote-sensing Instruments for relict landforms (e.g. sorted polygons, rock Stereo Mapping (PRISM) images from April 2008 glaciers, pingos, palsas) of the outer-Andes to identify and map intact and fossil rock glaciers Patagonia at a regional scale. He established the (Fig. 2). Both scenes lack any seasonal snow mountain permafrost altitudinal lower limit in a coverage, which is ideal for accurate landform latitudinal transect between 39° S and 55° S. identification. When rock glaciers were located in For the Monte San Lorenzo area, he mentioned cast shadows and therefore not visible in the ALOS the existence of rock glaciers and estimated the PRISM images, mapping was performed in freely lowermost limit of permafrost at 1600–1700 m a.s.l. available high-definition images on Google Earth. The geology of the Monte San Lorenzo area Intact rock glaciers include both active and comprises a Paleozoic low-grade metamorphic inactive landforms and contain ice-rich permafrost, basement of Devonian to Lower Carboniferous whereas in the latter case, permafrost thaw has age (R´ıo Lacteo´ Fm; Bell and Suarez´ 2000; led to rock glacier and permafrost degradation and Giacosa and Franchi 2001), and a thick volcanic therefore, fossil rock glaciers lack any interstitial and sedimentary sequence of Mezozoic age ice. Rock glacier identification and classification (El Quemado volcanic complex; Riccardi 1971). was based on a series of geomorphological The Patagonian Batholith (Upper Kretaceous) evidence. The following features were considered intruded the whole sequence in successive magma as characteristic of fossil rock glaciers: a subdued injections: the Monte San Lorenzo and Cerro surface where the typical rock glacier furrows Penitentes are composed chiefly of granite rocks and ridges topography is mostly eroded; a less (Suarez´ and Welkner 1999; Welkner 2000). pronounced front talus in comparison with intact Because of the interaction between the dominant rock glaciers; and accumulation of large boulders westerly winds and the N–S orientation of the at the rock glaciers’ front talus feet. Additionally, Patagonian Andes, a strong precipitation W– the frontal and lateral slopes of landforms may E gradient has developed in the San Lorenzo be colonized with variable amounts of vegetation area. Villalba et al. (2003) estimated that annual (Haeberli 1985; Ikeda and Matsuoka 2002; precipitation decreases from 10 m in the Southern Brenning 2005; Kaäb¨ 2007). Other morphological Patagonian Icefield to less than 300 mm within indicators, such as the presence of thermokarst and 100 km east of the main water divide at this ice outcrops, were not featured in the inventoried latitude. Hence, the area has a transitional climate landforms. Although the distinction between relict signature, ranging from maritime in the western and intact rock glaciers (i.e. the presence of flank of the main Chilean–Argentinean divide permafrost) must be determined by means of in towards continental in the eastern side. Wolff situ measurements (such as boreholes or thermal et al. (2013), citing Hijmans et al. (2005), locate datalogging), geomorphological mapping of rock the Argentinean flank of Monte San Lorenzo in glaciers can still provide a reliable estimation of a precipitation latitudinal strip between 400 and permafrost distribution at a regional scale (Roer 600 mm. Aravena (2007) reports a mean annual and Nyenhuis 2007). precipitation of 731 mm and 8.1°C mean annual Rock glacier outlines were manually digitized on temperature for the Cochrane weather station screen and the areas were calculated using the open- (47° 14 S, 72° 34 W, 180 m a.s.l.), 46 km to the source KOSMO GIS software (www.opengis.es, northwest of the study area. Temperature surveys 11 Aug., 2015) to produce the present polygon- between 2002 and 2013 by means of a HOBO based inventory. Landforms were mapped from the logger located in the western lateral moraine of San root zone to the bottom of the foot of the front Lorenzo Sur glacier (47° 42 S, 72° 19 W; Fig. 1) slope, including lateral and frontal slopes as part yielded a mean air annual temperature of 3.8°Cat of a landform’s outline. Additionally, rock glaciers 1140 m a.s.l. elevation. Based on these later data, a were classified into tongue, lobate and coalescent- theoretical temperature lapse rate of 0.65°C/100 m shaped landforms. An inventory map of the region

