EPJ Web of Conferences 232, 02002 (2020) https://doi.org/10.1051/epjconf/202023202002 HIAS 2019

The movements of Alpine throughout the last 10,000 years as sensitive proxies of temperature and climate changes

Walter Kutschera1,*, Gernot Patzelt2, Joerg M. Schaefer3, Christian Schlüchter4, Peter Steier1, and Eva Maria Wild1 1VERA Laboratory, Faculty of Physics, University of Vienna, A-1090 Vienna, Austria 2Gernot Patzelt, Patscher Strasse 20, A-6080 Innsbruck-Igls, Austria 3Joerg M. Schaefer, Geochemistry, Lamont-Doherty Earth Observatory, Pallisades, NY 10964-8000, USA 4Christian Schlüchter, Institute of Geological Sciences, University of Bern, CH-3012 Bern,

Abstract A brief review of the movements of Alpine glaciers throughout the Holocene in the Northern Hemisphere (European ) and in the Southern Hemisphere (New Zealand Southern Alps) is presented. It is mainly based on studies where 14C dating, dendrochronology and surface exposure dating with cosmogenic isotopes is used to establish the chronology of advances and retreats of glaciers. An attempt is made to draw some general conclusions on the temperature and climate differences between the Northern and Southern Hemisphere.

1 Introduction radiocarbon dating, surface exposure dating of rocks and with various cosmogenic radionuclides (10Be, It is well known that the Holocene, i.e. the geological time 14C, 26Al, 36Cl), and geomorphological considerations [10- period following the end of the Last , enjoyed 22]. relatively stable temperatures. But glaciers are sensitive The atmospheric CO2 concentration was remarkably proxies to even small temperature and/or climate changes. constant during the last 10,000 years, changing by only 20 Thus, the globally observed retreat of Alpine glaciers and ppm [1]. Such small CO2 variations are unlikely to trigger polar ices sheets since about 1850 AD (the end of the so- the observed glacier movements. It is therefore possible called and interrupted by three re-advances) that small solar activity variations [23], enhanced by has been linked to the temperature increase caused by (hitherto largely unknown) feed-back processes on Earth human activities, particularly due to the continuous caused the observed glacial fluctuations [24]. Whatever increase of the CO2 concentration in the atmosphere [1]. It the cause of these natural fluctuations, they constitute a is interesting to note that a change in the surface “background” which is now being modified in a complex temperature on Earth with increasing CO2 concentration in way by human activities. The challenge is then to evaluate the atmosphere was already discussed more than 100 years the human signal correctly, and try to make predictions ago by the Swedish chemist and Nobel Laureate Svante about the climate in the near future [25]. There is an Arrhenius [2]. Discussions are ongoing now to define a ongoing discussion about the certainties and uncertainties new geological period called “Anthropocene” [3, 4], of these predictions [e.g. 26]. In spite of great efforts where man’s influence on the environment is significant around the world to reduce anthropogenic CO2 emissions, and distinct [5, 6]. Even though the global retreat of it appears doubtful whether this is currently more than a glaciers since 1850 is considered to be a fingerprint of “pious wish” [27]. man’s influence on the climate, it is now evident that The current work cannot give an answer to these considerable glacial fluctuations occurred already much important questions. Rather it will present evidence for the earlier during the Holocene, when human impact was waxing and waning of Alpine glaciers in both the Northern negligible. and Southern Hemispheres throughout the Holocene. An In a way, the interest in Alpine glaciers of the past attempt will be made to explain at least some of these started with the accidental discovery of the famous Iceman fluctuations from first principles. Even though the Ötzi in 1991, a naturally mummified body which was well complexity of the climate may only allow crude estimates preserved for 5200 years in the icy environment of a high of those principles, we simply follow the advice of the late mountain pass (3210 m a.s.l.) in the Ötztal Alps [7-9]. Murray Gell-Man: “Nature is most easily described by a Since then, several forward and backward movements of sequence of approximations,” [28]. glaciers in the European Alps and in the New Zealand Southern Alps throughout the last 10,000 years have been established with the help of dendrochronology, ______*email: [email protected]

© The Authors, published by EDP Sciences. This is an open access article distributed under the terms of the Creative Commons Attribution License 4.0 (http://creativecommons.org/licenses/by/4.0/). EPJ Web of Conferences 232, 02002 (2020) https://doi.org/10.1051/epjconf/202023202002 HIAS 2019

