Lichenometry in the Cordillera Blanca, Peru: “Little Ice Age” moraine chronology

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Global and Planetary Change 59 (2007) 225 – 235 www.elsevier.com/locate/gloplacha

Lichenometry in the Cordillera Blanca, Peru: “Little Ice Age” moraine chronology Olga Solomina a,⁎, Vincent Jomelli b , Georg Kaser c , Alcides Ames d , Bernhard Berger c , Bernard Pouyaud e a b

Institute of Geography, Russian Academy of Science, Staromonetny-29, Moscow, 119017, Russia Centre National de la Recherche Scientifique, Laboratoire de Géographie Physique, Paris, France c Institut für Geographie, Universität Innsbruck, Austria d Huaraz, Peru e Institut de Recherche pour le Développement, Lima, Peru Available online 9 January 2007

Abstract This paper is a comparison and compilation of lichenometric and geomorphic studies performed by two independent teams in the Cordillera Blanca, Peru, in 1996 and 2002 on 66 “Little Ice Age” moraines of 14 glaciers. Using eleven new control points, we recalibrated the initial rapid growth phase of the previously established Rhizocarpon subgenus Rhizocarpon growth curve. This curve was then used to estimate the age of “Little Ice Age” moraines. The time of deposition of the most prominent and numerous terminal and lateral moraines on the Pacific-facing side of the Cordillera Blanca (between AD 1590 and AD 1720) corresponds to the coldest and wettest phase in the tropical Andes as revealed by ice-core data. Less prominent advances occurred between AD 1780 and 1880. © 2006 Elsevier B.V. All rights reserved. Keywords: “Little Ice Age” moraines; Cordillera Blanca; Lichenometry

1. Introduction The Cordillera Blanca of Peru is by far the most glaciated tropical mountain range, carrying about 25% of all tropical glaciers (Kaser and Osmaston, 2002). However, the chronology of the relatively recent “Little Ice Age” (“LIA”) glacier variations of the region is still poorly studied. Historical data on glacier fluctuations (Kinzl, 1942; Broggi, 1943; Clapperton, 1983; Seltzer, 1990; Kaser et al., 1990; Clapperton, 1993; Ames and Francou, 1995; Ames, 1998; Kaser, 1999; Kaser and Osmaston, 2002) are rather limited. Sievers (1914) observed general ⁎ Corresponding author. Tel.: +7 95 9390121; fax: +7 95 9590033. E-mail address: [email protected] (O. Solomina). 0921-8181/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.gloplacha.2006.11.016

glacier retreat in the Cordillera Blanca and in other areas of Peru and Ecuador during his journey in 1909. The timing of the beginning of this retreat is difficult to determine. Broggi (1943) quotes information from Raimondi indicating that the retreat of glaciers in the Cordillera Blanca started around 1862. However, this single observation cannot be considered as definitive proof of general glacier recession from advanced “LIA” positions. Scientific investigations of the glaciers in this area began in the 1930s when the first cartographic expedition organized by the Austrian-German Alpine Club explored the Cordillera Blanca. In 1932 Kinzl (1942) described the young moraines and attributed the outer ones to the “Little Ice Age” with the most recent moraine being built shortly before his visit.

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Geomorphological and geological data on “LIA” glacier variations in the Cordillera Blanca and adjacent areas are also rather scarce and generalized. In Southern Peru two stages of “LIA” moraines formation have been dated by Mercer and Palacios (1977) to 600–300 14C years BP (uncalibrated dates) and to 100–200 years ago. Rodbell (1992), who was the first to use lichenometry to date moraines in this region, reports four groups of moraines, deposited between 7000–6000, 3350–1800, 1250–400 years ago, and during the 20th century. However, all the radiocarbon control points except one used for growth curve construction were older than 1000 years. Therefore, the dates of moraines of the last millennium are rather uncertain. In essence, details of the “LIA” moraine chronology in the Cordillera Blanca are still unknown although these terrestrial “footprints” of glacier activity are very important as evidence of past climatic changes in the tropics. These chronologies are important for comparison and combination with high-resolution paleorecords such as ice cores from Huascaran and Quelccaya (Thompson et al., 1986, 1995, 2000; Seimon, 2003), and lacustrine sediments from Titicaca and other high-elevation lakes (Abbot et al., 1997; Binford et al., 1997). Lichenometric dating of “LIA” moraines could be of particular interest for archaeologists studying high-mountain civilizations, and for dating high-elevation archaeological sites, which are numerous and still poorly dated in Peru and Bolivia. Taking into consideration the importance of these studies, it is not surprising that two teams independently attempted to use lichenometry to date “Little Ice Age” moraines in the Cordillera Blanca in 1996 and 2002. In this paper we compile and compare these results. The aim of the paper is to reconstruct the chronology of “LIA” glacier advances focusing on their “LIA” maxima on the basis of an improved Rhizocarpon subgenus Rhizocarpon growth curve.

