PATICE

PATICE: The Patagonian Ice Sheet from 35,000 years ago to the present day

This page provides a brand-new reconstruction of the Patagonian Ice Sheet from 35,000 years ago to the present day (called PATICE).

PATICE is a new compilation of published ages and geomorphology, ranked and assessed and recalibrated, which we use to generate new empirical reconstructions of the ice sheet and its ice-dammed palaeolakes.

Citation: Davies et al., 2020. The Evolution of the Patagonian Ice Sheet from 35 ka to the Present Day (PATICE). Earth-Science Reviews. 

You can explore the PATICE data in the ArcGIS online interactive online map (no ArcGIS licence needed).

Resources

All the data used in the new PATICE reconstruction has been made available fully open-access, and the paper is available as gold open access.

• Download an A0 poster about the Patagonian Ice Sheet at the Last Glacial Maximum
• Interactive online map (ArcGIS Online)
• Download the published paper (Davies et al., 2020, Earth Science Reviews)
• Download the published data (Mendeley Data)

Below, there is more information about our map and database.

PATICE reconstruction

The Patagonian Ice Sheet formed during the last glaciation along the Andean mountain chain. It blocked the drainage of rivers to the Pacific, so large lakes formed in front of the ice margin as it receded. You can watch the ice sheet separate out into its different parts and the lakes draining and changing through time in the GIF below.

The Patagonian Icefields

Patagonia, in southernmost South America, is a region with accelerating glacier recession^1,2. Glaciers here are shrinking rapidly, which is enlarging glacial lakes, increasing flood risk³ and causing sea level rise⁴.

The Southern Andes region lost 1,208 billion tonnes of glacier ice from 1961 to 2016⁵, contributing 0.92 ± 039 mm per year to global sea level rise (27 mm from 1961 to 2016).

Today, there are four main icefields. The Northern Patagonian Icefield (46.4°S to 47.5°S), the Southern Patagonian Icefield (48.3°S to 52°S), the Gran Campo Nevado (52.8°S) and the Cordilleran Darwin Icefield (54.5°S)⁶.

The region also has numerous small glaciers and icefields, often centred on volcanoes. The lowest latitude Southern Hemisphere glacier that reaches the ocean is found in the Northern Patagonian Icefield (Glaciar San Rafael).

In total, the present-day icefields and glaciers cover a total area of 22,718 km², equating to a sea level equivalent of 15.1 mm⁷. These glaciers are all rapidly receding, as you can see in the GIF below.

Reconstructing past glacier change in Patagonia

We can use the past record of ice sheet behavior to understand how these glaciers interact with climate, and to better predict how they might behave in the future.

Patagonia has a rich record of glacial geomorphology that can help us to understand how glaciers might behave in the future⁸. In this study, we created a new GIS database of published glacial geomorphology and published ages to reconstruct the extent of the ice sheet from 35,000 years ago to the present day, in 5000-year time slices.

We also reconstructed the evolution of ice-dammed lakes. As the glacier ice shrank back, large and deep lakes formed in front of the ice margin. These lakes existed until the glacier ice shrank back past a col or spillway, allowing the lake to drain. These lakes may have been a key influence on the glaciers, as deep water would have encouraged the calving of icebergs.

The past behavior of the Patagonian Ice Sheet during different climate states and during rapid climate transitions could shed insights into ways in which the region is sensitive to changes and how it could respond to future change. The southern mid-latitudes are a particularly data-sparse region of the globe, and reconstructions of the Patagonian Ice Sheet provides a unique insight into the past terrestrial glacial and climatic change.

Last Glacial Maximum

The Patagonian Ice Sheet was centred on the central mountain chain of the Andes, and stretched from 38° to 56°S. During glacial maxima, the icefields coalesced to form a single large ice mass^9,10.

Its eastern margin comprised fast-flowing lobes of ice that extended out into the Argentinian steppe landscape, the largest fastest of which were ice streams. The western margin reached the continental shelf in the Pacific Ocean.

The Local Last Glacial Maximum occurred at 33,000 to 28,000 years ago from 38°S to 48°S, and earlier, at around 47,000 years ago from 48°S southwards.

At its maximum extent, the Patagonian Ice Sheet covered 492,600 km², with a sea level equivalent of 1,496 mm (the sea level amount locked up in the ice sheet). It was 350 km long and 2090 km wide.

It was comparable in size to the Antarctic Peninsula Ice Sheet today. For comparison, Sweden is 450,295 km² and the UK is 242,495 km².