2 Paleorecord of Temperature and CO2 currently goes back 800,000 years [29, 30], and shows the well-known Milankovic cycles of the ice ages first 18 The records of temperature and atmospheric CO2 discovered in the δ O record of foraminifera in deep-sea concentration on Earth show considerable variation over sediment cores [31]. Efforts to drill ice cores in the last 500 million years (Fig. 1). In particular, the CO2 back to 1.5 million years are under way [32, 33]. Except concentration during the time of the dinosaurs (~240 – 65 for the brief periods, the CO2 concentration million years ago) was around 1000 ppm, and the mean stays at its lowest value in this period and reaches about temperature about 6 oC warmer than today. From Fig. 1, 200 ppm at the glacial maximum 20,000 years ago. After one can see that during the last 10 million years, a gradual warming up to the Holocene some 10,000 years ago, both lowering of the temperature and of the CO2 concentration relatively stable temperature and CO2 concentrations occurred. persist. The rapid increase of CO2 during the last 60 years Around 2.5 million years ago (beginning of the from about 300 ppm to currently 415 ppm is well ), major glaciations in both Antarctica and documented [1]. Greenland set in. The record of ice cores from Antarctica

Temperature of Planet Earth https://en.wikipedia.org/wiki/File:All_palaeotemps.png ∆T ∆T (oC) (oC) +14 +14 +12 +12 +10 +10 +8 +8 +6 +6 +4 2100 +4 +2 2050 +2 0 0 -2 -2 -4 -4 -6 -6 500 400 300 200 100 60 50 40 30 20 10 5 4 3 2 1 800 600 400 200 20 15 10 5 1 Years before present (Myr) Years before present (kyr) CO2 CO2 (ppm) (ppm) 5,000 5,000 Atmospheric CO2 concentration of Planet Earth Foster et al., Nature Comm. 8:14845 (2017) 2,000 2,000

1,000 1,000

500 500

200 200

100 100

500 400 300 200 100 60 50 40 30 20 10 5 4 3 2 1 800 600 400 200 20 15 10 5 1

Figure 1. Summary of temperature and CO2 concentrations on Earth for the last 500 million years. The figure is a composite from the original temperature record [34] and the CO2 record [35]. The CO2 record was adjusted to the same time scale as the temperature record. Possible temperature increase in 2050 and 2100, respectively, predicted by some climate models are indicated with red dots on the rightmost temperature axis.

3 Movement of Alpine glaciers during evolved into a new field sometimes called “glacial the Holocene archaeology”. Besides the archaeological aspect in the study of glaciers, there is the sensitivity to small climate The accidental discovery of the Iceman “Ötzi” in 1991 [7- changes which became apparent through multiple periods 9], and the appearance of human artefacts and natural of forward and backward movements of glaciers materials released from receding Alpine glaciers [22], throughout the Holocene [11].

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3.1 The European Alps 3.1.1 The Pasterze glacier Glacier movements of the past have been studied in many The rapidly receding glaciers in our time sometimes mountain ranges of the European Alps [10,11, 13-15, 17, release well-preserved subfossil trees, which can be dated 19-22]. In 2001, 14C dating of a variety of organic with 14C and dendrochronology. Surprisingly, up to materials released from glaciers in the revealed 10,000-year old logs were found in the forefield of the eight Holocene phases of reduced glacier extent: 9910- Pasterze glacier, the largest glacier in the Austrian Alps 9550, 9010-7980, 7250-6500, 6170-5950, 5290-3870, [10], The situation of the finds is depicted in Fig. 2. This 3640-3360, 2740-2620, and 1530-1170 calibrated years indicated that in the Early Holocene these glaciers were BP (BP = before present = 1950 AD) [11]. In the following even smaller than today, allowing trees to grow in an area we discuss a few selected cases of glacier movements in still covered by ice today. the European Alps.

Johannisberg, 3463 m A Großglockner, 3798 m B

Johannisberg, 3463 m Pasterze Glacier (2000 AD)

Pasterze Glacier (1832 AD)

1462 GP-2174 D C PAZ-33 1462 1462 PAZ-28

PAZ-23 PAZ-26 PAZ-21 PAZ-27 PAZ-30 PAZ-16 PAZ-31 PAZ-4

PAZ-29 PAZ-22 PAZ-5

PAZ-25 PAZ-2 PAZ-1

8400 102008200 100008000 98007800 96007600 94007400 92007200 90007000 88006800 cal BC cal BP

Figure 2 Old wood found in the forefield of the Pasterze Glacier in the Austrian Alps. The dramatic loss of glacier mass can be assessed from the painted landscape of 1832 (A) and the photo of the twindling glacier in 2000 (B). Panel (D) zooms into the trapezoidal section of (B) and shows one log found in the forefield of the glacier. Panel (C) shows the age distribution of all wood specimens collected from the glacier forefield [10] by combining dendro dating (filled sections) with 14C measurements (error bars indicate 65% calibrated age ranges). The open sections depict missing inner tree-ring sections, and the hatched sections indicate compressed wood. This composite figure is reproduced from Ref. [22].

