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Landscape evolution in Antarctica
345
Earth-Science Reviews, 25 (1988) 345-353 Elsevier Science Publishers B.V., Amsterdam--Printed in The Netherlands
Landscape Evolution in Antarctica I.B. CAMPBELL and G.G.C. CLARIDGE N.Z. Soil Bureau, Department of Scientific and Industrial Research, Lower Hutt (New Zealand)
ABSTRACT Campbell, I.B. and Claridge, G.G.C., 1988. Landscape evolution in Antarctica. In: J. Firman (Editor), Landscapes of the Southern Hemisphere. Earth-Sci. Rev., 25: 345-353.
The extreme aridity of Antarctica means that very little water is available for weathering and geomorphic processes. The landscape everywhere has been shaped by ice, but land exposed after ice retreat has been modified by cold desert weathering processes. Wind is the main agent for removal and redeposition of weathered material, while fluvial action is insignificant as an eroding or transporting agent. There are few suitable deposits that allow reliable or accurate dating of events but the dates that are available show that landform evolution has been taking place very slowly. Soil weathering studies have provided a useful means of extrapolation showing relative ages of surfaces where no dates are available. The major weathering processes in Antarctica involve patterned ground movement, frost action, wind, salt weathering, and soil weathering. These combine to form distinctive landscape features. The complex nature of landform development is illustrated by considering events that have taken place within Wright Valley, one of the largest ice-free areas in Antarctica. Within this valley are land surfaces ranging from those found on Late Pleistocene up-valley tills, to high altitude extremely fretted and weathered land surfaces that have been ice-free possibly since the Early Miocene.
1 INTRODUCTION Antarctica has an area of about 14 million and contains about 90% of the world's ice. An understanding of the behaviour of the ice sheets, especially through the Pleistocene is crucial to predicting responses to future world climatic change. The study of Antarctic landscapes can provide information about past ice sheet behavior. The 2% of Antarctica that is ice-free (Fig. 1) comprises small scattered areas of exposed land about the edges of the continent and along the Transantarctic Mountains, which extend across the continent dividing it into two units, East Antarctica and West Antarctica. Most of the exposures of bare ground in Antarctica consist of bare rock, sometimes
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covered with accumulations of fretted rock particles or as loose unconsolidated tills. Unlike most parts of the world, the land surfaces have developed with an almost complete absence of vegetation, so there is no interaction between climate, vegetation, and landform development. In all but a few areas, the climate is too harsh to permit the existence of higher plants, b u t in favoured situations lichens and mosses are found, along with algae and other primitive forms of life.. Landforms in Antarctica have acquired their present form either through direct glacial sculpture or through subsequent modification b y arid land weathering processes. N o surfaces are k n o w n which on weathering criteria could be considered to pre-date the current glacial cycle.
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Fig. 1. Location map. The main ice-free areas are shown in black, although the actual ice-freeareas are much less than indicated. East Antarctica is on the fight of the Transantarctic Mountains and West Antarctica on the left.
The climate of Antarctica is unique. The very low temperatures and precipitation, together with strong winds, combine to produce an exceedingly a r i d environment in which the very restricted water availability limits physical, chemical, and biological processes. As will be shown later, some of the processes involved in landform development are also unique. 2 ENVIRONMENTAL FEATURES
2.1 Climate of Antarctica Climate is t h e most important factor in landscape evolution in Antarctica and its key attributes are temperature, precipitation and wind. Temperature. M e a n a n n u a l temperatures are everywhere below freezing point, rar~gingfrom - 1 0 ° C in coastal regions to - 5 5 " C in the centre (Weyant, 1966). However, temperatures may rise above freezing point for short periods in coastal areas or sheltered situations witMn the mountains. I n spi~e of the i n ~ s e cold, ground teml~xatures can b~mome relatively high for short periods during summer,
sometimes rising to 3 0 ° C at the surface of dark coloured rock or soils in sheltered situations. The soil can also remain unfrozen to a depth of 20 cm or more for short periods. A~ greater depths, it always remains below freezing and if there is enough moisture, ice-cemented permafrost is formed. Precipitation. Precipitation over the whole continent averages only 15 g cm -2 y-~ (Bull, 1971). As a result of the precipitation falling as snow, the atmosphere has very low humidity, while in the soils, water is present mainly as ice, or for short periods in summer, as liquid in small amounts (Campbell and Claridge, 1982). Water is therefore largely unavailable for weathering and erosional processes. Because of this, Antarctica is the world's coldest and most arid continent. Wind. Because of the presence of a large mass of ice at high altitude in the centre of the continent, the wind pattern of Antarctica is dominated by strong down-slope or katabatic winds. These winds have a direct effect on landscapes because of their erosive capacity; in particular through the creation of lag gravel or desert pavement by the removal of fine material, by the sculpturing and faceting of rocks through abrasion and by the movement of sand or gravel (McCraw, 1967).
2.2 Rock type Geology of Antarctica. Antarctic basement rocks (Anderson, 1965) are dominantly a folded complex of schist, gneiss, marble and sedimentary Precambrian to Early Palaeozoic rocks that were extensively intruded by granite about 500 m.y. ago. The basement complex was peneplaned between the Late Ordovician and the Devonian and glaciated in the Permian. A fiat-lying continental sequence of sandstones, siltstones, coalmeasures and tillites were deposited on the peneplaned surface during the Upper Palaeozoic. In J u r ~ times, basic igneous rocks were intruded within the basement rocks and the overlying sediments forming horizontal sills
347
up to 450 m thick with a very uniform composition. Since the Jurassic, the Transantarctic Mountains have been uplifted with only minor tilting, giving the originally flat-lying sediments a slight westward dip. Along the eastern scarp of the uplift, volcanic rocks, dating from about 10-15 million years to the present day, have been deposited. As their ages coincide with the present glaciation, they are important for dating weathering events and landscape development. Influence of rock type on landscapes. The younger volcanic rocks are important because of the glaciological and geomorphic interpretations which they allow, but many of the older rocks are also important because of their direct influence on landscape development. Most noteworthy is the dolerite which forms extensive cliffs or escarpments of distinctively columnar-jointed rock. Where it occurs as sills within the sediments, impressive stepped landscapes are developed. The harder basement rocks have eroded to produce massive forms, while softer argillites or sandstones give rise to landscapes with a strongly fluted appearance. The younger volcanic deposits (up to < 10 m.y.) for the most part show negligible signs of landscape modification which clearly suggest that geomorphic development in Antarctica is a very slow process. 3 AGE RELATIONSHIPS AND SURFACES
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3.1 Radiometric dating One of the most useful methods for determining the age of some surfaces in Antarctica has been through K / A r dating of various deposits, particularly small basaltic cones in the Taylor and Wright valleys. These volcanic deposits data from between about 2 and 4 m.y. ago and the position of the cones within the valleys show that they erupted after the major topographical features had formed. Debris from these eruptions lies on
some till-covered surfaces indicating that the surface is older than the eruption, and some debris has been included with till deposits, showing that they post-date the erupted material. Some young deposits in coastal areas with organic materials have been dated using 14C but this method is also of restricted value partly because deposits with datable materials are very limited in extent, but also because Antarctic ocean waters are depleted in 14C and dates obtained are much older than their true age.
