March 30, 2020
San Lorenzo Sur Glacier in Landsat images from 1986 and 2020. Yellow arrow is the 2020 terminus location, red arrow the 1986 terminus location, purple dots indicate the snowline. Point A indicates a location at 1400 m where debris cover has expanded.
San Lorenzo Sur Glacier is the main eastern outlet glacier of the Monte San Lorenzo range on the Chile-Argentina border. This Argentina glacier flows east from the border separating into an eastern (sometimes referred to as Rio Lacteo Glacier) and southern terminus tongue, that in 1986 terminated in two small developing proglacial lakes. From 1985 to 2005/2008 Monte San Lorenzo glaciers lost 18.6% of their area (Falaschi et al 2013). In this region glaciers thinned by ~0.5 m/year from 2000-2012 with most of the thinning on San Lorenzo Sur Glacier occurring on the lower sloped valley section below 1400 m (Falaschi et al 2017). The glacial history of the region has been documented with detailed visual depictions including dated Holocene moraines and trimlines encircling the developing proglacial lakes by Davies et al (2020), see below example. Here we examine the changes in San Lorenzo Sur Glacier from 1986-2020 using Landsat images.
In 1986 the southern terminus ended in a 0.5 km2 proglacial lake, while the eastern terminus ended in a developing proglacial lake of 0.2 km2. The snowline is at 1350 m and widespread debris cover begins below 1300 m, with Point A being above this elevation. By 1999 retreat has led to lake expansion at the southern terminus to 0.8 km2 and 1.1 km2 at the eastern terminus. The snowline is at 1600 m. In 2016 debris cover has expanded to Point A and the snowline is at 1800-1900 m. There are numerous small icebergs in the proglacial lake in front of the eastern terminus. By 2020 the southern terminus has retreated 2.5 km since 1986, and the proglacial lake now has an area of 3.3 km2. The eastern terminus has retreated 2.9 km since 1986 and the lake now has an area of 3.2 km2. Debris cover is now widespread at Point A indicating the ablation zone expansion. The lower 2 km of the southern terminus is a heavily debris covered relatively stagnant zone, poised for retreat. The eastern terminus has a stagnant zone that is 0.5 km wide indicating more limited near term potential for retreat and lake expansion. The snowline is again at 1800-1900 m. The elevation of snowline is not sufficient to maintain the larger valley tongues of the glacier. The expansion in area and elevation of debris cover has been observed on Northern Patagonia Icefeld glaciers as well (Glassser et al, 2016), for example on Exploradores Glacier.
San Lorenzo Sur Glacier in Landsat images from 1999 and 2016. Yellow arrow is the 2020 terminus location, red arrow the 1986 terminus location, purple dots indicate the snowline. Point A indicates a location at 1400 m where debris cover has expanded.
In the map above from Davies et al (2020) GIS app. The Black arrows indicate dated locations on moraines from ~2000-5000 years ago, while the green arrows indicate deglaciation dates from the Little Ice Age period. Yellow arrows on right hand image indicate Holocene moraines.