3.1.2 Ice cores at Mount Ortles and Monte Rosa interval in the European Alps during the Holocene” [36]. A somewhat different result was reported from an ice core A recent ice core from the summit of Mt. Ortles in South at Col Gnifetti (4455 m a.s.l.) in the Monte Rosa mountain Tyrol (3859 m a.s.l.) revealed that the deepest ice near range of Switzerland, where the deepest ice sample of this bedrock is about 7000 years old [36]. “Absence of older high-altitude glacier indicated an age of about 10,000 ice on the highest glacier of South Tyrol is consistent with years [37]. The higher altitude as compared to Mt. Ortles the removal of basal ice from bedrock during the Northern may have prevented the loss of basal ice during the Hemisphere Climatic Optimum (6-9 kyrs BP), the warmest Climatic Optimum mentioned above.

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3.1.3 The Rhone glacier 26Al (0.72 Myr) was also measured in the samples. Concordant exposure ages for 10Be and 26Al allowed one Exposure dating of rock surfaces on Earth by measuring to make the assumption that the Rhone Glacier removed in-situ produced cosmogenic radioisotopes [38, 39] with several meters of rock during the last Ice Age, leaving the Accelerator Mass Spectrometry (AMS) has made it sampled bedrock surfaces (Fig. 3) most likely free of possible to date glacially polished rock surfaces and glacial cosmogenic nuclides at the beginning of the Holocene moraines. Application of this method allowed one to period. Consequently, the measured cosmogenic nuclide reconstruct the dynamic history of many glaciers in the concentration in the sampled material integrates the European Alps, and in other glacial regions around the periods when these rocks were ice-free during the world. In particular, if the concentration of two Holocene. Since some erosion of the rocks happened also radioisotopes with very different half-lives are measured, during the Holocene, the 10Be exposure ages shown in the ratio of these radioisotopes allows one to reconstruct Fig. 3, decrease toward the lowest, central part of the the chronology of glaciers including erosion of the glacial trough, where ice velocity was greatest and thus underlying bedrock. Such measurements were performed erosion rate is expected to be fasted. The longest exposure 14 10 with C (t1/2 = 5.7 kyr) and Be (1.39 Myr) in quartz times (4.5 to 5.3 kyr) were found at the left-lateral margin grains of recently exposed rock surfaces of the Rhone zone of the glacier indicating the longest integral time Glacier [17], which was one of the dominant glaciers of when the glacier was smaller than today. the European Alps during the Last Ice Age. In addition,

Figure 3. Sampling location on exposed rocks (darker grey-shaded area) in front of the Rhone Glacier terminus (lighter grey-shaded area). Contour lines are labelled in meters a.s.l. The minimum 10Be exposure ages are shown. The figure is adopted from the work of Goehring et al. [17].

Figure 4. Schematic presentation of the time periods where European glaciers were smaller (red) and larger (blue) than today. The figure is adapted from the work of Goehring et al. [17]. “Nearby proxy records indicate that at least most of the time during which the Rhone glacier was smaller than today was during the early to mid-Holocene” [17]. In E the hypothetical Holocene advance and retreat scenario of the Rhone Glacier is displayed, indicating the glacier was smaller than today during the first part of the Holocene.

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There is also evidence that the ice of the Jostedalsbreen, 3.1.4 The Mer de Glace glacier the largest mountain glacier in Norway, is not a left-over A detailed radiocarbon and dendrochronological study was from the Last Ice Age, but only formed during the second performed by Le Roy et al. [41] at the Mer de Glace glacier half of the Holocene [40]. Although it is unlikely that the in the (Fig. 5A), the largest glacier of European Alps were completely ice-free in the early the French Alps. Subfossilized wood exposed at the right Holocene, the evidence for generally lower glaciation as lateral near the terminus of the glacier was compared to the second half of the Holocene is mounting, investigated (Fig. 5B). It revealed 10 glacial advances as indicated from the ice core result from Ortles mentioned during the last ~3000 years. These glacial fluctuations are above [36]. similar to those observed in the Swiss Alps, although not synchronous in all aspects.