3.2 Soil weathering studies Although the ages of some events or surfaces have been established by radiometric dating or biostratigraphy, it is not possible to date many surfaces and correlation between areas remains uncertain. However, it is possible to recognise soils with different degrees of weathering ranging from soils formed from fresh, unstained coarse and angular rock debris with little development of pedological features, to those consisting of deeply weathered and crumbled rock, strongly stained with yellowish-red to dark red colours and containing considerable quantities of soluble salts. Campbell and Claridge (1975) found that six weathering stages could be recognised between the extremes described here, and gave approximate ages to these by comparing them with soils that had formed on surfaces of known age. This study shows that some surfaces have remained stable for long periods of time, possibly since the Miocene. 4 ANTARCTIC GLACIAL SYSTEMS AND LANDFORM DEVELOPMENT
The major features of the Transantarctic Mountains were sculptured by glaciation at the onset of glaciation in Antarctica, which was possibly as early as the Eocene. Since that time the Transantarctic Mountains have been affected by three types of glacial events:
348 advances towards the Ross Sea and Ross Ice Shelf from East Antarctic ice flowing through valleys of the Transantarctic Mountains; advances of ice up the glacial valleys from the Ross Sea due to thickening of the Ross Ice Shelf and expansion of the West Antarctic ice sheet; and advances of small alpine glaciers in local mountains caused by increased precipitation in the neves. These have not necessarily been in phase. 5 WEATHERING PROCESSES A N D LANDFORM
DEVELOPMENT In Antarctica, modification of exposed landforms takes place primarily through frost action, salt weathering, fretting, wind action and minor fluvial action.
5.1 Frost action Because of the scarcity of water and the restricted number of freeze-thaw cycles, frost action is limited in its effect. Features formed by frost action include the formation of patterned ground, felsenmeer surfaces and scree slopes. Patterned ground. Patterned ground is a very widespread feature of the Antarctic landscape in which the ground surface is covered by a more or less symmetrical pattern of polygonal features. Polygons range from 2 to 20 m in diameter, but are usually of constant size in a particular locality. Patterned ground is formed by the expansion and contraction of subsurface ice, or of ice-cemented ground. The cracks between the polygons may be filled with ice or sand. Young tills develop strong polygonal patterns with considerable surface relief, but as the depth to ice-cement increases patterned ground movement diminishes and the landscape becomes smoother. Felsenmeer surfaces. Felsenmeer or block fields are surfaces consisting almost entirely of coarse rock derived from the underlying bedrock, They are best developed on surfaces of dolerite that have very strong natural joint patterns, but they may also form in hard sandstone. They are formed where snow that
accumulates in the bedrock joints of previously smoothed surfaces turns to ice and causes the rocks to be prized upwards by freeze and thaw. They usually persist as rubbly boulder fields. Scree slopes. Scree slopes are extensive in some of the ice-free areas, especially in places that have been deglaciated for very long periods (McCraw, 1967). They are formed by the accumulation of rock fragments prized from steep slopes by frost action. Rocks on the surface of a scree often show considerable weathering, frequently on their upper sides indicating that downslope movement is sometimes very slow. Scree sheets are generally best developed beneath outcrops of welljointed rocks such as dolerite.
5.2 Salt weathering, Salt weathering, caused by pressures exerted by crystallization of salts from solutions within pores and fine cracks, is a very important process contributing to rock decay and landscape evolution in Antarctica (Wellman and Wilson. 1965). Salts are widespread in the Antarctic environment because of the arid climate. Because of the tendency of salts to concentrate in hollows, salt weathering leads to the pitting or hollowing or rock surfaces. Large boulders and valley walls may be hollowed or pitted with holes several metres in diameter, the boulders usually having a casehardened and polished outer surface. Finegrained dolerite develops a distinctive micropit weathering pattern comprising pock marks that range from a few millimetres to several centimetres in diameter and depth, mainly on the upper surfaces of rocks. Salt weathering is not only a surface phenomenon however, as coarse grained rocks within the soil may become totally disaggregated by this process.
5.3 Influence of wind The mineral grains and rock fragments broken from large rocks by salt weathering provide material for the wind to transport.
