for peer review - holoantar · 2018. 10. 4. · for peer review 1 post-glacial rebound also have a...

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Coupling patterns between paraglacial and permafrost

degradation responses in Antarctica

Journal: Earth Surface Processes and Landforms

Manuscript ID: ESP-14-0216.R2

Wiley - Manuscript type: Paper

Date Submitted by the Author: n/a

Complete List of Authors: Oliva, Marc; University of Lisbon, Centre for Geographical Studies - Institute of Geography and Spatial Planning Ruiz-Fernández, Jesús; University of Oviedo, Department of Geography

Keywords: Elephant Point, Antarctica, climate warming, permafrost, paraglacial dynamics

http://mc.manuscriptcentral.com/esp

Earth Surface Processes and Landforms

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1. Introduction 1

2

The etymological origin of the term “paraglacial” comes from the Greek “para” (next to) and 3

the Latin “glacies” (ice), it thus means “next to the ice” (Mercier, 2008). The paraglacial 4

concept was first introduced by Church & Ryder (1972) to describe the non-glacial 5

processes directly conditioned by glaciation. Therefore, paraglacial dynamics only takes 6

place in recently deglaciated environments. The transition from glacier to non-glacier 7

conditions implies a readjustment to the new environmental setting. This phase is 8

characterized by very high sediment transfer between the slopes and the valley bottoms 9

through landslides, rockfalls, debris flows or fluvial and torrential activity (Ballantyne, 10

2002a). Paraglacial processes are a rapid response to the adjustment of rock slopes and 11

instability of the unconsolidated glacial deposits: terminal and lateral moraines, till, 12

glaciofluvial deposits and glaciolacustrine sediments (Ballantyne, 2008). Denudation as well 13

as production and redistribution rates of sediments are very high in the following decades 14

after deglaciation. Gradually, as sediments consolidate with time, rates decrease until 15

sediment transfer values typical of non-glaciated environments are reached. The relaxation 16

of the landscape to non-glacial conditions operates over timescales of 101-104 years, highly 17

conditioned by both process and spatial scale (Ballantyne, 2002a). The use of the concept 18

paraglacial period has been proposed to show evidence of the chronological implications of 19

these environmental shifts (Benn & Evans, 1998). Six different paraglacial land systems 20

have been identified: rock slopes, drift-mantled slopes, glacier forelands, and alluvial, 21

lacustrine and coastal systems (Ballantyne, 2002a). In these different geomorphological 22

contexts the paraglacial period is conditioned by the glacially conditioned sediment release 23

and the rate of sediment reworking (Ballantyne, 2002b; Slaymaker, 2011). The rates of 24

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post-glacial rebound also have a strong influence on the activity of mass movements at 1

regional scale (Cossart et al., 2014; Feuillet et al., 2014). 2

3

Research on paraglacial processes has been conducted in many cold-climate 4

environments. Most of the studies have focused on the geomorphological response and 5

landscape changes in recently deglaciated mountain environments in low and mid-latitude 6

ranges (e.g. Owen & Sharma, 1998; Palacios et al., 1999; Cossart, 2008; Iturrizaga, 2008; 7

Hart et al., 2010; Kellerer-Pirklbauer et al., 2010; Ballantyne & Stone, 2013). Some papers 8

have examined paraglacial activity in polar and subpolar regions, mainly in the Arctic (e.g. 9

Ballantyne & Benn, 1994; Mercier & Laffly, 2005; Mercier et al., 2009; Rachlewicz, 2010). 10

However, there is a lack of studies focusing particularly on paraglacial processes in 11

Antarctica. Fitzsimons (1996) analysed the redistribution of glacial sediments and the post-12

glacial environmental evolution in Vestfold Hills (East Antarctica). Davies et al. (2013) 13

analysed the paraglacial assemblage in the context of the landscape evolution in James 14

Ross Island in the NE of the Antarctic Peninsula (AP). Zwolinski (2007) and Francelino et 15

al. (2011) studied the distribution of processes, landforms and soils under paraglacial 16

conditions in King George island, in the archipelago of the South Shetland Islands (SSI). 17

18

The land system studied in this paper corresponds to a maritime permafrost environment in 19

Antarctica, namely in Elephant Point (Livingston island, SSI). Part of this ice-free peninsula 20

is under a paraglacial regime. The retreat of the ice cap in Livingston over the last decades 21

has exposed the land surface in several margins of the westernmost margin of this island, 22

such as it is the case of Elephant Point. The declining rate of the glaciated surface in this 23

island during the second half of the XX century has been in the order of ca. 0.8 km2 per 24

year as average. The glacierized area totalled 734 km2 in 1956, 703 km2 in 1996 (Calvet et 25

