Patagonian Ice Sheet thinning during a changing climate

J.Boex, C. Fogwill, S. Harrison, N.F. Glasser, A. Hein, C. Schnabel and S. Xu.  Rapid thinning of the Late Pleistocene Patagonian Ice Sheet followed migration of the Southern Westerlies. Scientific Reports 3: 2118, p. 1-6

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The Patagonian Ice Sheet

Patagonian mountains east of the North Patagonian Icefield. Credit: Stephen Harrison

Patagonian mountains east of the North Patagonian Icefield. Credit: Stephan Harrison

This recent open-access paper in the new journal Science Communications, which is part of the Nature group, has demonstrated that the during the deglacial period (~19,000 years ago), the Patagonian Ice Sheet in South America responded rapidly in response to changing precipitation patterns and warming during the last deglaciation. The fact that the Patagonian Ice Sheet responded so quickly to changes in precipitation and temperature has vivid implications for the current, and future, behaviour of the current North Patagonian Icefield  and South Patagonian Icefield. We already know that the shrinkage of the North and South Patagonian ice fields was faster over the last decade or so than at any point in the last couple of centuries. Understanding on a broader scale how these sensitive, high-latitude ice masses are dependent on small changes in atmospheric circulation means that we will better be able to predict the future behaviour of these ice sheets. Reconstructing rates of ice-sheet decay since the Last Glacial Maximum means that we can better assess the mechanisms of climate change (including changing wind patterns) during a major climate transition.

Map of the glaciers of the Patagonian Andes. Note the precipitation gradient west and east across the mountain range.

Map of the glaciers of the Patagonian Andes. Note the precipitation gradient west and east across the mountain range. From: Davies and Glasser, 2012

The North Patagonian Icefield, in the remote and high Andes of Chile and Argentina, is the highest-latitude (i.e. closest to the equator) ice sheet in the world. The North and South Patagonian icefields exist only because of the high volumes of snowfall that they receive. Snow from predominant Westerlies is dumped on the high Andes mountains, which are a topographic barrier. East of the Andes is a large semi-arid desert, and glaciers on the west of the Andes are far larger than glaciers on the east. These icefields are therefore currently very sensitive to changes in atmospheric wind patterns. Many thousands of people currently live in this glacierised region, and their lives are dominated by the large icefields, which provide water for irrigation, and where glacier lakes, formed by retreating glaciers behind large moraines, pose a significant hazard. Increased recession of the North and South Patagonian Icefields could increase the risk of catastrophic glacial lake outburst floods and raise global sea levels, as well as being a sensitive indicator of climate change.

The Last Glacial Maximum

During the Last Glacial Maximum, circa 20,000 years ago, the Andes were covered in the huge Patagonian Ice Sheet (formed from the conjoined North and South Patagonian icefields and their smaller satellites). Over the last 80 years, researchers mapped and dated the limits of the ice sheet.  The ice sheet was drained by fast-flowing outlet glaciers, which drew down the central ice-mass with outlet lobes with low surface gradients. This was a dynamic, temperate, low-angled ice sheet, drained by large ice streams both west and east of the Andes. It was, on average, 1130 m thick.

A. Patagonian Ice Sheet, today (dark grey) and during the Last Glacial Maximum (dark grey). B: Modelled ice thicknesses during the LGM. From: Boex et al., 2013

A. Patagonian Ice Sheet, today (dark grey) and during the Last Glacial Maximum (dark grey). B: Modelled ice thicknesses during the LGM. From: Boex et al., 2013

Fieldwork

In order to constrain rates of thinning, the authors of this study scaled high mountains to create dated transects, capturing the deglaciation. Cosmogenic nuclide ages on erratic boulders provided these age constraints.  Professor Neil Glasser said,

“The great thing about this study is that we were able to combine geomorphological mapping of moraines, trimlines and glacial surfaces with cosmogenic isotope dating. This new technique is very powerful because we can use the geomorphological mapping to understand patterns of erosion and deposition beneath the former ice sheet. The geomorphology also tells us about former glaciological conditions such as ice movement direction. Then we can use the cosmogenic isotope dating to gain some insight into the ages of these events”.

The fieldwork was, high, remote, and set in the beautiful Patagonian landscape. Stephan Harrison said,

Jake Boex sampling glacially-transported granite boulders at 1500 m up on Cerro Tamango. These boulders were deposited here around 16,000 years ago. Credit: Stephen Harrison

Jake Boex sampling glacially-transported granite boulders at 1500 m up on Cerro Tamango. These boulders were deposited here around 16,000 years ago. Credit: Stephan Harrison

“We deliberately targeted the mountains east of the North Patagonian Icefield because we knew there were moraines and glacial erratic boulders at high elevations there. These boulders could only have been transported eastward by an expanded North Patagonian Icefield.  So we knew that if we could date the age of the erratic boulders at successively lower elevations, we could estimate the thinning of the ice sheet. Fieldwork in Patagonia is always challenging given that it is remote and the weather is so changeable. It is one of the windiest places on Earth. Conducting the mapping and looking for erratic boulders at high elevations next to the North Patagonian Icefield allowed us to get to places that few people have ever visited”.

The results

Jake Boex sampling a glacially transported granite boulder

Jake Boex sampling a glacially transported granite boulder for cosmogenic nuclide analysis. Credit: Stephan Harrison.

This combination of cosmogenic nuclide dating from boulders on mountain tops and careful mapping showed that the Patagonian Ice Sheet began to recede at around 29,000 years ago, but remained close to its current configuration until ~19,000 years ago. Between 19,000 and 18,000 years ago there was stepped thinning, but after 18,000 years ago the rate of thinning rapidly increased. This was a decaying ice sheet, melting much faster than it received accumulation from snow fall. By 15,600 years ago, the ice sheet was around 10 to 15 km from its present extent. Fast-flowing outlet glaciers played a key role in drawing down ice, thinning the ice sheet and enhancing melting at lower altitudes. There is no evidence of a readvance during the Antarctic Cold Reversal at 13,000 years ago in this region (which is often seen in Antarctic records), and no evidence of a readvance during the Younger Dryas (11,000 years ago), as seen in Scotland and much of Europe (although moraines ascribed to the Younger Dryas were found in other valleys nearby).

This rapid, large scale thinning indicates that the Patagonian Ice Sheet was very sensitive to a changing climate. This thinning coincides with changes in the Southern Hemisphere oceanic and atmospheric systems. This thinning occurs during a period when the ocean thermohaline circulation changed, causing warming in the mid-latitudes. The Southern Hemisphere Westerlies, those winds upon which the Patagonian Ice Sheet was (and the icefields still are) so dependent, were shifted southwards, reducing the amount of precipitation falling on the ice sheet.

Digital elevation model of the study area, showing the transect, and reconstructed ice surfaces at different times. From Boex et al., 2013

Digital elevation model of the study area, showing the transect, and reconstructed ice surfaces at different times. From Boex et al., 2013

Conclusions

This study shows, in agreement with recent work focused on current recession, that the Patagonian Ice Sheet was very sensitive to climate change during the Pleistocene. This climate change included regional warming and shifts in the latitude of Southerm Hemisphere Westerlies, which brought the ice sheet much of its precipitation. These westerlies controlled the extent and magnitude of glaciation at these latitudes. The westerly airflow that sustains the current North and South Patagonian ice sheets is predicted to continue to move southward, towards the pole as a result of the current warming climate – and this has significant implications for the viability and longevity of these ice sheets.

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