Fig. 2. . Tongue-shaped (t) and coalescent (c) rock glacier outlines mapping examples on ALOS PRISM imagery. was produced, including all of the identified rock station (GPS) base station in Cochrane, Chile. The glaciers as well as the glacier covered area as coordinates of the GCP were compared with those retrieved by Falaschi et al. (2013); the projection in the DSM and showed minor differences; we used was UTM zone 18 with WGS84 datum. found a negligible horizontal shift and a mean To investigate the influence of lithology on vertical bias of 6.7 m, with a standard deviation rock glacier size, the rock type of each rock of 6.6 m. These values were considered acceptable glacier was retrieved by combining fieldwork with given the purposes of this study. the interpretation of very high resolution images available from Google Earth and the 1:250.000 scale geological maps from the Argentinean Results National Geographic Institute (Giacosa and Franchi We inventoried a total of 177 rock glaciers, 2001). accounting for 11.31 km2. A total of 130 landforms An ALOS PRISM stereopair-generated, 10 m (9.86 km2, 87%) correspond to intact ones (Fig. 3), resolution digital surface model (DSM)was whereas the remaining 47 (1.45 km2, 13%) are generated, with the DSM and Ortho-image relict. A summary of the rock glacier inventory is Generation Software specially designed for PRISM presented in Table 1. data by the Japan Aerospace Exploration Agency. The largest intact inventoried landform has an Topographic parameters for each landform (maxi- area of 1.21 km2, which is comparable to the mum, minimum and mean elevation, mean slope rock glaciers found in the Tien Shan mountains and aspect) were derived from this model. The (Bolch and Gorbunov 2014), which are said to horizontal accuracy of ALOS PRISM DSMs was host the largest rock glaciers on the Earth’s surface reported as less than 6 m and the vertical error (Dario Trombotto 2014, pers. com.). It must be was 2–18 m without the use of ground control noted, however, that this landform is in fact a two- points (GCPs) (Takaku and Tadono 2009). During lobe coalescent unit. The largest non-compound successive field campaigns 16 Global Navigation rock glacier covers an area of 0.6 km2.Table1 Satellite System (GNSS) points were surveyed also shows that intact rock glaciers have a larger using a Trimble 5700 receive (Trimble Navigation, mean size (0.092 km2) than fossil rock glaciers Sunnyvale, California, USA) and used as GCPs (0.036 km2). to validate the DSM. Coordinates and elevation At least 55% of the intact rock glaciers (and 65% of these points were differentially corrected using in the case of fossil ones) were classified as debris data from a private permanent global positioning rock glaciers (Barsch 1996). Regarding rock glacier

Fig. 3. Tongue-shaped rock glaciers in the Rio Oro (ﬁeld photograph a-1 and Google Earth imagery a-2); multi-unit rock glacier in the R´ıo Lacteo´ Valley (ﬁeld photograph b-1 and ALOS PRISM image b-2).

Table 1. Summary of the Monte San Lorenzo region rock glacier A striking feature is that the prevailing inventory orientation of rock glaciers in the area is towards west to southwest (see Fig. 4a), and thus at odds Rock glaciers Total with the expected east to southeast aspect that Intact Fossil is commonly found in other sites in the South American Andes (e.g. Brenning and Trombotto Number 130 47 177 Area (km2) 9.86 1.45 11.31 2006; Perucca and Esper Angillieri 2008; Esper Mean size (km2) 0.092 0.036 – Angillieri 2009; Martini et al. 2013; Rangecroft Maximum size (km2) 1.21 0.3 – et al. 2014). Maximum elevation (m) 2155 2030 – The front talus of intact rock glaciers in the area Mean elevation (m) 1742 1590 – Minimum elevation (m) 1335 1267 – are located at a minimum altitude of 1400 m a.s.l. Rock glacier spatial density was calculated as the ratio between the total rock glacier area and the area above 1400 m a.s.l. in 1.4% (Fig. 4b). This elevation of 1400 m a.s.l. can be regarded as the lowermost morphology, most of them are tongue shaped (63%) limit of the discontinuous mountain permafrost at compared with lobate (17.5%) and coalescent a regional scale (Barsch 1978; Brenning 2005). (19.5%). Polymorphic (Frauenfelder and Kaäb¨ With respect to rock glacier lithologic com- 2000) rock glaciers, where several generations of position, it can be said that, in general terms, rock glaciers form composite landforms (with the the rock types involved in rock glacier formation newer rock glacier lobes flowing above older ones) belong to three distinct groups: plutonic (mainly were not detected. granitic), volcanic (mainly volcanic agglomerates Two-thirds of the identified rock glaciers were and breccias) and metasedimentary (mainly grey- classified as debris rock glacier type, including waque) (see Table 2). Since the most extensively the largest landforms. They are located in glacier cirques and niches, flowing parallel to the talwegs. This probably reflects the importance of glacier cirques as sites of cryosediment and snow avalanche Table 2. Rock glacier morphology accumulation, providing both the debris and ice source for rock glacier formation. Both intact Rock glaciers Total and fossil rock glaciers were identified, forming Form Tongue Lobate Coalescent from cryogenized lateral moraines, such as in the Number 111 31 35 177 San Lorenzo Sur and Penitentes glaciers lateral 2 moraines. Area (km ) 7.04 0.5 3.77 11.31

