MASS BALANCE CHANGE OF MANSO GLACIER SINCE THE LITTLE ICE AGE

LUCAS RUIZ; VALENTINA ZORZUT; MELAINE LE ROY; JUAN CRUZ GHILARDI

Congreso; XXIX Reunión Científica AAGG; 2022

Asociación Argentina de Geofísicos y Geodestas

Since the end of the Little Ice Age (~1870), glaciers of the Northern Patagonian Andes haveretreated markedly, with some short periods of readvance, but an accelerating rate in the lastdecades. Nevertheless, these changes cannot be directly related to climatic forcings, as theresponse of glaciers is also affected by topographic features (e.g., glacier size, glacier surfaceslope, glacier bedrock slope), surroundings characteristics (e.g., proglacial lake, type of bedrocksediment), supraglacial features (debris-cover, ponds, or ice cliffs), subglacial hydrology, andcalving dynamics (in the case tidewater or lake terminating glaciers). As a result, changes inglacier size through advance or retreat are an indirect, delayed, and filtered signal to changes inclimate. However, the glacier mass balance change is a direct and undelayed response to thechanges in atmospheric conditions. Unfortunately, mass change of glaciers in the NorthPatagonian Andes is only available for the last two decades, limiting our understanding of theimpact of climate change in more prolonged periods. Here we present an assessment of themass change of the Manso glacier in the Monte Tronador, Northern Patagonian Andes, sincethe Little Ice Age. Manso glacier is a debris-covered tongue valley glacier, which has shown anincreasing calving activity in the last 30-40 yrs. Due to the retreat of the glacier front, aproglacial lake started to develop in the mid-1980s and has continually grown until 2009 when aGlacial Lake Outburst flood (GLOF) partially emptied it. Since 2009, the proglacial lake hascontinued growing, and it is now twice larger than before the GLOF. Previousdendrogeomorphological studies (Masiokas and others, 2010) were complemented with aerialphotos and satellite images to reconstruct the extent of the glaciers from 1840 to 2020 (Figure 1A). Glacier ice thickness and volume between 1840 and 2000 were obtained using the perfectplastic glacier model (Benn and Hulton, 2010) . Then, ice thickness changes were computed bydifferencing the ice thickness between different periods and complemented with the geodeticmass balance from differencing the Digital elevation Model since 2000 (Ruiz and others, 2017)to compute the mass change 1840 to 2019. The glacier ice thickness model was calibratedusing the contour of the glacier and validated using bathymetric and ice thicknessmeasurement. Ice thickness estimations have an uncertainty of 10-20 m. To compute theglacier's mass change, we assume that glacier ice thickness change above the Equilibrium LineAltitude (above 1900-2000 m) is similar to the measured between 2000 and 2019. Glacier massloss between 1840 and 1981 was minimal (0.1 ± 0.3 m w.e. yr -1 ), following almost no glacierextent changes. Since the mid-1980s, glacier mass loss has increased rapidly and achievedaround 0.5 ± 0.3 m w.e. yr -1 after the 2000s (Figure 1 B). Also, a discernible change in thesurface characteristics of the glacier since the mid-1980s is observed, when mass wastingdominated features associated with a compressive ice flux were replaced with transversalcrevasses suggesting a change towards an extensional stress field. Minimal glacier masschange during the 20 th century is attributed to the delay response to the amelioration of climateafter the Little Ice Age due to a thick debris cover at the glacier tongue. However, more recentglacier mass loss increase is attributed to the dynamic response of the glacier to changes in theforce balance near the front, which initiates positive feedback between thinning andacceleration, increasing the calving rate, and as a consequence, also increase the proglaciallake surface and volume. In the last decades, the Manso glacier mass change is dominating theglacier mass loss of Monte Tronador and due to its ice volume is expected to be the case in thenear future. The reconstructed glacier mass balance of Manso glacier will help calibrate andvalidate glacier mass balance and ice dynamics models, crucial to elucidate the links betweenclimate, mass balance, and glacier dynamics. A better understanding of the glacier-climaterelationships and the ice dynamics can thus help improve our current understanding of theseissues but will also prove essential for the development of reliable projections of future glacierchanges in the North Patagonian Andes.