After the Last Glacial Maximum

The Patagonian Ice Sheet began to retreat and shrink by 25,000 years ago. The ice sheet stabilized and formed large moraines during the period 21,000 to 18,000 years ago, which was then followed by rapid deglaciation, especially between 18,000 and 15,000 years.

By 15,000 years ago, the Patagonian Ice Sheet had separated into several disparate ice masses, draining into large ice-dammed lakes along its eastern margin. These lakes probably encouraged the calving of icebergs, and facilitated rapid melting.

Glacial readvances or stabilisations occurred at least at 14,000 to 13,000 years ago, 11,000 years ago, 6000 to 5000 years ago, 2000 to 1000 years ago, and 500 to 200 years ago.

Glacier lakes

On the eastern side of the Andes, the moraines from these glacier outlet lobes often mark the present-day continental watershed drainage divide. Today, many of the glacial lakes of Patagonia drain westwards, into the Pacific Ocean. The Patagonian Ice Sheet dammed this drainage route, forcing higher lake levels and drainage to the Atlantic Ocean.

During glacier recession, a series of large proglacial lakes formed along the eastern margin, dammed between the ice sheet and higher ground or moraines^11–15. As the ice sheet receded, continental scale drainage reversals occurred^14,16,17.

In the GIF above, you can see the glacial lakes forming as the ice sheet retreats back towards the high ground of the Andes. By about 10,000 years ago, most lakes have either drained completely or dropped to their current levels.

This figure shows how the ice dammed glacial lakes evolved as the glacier ice receded. As the glaciers shrank, new cols and spillways became available, resulting in the lakes dropping to a new level.

Rates of recession

The Patagonian Ice Sheet was relatively stable from 35,000 to 30,000 years ago. Recession from the Local Last Glacial Maximum began by 25,000 years ago, predating the global Last Glacial Maximum. This may be because the Patagonian Ice Sheet was smaller and more dynamic than the larger global ice sheets.

Very rapid recession and widespread deglaciation began after 18,000 years ago, during a period of rapid warming highlighted in the Antarctic ice cores¹⁸ and rapid global sea level rise. It may also have been driven by a southwards shift in the Southern Westerly Winds.

The Patagonian Ice Sheet probably contributed ~615 mm to global sea level rise between 20,000 and 15,000 years ago, when it shrank from 359,600 to 121,800 km².

Glaciers stabilized or re-advanced during the Antarctic Cold Reversal, but the Patagonian Ice Sheet was much smaller at this time (116,700 km²).

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Accelerating rates of glacier recession

Rates of recession were slow through the Holocene until the last few decades (with the caveat that some time periods are highly uncertain). Absolute recession rates (km² per year; km² a^-1) over recent decades rival those seem between 20,000 and 15,000 years ago for an ice sheet that was more than two orders of magnitude larger.

Relative rates of recession (percentage change per year; % a^-1) are higher between 200 years ago and 2011 AD than at any time observed in our reconstruction. It is likely that there were periods of time with especially rapid recession during the last glacial-interglacial transition, when many outlet lobes were calving into large, ice-dammed lakes, but our compilation is unable to capture this.

There are fewer degrees of freedom for ice extent and volume changes during the Holocene (last 10,000 years). Ice margins stabilized not far from present-day positions by the Early Holocene, and dated moraines suggest that readvances were similar to the advance at 200 years ago in size.

Thus we can argue that average rates of ice-marginal recession are currently faster than at any time observed in the Holocene, in line with the recent temperature changes observed in Antarctica and Patagonia, following a sustained period of relative stability, and when glacial lake area remains fairly constant.

Since observations indicate that rates of recession have accelerated in Patagonia over recent decades, from 34.2 km² a^-1 (0.14 % a^-1) for 1986 to 2001 AD to 51.2 km² a^-1 (0.22 % a^-1) for 2001-2011 AD⁶, this is especially concerning.

Ice-dammed palaeolakes

Glacial lake area peaked at 13,000 years ago, with an estimated area of 13,999 km². These large lakes would have accelerated glacial recession at this time by encouraging the outlet glacial lobes to calve icebergs.

Glacial lake area minimum was reached by about 10,000 years ago. Rapid recession from 13,000 to 10,000 years ago led to many cols and spillways opening, and the lakes reached their current spatial extent and volume by about 10,000 years ago.

As the ice dams receded, this cold, fresh water may have been released suddenly into the Pacific Ocean, possibly affecting regional climate^14,16.