349 The removal of these fragments produces the desert pavement, while the transported debris can cause faceting, tippling and polishing of clasts to form ventifacts or the formation of wind scoops or erosion hollows while the eroded debris may form sand or gravel dunes. Wind not only removes and transports rock particles effectively but also contributes to rock disintegration because the particles of ice, snow or sand that it carries abrade the rocks on which they impact. The main result of wind and other erosional processes is not so much a marked lowering of the land surface, as the smoothing of the surface producing a stable landform. Clast modification. Abrasion, especially of hard rocks such as dolerite and case-hardened sandstone, generally forms faceted rather than rounded rocks. Wind passing over flat slabs of rock may form a distinctive pattern of ripples on the trailing surfaces. The youngest glacial surfaces show little sign of modification by abrasion whereas the oldest surfaces show greater modification with strongly developed desert pavements. Abrasion is commonly greatest in places where the wind flow is highly turbulent, for example around protruding boulders. The upper edges of terrace scarps also frequently have very severe turbulent wind flow across their surfaces and may be severely abraded. Many of the desert pavement surfaces are very stable, because the closely packed pebble pavement affords little opportunity for the free movement of sand particles. As wind velocities rise, the capacity for abrasion by sand particles can diminish as surfaces may quickly become coated with drifting snow. Wind erosion hollows. These are minor surface irregularities formed around the sides of boulders where severe wind turbulence has caused the erosion of the underlying till or bedrock. Fine material is usually removed from the sides and upwind end of the boulder to be redeposited in the lee as a 'tail' of fine gravel. Dunes. Dunes are found only in a few isolated areas where sand is trapped. They have low
relief and show little sign of instability or migrational behaviour. Their scarcity is related to the small amounts of sand-sized material available for transport. Gravel dunes up to about 1 m high occur in some places and are best developed on very old exposed surfaces where they may form extensive wind ripple patterns. Sand assimilation. The absence of aeolian deposits in the cold desert landscape, where sediment supply and means of transport are both available, can be accounted for by a process here termed sand assimilation, in which sand is incorporated into the regolith. On many surfaces, harder rocks commonly form the surface pavement and make up the bulk of the soil. However the sand fraction often consists largely of rounded quartz grains derived from sandstones. Sand-sized particles, which are broken down by erosion and transported by wind, are trapped in the depressions and cracks formed by patterned ground development and assimilated into the soil by patterned ground movements. Thus, tills near a sandstone outcrop may contain significant amounts of rounded sand grains, whereas there are little or no sandstone clasts within the till. The assimilation of sand into the regolith may account for the scarcity of dunes. 5. 4 Fluvial features Fluvial landforms are confined to minor frozen lakes with lake benches marking the position of former lake levels, small intermittently flowing streams with weakly developed terraces, and small fans. The most extensive fluvial features are proglacial outwash plains with thin deposits of fluvially reworked glacial sediments or sands and fine gravels. Where an ice front with a proglacial lake has retreated over a considerable distance, fluvially modified sediments and landscape may be locally extensive. Fluvial features are mostly confined to the warmer dry valley area and their limited extent demonstrates both the scarcity of water and the persistence of a cold arid climate over time.
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6 LANDFORM DEVELOPMENT AND GLACIAL H I S T O R Y IN T H E W R I G H T V A L L E Y
The complexity of landform development can perhaps best be shown by considering a specific area, the Wright Valley (Fig. 2), one of the largest ice-free valleys in Antarctica, which is free of ice for some 55 km of its length. The Wright Valley may be taken as a model for many valleys that are currently occupied by relatively slow-moving or stagnant ice, but which at one time acted as outlets for the inland ice. These valleys have probably become ice-free by being cut off from their ice source through uplift of the Transantarctic Mountains. The general features of this west-east lying valley include a broad, deep (1600 m) Ushaped valley form over most of its length, a valley-head glacier (the Wright Upper Glacier) and set of canyons (the Labyrinth) at its western end, a glacier at its seaward end (the Wright Lower Glacier), a stream flowing inland to a thermally stratified lake (Lake Vanda), and mountain ranges on either side.
6.1 Valley floor deposits The oldest deposits occur on the valley floor, mainly in the central part of the valley
and are deposits of till, overlain by silts and gravels. The deposits include a prominent bed with abundant pecten shells, as well as other microfossils and are called Prospect Formation (Vucetich and Topping, 1972). The fossils were deposited on the floor of a fiord during Pliocene times or possibly earlier. Prospect Formation deposits give a minimum age for the last through-valley glaciation and valley cutting event, and show that uplift of about 300 m has occurred since that time, but the change from a fiord to a dry valley must have been completed prior to the deposition of the oldest moraine at the Meserve Glacier (2.2-3.9 m.y. ago).
6.2 Moraines of the Wright Lower Glacier At the eastern end of Wright Valley, a set of moraines on the valley floor are derived from the Wright Lower Glacier, a lobe of the Wilson Piedmont Glacier which advanced westward up the valley from McMurdo Sound, The tills of these moraines, which are the most extensive younger glacial deposits in the valley, show increasing weathering and soil development (Bock heim, 1979) with increas ing distance from the coast, and their ages have been estimated to range between 18,000
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351 and 900,000 years. Apart from the deposition of these tills, the ice that advanced into the valley appears to have had little effect on the overall topography of the valley. Evidence of similar ice advances can be found in many other valleys along the Transantarctic Mountains.
6.3 Moraines of alpine glaciers in the lower Wright Valley At the eastern end of the Olympus and Asgard ranges, precipitation is greater than at the western end and the mountains carry extensive snowfields with glaciers flowing from some of them. Some of these glaciers have in the past reached the valley floor and one of them, the Meserve Glacier, has been intensively studied (Everett, 1971). The Meserve Glacier has a lateral moraine complex of three main tills. The outer one has subdued relief with moderately weathered soils and contains scoria derived from a volcanic cone in its neve. The time of formation of this moraine is believed to be between 2.2 and 3.9 m.y. (Denton et al., 1970). The younger moraines have greater relief and less weathered soils. The tills are not extensive, indicating that glacial erosion is comparatively slow. For example, these alpine glaciers at the east of the Asgard Range have not cut into the valley walls and the tills are derived from rock outcrops above the neves. Landscape modification by these alpine glaciers over the last few million years has therefore been slight.
6.4 Fluvial valley floor deposits Modification of the Wright Valley floor by fluvial action has occurred since the valley was uplifted and cut off from the sea. Lake Vanda occupies the lowest part of an enclosed drainage basin. Around the lake a series of benches, cut into Prospect Formation deposits when the lake was at a higher level, are found, covered with reworked valley floor sediments. The Onyx River, which feeds the
lake, carries little sediment and has modified the valley floor sediments, forming small terraces and flood plains. A number of small fans have formed on the foot slopes of the valley sides and these are considered to form through relatively rapid meltout of small residual cirque glaciers during their final stages of decay. It thus seems that fluvial action in Wright Valley since it was uplifted has largely been confined to the reworking of sediments with slight modification of existing landforms by stream cutting, terracing, and alluvial deposition.