Fig. 4. (a) Radial plot showing intact rock glacier number (grey) and area (black) with respect to eight orientation classes. (b) Rock glacier spatial density as a function of elevation.

Table 3. Distribution of rock glacier lithologic composition. Hence, the lower limit of mountain permafrost San Lorenzo Fm: granites, diorites and tonalites. R´ıo Lacteo´ occurs at lower elevations in dryer climate Fm: metagreywaques, slates and filites. El Quemado Group: ignimbrites and breccias regions. Guodong and Dramis (1992) suggested that in Rock glaciers high latitudes, the limit of mountain permafrost Lithology San Lorenzo R´ıo Lacteo´ El Quemado descends with continentality. Similar findings were granites metamorphites vulcanites made by Barsch (1978) and King (1986), who stated that the permafrost boundary is located at lower Number 36 58 36 elevations in dry, continental areas. If a similar Area (km2) 4.623 2.735 2.514 pattern is assumed for the Monte San Lorenzo area, a lower elevation of the rock glaciers’ front talus should be expected in the Monte Zeballos (47° 02 S, 71° 41 W), 50 km east of Monte San Lorenzo and therefore in a drier and more continental distributed outcrops in the area correspond to the environment. Fittingly, preliminary analyses led to metamorphic rocks of the R´ıo Lacteo´ Fm, most an estimation of 1750 m as the lower limit of rock of the landforms (44%) are composed of this glaciers in the Monte Zeballos area. Here, however, rock type. However, their mean size is smaller in a strong topographic control probably exists, since comparison to landforms made of other lithologies, rock glaciers are developed directly at the foot of granite rock glaciers being the largest on average. the 500 m high vertical rock walls that delimitate The median size of the granite (San Lorenzo the eastern extent of the Lago Buenos Aires Fm) intact rock glaciers is 0.036 km2, whereas plateau. metamorphic rock glaciers (R´ıo Lacteo´ Fm) and Debris rock glaciers flow downslope from the vulcanites (El Quemado Fm) have median sizes of inside of glacier cirques, originating from basal or 0.028 km2 and 0.039 km2, respectively. lateral moraines affected by cryogenic processes. Their large size and abundance probably reflect Discussion the importance of the previous glacier processes The estimated rock glacier spatial density of 1.4% and the impact on landscape evolution for rock for the study area is lower than in other, drier glacier formation and growth. Glacier cirques serve Andean sites such as the Andes of Santiago and as catchment areas of snow avalanches and frost- Mendoza (4%; Azocar´ and Brenning 2010), or the weathered material, and also provide sun-sheltered Valles Calchaqu´ıes region (2.2%; Falaschi et al. slopes, which in addition to the above, make 2014). This comparison may lead to the conclusion them perfectly suited for rock glacier formation. that rock glacier spatial density increases with However, glacier cirques may have the proper slope decreasing precipitation, an assertion recently made necessary to exceed the shear stresses required for by Boeckli et al. (2012) for several areas in the Alps. permafrost creep (Barsch 1996).

