Periglacial environments

Introduction to periglacial environments | James Ross Island | Patriot Hills, Ellsworth Mountains | Vestfold Hills, East Antarctica | Terra Nova Bay, Northern Victoria Land | References | Comments |

Introduction  to periglacial environments

Periglacial, Paraglacial and Permafrost

Periglacial environments are those that are in a cold climate, typically near glacierised regions. Permafrost environments are those where the ground is frozen for more than two years in a row[1]. In contrast, paraglacial processes, landforms and landscapes are those that are directly conditioned by former glaciation and deglaciation[2].

Antarctic environments and landscapes are conditioned by both periglacial and paraglacial processes, with landforms related to frozen ground, and with the redistribution of glacigenic sediments by fluvial, coastal and aeolian processes. Rock wall relaxation following the removal of buttressing glaciers is also a common paraglacial process in Antarctica.

The periglacial environment is a cold climate, frequently marginal to the glacial environment, and is characteristically subject to intense cycles of freezing and thawing of superficial sediments. Permafrost commonly occurs within this periglacial environment. However, processes that involve the freezing, unfreezing, and movement of water are considered to be periglacial; processes associated with the presence of perennially frozen ground are permafrost. Permafrost is therefore closely associated with the periglacial environment, and usually permafrost processes take place within a periglacial environment[1].

Periglacial environments in Antarctica

Most of the ice-free ground in Antarctica is underlain by frozen ground[3]. Rock glaciers are common in sub-Antarctic islands and coastal areas[4], in the Transantarctic Mountains and in the McMurdo Dry Valleys[5].

Climate change is likely to result in changes in periglacial areas, such as large hydrological changes, increased methane release to the atmosphere, changes in vegetation composition, and increases in dissolved material to rivers and oceans[3]. Slope instability is likely to increase under a warming climate, and processes involving changing wind regimes, freeze-thaw cycles and related landforms will also be affected.

We will now look at a few case studies of Antarctic periglacial and permafrost environments. Antarctic periglacial environments are very variable, and the processes and landforms active are dependent on the amount of seasonal meltwater available.

James Ross Island

Climate

James Ross Island has a cold, polar-continental climate, with mean annual temperatures of -7°C and summer highs reaching +8°C in January[4]. It has a semi-arid climate, with precipitation (mainly as snow) between 200 and 500 mm per annum (water equivalent)[4, 6].

During the Last Glacial Maximum, James Ross Island was covered by ice, with the Prince Gustav Ice Stream depositing glacial erratics on the north-western parts of the island[7].

Paraglacial landforms

Paraglacial processes are well developed on James Ross Island. Here, mass wasting of rock walls, movement on debris-mantled slopes, wind processes and some limited fluvial (stream) transportation dominate sediment transfers, which are limited to the short summer season. Paraglacial sediments and landforms overprint older glacial landforms, and understanding these is important for unravelling the glacial stratigraphy.

The principle paraglacial landforms include marine terraces and raised beaches, which are well developed here. There is a prominent marine limit at 30 m on Brandy Bay[14]. Coastal processes rework exposed glacial sediments, re-depositing them in spits and modern beaches. The windy climate results in finer grained material being removed from surficial sediments; these aeolian processes result in cobble-boulder armour overlying finer grained sediments across the Ulu Peninsula. Scree slopes are common, and provide input for rock glaciers, protalus ramparts and glaciers. Freeze-thaw weathering, over-steepening and rock-slope relaxation following the removal of buttressing ice masses encourages scree to form. Finally, James Ross Island is characterised by large-scale mass movements, where volcanic rocks resting on sloping soft Cretaceous sediments become detached from their rock walls and slip downslope.

Periglacial landforms

Periglacial landforms are well developed on James Ross Island[8-13], facilitated by relatively warm summer temperatures, which allow melting and surface water. These observations are based on fieldwork to the Ulu Peninsula in January to February 2011.

Rock glaciers are a common feature on James Ross Island. Some of these are located at the end of an ice-cored moraines, in front of stagnating glacier ice[4, 11, 15, 16]. Other rock glaciers form beneath steep cliffs, where active scree slopes result in abundant rock debris being available. This scree, combined with perennial snow banks, also results in the development of protalus ramparts. Protalus ramparts here are curved, relatively flat features. They form by stones rolling down snow banks, and accumulating at their base.

Slope processes are very active on James Ross Island, and there are solifluction lobes on many of the medium- to low-gradient slopes. Some of these have boulders ploughing into the sediment. Alluvial fans and valleys fills are major sediment sinks on the island. These periglacial slope processes are controlled by freeze-thaw activity, rock weathering, frost heave and thaw consolidation. Surficial sediments are saturated as the frozen permafrost table inhibits water draining away and being absorbed into the soil. Frost creep is also important, with the slow downslope gravitational deformation of surficial sediments through freeze-thaw activity[cf. 17].

Finally, freeze-thaw weathering results in frost-shattered boulders, snow hollows, sorted stone polygons and stripes, and surface cracks. The sorted polygons [cf. 18] comprise sand and fine to coarse gravel, surrounded by angular coarse gravel and cobbles.  These polygons are formed by the development of vertical ice wedges in the ground, or through regular needle ice-growth[18-20].

Patriot Hills, Ellsworth Mountains

The Patriot Hills are located in the Ellsworth Mountains (80°S), 50 km inland from the Ronne Ice Shelf grounding line[5]. There are seven deglaciated valleys, and two glacierised (ice-filled) valleys. This is a dry, windy area, with strong southerly katabatic winds. The mean annual temperature is -28°C, and the summer high temperatures are around -15°C.

The periglacial landforms here comprise rock glacier-like landforms, slightly creeping debris slopes, and rock falls[5]. The rock glacier-like landforms has a lobed shape, abrupt fronts and sides, and a relatively flat top. This extends 400 m downslope from the cirque above as a tongue of rock debris. It is arcuate with small ridges on the downslope upper surface. It is connected with ice-cored moraines. Although this is very like a rock glacier, the lack of information available on the internal structure means that it is difficult to interpret it as such[5].

This area is far colder than James Ross Island, and the lack of meltwater available means that periglacial landforms are less well developed.

Vestfold Hills, East Antarctica

Vestfold Hills is a 200 km2 oasis in East Antarctica, and is the third largest ice-free area in Antarctica after the Dry Valleys in southern Victoria Land and the Bunger Hills of Wilkes Land[21].  There is a cool periglacial climate with a mean annual temperature of -10.2°C. Rainfall is very rare, and precipitation is light, making this a semi-arid environment. Snow and ice melts from December to February, resulting in limited surface water being available.

Surficial sediments comprise thin mantles and scattered erratic boulders as isolated patches, ridges of glacial deposits and filled valleys[21]. The lack of surficial sediments here is a result of low volumes of debris in glacier ice, and paraglacial processes that transport material downslope into sediment sinks in the valley floors. Ice-cored moraines have a poor preservation  potential, with meltwater rapidly redistributing sediments via mass movements and debris flows[21].

Terra Nova Bay, Northern Victoria Land

This is an ice-marginal, high-latitude periglacial environment, characterised by cold, arid and windy conditions[22]. In these environments again, freeze-thaw and solifluction processes are limited because of the lack of moisture and the shallow active layer (depth that seasonally thaws and refreezes). The main processes here are mass wasting through rock disintegration and gravitationally-driven mass movements. Wind erosion, however, is significant, eroding the rocks into various shapes with tafoni and honeycomb weathering.

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