6.5 Tills deposited by the Wright Upper Glacier In the western Wright Valley, a number of tills, marking either separate advances, or stages of retreat of the Wright Upper Glacier, have been recognised between the present ice margin and Lake Vanda (Calkin et al., 1970). The oldest of these tills has not been benched by Lake Vanda and therefore must be younger than the older Wright Lower Glacier moraines. During the later stages of its retreat, the Wright Upper Glacier has deposited very little till. The surface on which it flows is a dolerite sill, which has been cut to form a system of canyons that descend into both forks of the Wright Valley around Dais. The sculpturing and weathering on the rocks emerging beneath the retreating glacier show that it is a relict geomorphic feature, with little sign of modification by the latest glacial event. It is not known when this topography was formed but it is assumed that it was during early valley sculpturing. Post-glacial landform modification of the Labyrinth and Dais area includes patterned ground formation and moderate weathering of soils, accumulation of sand, the formation of felsenmeer surfaces and scree accumulation on steep slopes.
6.6 Cirques of the western Asgard and Olympus ranges In the mouths of dry cirques of the western Asgard and Olympus ranges we have found
352 deposits of till containing erratics believed to have come from outcrops of basement rocks closer to the coast. Their degree of weathering indicates that they are considerably older than those on the oldest moraines in the eastern Wright Valley. Their position suggests that they may have been deposited by ice that filled the Wright Valley and moved inland from the coast (Campbell and Claridge, 1978). The cutting of the alpine valleys, however, must have occurred prior to this. On higher surfaces above these tills, the most strongly oxidised soils that we have observed in Antarctica are found. Associated with the old tills and weathered soils are intensively fretted landscapes and strongly pitted rocks. The Beacon Sandstone outcrops in particular, fret and produce landscapes at times resembling ancient ruins, while the dolerite may weather to produce a tor landscape with bedrock crumbling and oxidising to a depth of 50 cm. The dry alpine valleys of the Asgard and Olympus ranges become progressively smaller and more cirque-like in appearance towards the west of Wright Valley. They give the impression that scarp retreat may have taken place in a westerly direction in an erosion cycle that is lowering and planing the mountain tops. If such a cycle of erosion has occurred, it must have taken place over a very long time, given the slow rate of landform modification. 7 CONCLUSION The landscape of the present-day ice-free areas of Antarctica has everywhere been shaped by ice, but land exposed after ice retreat has been modified by cold desert weathering processes, in particular frost action, soil weathering and wind. Some bedrock surfaces crumble by frost action producing felsenmeer and scree slopes although others from coarse grained rocks crumble through salt weathering but with time, landscape rounding, fretting and smoothing, accompanied by prolonged weathering and oxidation,
take place. Wind is the main agent for removal of weathered material, much of which is reduced to sand, and is assimilated into the soil. Fluvial action is insignificant as an eroding or transporting agent. Although only limited dating of events is possible, everything points to events having occurred over a great length of time because of the very slow rate at which processes are operating. Using the Wright Valley as an example, it was shown that landform genesis is complex and results from the interaction of many events over a prolonged period of time. Major fluctuations of the West Antarctic ice sheet with consequent ice invasion into valleys of the Transantarctic Mountains have not markedly influenced landform development, and the influence of alpine glaciers over the last 4 m.y. has also been relatively minor. In the Wright Valley and elsewhere, there is a general landscape development sequence consisting of young landscapes with minimal weathering and surface modification on the younger-aged surfaces, mostly found on valley floors, progressing to very old higher-altitude landscapes formed over a very long time as a result of cyclic weathering by freeze and thaw, salt weathering and slow fretting with removal of weathering products by wind: We believe that these ancient landscapes have been forming since the retreat of one of the earliest Antarctic ice floods and escaped burial by later expansions of the West Antarctic ice sheet. The distinctive character of the processes operating and the landform produced justify Antarctica being considered as a separate morphogenetic region when compared with other regions in the world. 8 REFERENCES Anderson, J.D., 1965. Bedrock geology of Antarctica: a summary of exploration, 1831-1962, In: J.B. Hadley (Editor), Geologyand Palaeontologyof the Antarctic. Am. Geophys. Union Antarct~ Res. Ser., 6: 1-70. Bockheim,J.G., 1979. Relative age and origin of soils in eastern Wright Valley, Antarctica. Soil Sci., 128: 142-152. Bull, C.. 1971. Snow accumulation in Antarctica. In:
353 L.O. Quam (Editor), Research in the Antarctic. Am. Assoc. Advan. Sci., Pub. No. 93, Washington, D.C., pp. 367-424. Calkin, P.E., Behling, R.E. and Bull, C., 1970. Glacial history of Wright Valley, southern Victoria Land, Antarctica. Antarct. J. U.S., 5: 22-27. Campbell, I.B. and Claridge, G.G.C., 1975. Morphology and age relationships of Antarctic soils. In: R.P. Suggate and M.M. Cresswell (Editors), Quaternary Studies. R. Soc. N.Z. Bull., 13: 83-88. Campbell, I.B. and Claridge, G.G.C., 1978. Soils and Late Cenozoic history of the Upper Wright Valley area, Antarctica. N . Z . J . Geol. Geophys., 21: 635-643. Campbell, I.B. and Claridge, G.G.C., 1982. The influence of moisture on the development of soils of the cold deserts of Antarctica. Geoderma, 28: 221-238. Denton, G.H., Armstrong, R.L. and Stuiver, M., 1970. Late Cenozoic glaciation in Antarctica: the record in
the McMurdo Sound region. Antarct. J. U.S., 5: 15-21. Everett, K.R., 1971. Soils of the Meserve Glacier area, Wright Valley, Southern Victoria Land, Antarctica. Soil Sci., 112: 425-438. McCraw, J.D., 1967. Some surface features of McMurdo Sound region, Victoria Land, Antarctica. N.Z. J. Geol. Geophys., 10: 394-417. Vucetich, C.G. and Topping, W.W., 1972. A fiord origin for the pecten deposits, Wright Valley, Antarctica. N.Z.J. Geol. Geophys., 15: 600-673. Weyant, W.S., 1966. The Antarctic climate. In: J.C.F. Tedrow (Editor), Antarctic Soils and Soil Forming Processes. Am. Geophys. Union Antarct. Res. Ser., 8: 47-59. Wellman, H.W. and Wilson, A.T., 1965. Salt weathering, a neglected geological erosive agent in coastal and arid environments. Nature, 205: 1097-1098. [Accepted for publication July 22, 1988]
Earth-Science Reviews, 25 (1988) 345-353 Elsevier Science Publishers B.V., Amsterdam--Printed in The Netherlands
Landscape Evolution in Antarctica I.B. CAMPBELL and G.G.C. CLARIDGE N.Z. Soil Bureau, Department of Scientific and Industrial Research, Lower Hutt (New Zealand)
ABSTRACT Campbell, I.B. and Claridge, G.G.C., 1988. Landscape evolution in Antarctica. In: J. Firman (Editor), Landscapes of the Southern Hemisphere. Earth-Sci. Rev., 25: 345-353.