windward and lee-ward effects (Kaser and Georges, 1997). In the Rio Santa valley west of the Cordillera Blanca, precipitation ranges from an annual mean value of 160 mm at Hidroelectra station (Fig. 1; 1386 m asl) and 180 mm in Caraz (2286 m asl) up to a maximum of 750 mm in Ticapampa (3480 m asl) (1950s–1980s) (Niedertscheider, 1990). In contrast, the eastern slopes of the Cordillera receive more than 3000 mm per year of which 70–80% occurs between October and April (Kaser et al., 2003). Annual temperature changes are minor. For example, at Querocha (3955 m asl) mean annual temperature variation is less than 2 °C (1954–1987) (Kaser and Osmaston, 2002). Most glaciers exist on fairly steep slopes above 4500 m asl. The few valley-type glaciers are heavily debris covered in their lower parts and surrounded by fresh-looking, prominent end and lateral moraines. In many cases, lakes contained by these

2. Study area As part of the South American Andes, the Cordillera Blanca stretches over about 180 km from 8°30′ S to 10°8′ S. Twenty-seven peaks reach elevations of over 6000 m asl, and more than 200 peaks exceed 5000 m asl. The Cordillera Blanca forms the Continental Divide draining into Rio Maranon and the Atlantic to the east, and into Rio Santa and the Pacific to the west. The shape and setting of the tropical Cordillera make them a pronounced barrier to the dominant and persistent easterly atmospheric flow separating the wet Amazon side from the dry Pacific side and producing pronounced

Fig. 1. Study area. Circles — sites investigated in 1996 and 2002, stars — sites investigated by Rodbell (1992), squares — meteorological stations.

O. Solomina et al. / Global and Planetary Change 59 (2007) 225–235

moraines have frequently been subject to sudden, and in some cases catastrophic, flood outbreaks (Ames, 1998). In general glaciers of the Cordillera Blanca have suffered significant retreat since the “LIA”. Major advances occurred in the 1920s, but drastic recession in the 1930s and 1940s counteracted those gains. During the following two decades glacier extent remained approximately constant (Kaser et al., 1990) followed by slight advance in the 1970s (Ames, 1998). Since the 1980s significant and accelerating retreat has been documented for all glaciers of Cordillera Blanca. Although recently some glaciers have begun to advance (Kaser and Osmaston, 2002). The records from fourteen glaciers are presented here (Table 1). Most of them are of cirque type, rather small (1–1.5 km2), and located between ca 4500 and 5100/ 6000 m asl. Nine glaciers are situated on the western slope (Pacific-facing slope) of the Cordillera Blanca, and five on the eastern slope, facing the Atlantic. 3. Methods The team led by Solomina in 1996 and the team led by Jomelli in 2002 measured lichen (Rhizocarpon subgenus Rhizocarpon) on moraines and other natural and anthropogenic surfaces in the Cordillera Blanca using field techniques similar to those recommended by Innes (1985) and McCarroll (1993, 1994). Each moraine was considered as one site without separation into segments or plots, and was thoroughly searched for the largest lichen.

227

Measurements were taken on top of end moraines and on flat portions of lateral moraines. The axis of the largest lichen on each large block (over 50 cm2) was measured with a flexible, transparent plastic ruler. On each site as many measurements (up to 250) as possible were made. The measurement error is 0.5 mm. The largest lichen was considered “anomalous” if its size exceeded the second largest one by 20% or more. On each site the mean of five axes was used as the age predictor. Measurements were performed on 40 lateral and 26 end moraines. The main difference in measuring techniques used in 1996 and 2002 was that the minimum axis was recorded in 2002 and the maximum one in 1996. In spite of the differences in choice of axes at Yanamarey Glacier, values vary by less than 10%. A number of factors affecting the time of colonization and lichen growth rates may explain the differences in lichen size on synchronous surfaces. The most important in the Cordillera Blanca is the contrast between the dry western and moist eastern slopes. For this reason the sites from the western and eastern slopes are treated separately. We assume that our moraine data set can be considered as homogenous with respect to temperature, light intensity, substrate stability, altitude, influence of vegetation and lichen species measured. The effect of substrate lithology in the Cordillera Blanca was discussed by Rodbell (1992), who noticed that lichens are larger on young granodiorite moraines than on fine-grained metasedimentary substrates. On the moraines of all of our sites there were enough granitic rocks to use for lichenometric purposes.