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al., 1999) and 697 km2 in 2004 (Osmanoglu et al., 2013). Glacier shrunk in this archipelago 1

has been parallel to a sharp increase of the mean annual temperatures of about 2.5ºC 2

since the late 50’, one of the largest temperature increases in Earth (Turner at al., 2005; 3

Steig et al, 2009). Nevertheless, a deceleration of the volume loss rates has been recorded 4

since 2002 in this island (Navarro et al., 2013). 5

6

This rapid deglaciation process has led to profound implications on the environmental 7

dynamics of the terrestrial ecosystem in these recently deglaciated areas. In Elephant Point 8

the glacier retreat has cleared of glacial ice the frontal moraine, resulting in the exposure of 9

glacially-conditioned sediment sources that are being intensely reworked by a wide range of 10

geomorphic processes. 11

12

The purpose of the present paper is to: 13

(a) Map the spatial distribution of the landforms and processes affected by paraglacial 14

dynamics in Elephant Point. 15

(b) Identify the driving factors responsible for the moraine degradation in this area. 16

(c) Calculate the surface affected by recent mass-wasting in the moraine. 17

(d) Identify the main characteristics of paraglacial processes in Antarctic settings. 18

(e) Discuss how these geomorphological processes fit within the recent climate trends 19

occurred in the northern AP and the probable future environmental response to 20

changing climate conditions. 21

22

2. Study area 23

24

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Up to 16% of the surface in Livingston island is ice-free terrain (128 of the total 818 km2). 1

Most of the deglaciated areas in Livingston are distributed in the relatively flat surfaces in 2

the westernmost part of this island. This is the case of Byers Peninsula - which accounts for 3

almost the half of the ice-free extension of Livingston and constitutes the largest ice-free 4

area in the SSI - as well as several other minor deglaciated enclaves along the margins of 5

this island, such as Elephant Point. Here, in the SW fringe of Livingston, the retreat of the 6

Rotch Dome glacier has exposed the land surface in an area comprising 1.16 km2. 7

8

Most of the glaciers in the SSI are polythermal, combining both cold and warm-based 9

surfaces (Navarro et al., 2013). In the case of the Rotch Dome, data generated by seismic 10

and ground-penetrating radar suggests that it corresponds to a cold-based glacier in 11

contact with the ice-free area in Byers Peninsula (Navarro, personal communication), 12

though no specific data exists for Elephant Point. The age and type of permafrost after 13

deglaciation is conditioned by the basal temperature of the retreating glacier (Harris & 14

Murton, 2005). If the terminal area of the Rotch Dome in this peninsula constituted a warm-15

based glacier, permafrost may have formed after the deglaciation or due to the gradual 16

thinning of the ice in this dome-shaped glacier. If it was the case of a cold-based glacier it 17

would mean that permafrost already pre-existed in this area before the glacial retreat, 18

therefore the basement of the glacier was already frozen before its exposure to the 19

atmosphere. 20

21

In Elephant Point frozen ground conditions have been found widespread, probably related 22

to a permafrost thermal regime even at elevations near sea level (Oliva & Ruiz-Fernández, 23

2014). The environmental dynamics in this ice-free area is controlled by a wide range of 24

periglacial processes, which are very active in the four main different geomorphological 25

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units existing in Elephant Point: proglacial area, moraine system, bedrock plateaus and 1

marine terraces (Figure 1). 2

3

This research focuses on the first two units, which allow a better and an easier 4

reconstruction of past-geomorphic evolution. The other two are located in the southern area 5

next to the coastline, which constitutes the oldest ice-free environment. Here, the regional 6

isostatic uplift affecting the SSI during the Holocene (Fretwell et al., 2010; Watcham et al., 7

2011) has generated a succession of five raised beaches at elevations between 2 and 10 m 8

a.s.l. In the nearby Byers Peninsula this sequence of marine terraces formed only during 9

the last 1.8 ka BP (Hall & Perry, 2004). The bedrock plateaus are rock surfaces located at a 10

higher elevation than the Holocene marine terraces. 11

12

Figure 1 13

14

Climate conditions in Elephant Point are typical of maritime Antarctic environments from the 15

northernmost tip of the AP region. In the nearby Byers Peninsula, mean annual 16

temperatures for the last decade showed an average of -2.8ºC at 70 m (Bañón et al., 2013). 17