The extreme aridity of Antarctica means that very little water is available for weathering and geomorphic processes. The landscape everywhere has been shaped by ice, but land exposed after ice retreat has been modified by cold desert weathering processes. Wind is the main agent for removal and redeposition of weathered material, while fluvial action is insignificant as an eroding or transporting agent. There are few suitable deposits that allow reliable or accurate dating of events but the dates that are available show that landform evolution has been taking place very slowly. Soil weathering studies have provided a useful means of extrapolation showing relative ages of surfaces where no dates are available. The major weathering processes in Antarctica involve patterned ground movement, frost action, wind, salt weathering, and soil weathering. These combine to form distinctive landscape features. The complex nature of landform development is illustrated by considering events that have taken place within Wright Valley, one of the largest ice-free areas in Antarctica. Within this valley are land surfaces ranging from those found on Late Pleistocene up-valley tills, to high altitude extremely fretted and weathered land surfaces that have been ice-free possibly since the Early Miocene.
1 INTRODUCTION Antarctica has an area of about 14 million and contains about 90% of the world's ice. An understanding of the behaviour of the ice sheets, especially through the Pleistocene is crucial to predicting responses to future world climatic change. The study of Antarctic landscapes can provide information about past ice sheet behavior. The 2% of Antarctica that is ice-free (Fig. 1) comprises small scattered areas of exposed land about the edges of the continent and along the Transantarctic Mountains, which extend across the continent dividing it into two units, East Antarctica and West Antarctica. Most of the exposures of bare ground in Antarctica consist of bare rock, sometimes
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covered with accumulations of fretted rock particles or as loose unconsolidated tills. Unlike most parts of the world, the land surfaces have developed with an almost complete absence of vegetation, so there is no interaction between climate, vegetation, and landform development. In all but a few areas, the climate is too harsh to permit the existence of higher plants, b u t in favoured situations lichens and mosses are found, along with algae and other primitive forms of life.. Landforms in Antarctica have acquired their present form either through direct glacial sculpture or through subsequent modification b y arid land weathering processes. N o surfaces are k n o w n which on weathering criteria could be considered to pre-date the current glacial cycle.
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180
Fig. 1. Location map. The main ice-free areas are shown in black, although the actual ice-freeareas are much less than indicated. East Antarctica is on the fight of the Transantarctic Mountains and West Antarctica on the left.
The climate of Antarctica is unique. The very low temperatures and precipitation, together with strong winds, combine to produce an exceedingly a r i d environment in which the very restricted water availability limits physical, chemical, and biological processes. As will be shown later, some of the processes involved in landform development are also unique. 2 ENVIRONMENTAL FEATURES
2.1 Climate of Antarctica Climate is t h e most important factor in landscape evolution in Antarctica and its key attributes are temperature, precipitation and wind. Temperature. M e a n a n n u a l temperatures are everywhere below freezing point, rar~gingfrom - 1 0 ° C in coastal regions to - 5 5 " C in the centre (Weyant, 1966). However, temperatures may rise above freezing point for short periods in coastal areas or sheltered situations witMn the mountains. I n spi~e of the i n ~ s e cold, ground teml~xatures can b~mome relatively high for short periods during summer,
sometimes rising to 3 0 ° C at the surface of dark coloured rock or soils in sheltered situations. The soil can also remain unfrozen to a depth of 20 cm or more for short periods. A~ greater depths, it always remains below freezing and if there is enough moisture, ice-cemented permafrost is formed. Precipitation. Precipitation over the whole continent averages only 15 g cm -2 y-~ (Bull, 1971). As a result of the precipitation falling as snow, the atmosphere has very low humidity, while in the soils, water is present mainly as ice, or for short periods in summer, as liquid in small amounts (Campbell and Claridge, 1982). Water is therefore largely unavailable for weathering and erosional processes. Because of this, Antarctica is the world's coldest and most arid continent. Wind. Because of the presence of a large mass of ice at high altitude in the centre of the continent, the wind pattern of Antarctica is dominated by strong down-slope or katabatic winds. These winds have a direct effect on landscapes because of their erosive capacity; in particular through the creation of lag gravel or desert pavement by the removal of fine material, by the sculpturing and faceting of rocks through abrasion and by the movement of sand or gravel (McCraw, 1967).
2.2 Rock type Geology of Antarctica. Antarctic basement rocks (Anderson, 1965) are dominantly a folded complex of schist, gneiss, marble and sedimentary Precambrian to Early Palaeozoic rocks that were extensively intruded by granite about 500 m.y. ago. The basement complex was peneplaned between the Late Ordovician and the Devonian and glaciated in the Permian. A fiat-lying continental sequence of sandstones, siltstones, coalmeasures and tillites were deposited on the peneplaned surface during the Upper Palaeozoic. In J u r ~ times, basic igneous rocks were intruded within the basement rocks and the overlying sediments forming horizontal sills
347
up to 450 m thick with a very uniform composition. Since the Jurassic, the Transantarctic Mountains have been uplifted with only minor tilting, giving the originally flat-lying sediments a slight westward dip. Along the eastern scarp of the uplift, volcanic rocks, dating from about 10-15 million years to the present day, have been deposited. As their ages coincide with the present glaciation, they are important for dating weathering events and landscape development. Influence of rock type on landscapes. The younger volcanic rocks are important because of the glaciological and geomorphic interpretations which they allow, but many of the older rocks are also important because of their direct influence on landscape development. Most noteworthy is the dolerite which forms extensive cliffs or escarpments of distinctively columnar-jointed rock. Where it occurs as sills within the sediments, impressive stepped landscapes are developed. The harder basement rocks have eroded to produce massive forms, while softer argillites or sandstones give rise to landscapes with a strongly fluted appearance. The younger volcanic deposits (up to < 10 m.y.) for the most part show negligible signs of landscape modification which clearly suggest that geomorphic development in Antarctica is a very slow process. 3 AGE RELATIONSHIPS AND SURFACES
DATING
OF
3.1 Radiometric dating One of the most useful methods for determining the age of some surfaces in Antarctica has been through K / A r dating of various deposits, particularly small basaltic cones in the Taylor and Wright valleys. These volcanic deposits data from between about 2 and 4 m.y. ago and the position of the cones within the valleys show that they erupted after the major topographical features had formed. Debris from these eruptions lies on
some till-covered surfaces indicating that the surface is older than the eruption, and some debris has been included with till deposits, showing that they post-date the erupted material. Some young deposits in coastal areas with organic materials have been dated using 14C but this method is also of restricted value partly because deposits with datable materials are very limited in extent, but also because Antarctic ocean waters are depleted in 14C and dates obtained are much older than their true age.
