MASS BALANCE CHANGE OF MANSO GLACIER SINCE THE LITTLE ICE AGE

RUIZ, L.; RIVERA, ANDRES; ZORZUT, VALENTINA.; PITTE P.

Mendoza

Conferencia; XXIX Reunión Científica AAGG; 2021

Since the end of the Little Ice Age (~1870), glaciers of the Northern Patagonian Andes have retreated markedly, with some short periods of readvance, but an accelerating rate in the last decades. Nevertheless, these changes cannot be directly related to climatic forcings, as the response of glaciers is also affected by topographic features (e.g., glacier size, glacier surface slope, glacier bedrock slope), surroundings characteristics (e.g., proglacial lake, type of bedrock sediment), supraglacial features (debris-cover, ponds, or ice cliffs), subglacial hydrology, and calving dynamics (in the case tidewater or lake terminating glaciers). As a result, changes in glacier size through advance or retreat are an indirect, delayed, and filtered signal to changes in climate. However, the glacier mass balance change is a direct and undelayed response to the changes in atmospheric conditions. Unfortunately, mass change of glaciers in the North Patagonian Andes is only available for the last two decades, limiting our understanding of the impact of climate change in more prolonged periods. Here we present an assessment of the mass change of the Manso glacier in the Monte Tronador, Northern Patagonian Andes, since the Little Ice Age. Manso glacier is a debris-covered tongue valley glacier, which has shown an increasing calving activity in the last 30-40 yrs. Due to the retreat of the glacier front, a proglacial lake started to develop in the mid-1980s and has continually grown until 2009 when a Glacial Lake Outburst flood (GLOF) partially emptied it. Since 2009, the proglacial lake has continued growing, and it is now twice larger than before the GLOF. Previous dendrogeomorphological studies (Masiokas and others, 2010) were complemented with aerial photos and satellite images to reconstruct the extent of the glaciers from 1840 to 2020 (Figure 1 A). Glacier ice thickness and volume between 1840 and 2000 were obtained using the perfect plastic glacier model (Benn and Hulton, 2010). Then, ice thickness changes were computed by differencing the ice thickness between different periods and complemented with the geodetic mass 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 calibrated using the contour of the glacier and validated using bathymetric and ice thickness measurement. Ice thickness estimations have an uncertainty of 10-20 m. To compute the glacier's mass change, we assume that glacier ice thickness change above the Equilibrium Line Altitude (above 1900-2000 m) is similar to the measured between 2000 and 2019. Glacier mass loss between 1840 and 1981 was minimal (0.1 ± 0.3 m w.e. yr-1), following almost no glacier extent changes. Since the mid-1980s, glacier mass loss has increased rapidly and achieved around 0.5 ± 0.3 m w.e. yr-1 after the 2000s (Figure 1 B). Also, a discernible change in the surface characteristics of the glacier since the mid-1980s is observed, when mass wasting dominated features associated with a compressive ice flux were replaced with transversal crevasses suggesting a change towards an extensional stress field. Minimal glacier mass change during the 20th century is attributed to the delay response to the amelioration of climate after the Little Ice Age due to a thick debris cover at the glacier tongue. However, more recent glacier mass loss increase is attributed to the dynamic response of the glacier to changes in the force balance near the front, which initiates positive feedback between thinning and acceleration, increasing the calving rate, and as a consequence, also increase the proglacial lake surface and volume. In the last decades, the Manso glacier mass change is dominating the glacier mass loss of Monte Tronador and due to its ice volume is expected to be the case in the near future. The reconstructed glacier mass balance of Manso glacier will help calibrate and validate glacier mass balance and ice dynamics models, crucial to elucidate the links between climate, mass balance, and glacier dynamics. A better understanding of the glacier-climate relationships and the ice dynamics can thus help improve our current understanding of these issues but will also prove essential for the development of reliable projections of future glacier changes in the North Patagonian Andes.