Table 1 Properties of investigated glaciers (Ames et al., 1989) Name of glacier

Number of glacier

Latitude S

Longitude W

Area (km2)

Orientation accumulation/ ablation zones

Front elevation (m)

Max elevation (m)

Pacific-facing slope Broggi 1POO5CIF03 Cancahua 1P005CKG01 Yanamarey 1P005CVAC2 Uruachraju 1P005CTCA2 Artesonraju 1P005CGH01 Llaca 1P005CPD01 Pisco 1P005C1E02 Checouiacraju 1P005CKKO3 Huandoy 1P005CGB01 (Paron)

8°59.95′ 9°05.16′ 9°39.26′ 9°35.20′ 8°57.48′ 9°24.90′ 9°00.91′ 9°10.00′ 8°58.28′

77°35.06′ 77°32.93′ 77°16.20′ 77°18.91′ 77°37.58′ 77°25.90′ 77°37.91′ 77°32.07′ 77°41.93′

1 3.51 1.35 2.15 5.97 4.63 2.21 2.29 2.71

NW/NW E/SE SW/SW SW/SW W/W SW/SW S/S SW/SW W/SW

4575 4525 4590 4600 4850 4490 4925 4700 4600

5100 6354 5200 5700 6000 6168 5300 5600 5850

Atlantic-facing slope Allicocha 1D371GGBA3 Atlante 1D371GGAA4 Cancaraca-1 1D371GHC03 Cancaraca-2 1D371GHC04 Pucajirca Norte 1P005CGJE2 (Safuna)

9°14.70′ 9°15.97′ 9°08.30′ 9°07.70′ 8°51.28′

77°28.28′ 77°25.70′ 77°30.90′ 77°30.20′ 77°36.34′

1.40 0.97 1.48 0.46 4.69

E/E N/NE E/E S/S NW/NW

4875 4625 4700 4650 4360

5900 5751 5760 5250 6050

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We consider the lichenometric dates of moraines as the minimum age of respective glacier advances.

potentially useful for this purpose. These sites range from 2500 to 4700 m in altitude (Fig. 1) and from 900 BC to AD 1996 in age (Table 2).

4. Results 4.1. Control points The only growth curve of Rhizocarpon subgenus Rhizocarpon in the Cordillera Blanca is based on eight prehistoric control points and one young flood deposit (AD 1941) (Rodbell, 1992). The absence of control points between AD 1941 and 1575 ± 170 14C years BP makes dating in this interval very uncertain. In order to assure better chronological control for the growth curve we studied several natural and anthropogenic sites

Table 2 List of anthropogenic and natural sites studied in the Cordillera Blanca in 1996 and 2002 Sites

Altitude asl Age (m)

Reference

Archeological monuments in Chavin Tombstones of the “Parque litico”, Archeological Museum, Huaraz, Recuay and Wari (Huari) cultures Tombstones of Recuay culture, near Huaraz Ruins Willka Wain, near Huaraz Ruins Huacramarca, left side of Putaca valley Ruins near Vetahirca, right side of Putaca valley Mudflow deposits, near Huaraz

3140

Stingl, 1980; Fiedel, 1987

3050

3100 3400 4500

900 BC to 250 BC; AD 1919 a, AD 1945 b 300 BC to AD 700, AD 500 to AD 1000

300 BC to AD 700 AD 1000-AD 1200 Pre-Colombian time?

Fiedel, 1987; Auroi, 1988

Auroi, 1988 Lumbreras, 1974

4400

Pre-Colombian time?

3100

AD 1941

Mudflow deposits, Ranrahirca

2500

AD 1970

Mudflow deposits, Masac

2500

AD 1725

Rockfall deposits in Cancaraca valley Cemetery in Huaraz Cemetery in Pompey

4200

AD 1970s

3050 3415

AD 1917–1996 AD 1934, AD 1955 AD 1923–1924 Kinzl, 1942

Mina Atlante Forefields of 14 glaciers (Table 1) a b

Beginning of excavations in 1919. New phase of excavation after the mudflow.

Ames and Francou, 1995 Ames and Francou, 1995 Ames and Francou, 1995 Ames, pers. comm.