Permafrost distribution in mountain terrain, at not dissimilar to those found at glaciers’ ELA. a local scale, is largely affected by topography, As a result, it can be argued that the location of by controlling the temperature, air circulation and glaciers and rock glaciers is mainly a consequence incoming solar radiation. Haeberli (1983) and of particular topoclimatic factors, rather than the Haeberli and Burn (2002) found that in the Alps, product of regional climatic differences. Hence, MAATs of –1°Cto–2°C are necessary for the the required conditions in the Andes mountains formation and development of active rock glaciers for rock glacier development are not strictly dry or and discontinuous permafrost. In the Andes, continental, as has often been seen for the Central however, other studies have suggested the presence and Arid Andes (Barsh 1992, 1996), but may be dry of active rock glaciers under positive present-day to moderately humid with cool summers for other MAAT regimes (Azocar´ and Brenning 2010). regions further south. In the San Lorenzo Sur glacier, air temperature was surveyed by means of a HOBO datalogger Conclusions (“San Lorenzo Sur”, 47° 42 S, 72° 19 W, 1140 m The inventory presented here represents the first a.s.l.; see Fig. 1) between 2002 and 2013. From comprehensive and detailed rock glacier inventory these data, and using a theoretical lapse rate for the Monte San Lorenzo area in southern of 0.65°C/100 m, the present-day –1°CMAAT Patagonia. The use of ALOS PRISM images high- isotherm was calculated at 1870 m a.s.l. If this lights the importance of relying on high-resolution elevation is to be trusted, and keeping in mind optical imagery for the clear identification of that the intact rock glaciers occur between 1400 rock glaciers and their geomorphological features. and 2600 m a.s.l. in the area, this would mean Within the intact rock glacier group, more detailed that at least some of these landforms might be methods such as photogrammetric or differential above present-day 0°C MAAT conditions. Hence, GPS surveys are required for determining rock it can be argued that the occurrence of these glacier flow and the differentiation between active intact rock glaciers is in fact indicative of past or inactive landforms. The major drawback of these thermal conditions and rock glaciers are resilient to techniques lies in the limited number of landforms present-day climate conditions. The slow response that can be investigated. rate of the permafrost’s temperature field can However, this inventory provides the first well result in permafrost subsisting decadal to century documented evidence of rock glacier occurrence scale periods under positive temperature conditions in the Patagonian Andes published so far, and (Gruber and Haeberli 2007; Kaäb¨ et al. 2007). identifies research avenues for future regional Alternatively, rock glaciers might reach a thermal geocryological studies. Because of the small equilibrium state owing to local topoclimatic amount of geocryological research that has factors (Haeberli 1985), such as topographic previously been carried out in Patagonia comprising shading and circulation of cold air originating from rock glaciers, there is still a vast, unexplored large glaciers (Brenning 2005). research field in this regard. Basic questions such as Increasing air temperature can lead to rock rock glacier flow and velocity, permafrost content, glacier flow acceleration and permafrost degrada- rock glacier–climate interactions, hydrology and its tion, eventually triggering rock glacier destabiliza- significance as a water source, just to name a few, tion and collapse (Kaäb¨ et al. 2007; Roer et al. 2008; need to be answered. Iribarren Anacona and Bodin 2010). However, To improve our knowledge of the current state of no visible signs of rock glacier acceleration such these frozen water resources in these remote areas, as crevasses or rupture were identified in the further effort needs to be aimed at the establishment inventoried landforms. of high-altitude, in situ hydro-climatic data records. Another interesting issue is the relationship Presently, however, with no available in situ between the local glacier equilibrium line altitude ground temperature surveys, the present mountain (ELA) and the discontinuous mountain permafrost permafrost distribution cannot be anything but limit. In the Monte San Lorenzo region, the ELA approximated at a regional scale by means of rock was estimated at 1800–1900 m a.s.l. (Wenzens glacier inventories. 2002; Condom et al. 2007; Falaschi et al. 2013). In accordance with the above-mentioned points with regard to the discontinuous mountain permafrost Acknowledgements limit, it may be concluded that climate conditions This study was funded by the Agencia Nacional de found at rock glacier sites are, to a large extent, Promocion´ Cient´ıfica y Tecnica´ (grant PICT 2007-

0379). The authors thank Dr. Ricardo Villalba, equilibrium-line altitudes on Andean glaciers (10°N– Dr. Lucas Ruiz, Alberto Ripalta and Ernesto 55°S). Global and Planetary Change, 59, 189–202. Esper Angillieri, Y., 2009. A preliminary inventory of rock Corvalan´ for their field support. Special thanks ° also to Dr. Dario Trombotto, the Inventario glaciers at 30 S latitude, Cordillera Frontal of San Juan, Argentina. Quaternary International, 195, 151– Nacional de Glaciares initiative from IANIGLA, 157. doi:10.1016/j.quaint.2008.06.001 Leo Montenegro and crew from the Perito Moreno Falaschi, D., Bravo, C., Masiokas, M., Rivera, A. and National Park, and Mario Sar from Estancia Los Villalba, R., 2013. First glacier inventory and recent Nires.˜ The authors are very grateful to the reviewers changes in glacier area in the Monte San Lorenzo Region for providing valuable suggestions that helped to (47°S), Southern Patagonian Andes, South America. significantly improve this manuscript. 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