Box 22 The Patagonian Icefields

There are many glaciers in the Andes south of 46°S in Patagonia. Two major ice masses exist in the region. First is the ^4200 km2 North Patagonian Icefield (47°00'S, 73°39'W), which is some 120 km long and 40-60 km wide, capping the Andes between altitudes of 700-2500 m a.s.l. Annual precipitation on the western side of the icefield increases from 3700 mm at sea level to an estimated maximum of 6700 mm at 700 m a.s.l. Precipitation decreases sharply on the eastern side of the icefield. The North Patagonian Icefield contains the lowest latitude marine-terminating glacier on Earth, the San Rafael Glacier, which descends through rainforest to calve icebergs into a coastal Laguna as illustrated below. Second is the larger (^13000 km2) South Patagonian Icefield, which stretches north to south for 360 km between 48°50' and 51°30'S with a mean width of ^40 km. The southern icefield again has very strong west to east precipitation contrasts, as well as some very large tidewater and lake-calving glaciers (Aniya, 1999). There are also many smaller satellite glaciers on the mountains surrounding the two icefields. There is strong evidence that the glaciers of the two icefields are very sensitive to recent climate change. As a result, they have been receding and thinning (Rivera et al, 2007). The glaciers of the two icefields have therefore contributed to recent sea-level rise. Rignot et al. (2003) estimated that the two icefields jointly contributed 0.042 ± 0.002 mm yr-1 to global sea level in the period 1968/1975 to 2000 but that this has doubled to 0.105 ± 0.011 mm yr-1 in the more recent years 1995-2000.

Sources: Aniya, M. (1999) Recent glacier variations of the Hielos Patagonicos, South America, and their contribution to sea-level change. Arctic, Antarctic and Alpine Research, 31, 165-73. Rignot, E., Rivera, A. and Casassa, G. (2003) Contribution of the Patagonia Icefields of South America to sea level rise. Science, 302, 434-7.

Sources: Aniya, M. (1999) Recent glacier variations of the Hielos Patagonicos, South America, and their contribution to sea-level change. Arctic, Antarctic and Alpine Research, 31, 165-73. Rignot, E., Rivera, A. and Casassa, G. (2003) Contribution of the Patagonia Icefields of South America to sea level rise. Science, 302, 434-7.

Rivera, A., Benham, T., Casassa, G. et al. (2007) Ice elevation and areal changes of glaciers from the Northern Patagonia Icefield, Chile. Global and Planetary Change, 59, 126-37. [Photograph: N.F. Glasser]

Figure 2.6 ASTER satellite image of Glaciers Colonia and Arenales on the eastern side of the North Patagonian Icefield. Note the increase in surface debris cover down-glacier as a result of surface ablation. [Image courtesy of: Krister Jansson]

1. In both cases the glaciers are fed by large amounts of precipitation that falls as snow at higher elevations from southwesterly weather systems, with very strong precipitation gradients between the west (maritime) and the east (continental).

2. The glaciers have steep mass balance gradients (see Section 3.2), with extremely high rates of snow accumulation and high ablation rates.

3. Both regions are dominated by temperate ('warm') glaciers, where the ice at the bed is at the pressure-melting point throughout.

4. Outlet glaciers terminate in a wide variety of environments, with examples of land-terminating glaciers (both Patagonia and New Zealand), lake-calving glaciers (both Patagonia and New Zealand) and marine-terminating glaciers (Patagonia).

5. Glaciers in both regions have been receding and thinning dramatically in recent years. The volume of ice in the Southern Alps of New Zealand has reduced by about 5.8 km3, or almost 11%, in the past 30 years. More than 90% of this loss is from 12 of the largest glaciers and is inferred to be a direct response to rising temperatures over the twentieth century.

6. Most of the mass loss has been in the terminal sections of the glaciers, where rates of down-wasting are measured in metres per year. Glaciers to the east of the Main Divide in New Zealand and to the east of the Andes in Patagonia are bordered by 100 m high lateral moraines formed as the glaciers down-waste.

7. As the glaciers down-waste, large proglacial melt lakes form around their termini, for example the Tasman and Hooker Lakes in New Zealand. Large areas of the glaciers may begin to float where they terminate in these lakes, promoting rapid subaqueous melting and further mass loss due to iceberg calving.

Figure 2.7 The snout of the Tasman Glacier in New Zealand, showing the extent of recent (twentieth and twenty-first centuries) down-wasting. Note the debris-covered snout and large lateral moraines, which impound the proglacial lake. [Photograph: N.F. Glasser]

There are also similarities in the typical subglacial processes operating beneath these glaciers and the landforms and sediments that they produce.

1. The presence of basal meltwater means that glaciers in both areas move by glacier sliding, subsole deformation or a combination of both (see Section 3.3). As a result, they

2.4 Northern Hemisphere Temperate Glaciers: Alaska and Iceland 21

carry little debris at the bed. Basal debris-rich layers are thin, of the order of centimetres to tens of centimetres, because of the substantial volumes of meltwater flowing there. These glaciers are also fast-flowing and slide over their beds, and are therefore normally regarded as efficient agents of glacial erosion.

2. The glacier surfaces are commonly debris-mantled, with extensive areas of supraglacial debris derived from rockfall activity on surrounding valley walls and slopes. This supraglacial debris is either organised into medial and lateral moraines, both of which are important features of high-level sediment transport, or forms a continuous cover on the ice surface. Outburst floods (jokulhlaups) from englacial or subglacial lake drainage can also reach the glacier surface to contribute to the supraglacial debris load.

3. Glacial landforms and sediments are varied, reflecting the complexity of the sediment transport and depositional processes operating. Supraglacial debris supply and sedimentation are important, particularly where the debris becomes concentrated on the ice surface during down-wasting. A recent study of New Zealand glaciers highlights the importance of glaciofluvial sediment transfer, suggesting that the dominant sediments around the glaciers are glaciofluvial, transported by supraglacial, subglacial and proglacial streams. Reworking, particularly by proglacial streams, dominates the final depositional products. Consequently many of the 'glacial' sediments are actually glaciofluvial and therefore lack a distinct glacial imprint.

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