3.2 Soil weathering studies Although the ages of some events or surfaces have been established by radiometric dating or biostratigraphy, it is not possible to date many surfaces and correlation between areas remains uncertain. However, it is possible to recognise soils with different degrees of weathering ranging from soils formed from fresh, unstained coarse and angular rock debris with little development of pedological features, to those consisting of deeply weathered and crumbled rock, strongly stained with yellowish-red to dark red colours and containing considerable quantities of soluble salts. Campbell and Claridge (1975) found that six weathering stages could be recognised between the extremes described here, and gave approximate ages to these by comparing them with soils that had formed on surfaces of known age. This study shows that some surfaces have remained stable for long periods of time, possibly since the Miocene. 4 ANTARCTIC GLACIAL SYSTEMS AND LANDFORM DEVELOPMENT
The major features of the Transantarctic Mountains were sculptured by glaciation at the onset of glaciation in Antarctica, which was possibly as early as the Eocene. Since that time the Transantarctic Mountains have been affected by three types of glacial events:
348 advances towards the Ross Sea and Ross Ice Shelf from East Antarctic ice flowing through valleys of the Transantarctic Mountains; advances of ice up the glacial valleys from the Ross Sea due to thickening of the Ross Ice Shelf and expansion of the West Antarctic ice sheet; and advances of small alpine glaciers in local mountains caused by increased precipitation in the neves. These have not necessarily been in phase. 5 WEATHERING PROCESSES A N D LANDFORM
DEVELOPMENT In Antarctica, modification of exposed landforms takes place primarily through frost action, salt weathering, fretting, wind action and minor fluvial action.
5.1 Frost action Because of the scarcity of water and the restricted number of freeze-thaw cycles, frost action is limited in its effect. Features formed by frost action include the formation of patterned ground, felsenmeer surfaces and scree slopes. Patterned ground. Patterned ground is a very widespread feature of the Antarctic landscape in which the ground surface is covered by a more or less symmetrical pattern of polygonal features. Polygons range from 2 to 20 m in diameter, but are usually of constant size in a particular locality. Patterned ground is formed by the expansion and contraction of subsurface ice, or of ice-cemented ground. The cracks between the polygons may be filled with ice or sand. Young tills develop strong polygonal patterns with considerable surface relief, but as the depth to ice-cement increases patterned ground movement diminishes and the landscape becomes smoother. Felsenmeer surfaces. Felsenmeer or block fields are surfaces consisting almost entirely of coarse rock derived from the underlying bedrock, They are best developed on surfaces of dolerite that have very strong natural joint patterns, but they may also form in hard sandstone. They are formed where snow that
accumulates in the bedrock joints of previously smoothed surfaces turns to ice and causes the rocks to be prized upwards by freeze and thaw. They usually persist as rubbly boulder fields. Scree slopes. Scree slopes are extensive in some of the ice-free areas, especially in places that have been deglaciated for very long periods (McCraw, 1967). They are formed by the accumulation of rock fragments prized from steep slopes by frost action. Rocks on the surface of a scree often show considerable weathering, frequently on their upper sides indicating that downslope movement is sometimes very slow. Scree sheets are generally best developed beneath outcrops of welljointed rocks such as dolerite.
5.2 Salt weathering, Salt weathering, caused by pressures exerted by crystallization of salts from solutions within pores and fine cracks, is a very important process contributing to rock decay and landscape evolution in Antarctica (Wellman and Wilson. 1965). Salts are widespread in the Antarctic environment because of the arid climate. Because of the tendency of salts to concentrate in hollows, salt weathering leads to the pitting or hollowing or rock surfaces. Large boulders and valley walls may be hollowed or pitted with holes several metres in diameter, the boulders usually having a casehardened and polished outer surface. Finegrained dolerite develops a distinctive micropit weathering pattern comprising pock marks that range from a few millimetres to several centimetres in diameter and depth, mainly on the upper surfaces of rocks. Salt weathering is not only a surface phenomenon however, as coarse grained rocks within the soil may become totally disaggregated by this process.
5.3 Influence of wind The mineral grains and rock fragments broken from large rocks by salt weathering provide material for the wind to transport.
