What was the former Patagonian Ice Sheet?
The Patagonian Ice Sheet was a large, elongated mountain ice mass that developed over the Andes mountains of southern South America during cold periods[1]. The Patagonian Ice Sheet has advanced and retreated at least 5 times in the last million years[2] in response to changes in global climate (i.e. cooling and warming).
What evidence is there for this former mountain ice sheet?
The evidence for past glaciations of the Patagonian Ice Sheet is preserved in the landscape in the form of landforms (such as moraines) and sediments (such as fine-grained lake sediments and coarse, poorly sorted glacial sediments). The great quantity and variety of glacial landforms in Patagonia[3] make the record of Patagonian Ice Sheet activity one of the longest and most complete anywhere in the world[4].
Last Glacial Maximum (21,000 years ago)
During the global Last Glacial Maximum (LGM) around 21,000 years ago, the Patagonian Ice Sheet almost completely submerged the Patagonian Andes between around 38 to 56°S[1]. In length, the distance between the most northern and southern tips of the ice sheet was around 2000 km (see map above).
How big was the Patagonian Ice Sheet?
Computer simulations estimate that, when at its largest, the Patagonian Ice Sheet had a volume of ~525,000 km3[5]. However, as climate warmed after the LGM, the Patagonian Ice Sheet rapidly thinned and retreated[6,7,8,9]. In all, the ice sheet shrank by ~500,000 km3 in ~8-9000 years, and contributed ~1.2 m to global sea level[5].
What is left of the Patagonian Ice Sheet?
Today, small remnants of the Patagonian Ice Sheet exist in the form of three main mountain icefields, these are: the Northern Patagonian Icefield (NPI; shown in the GIF below), Southern Patagonian Icefield (SPI), and the Cordillera Darwin Icefield (CDI). These ice masses are currently rapidly retreating under the influence of global warming.
The structure of the former Patagonian Ice Sheet
The Patagonian Ice Sheet was divided into two main parts: a western part and an eastern part that spread out from a central ice-divide along the Andean mountains (see map at top of page). The Patagonian Ice Sheet was drained by at least 66 major outlet glaciers[1]. These outlet glaciers transported ice from the interior parts of the ice sheet to the margins and, in doing so, they controlled the overall form of the ice sheet[1].
But rather than simply flowing east or west from the main ice-divide, these outlet glaciers were strongly influenced by topography (see the arrows showing former ice flow directions in the diagram below), being funnelled through a complicated network of bedrock valleys[10,11].
Pacific Ocean fjords
On the west side of the ice sheet, most outlet glaciers flowed into Pacific Ocean fjords (see the present-day example in the satellite image below). We cannot currently be sure how far these outlet glaciers advanced, because the seafloor has not yet been explored for glacial landforms.
However, computer simulations suggest that most outlet glaciers would have reached the continental shelf edge at the LGM[5]. These simulations also show that, on the west side of the ice sheet, glaciers were fast-flowing, with ice velocities of up to 400 m per year due to plentiful snowfall over the Andean mountains.
Piedmont lobes
In the northern parts of the former Patagonian Ice Sheet, such as the Chilean Lake District (see map below), west-flowing glaciers did not extend to the Pacific Ocean, but instead formed large piedmont lobes that remained on land[8,12].
The geomorphological map below shows moraines (red lines) that delimit the maximum extent of former outlet glaciers in the Chilean Lake District. Note the piedmont lobe glaciers that spilled out on to flat coastal plains, with their source areas high in the mountains.
Below is an example of a present-day piedmont lobe glacier in Alaska.
Moraines and glacial lakes
On the eastern side of the ice sheet, outlet glaciers flowed along large valleys that emerged on the flat Argentinian plains. At the LGM, the largest outlet glaciers advanced more than 150 km east of the modern icefield limits[13]. When they moved on to the flat plains they stabilised, and constructed arcuate terminal moraines[14,15,16,17,18], such as those shown in the photograph below.
During periods of Patagonian Ice Sheet retreat (such as after the LGM) many valleys were flooded with glacial lakes (see the shorelines photographed below, which provide evidence for these former lakes) as meltwater was trapped between terminal moraines and the receding glacier margins[15,19,20,21]. These lakes, which in some valleys were more than 500 m deep[21], had an important role on ice dynamics, likely increasing the rate of glacier retreat through the calving of icebergs[22].
Why is it important to study the Patagonian Ice Sheet?
Ice sheets are sensitive to changes in the temperature and circulation patterns of the atmosphere and oceans. This means that, firstly, the reconstruction and dating of former ice sheet activity can be used to better understand ice sheet-climate interactions[23]. Such information may be critical in understanding how modern ice sheets will respond to continued global warming[24].
Secondly, in the Southern Hemisphere, which is dominated by oceans, Patagonia is one of only a few areas of land from which scientists can develop records of past environmental change. Such records, which include records of long-term glacial change, allow us to more fully understand how the Southern Hemisphere climate system works, and how it may interact with climate changes happening at the global scale[25].