(A) (B)

Figure 5 The lower section of the Mer de Glace glacier in the Mont Blanc mountain range, with the Grandes Jorasse (4208 m a.s.l.) in the background (A). The right figure (B) from the work of [41] shows the sampling sites at the right lateral moraine near the glacier terminus. Two positions of the terminus in 1995 and 2013, respectively, are marked with black dashed curves. For a detailed description of the sampling sites and the material recovered see Ref. [41].

3.1.5 Timberline An important proxy for temperature changes in the Alps during the past is the timberline (sometimes also called tree line). The timberline is the edge of the habitat at which trees can grow (Fig. 6). Since precipitation in the Alps is abundant, the movement of the timberline is primarily affected by the change in summer temperature during the main growing season. From studies of subfossil trees preserved at different altitudes, periods of higher and lower temperatures, respectively, can be reconstructed by 14C measurements and dendrochronology. From studies like this (e.g. [12]), it

has been estimated that the timberline moves Figure 6. Typical timberline in an Alpine landscape. The approximately 100 m up or down for a summer transition from the forested region to grass land can be clearly temperature change of ±0.6 oC. seen in this picture, which is reproduced from a free download (https://unsplash.com/s/photos/timberline).

3.1.6 Temperature change during the Holocene Holocene Thermal Maximum (HTM), and continuously decreased in the second half of the Holocene approaching In Figure 7, information on glacier and tree-line its lowest values during the Little Ice Age (LIA) between movements are summarized and converted into 1300 and 1850 AD. This period also resulted in the largest approximate temperature variations during the entire glacier advance during the Holocene. The Holocene Holocene [22]. Glacial retreats are usually accompanied temperature trend is also supported by other temperature- with higher timberlines indicating higher temperatures. sensitive proxies, e.g. by the study of chronomids in a Overall, the mean temperature went through a maximum high-altitude lake (2796 m a.s.l.) in the Austrian Alps [42]. during the first half of the Holocene, sometimes called the

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+0.5 Summer temperature, (May to September)

) C

o 0 (

∆T -0.5 -1.0 Iceman

Tree line, higher than today (movement: 100m/0.6oC)

Glacier periods of retreat, Hormes 2001 Joerin 2006

Glacier extent, Eastern Alps Smaller than today Advance as in modern times BA

Mesolithic Neolithic BA H L RT MA MT 10 9 8 7 6 5 4 3 2 1 BC / AD 1 2 Calibrated date (kiloyears)

Figure 7. Schematic presentation of glacier and tree-line movements in the European Alps during the Holocene [22]. The periods of smaller glaciers and higher tree lines are indicated with the box symbols. Glacial advances are indicated with filled triangles and curves. The largest advances took place during the Little Ice Age (~AD 1300 to 1850). The top curve depicts the relative summer temperature variations deduced mainly from tree-line movements. The mean temperature between AD 1900 and 2000 is used as reference (ΔT = 0 oC). The red vertical line marks the time where the Iceman Ötzi died [43].

3.2 The New Zealand Southern Alps 3.2.1 The Mueller glacier

Most of the information on Alpine glacier movements The chronology of glacier advances through the past 7000 through the Holocene comes from the Northern years were reconstructed from measurements of Hemisphere. However, the New Zealand Southern Alps cosmogenic in-situ produced 10Be in moraines of the allow one to get information on glacier movements in the Mueller glacier, which is located at Mt. Sefton in the Mt. Southern Hemisphere at comparable latitudes. While Mont Cook mountain range [16]. Figure 8 is reproduced from Blanc, the highest mountain of the European Alps (4810 m this work and shows the area where the extensive 10Be a.s.l.) is located at 45.8o N, Aroaki/Mount Cook, the measurements were performed. The deduced exposure highest mountain of the NZ Southern Alps (3927 m a.s.l.) ages are listed which allow one to draw some conclusions is located at 43.5o S. Because of the closeness of the Pacific for the glacier advances with respect to the ones in the Ocean to the East and the Tasman Sea to the West of the European Alps. Interestingly, glacier advances seem not to NZ Southern Island, the glaciation of the NZ Southern be synchronous between the two hemispheres. Simply Alps is more intense as compared to the European Alps, speaking, during some time periods glaciers advanced in even though the latter reach up to considerably higher the Southern Hemisphere (SH) whilst they simultaneously altitudes. Particularly the western slopes of the NZ retreated in the Northern Hemisphere (NH), and vice Southern Alps, which are under the influence of the versa. Tasman Sea and westerlies, are the home of large glaciers As discussed by Schaefer et al. [16], it is possible (e.g. Fox and Franz Josef glacier), which still reach down that regional effects influenced the pattern of glacier to about 300 m above sea level. Due to regional cooling, movements in this region. There is, however, some these two glaciers even advanced during a period of global indication that the oldest ages are found for the most warming [44]. advanced stages of the glacier. This is more clearly seen in Here, we describe two rather detailed studies of the chronology of moraines for the Cameron glacier glacier movements on the eastern side the NZ Southern discussed in the next section (3.2.2). Alps.

