The rapid retreat of two lake-terminating outlet glaciers of the Northern Patagonian Ice Sheet and underestimation of mass loss due to subaqueous ice volume

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Abstract
The first-ever field measurements conducted at the outlet glaciers Gualas and Reichert at the Northern Patagonian Ice Sheet, Chile, provide the basis for this work. Both glaciers currently terminate in large (2 km resp. 9 km length) proglacial lakes. The glaciers have retreated rapidly over the past four decades, whereby Reichert glacier retreated by over 100 m per year. Our bathymetry measurements of these lakes make it possible to estimate the mass loss and make assumptions about floatation of these glaciers during retreat. The lakes have depths of up to 250 resp. 350 m and therefore the volume previously occupied by the glaciers is significant. Recent UAV surveys of the final 3 km of the tongue of both glaciers and satellite data provide high-resolution elevation models and are employed to estimate mass loss since the 2000s. With this preliminary study we aim to investigate whether mass loss of these glaciers has been underestimated due to neglected subaqueous ice mass loss. Knowledge from this study will contribute to improving past, present and future mass change estimates of similar glaciers, with relevance for, e.g., sea level rise contribution from ice sheets and their outlet glaciers.