4.1.1. Anthropogenic sites In 1996 we visited the archaeological site at Chavin; the Willka Wain monuments near Huaraz; tombstones of the Recuay and Huari (Wari) cultures, exposed in the open air in the archaeological museum in Huaraz, and modern gravestones in the cemeteries of Huaraz and Pompey. Rhizocarpon thalli found on these surfaces are sparse and too small to be considered. The Rhizocarpon thalli in the ruins at Huacramarca and Vetahirca in Putaca valley, also visited in 1996, are numerous and large (with maxima in 52 mm and 47 mm respectively), however the age of these ruins is unknown. Two recent anthropogenic sites were taken into account to constrain the growth curve — the entrance to Mina Atlante, covered by a glacier advance in 1923– 1924 and rock debris from road construction in the 1970s at the Cancaraca-2 Glacier. 4.1.2. Natural surfaces Both teams studied 23 natural sites (Tables 2 and 3): moraines, outwash plains, roche moutonnées, mudflows, rockfall deposits, and paleo-shorelines. The ages of these surfaces, dated by old documents, reports, terrestrial and aerial photographs, and topographic maps, range from the early 18th century to the 1970s. However, we were unable to identify Rhizocarpon subgenus Rhizocarpon thalli on the surface of the oldest mudflow deposits in AD 1725 at Masac, or Ranrahirca (AD 1970). Thus, all sites, which provided control points to constrain the curve, were deposited or exposed between the 1930s and the 1980s. The oldest historically dated glacier advance in the Cordillera Blanca occurred in 1923–1924 at the Atlante glacier (Kinzl, 1942). The miners of Mina Atlante reported that Atlante Glacier advanced in 1923/ 24. However, by 1939 it had retreated by more than 100 meters (Kinzl, 1942). At the time of the advance of Atlante Glacier, miners in other Peruvian mines also had problems with advancing glaciers (Oppenheim, 1945). In several cases the glaciers attained almost the same position reached during the “LIA” maximum in the early 1920s, marked by fresh-looking huge terminal and lateral moraines (Kaser and Osmaston, 2002). Unfortunately, only one moraine at Atlante Glacier can be traced to this advance. One can speculate, however, that the other sharp crested moraines surrounding the surfaces deglaciated in 1930s, when the first photos and maps of

O. Solomina et al. / Global and Planetary Change 59 (2007) 225–235

229

Table 3 Axiss of the largest Rhizocarpon subgenus Rhizocarpon lichen on surfaces of known age in the Cordillera Blanca Location

Year of measurements

Age, years AD

Number of measured lichens

Largest diameter (mm)

Mean of five largest thalli (mm)

Reference

Pacific-facing slope Broggi Proglacial till surface Broggi Proglacial till surface Broggi Rockfall deposits Yanamarey End moraine

1996 1996 1996 1996

1948–1962 1932–1948 1970s 1975/1976

11 50 25 None

9 12 8

6 10 7

Yanamarey

Proglacial till surface

1996

1939–1948

10

9

8

Yanamarey

Moraine

2002

1939

5

15

12

Moraine Proglacial till surface Moraine Moraine Moraine Moraine Moraine Moraine Debris flow Lake Debris flow

2002 2002 2002 2002 2002 2002 2002 2002 2002 2002 2002

1948 1970 1932 1968 1985 1932 1948 1964 1961 1972 1970

18 None None 11 7 10 None None 6 None 3

8

7

7 5 14

7 4 12.2

7

6.2

6

6

Ames, 1998 Ames, 1998 Ames, pers. comm. Hastenrath and Ames, 1995 Hastenrath and Ames, 1995 Hastenrath and Ames, 1995 Aerial photographs Aerial photographs Aerial photographs Aerial photographs Aerial photographs Aerial photographs Aerial photographs Aerial photographs Aerial photographs Aerial photographs Aerial photographs

1996 1996 1996

1970s 1932–1948 N1932 (1923–1924?)

10 100 120

4 11 17

3.4 10.2 16.4

Ames, pers. comm. Ames, 1998 Ames, 1998

1996

1970/71

120

11

10

1996 1996

1948–1962 1923–1924

175 10

16 15

16 9.8

1996

1923–1924

15

15

12.6

1996

1970s

10

6

6.5

Lliboutry et al., 1977 Aerial photographs Kinzl, 1942; Ames, 1998 Kinzl, 1942; Ames, 1998 Aerial photographs