349 The removal of these fragments produces the desert pavement, while the transported debris can cause faceting, tippling and polishing of clasts to form ventifacts or the formation of wind scoops or erosion hollows while the eroded debris may form sand or gravel dunes. Wind not only removes and transports rock particles effectively but also contributes to rock disintegration because the particles of ice, snow or sand that it carries abrade the rocks on which they impact. The main result of wind and other erosional processes is not so much a marked lowering of the land surface, as the smoothing of the surface producing a stable landform. Clast modification. Abrasion, especially of hard rocks such as dolerite and case-hardened sandstone, generally forms faceted rather than rounded rocks. Wind passing over flat slabs of rock may form a distinctive pattern of ripples on the trailing surfaces. The youngest glacial surfaces show little sign of modification by abrasion whereas the oldest surfaces show greater modification with strongly developed desert pavements. Abrasion is commonly greatest in places where the wind flow is highly turbulent, for example around protruding boulders. The upper edges of terrace scarps also frequently have very severe turbulent wind flow across their surfaces and may be severely abraded. Many of the desert pavement surfaces are very stable, because the closely packed pebble pavement affords little opportunity for the free movement of sand particles. As wind velocities rise, the capacity for abrasion by sand particles can diminish as surfaces may quickly become coated with drifting snow. Wind erosion hollows. These are minor surface irregularities formed around the sides of boulders where severe wind turbulence has caused the erosion of the underlying till or bedrock. Fine material is usually removed from the sides and upwind end of the boulder to be redeposited in the lee as a 'tail' of fine gravel. Dunes. Dunes are found only in a few isolated areas where sand is trapped. They have low
relief and show little sign of instability or migrational behaviour. Their scarcity is related to the small amounts of sand-sized material available for transport. Gravel dunes up to about 1 m high occur in some places and are best developed on very old exposed surfaces where they may form extensive wind ripple patterns. Sand assimilation. The absence of aeolian deposits in the cold desert landscape, where sediment supply and means of transport are both available, can be accounted for by a process here termed sand assimilation, in which sand is incorporated into the regolith. On many surfaces, harder rocks commonly form the surface pavement and make up the bulk of the soil. However the sand fraction often consists largely of rounded quartz grains derived from sandstones. Sand-sized particles, which are broken down by erosion and transported by wind, are trapped in the depressions and cracks formed by patterned ground development and assimilated into the soil by patterned ground movements. Thus, tills near a sandstone outcrop may contain significant amounts of rounded sand grains, whereas there are little or no sandstone clasts within the till. The assimilation of sand into the regolith may account for the scarcity of dunes. 5. 4 Fluvial features Fluvial landforms are confined to minor frozen lakes with lake benches marking the position of former lake levels, small intermittently flowing streams with weakly developed terraces, and small fans. The most extensive fluvial features are proglacial outwash plains with thin deposits of fluvially reworked glacial sediments or sands and fine gravels. Where an ice front with a proglacial lake has retreated over a considerable distance, fluvially modified sediments and landscape may be locally extensive. Fluvial features are mostly confined to the warmer dry valley area and their limited extent demonstrates both the scarcity of water and the persistence of a cold arid climate over time.
3>0
6 LANDFORM DEVELOPMENT AND GLACIAL H I S T O R Y IN T H E W R I G H T V A L L E Y
The complexity of landform development can perhaps best be shown by considering a specific area, the Wright Valley (Fig. 2), one of the largest ice-free valleys in Antarctica, which is free of ice for some 55 km of its length. The Wright Valley may be taken as a model for many valleys that are currently occupied by relatively slow-moving or stagnant ice, but which at one time acted as outlets for the inland ice. These valleys have probably become ice-free by being cut off from their ice source through uplift of the Transantarctic Mountains. The general features of this west-east lying valley include a broad, deep (1600 m) Ushaped valley form over most of its length, a valley-head glacier (the Wright Upper Glacier) and set of canyons (the Labyrinth) at its western end, a glacier at its seaward end (the Wright Lower Glacier), a stream flowing inland to a thermally stratified lake (Lake Vanda), and mountain ranges on either side.
6.1 Valley floor deposits The oldest deposits occur on the valley floor, mainly in the central part of the valley
and are deposits of till, overlain by silts and gravels. The deposits include a prominent bed with abundant pecten shells, as well as other microfossils and are called Prospect Formation (Vucetich and Topping, 1972). The fossils were deposited on the floor of a fiord during Pliocene times or possibly earlier. Prospect Formation deposits give a minimum age for the last through-valley glaciation and valley cutting event, and show that uplift of about 300 m has occurred since that time, but the change from a fiord to a dry valley must have been completed prior to the deposition of the oldest moraine at the Meserve Glacier (2.2-3.9 m.y. ago).
6.2 Moraines of the Wright Lower Glacier At the eastern end of Wright Valley, a set of moraines on the valley floor are derived from the Wright Lower Glacier, a lobe of the Wilson Piedmont Glacier which advanced westward up the valley from McMurdo Sound, The tills of these moraines, which are the most extensive younger glacial deposits in the valley, show increasing weathering and soil development (Bock heim, 1979) with increas ing distance from the coast, and their ages have been estimated to range between 18,000
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351 and 900,000 years. Apart from the deposition of these tills, the ice that advanced into the valley appears to have had little effect on the overall topography of the valley. Evidence of similar ice advances can be found in many other valleys along the Transantarctic Mountains.
6.3 Moraines of alpine glaciers in the lower Wright Valley At the eastern end of the Olympus and Asgard ranges, precipitation is greater than at the western end and the mountains carry extensive snowfields with glaciers flowing from some of them. Some of these glaciers have in the past reached the valley floor and one of them, the Meserve Glacier, has been intensively studied (Everett, 1971). The Meserve Glacier has a lateral moraine complex of three main tills. The outer one has subdued relief with moderately weathered soils and contains scoria derived from a volcanic cone in its neve. The time of formation of this moraine is believed to be between 2.2 and 3.9 m.y. (Denton et al., 1970). The younger moraines have greater relief and less weathered soils. The tills are not extensive, indicating that glacial erosion is comparatively slow. For example, these alpine glaciers at the east of the Asgard Range have not cut into the valley walls and the tills are derived from rock outcrops above the neves. Landscape modification by these alpine glaciers over the last few million years has therefore been slight.
6.4 Fluvial valley floor deposits Modification of the Wright Valley floor by fluvial action has occurred since the valley was uplifted and cut off from the sea. Lake Vanda occupies the lowest part of an enclosed drainage basin. Around the lake a series of benches, cut into Prospect Formation deposits when the lake was at a higher level, are found, covered with reworked valley floor sediments. The Onyx River, which feeds the
lake, carries little sediment and has modified the valley floor sediments, forming small terraces and flood plains. A number of small fans have formed on the foot slopes of the valley sides and these are considered to form through relatively rapid meltout of small residual cirque glaciers during their final stages of decay. It thus seems that fluvial action in Wright Valley since it was uplifted has largely been confined to the reworking of sediments with slight modification of existing landforms by stream cutting, terracing, and alluvial deposition.