The remaining lake water today, from lakes that were in the footprint of the palaeolakes, is 6,824 km². Overall, between the maximum lake extent at 13,000 years ago and today, there has been a reduction in lake area of 7,176 km².

How was the reconstruction made?

There is a large volume of published ages and glacial geomorphology that help us to reconstruct the past ice sheet extent and dynamics in Patagonia. The geomorphological data provide information on former ice sheet margins, ice-dammed palaeolake evolution, and ice-flow direction. Our new GIS database includes 58,823 landforms and 1,669 published ages.

Glacial Geomorphology

Landforms were mapped from satellite images and digital elevation models, most commonly LANDSAT 7 and LANDSAT 8 images, as well as high resolution satellite imagery available in Google Earth.

Our compilation includes moraines, trimlines, glacial lineations (bedrock and sedimentary drumlins or flutes), meltwater palaeochannels, outwash plains, shorelines, deltas and cirques. Our compiled maps also show related topographic landforms such as rivers, lakes and volcanoes.

In total, we mapped 58,823 landforms, including 25,009 moraines, 2,507 shorelines, 3,926 lineations, 4,309 empty cirques and 4,536 palaeochannels.

Moraines and trimlines give us information about former ice sheet margins, whilst lineations and the pattern of moraines tells us about ice-flow direction. Cirques tell us about regions that were previously glaciated, but now are ice-free. Palaeochannels, outwash plains, shorelines and deltas give us information on ice-dammed palaeolakes.

There is more information on Patagonian Glacial Geomorphology here.  In the view below, you can use Google Earth to explore Patagonian moraines around the North Patagonian Icefield. The arcuate ridges denote the position of the former ice margin.

In general, there are four distinct temperate glacial landsystems in Patagonia.

1. An upland glacier landsystem, with an assemblage of cirques, lateral and terminal moraines, mountain glaciers and snow patches, flutes, and lakes;
2. In the lowlands, a land-terminating glacial landsystem, with moraine arcs, outwash plains, meltwater channels, drumlins, and hummocky moraine;
3. A lowlands glaciolacustrine landsystem, with deltas and shorelines, and ice-contact glaciofluvial landforms;
4. An offshore glaciomarine landsystem, with fjords, offshore moraine ridges, drumlins, raised fluvial deltas and slope failures, and turbidity current channels.

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Chronology

There are many ways to fix in time particular glacial landforms. Each technique dates something slightly differently, which makes them hard to compare directly. Our GIS database includes 1,669 ages relevant to the timing of deglaciation. Each age is scrupulously checked and recalibrated according to the latest protocols.

Our database includes cosmogenic nuclide exposure-age dating of boulders, ideally situated on moraines to give a time of formation for that moraine, radiocarbon dating of organic material, tephrochronology (dating of volcanic ash layers), lichenometry (measuring the size of specific species of lichens to derive an exposure age), dendrochronology (tree-ring dating), historical sources (archival maps and photographs), varve ages (annually laminated lake sediments), and optically stimulated luminescence.

Each age is assessed according to our protocols and given a reliability assessment. The most reliable cosmogenic nuclide ages are used to give an average age for moraine formation.

Reconstructing the ice extent

We used the moraines, dated by various methods, to reconstruct the ice margin. At places where we were confident of the age of the moraines, we could draw short, isolated isochrones.

Secondly, we interpolated between the isochrones to reconstruct overall ice-sheet limits, using moraines and topography.

Thirdly, we provided an assessment of our degree of confidence in each ice margin, from high confidence to medium confidence and low confidence. High confidence ice limits have both well defined glacial geomorphology and a well constrained chronology.

Medium confidence ice limits are defined by geomorphology and are near to published ages, but are less well constrained.

Low confidence limits have no well-defined geomorphology, lie far from published ages, and are first interpretations that require further investigation.

The PATICE Team

Authors are in alphabetical order.

Bethan J. Davies, Christopher M. Darvill, Harold Lovell, Jacob M. Bendle, Julian A. Dowdeswell, Derek Fabel, Juan-Luis García, Alessa Geiger, Neil F. Glasser, Delia M. Gheorghiu, Stephan Harrison, Andrew S. Hein, Michael R. Kaplan, Julian R.V. Martin, Monika Mendelova, Adrian Palmer, Mauri Pelto, Ángel Rodés, Esteban A. Sagredo, Rachel Smedley, John L. Smellie, Varyl R. Thorndycraft.