Uruachraju

Artesonraju

Huascaran

Site

Atlantic-facing slope Cancaraca-2 Artificial rockfall deposits Cancaraca-2 Second terrace of the lake Cancaraca-2 Terminal moraine dam of the lake Allicocha End moraine, partly in the lake Allicocha Moraines of hanging glacier Atlante Rocks and debris near the entrance to the mine Atlante End moraine corresponding to the same advance Atlante Youngest moraine ridge

the glaciers in the Cordillera Blanca were produced, were deposited during this glacier advance. This is likely the case with the Cancaraca-2 terminal moraine damming the proglacial lake. 4.2. Growth curve construction Fig. 2 displays two sets of control points from the western and eastern slopes of the Cordillera Blanca. It is important to notice that the two data sets obtained independently by two different teams in 1996 and 2002 support each other. The youngest control point used by Rodbell (AD 1941, 20 mm) does not fit into the general pattern of our distribution (Fig. 2), though Rodbell (1992) himself suggested that lichen sizes on

ice-cored moraines of comparable age might be smaller than on these debris flow deposits due to slower surface stabilization. The wide scatter of control points, their limited number, and the absence of data older than the 1920s makes it impossible to construct a growth curve for the Atlantic-facing slope of the Cordillera Blanca. The Pacific data set is more consistent and clearly indicates a general trend of lichen growth in the 20th century. For this reason the moraines from the Pacific-facing slope are the main focus of our further discussion. We used eleven new control points (Table 3) and the most reliable radiocarbon date of 1575 ± 170 14C years BP (equal to a maximum axial Rhizocarpon measurement of 55 mm) obtained by Rodbell (1992) from the

230

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Quilloc moraine to calibrate the growth curve of Rhizocarpon subgenus Rhizocarpon for the western slope of the Cordillera Blanca (Fig. 3). The calibration of the radiocarbon date of 1575 ± 170 14C years BP yields a wide range of calendar ages from 1630 to 1310 cal years BP (Ramsey, 1995). In order to avoid subjectivity in choosing a calibrated age value for this crucial control point we constructed two curves (Figs. 3, 1a and b,) based on the two extreme age estimates. The lichenometric ages lie within the corridor limited by the two curves. The curves were constructed using the method proposed by Matthews (1974). The curves are applicable only to surfaces located higher than 3500 m asl since lower elevation sites do not support enough yellow Rhizocarpon to be used for lichenometry. In contrast to Rodbell (1992), we have focused on the younger part of the growth curve and on moraine dating over the last millennium. Control points between ca 80 ago and ca 1500– 1600 years BP are lacking, therefore, our growth curve remains preliminary, and all age estimates presented are tentative. 4.3. Relative and absolute age of moraines Using the growth curve (Fig. 3) we provisionally estimated the numeric age of moraines on the Pacific side of the Cordillera Blanca. These estimates, rounded to the nearest decade are given as intervals of maximum and minimum ages (Table 4i). The absence of age estimates for some deposits is explained by the limitation of the growth curve to measurements smaller than 55 mm. Due to the lack of a growth curve for the Atlantic side of the Cordillera

Fig. 3. Control points and growth curves for the Rhizocarpon subgenus Rhizocarpon in the Cordillera Blanca. The growth curves for maximum and minimum age estimates are constructed using the formula: log( y + c) = a + bx, where y is moraine age (years), x is lichen size (mm), a, b, c — constants. “ c ” is computed using the iterative procedure with one-year increment to maximize the correlation coefficient between lichen size and moraine age (Matthews, 1974). The two curves limit the interval of the oldest (1630 cal years BP) and youngest (1340 cal years BP) absolute age estimates of the radiocarbon date 1575 ± 170 14C years BP, obtained by Rodbell (1992). Dashed lines display the 20% error intervals for both curves.

Blanca we provide no age estimates for these glaciers (Table 4ii). 5. Discussion 5.1. Moraines and glacier advances We consider the age of moraines to be very close to the time of corresponding glacier advances. Tropical glaciers react faster to climatic changes than those in mid-latitudes. Kaser et al. (1990) analysed the 20-yearlong period of variations of four small glaciers in the Cordillera Blanca and found correlations with annual temperature and precipitation lagging by 4 years. Taking into consideration the relatively low precision of our moraine dating this lag is not important for our estimates. All glaciers studied are of the same cirque or cirque-valley types and of similar areas, therefore we suggest that their advances marked by synchronous moraines are related to the same climatic signals. 5.2. Relative dates of moraines

Fig. 2. Control points of the Rhizocarpon subgenus Rhizocarpon. 1 — Pacific-facing slopes of the Cordillera Blanca, 2 — Atlanticfacing slopes of the Cordillera Blanca, 3 — by Rodbell (1992).