6.5 Tills deposited by the Wright Upper Glacier In the western Wright Valley, a number of tills, marking either separate advances, or stages of retreat of the Wright Upper Glacier, have been recognised between the present ice margin and Lake Vanda (Calkin et al., 1970). The oldest of these tills has not been benched by Lake Vanda and therefore must be younger than the older Wright Lower Glacier moraines. During the later stages of its retreat, the Wright Upper Glacier has deposited very little till. The surface on which it flows is a dolerite sill, which has been cut to form a system of canyons that descend into both forks of the Wright Valley around Dais. The sculpturing and weathering on the rocks emerging beneath the retreating glacier show that it is a relict geomorphic feature, with little sign of modification by the latest glacial event. It is not known when this topography was formed but it is assumed that it was during early valley sculpturing. Post-glacial landform modification of the Labyrinth and Dais area includes patterned ground formation and moderate weathering of soils, accumulation of sand, the formation of felsenmeer surfaces and scree accumulation on steep slopes.
6.6 Cirques of the western Asgard and Olympus ranges In the mouths of dry cirques of the western Asgard and Olympus ranges we have found
352 deposits of till containing erratics believed to have come from outcrops of basement rocks closer to the coast. Their degree of weathering indicates that they are considerably older than those on the oldest moraines in the eastern Wright Valley. Their position suggests that they may have been deposited by ice that filled the Wright Valley and moved inland from the coast (Campbell and Claridge, 1978). The cutting of the alpine valleys, however, must have occurred prior to this. On higher surfaces above these tills, the most strongly oxidised soils that we have observed in Antarctica are found. Associated with the old tills and weathered soils are intensively fretted landscapes and strongly pitted rocks. The Beacon Sandstone outcrops in particular, fret and produce landscapes at times resembling ancient ruins, while the dolerite may weather to produce a tor landscape with bedrock crumbling and oxidising to a depth of 50 cm. The dry alpine valleys of the Asgard and Olympus ranges become progressively smaller and more cirque-like in appearance towards the west of Wright Valley. They give the impression that scarp retreat may have taken place in a westerly direction in an erosion cycle that is lowering and planing the mountain tops. If such a cycle of erosion has occurred, it must have taken place over a very long time, given the slow rate of landform modification. 7 CONCLUSION The landscape of the present-day ice-free areas of Antarctica has everywhere been shaped by ice, but land exposed after ice retreat has been modified by cold desert weathering processes, in particular frost action, soil weathering and wind. Some bedrock surfaces crumble by frost action producing felsenmeer and scree slopes although others from coarse grained rocks crumble through salt weathering but with time, landscape rounding, fretting and smoothing, accompanied by prolonged weathering and oxidation,
take place. Wind is the main agent for removal of weathered material, much of which is reduced to sand, and is assimilated into the soil. Fluvial action is insignificant as an eroding or transporting agent. Although only limited dating of events is possible, everything points to events having occurred over a great length of time because of the very slow rate at which processes are operating. Using the Wright Valley as an example, it was shown that landform genesis is complex and results from the interaction of many events over a prolonged period of time. Major fluctuations of the West Antarctic ice sheet with consequent ice invasion into valleys of the Transantarctic Mountains have not markedly influenced landform development, and the influence of alpine glaciers over the last 4 m.y. has also been relatively minor. In the Wright Valley and elsewhere, there is a general landscape development sequence consisting of young landscapes with minimal weathering and surface modification on the younger-aged surfaces, mostly found on valley floors, progressing to very old higher-altitude landscapes formed over a very long time as a result of cyclic weathering by freeze and thaw, salt weathering and slow fretting with removal of weathering products by wind: We believe that these ancient landscapes have been forming since the retreat of one of the earliest Antarctic ice floods and escaped burial by later expansions of the West Antarctic ice sheet. The distinctive character of the processes operating and the landform produced justify Antarctica being considered as a separate morphogenetic region when compared with other regions in the world. 8 REFERENCES Anderson, J.D., 1965. Bedrock geology of Antarctica: a summary of exploration, 1831-1962, In: J.B. Hadley (Editor), Geologyand Palaeontologyof the Antarctic. Am. Geophys. Union Antarct~ Res. Ser., 6: 1-70. Bockheim,J.G., 1979. Relative age and origin of soils in eastern Wright Valley, Antarctica. Soil Sci., 128: 142-152. Bull, C.. 1971. Snow accumulation in Antarctica. In:
353 L.O. Quam (Editor), Research in the Antarctic. Am. Assoc. Advan. Sci., Pub. No. 93, Washington, D.C., pp. 367-424. Calkin, P.E., Behling, R.E. and Bull, C., 1970. Glacial history of Wright Valley, southern Victoria Land, Antarctica. Antarct. J. U.S., 5: 22-27. Campbell, I.B. and Claridge, G.G.C., 1975. Morphology and age relationships of Antarctic soils. In: R.P. Suggate and M.M. Cresswell (Editors), Quaternary Studies. R. Soc. N.Z. Bull., 13: 83-88. Campbell, I.B. and Claridge, G.G.C., 1978. Soils and Late Cenozoic history of the Upper Wright Valley area, Antarctica. N . Z . J . Geol. Geophys., 21: 635-643. Campbell, I.B. and Claridge, G.G.C., 1982. The influence of moisture on the development of soils of the cold deserts of Antarctica. Geoderma, 28: 221-238. Denton, G.H., Armstrong, R.L. and Stuiver, M., 1970. Late Cenozoic glaciation in Antarctica: the record in
the McMurdo Sound region. Antarct. J. U.S., 5: 15-21. Everett, K.R., 1971. Soils of the Meserve Glacier area, Wright Valley, Southern Victoria Land, Antarctica. Soil Sci., 112: 425-438. McCraw, J.D., 1967. Some surface features of McMurdo Sound region, Victoria Land, Antarctica. N.Z. J. Geol. Geophys., 10: 394-417. Vucetich, C.G. and Topping, W.W., 1972. A fiord origin for the pecten deposits, Wright Valley, Antarctica. N.Z.J. Geol. Geophys., 15: 600-673. Weyant, W.S., 1966. The Antarctic climate. In: J.C.F. Tedrow (Editor), Antarctic Soils and Soil Forming Processes. Am. Geophys. Union Antarct. Res. Ser., 8: 47-59. Wellman, H.W. and Wilson, A.T., 1965. Salt weathering, a neglected geological erosive agent in coastal and arid environments. Nature, 205: 1097-1098. [Accepted for publication July 22, 1988]