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Climate Modeling in Las Leñas, Central Andes of Argentina

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Glacier - climate modeling in Las Leñas, Central Andes of Argentina Master’s Thesis Faculty of Science University of Bern presented by Philippe Wäger 2009 Supervisor: Prof. Dr. Heinz Veit Institute of Geography and Oeschger Centre for Climate Change Research Advisor: Dr. Christoph Kull Institute of Geography and Organ consultatif sur les changements climatiques OcCC Abstract Studies investigating late Pleistocene glaciations in the Chilean Lake District (~40-43°S) and in Patagonia have been carried out for several decades and have led to a well established glacial chronology. Knowledge about the timing of late Pleistocene glaciations in the arid Central Andes (~15-30°S) and the mechanisms triggering them has also strongly increased in the past years, although it still remains limited compared to regions in the Northern Hemisphere. The Southern Central Andes between 31-40°S are only poorly investigated so far, which is mainly due to the remoteness of the formerly glaciated valleys and poor age control. The present study is located in Las Leñas at 35°S, where late Pleistocene glaciation has left impressive and quite well preserved moraines. A glacier-climate model (Kull 1999) was applied to investigate the climate conditions that have triggered this local last glacial maximum (LLGM) advance. The model used was originally built to investigate glacio-climatological conditions in a summer precipitation regime, and all previous studies working with it were located in the arid Central Andes between ~17- 30°S. Regarding the methodology applied, the present study has established the southernmost study site so far, and the first lying in midlatitudes with dominant and regular winter precipitation from the Westerlies. However, only slight adjustments turned out to be necessary to fit the model from Kull (1999) to a regular winter precipitation regime. The model reliably reproduced modern glacio- climatological conditions, and therefore, it was found to be also suitable to investigate paleo glacio- climatological conditions in the study area. The modeling results are robust even when important input parameters are strongly varied. The theoretically possible climate conditions that could have triggered the advance range from extremely cold and dry to temperate and extremely humid. However, by evaluating the glacier geometry and by consulting reconstructed late Pleistocene climate conditions from other proxies in the region of the study area, a rather narrow range could be defined, delineating the most probable precipitation / temperature combinations. This range implies that the LLGM in Las Leñas was triggered in a cold- humid climate. For the scenario with best guess input parameters, the most probable climate conditions range from a temperature depression of 8°C in combination with a precipitation increase of 500mm, to a temperature depression of 5°C in combination with a precipitation increase of 1840mm. i From the modeling perspective, it can not be excluded that the precipitation increase could also have been caused by increased summer precipitation. However, all evidence, emerging from the exposition of the catchment, the glacier geometry and many proxies in Central Chile, points to intensified Westerlies causing more winter precipitation in the study area during late Pleistocene times. The reconstructed cold-humid paleoclimate in this study points to an LLGM advance in Las Leñas during full glacial times, but ultimate constraints about the exact timing during the last glacial cycle are not possible. Still, an LLGM advance during Marine isotope stage 4 (MIS4, ~65 ka BP) is less probable due to the relative freshness of the moraines. Several periods during MIS2 (LGM) and MIS3 would fulfill both a strong temperature depression and humid conditions in Central Chile. However, the regional glacier chronologies available and especially the humid conditions reported for Central Chile provide more arguments for an advance prior to the LGM, around ~40-28 ka BP during MIS3, but this needs confirmation. The 10Be surface exposure dates of the boulders, that were sampled during the field campaign in December 2008, will shortly be available and are expected to bring the timing issue forward. A pre-LGM advance in Las Leñas would be in agreement with most of the few glacier chronologies available so far in the Southern Central Andes between 31-39°S, and would highlight the sensitivity of the late Pleistocene glaciers to precipitation changes. ii iii Contents Abstract................................................................................................................................................... i Contents................................................................................................................................................. iv List of Figures...................................................................................................................................... vii List of Tables......................................................................................................................................... ix Abbreviations......................................................................................................................................... x 1 Introduction and Motivation ......................................................................................................... 1 2 Location and climate of the study area......................................................................................... 3 2.1 Location................................................................................................................................... 3 2.2 Modern climate........................................................................................................................ 6 2.2.1 South America ........................................................................................................... 6 2.2.1.1 Atmospheric circulation and precipitation................................................. 7 2.2.1.2 Interseasonal variability of precipitation ................................................... 8 2.2.1.3 Interannual variability of precipitation ...................................................... 9 2.2.2 Central Andes........................................................................................................... 10 2.2.2.1 Atmospheric circulation and precipitation............................................... 10 2.2.2.2 Intraseasonal variability of precipitation ................................................. 12 2.2.2.3 Interannual variability of precipitation .................................................... 13 2.2.2.4 Characterization of the precipitation regime in Las Leñas ...................... 13 2.3 Equilibrium line and snow line altitudes............................................................................... 15 2.3.1 Central Andes between 27 – 36°S............................................................................ 15 2.3.2 Around Las Leñas .................................................................................................... 16 2.4 Late Pleistocene climate in the Central Andes ...................................................................... 19 2.4.1 Central Andes from 17 – 31°S ................................................................................. 20 2.4.2 Southern Central Andes from 32 – 43°S.................................................................. 22 2.4.3 Paleoclimatic implications ....................................................................................... 23 2.4.3.1 Tropical Easterlies ................................................................................... 23 2.4.3.2 Extratropical Westerlies........................................................................... 24 2.4.3.3 Paleoclimate conditions in the broader study area................................... 25 2.4.3.4 The global context ................................................................................... 27 iv 3 Glaciological background............................................................................................................. 31 3.1 Mass balance ......................................................................................................................... 32 3.2 Ice flow.................................................................................................................................. 35 3.2.1 Internal deformation................................................................................................. 35 3.2.2 Basal sliding............................................................................................................. 36 3.3 Glaciers as climate proxies.................................................................................................... 37 4 Methods ......................................................................................................................................... 41 4.1 Glacier-climate model ........................................................................................................... 41 4.1.1 Climate model.......................................................................................................... 42 4.1.2 Mass balance model................................................................................................. 43 4.1.3 Dynamical ice flow model ......................................................................................
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