Broad agreement of our measurements of lichens on moraines with those of Rodbell (1992) as well as the reasonable agreement of the independent measurement

O. Solomina et al. / Global and Planetary Change 59 (2007) 225–235 Table 4i Lichenometric dating estimates and sample data for moraines in the Cordillera Blanca (Pacific-facing slope) Sites

Number of Five largest diameters Age measured (mm) (years lichens ago) Single Mean Median

Pacific-facing slope Broggi Moraine and 11 sandur, AD 1948–1962 Moraine and 50 sandur, AD 1932–1948 End moraine 75 75

End moraine of hanging glaciers Cancahua End moraine in the middle of the lake End moraine

Age (years AD)

9

6

6

30

1970

12

10

10

60

1940

20

18

17

34

29

27

140– 150 300– 340

1850– 1860 1660– 1700

None

120

31

29

29

End moraine

75

24

18

15

Left lateral moraine End moraine overlayed by 3 Uppermost lateral moraine External lateral moraine

100

26

23

23

50

27

23

22

120

40

37

37

50

53

Yanamarey (measurements of 1996) Roches 25 5 moutonnées End moraine 10 9 Left lateral 50 23 moraine Left lateral 100 34 moraine End moraine 100 60 End moraine 100 68

50

50

4

4

8 17

8 16

32

33

59 64

58 65

Yanamarey (measurements of 2002) Left lateral 57 33 31 moraine Left lateral 184 23 22 moraine Left lateral 197 61 59 moraine

30 20 59

300– 340 140– 150 200– 220 200– 220

1660– 1700 1850– 1860 1780– 1800 1780– 1800

490– 570

1430– 1510

Table 4i ( continued) Sites

Llaca Right lateral moraine Right lateral moraine Left lateral moraine End moraine

29 23

Age (years AD)

320– 360 200– 220

1640– 1680 1780– 1800

300– 340 120– 130 280– 320 320– 360

1660– 1700 1870– 1880 1680– 1720 1640– 1680

300– 340 320– 360 150– 160

1660– 1700 1640– 1680 1840– 1850

280– 320 170– 190

1680– 1720 1810– 1830

320– 360 300– 340

1640– 1680 1660– 1700

184

33

29

29

56

19

17

16

49

32

28

27

40

33

30

30

199

32

29

28

178

32

30

29

127

22

19

18

105

31

28

27

17

22

21

21

38

32

30

28

42

30

29

29

33

33

31

30

340– 390

1610– 1660

Huandoy (paron) Right lateral 21 moraine

30

29

28

300– 340

1660– 1700

Uruachraju Left lateral moraine Right lateral moraine Left lateral moraine Artesonraju Left lateral moraine Left lateral moraine

1000– 800– 1200 1010

20

1980

Pisco Right lateral moraine

50 120– 130 360– 410

1950 1870– 1880 1590– 1640

1610– 1660 1800– 1810

Number of Five largest diameters Age measured (mm) (years lichens ago) Single Mean Median

Yanamarey (measurements of 2002) 38 31 30 Right lateral moraine Right lateral 30 25 23 moraine

Checouiacraju Left lateral moraine Right lateral moraine

340– 390 190– 200

231

results for our work in 1996 and 2002 shows that lichen in the high mountains of the Cordillera Blanca can be used successfully, at least for relative dating of natural and anthropogenic deposits and landforms above the elevation of 3500 m asl. Moraines of the last millennium on the Pacific side can be divided into three groups by their morphology and lichen sizes: those with lichen axis equal to 10 mm or less (the most recent recessional moraines); numerous,

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Table 4ii Lichenometry of moraines in the Cordillera Blanca (Atlantic-facing slope) Sites

100 60 50 120

Cancaraca-2 Cone of artificial rockfall 10 First terrace Absent Second terrace 100 Moraine dam closing 60 the lake Same stage, adjacent 60 wall End moraine crossed 20 by the road Moraine combined 50 of 3 ridges 120 Lateral moraine on the outer slope of the left lateral moraine of Cancaraca-1 Allicocha End moraine, partly in the lake Upper part of the right lateral moraine Right lateral moraine Right lateral moraine, 10 m lower the top of the highest ridge End moraine Right lateral moraine Right lateral moraine Inner part of moraine complex Outer part of moraine complex

Sites

Number of Five largest diameters measured lichens (mm)

Number of Five largest diameters measured lichens (mm) Single Mean Median

Atlantic-facing slope Cancaraca-1 Left lateral moraine Top of the end moraine Left lateral moraine Outer part of the lateral moraine

Table 4ii ( continued)