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Mt. Sefton (3151 m a.s.l.)

Figure 8. This figure is reproduced from Ref. [16] and shows the results of the 10Be ages (given in years with 2σ uncertainties) with the assignment to the Holocene moraines of the Mueller glacier. The ages marked in purple colour are from moraines which were deposited during mid- to late-19th century.

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3.2.2 The Cameron glacier interesting conclusion. While in the NH the temperature in the first half of the Holcene is higher than in the second 10 Another detailed study of Be ages of moraines was half, the opposite seems to be the case in the SH. It is performed by Putnam et al. [18] at the Cameron glacier in interesting that this trend is supported by the insolation the Arrowsmith Range, which is a smaller mountain range curves (red curves of panels c and h) calculated from the parallel to the Southern Alps about 85 km northeast of Milankovic theory of the Earth’s orbital parameter Arioka/Mt. Cook. The altitude of the Arrowsmith Range is variations [45]. Although the insolation changes only by 4 ~1000 m below the highest ones of the Southern Alps, but to 6 % over the Holocene, the general temperature trend is glaciers are still abundant. Figure 9A show the main results reproduced. A recent study to take possible effects of from the work of Putnam et al. [18], where the moraines interannual climate variability on the observed glacier of the Cameron glacier were dated. fluctuations into account, confirms the robustness of the The detailed comparison of temperature proxies of glacier fluctuations against this short-term climate the NH and SH displayed in Fig. 9B [18] leads to an fluctuations [46].

(A) Couloir Peak (B) (2624 m a.s.l.)

1930 AD

520 ± 410 650 ± 30

6,890 ± 410 9,200 ± 100

8,100 ± 200

9,840 ± 470

10,690 ± 410

Figure 9. In Fig 9A the mean 10Be ages in years (uncertainties 1σ) resulting from the work of Putnam et al. [18] are displayed at the Holocene moraines from the Cameron glacier. The oldest age signals the largest glacier advance at the Early Holocene. Fig. 9B shows an interesting comparison of various temperatures proxies for the NH (panels a-e) and the SH (panels f-h). For a detailed description of Fig. 9B, see Ref. [18].

4. Conclusions [18]. The partly asynchronous movements of glaciers in the two hemisphere is probably influenced by regional The comparison of glacier movements throughout the effects [16]. Holocene from Alpine mountain ranges in the NH and SH This is not surprising considering that the European Alps revealed some interesting differences. The opposite are largely imbedded into a continental climate, whereas temperature development from the Early to the Late the NZ Southern Alps are subject to a maritime Holocene in the two hemispheres is established by a environment. number of proxies and supported by the Milankovic theory

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The increasing number of reports about glacier to draw more definite conclusions about the influence of dynamics in the past from other glaciated regions of the the sun on the glacier movement during the Holocene. Northern and Southern Hemispheres, e.g. from North and One thing can be said with some certainty: The South America, will help to develop a more concise picture complex moving patterns of glaciers in both hemispheres about climatic variations around the globe during the of the past have now given way to an accelerated retreat of Holocene. Concerning natural causes of the observed all glaciers around the globe, most likely due to the variations, it is unlikely that they were triggered by anthropogenic impact on our climate. atmospheric CO2 because it was remarkably constant throughout the Holocene (Fig. 1 and Ref. [1]). The possible influence of solar activity variations has been Acknowledgement discussed [47-49], but since the total solar irradiance varies by only ~0.1 % during the Holocene [23], other We have benefitted from many discussions with effects such as volcanic eruptions may have some colleagues around the world on the issue of glacier influence too. In general, it seems that some amplifying movements. The wisdom of the late Wally Broecker on terrestrial effect are needed to explain the magnitude of the climate issues is greatly appreciated as an important input observed glacier movements. It has to be seen whether the into the current work. recent detailed analysis of solar activity [50] will allow one

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