30 36 36 52

27 35 31 46

27 36 31 47

4

3

3

11 15

10 14

10 14

17

16

17

20

16

15

30

27

26

85

65

60

120

11

10

10

100

14

11

11

50 100

18 25

15 23

15 23

130 75 75 75

36 42 54 13

33 40 48 12

33 40 47 12

100

16

16

16

12

9

8

30

27

27

41

39

38

68

65

64

98

82

81

Hanging glaciers in Allicocha valley Moraine and rocks 15 (4700–4730 m) Moraine and rocks 50 (4680–4700 m) Moraine and rocks 75 (4650–4680 m) Moraine and rocks 75 (4520–4650 m) Old moraine-dam 100 (4400 m)

Single Mean Median Atlante Rocks near the entrance to the mine Youngest end moraine Young moraine, partly overlayed by moraine (2) Right lateral moraine upper the lake Right lateral moraine Right lateral moraine Right lateral moraine Left lateral moraine Large blocks below the road Pucajirca norte (safuna) End moraine Left lateral moraine Left lateral moraine Left lateral moraine Left lateral moraine Left lateral moraine

10

15

10

9

10 10

6 13

6 7

6 6

15

15

13

13

110 50 100 60 150

31 28 47 45 56

29 25 44 41 52

28 25 45 41 53

16 148 127 22 66 15

22 35 42 50 61 83

21 32 40 48 59 80

21 30 39 48 58 78

prominent, and tall end and lateral moraines supporting lichen between 17–23 mm and 28–32 mm; and single moraines with lichen of about 37 mm, 50 mm, 59 mm and 64 mm (Fig. 4). This means that some moraines, at least the eleven with lichen between 17–23 mm and the fifteen with lichen between 28–32 mm were deposited during glacier advances triggered by two separate regional climatic signals. The lichen sizes measured on the moraines of glaciers on the eastern slopes of the Cordillera Blanca are not as clearly grouped (Fig. 4). There are several possible explanations for this dispersion of ages. We believe that variations in lichen size on moraines reflect true differences in moraine ages and, therefore, indirectly represent the different dynamics of glaciers on the eastern and western sides of the Cordillera Blanca. 5.3. Calibration of the growth curve The growth curve is crucial for lichenometric dating. Due to the absence of control points between ca 80 ago and 1500–1600 years BP in the Cordillera Blanca, the curve presented here is considered preliminary. However, growth rates estimated for the last century in the Cordillera Blanca are now better constrained due to the eleven new control points obtained for the Pacific slope and five on the Atlantic one. The scatter of lichen measurements vs. time on

O. Solomina et al. / Global and Planetary Change 59 (2007) 225–235

Fig. 4. Histogram of lichenometric dates of moraines on the eastern and western slopes of the Cordillera Blanca.

the Atlantic side of the Cordillera Blanca is wide, but it is evident that the growth rate of Rhizocarpon subgenus Rhizocarpon tends to be higher on the more humid Atlantic side (Fig. 2). The size of lichen on surfaces 100 years old on the Pacific side is 15 mm. Further back in time the growth rate of Rhizocarpon subgenus Rhizocarpon decreases. Unfortunately, it is neither known when exponential growth is replaced by linear growth in this case, nor can a comprehensive theoretical model suggest a growth gradient. The final choice of the shape of the curve will be possible only when new and older control points can be obtained in this region. 5.4. 14C and historical dates of “LIA” moraines in tropical South America Radiocarbon dates of moraines in the Cordillera Blanca of the last millennium are rare. Based on the radiocarbon dates (630 ± 65 years BP, i.e. AD 1290– 1400 in the Cordillera Vilcanota and 270 ± 80 years BP,

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i.e. AD 1510–1800 at Quelccaya) Mercer and Palacios (1977) concluded that the “LIA” culminated here between 600 and 300 years BP. Röthlisberger (1987) dated a buried soil horizon at 440 ± 185 years BP (i.e. AD 1300–1660) in the Cordillera Blanca (Ocshapalca Glacier) and suggested that the advance occurred sometime later. Using historical data, Hastenrath (1981) demonstrated that glaciation in the Equadorian Andes, near Quito, was more extensive than at present during the first half of the 18th century and the 16th centuries. Our age estimates of the moraines do not contradict these data with lichenometry able to provide a more detailed although tentative picture for the timing and scale of glacier variations over the last millennium in the Cordillera Blanca. 5.5. Climatic proxies and lichenometric dates The most prominent peak in the distribution of moraines on the Pacific-facing slope of the Cordillera Blanca is between AD 1590–1720 (15 moraines). All nine glaciers studied on the Pacific slope of the Cordillera Blanca have moraines of this age. The size of lichen varies little (29–30 mm). This peak marks the period of the most prominent glacier advance of the last millennium in this region and coincides with maximum net accumulation in AD 1500–1720 recorded in the Quelccaya ice core — the wettest period of the millennium (Thompson et al., 1986). At both, Quelccaya and Huascaran, relatively depleted delta 18 O values display cooling between AD 1490 and 1900. Thus, judging by these climatic proxies, glacier advances in the 17th century were triggered both by a decrease in temperature and an increase in snow accumulation (Fig. 5). It is remarkable that the peak of moraine deposition in the early part of the 17th century closely

Fig. 5. Comparison of the distribution of moraines dated by lichenometry, with temperature and net accumulation reconstruction from the Quelccaya ice cores (Thompson, 1996; Thompson et al., 2000). The arrow shows the timing of the climatically effective eruption of Huaynaputina (AD 1600).

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coincides with marked cooling following the Huaynaputina eruption in AD 1600 (Silva and Zielinsky, 1998). Eleven moraines deposited between AD 1780–1880 are lichenometrically identified in the forefields of seven glaciers on the Pacific-facing slope of the Cordillera Blanca. They are of smaller extent and it is likely that their advances were triggered by a decrease in temperature, rather than increasing precipitation (Fig. 5). There are no moraines dated by lichenometry between ca AD 1000 and ca AD 1500. This time period coincides with a period of negative values of net accumulation indicated by the Quelccaya Core 1 (Thompson et al., 1986). The low stand of Lake Titicaca between 900 and 500 cal years BP also points to dry conditions (Abbot et al., 1997; Cross et al., 2000). The fall of the Tiwanaku culture at ca AD 1150 is thought to be connected to the beginning of this dry period: the culture, depending on raised field agricultural technology, and declined as a consequence of agricultural collapse (Ortloff and Kolata, 1993). Thus, good climatic explanations can be given for glacier advances dated by lichenometry in the proxy records. However, we realize that new control points for the growth curves and other independent climate proxy data are needed to provide more confidence in the accuracy and reliability of these lichenometric dates. Our data support the conclusion of Luckman and Villalba (2001) that mountain glacier variations in the Southern Hemisphere during the “LIA” are broadly synchronous with those in the Northern Hemisphere. They found that land-based glaciers in Patagonia reached their maximum extents between AD 1600 and 1700, at the same time as those in the Cordillera Blanca according to our lichenometric estimates. Advances of the 19th century are also synchronous along the transect from Alaska to Patagonia (Luckman and Villalba, 2001).

(17–23 mm and 28–32 mm). Relative lichenometric dates of moraines of the Atlantic-facing slope are less regular and internally inconsistent. 3. The peak “LIA” advance occurred between AD 1590 and AD 1720: moraines at all nine glaciers studied on the Pacific side were deposited during this period. These advances were triggered by positive net accumulation and cooling according to the ice-core data from Huascaran and Quelccaya (Thompson et al., 2000). Less extensive, younger advances between AD 1780 and 1880, coincide with cooler temperatures in the ice cores. 4. Agreement of the 1996 and 2002 lichenometric records obtained independently by two teams adds confidence to the reliability of the younger part of the lichenometric curve, presented here. Besides the moraine dating, the improved growth curve might be of interest for archaeologists, working on highelevation sites in Cordillera Blanca. Acknowledgements We are grateful to John Andrews and James Benedict for the comments on the earlier version of this manuscript. We appreciate the help of Christian Georges who provided the map in Fig. 1. The reviewers — Johannes Koch, Vanessa Winchester and Stephan Harrison and the Editor Christoph Schneider were most helpful in improving the paper. Gregory Wiles edited text and provided valuable comments. The French team was supported by Great Ice, IRD and Eclipse of CNRS. The contribution of the Innsbruck group was supported by the Austrian Science Foundation Projects, project-FWF P-13567 and P16113. References

6. Conclusions 1. Eleven new control points allow us to estimate the Rhizocarpon subgenus Rhizocarpon growth rate during the initial rapid growth phase, and to improve the younger portion of the previous growth curve (Rodbell, 1992). Further back in time the lichen growth rate decreases, but the timing of the transition to a slower growth rate is still unknown with only one control point available to constrain the older portion of the curve. This suggests caution when interpreting lichenometric measurements in terms of calendar ages, especially with regard to periods before AD 1590. 2. Relative age estimates of moraines reveal two main peaks of glacier advances on the Pacific-facing slope of the Cordillera Blanca during the last millennium

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