Precipitations in this archipelago are highly variable - oscillating between 500 to 800 mm -, 18

mostly concentrated during the summer season, falling either as rain or snow. This 19

seasonal concentration of the precipitations coincides in time with the period when more 20

freeze-thaw cycles are recorded in the ground (de Pablo et al., 2013). This favours the 21

effectiveness of physical weathering as well as mass wasting processes. The highly 22

weathered bedrock mainly composed of basalts enhances also the potential for sediment 23

mobilization down-valleys and subsequent deposition. Besides the basaltic rocks, the 24

moraine sediments are also constituted by some granodiorites and shales. 25

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1

Vegetation cover is very sparse and geographically restricted to the flat marine terraces and 2

enclaves with abundant birdlife activity. It basically consists of several species of mosses 3

(e.g. Andreaea gainii, Calliergon sarmentosum, Calliergidium austro-stramineum) and 4

lichens (e.g. Usnea antarctica, Usnea aurantiaco-atra, Rhizocarpon geographicum). 5

6

3- Materials and methods 7

An accurate geomorphological mapping of this area was conducted in late January 2014, 8

when the snow had almost melted and the landscape features were recognizable in the 9

field. Based on this geomorphological approach, we identified the processes and landforms 10

existing in Elephant Point. The presence of specific landforms can be indicative of the 11

recent geomorphic evolution in the area. This is the case for example of thermokarst lakes, 12

which can be indicative of permafrost degradation (French, 2007). 13

14

The environment affected by paraglacial dynamics was mapped in the field in January 2014 15

based on the most recent imagery of Google Earth from Elephant Point (6-2-2010), where 16

the area appears completely snow-free. The edition of this geomorphological map followed 17

the RCP 77 system (Joly, 1997). The image of 2010 was compared with the aerial image of 18

the U.S. Geological Survey (17-12-1956) in order to infer the deglaciated surface over the 19

last decades. However, no aerial or satellite images were found from the area between 20

these years, which impede extending the comparison to other periods in between. 21

22

The approach used in this research has allowed to calculate the surface affected by mass 23

movements in the moraine area, but not to measure activity rates of geomorphic processes. 24

This surface has been calculated using the software QGIS 2.4 and the Open Layers plugin 25

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that provides satellite images already georeferenced (Google Earth web service). We have 1

calculated the total surface of each mass movement landform (mudflows, landslides and 2

debris flows) as well as the relative surface occupied by these typologies within their slope 3

and within the moraine. The area affected by thermokarst processes in the western alluvial 4

fan has also been calculated. Statistical comparisons among slope aspects were carried 5

out using the non-parametric Kruskal-Wallis test. If significant differences were observed at 6

a p

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near the front. This accelerated receding firstly affected the upper part of the inner slope of 1

the moraine, which rapidly remained ice-free. This trend extended subsequently to the 2

lower areas of the moraine and to the flat surfaces geographically distributed between the 3

moraine and the glacier front. In fact, this rapid deglaciation has exposed a new proglacial 4

environment, with a plateau in the central part of the peninsula and new land surfaces in 5

both margins. From this central plateau the runoff originated from the glacial and snow 6

melting waters drains towards the W and E of the peninsula, and has formed new alluvial 7

fans. 8

9

10

Table 1 11

12

Today, the moraine is constituted by two main ridges that run parallel across 1.2 km from 13

the W to the E of the peninsula. The moraine ridges reach the maximum elevation at the 14

central part of the peninsula, ranging between 50-55 m a.s.l. in the inner ridge and 45-50 m 15

a.s.l. in the outermost arch. 16

17

A wide range of geomorphological processes occur in the moraine system and proglacial 18

environment (Table 1). However, the typology of processes affecting the northern and 19

southern sides is significantly different. For example, this can be seen in the large number 20

of frost mounds distributed in the northern slope, where fine-grained sediments are present, 21

in contrast to the southern slope, where only two were observed in this side with coarser 22

material. 23

24

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Frost shattering is widespread across the moraine, especially effective on the highly 1

weatherable rocks. The fractured rock fragments are being reworked by cryoturbation 2

processes in flat and gently-sloping areas, resulting in pattern ground surfaces (sorted-3

circles and stone stripes). In both slopes of the moraine widespread solifluction processes 4

are mobilizing shattered rock fragments down-slope (Figure 2). 5

6

Figure 2 7

8

The southern slope of the moraine falls steeply to the upper marine terraces with a slope 9

angle of 35º, while in the northern side the moraine connects more gently with the present-10

day glacier front, with an average slope of 22º. The altitudinal difference between the 11

moraine ridges and the adjacent flat surfaces is larger in the southern slope (from 45-50 to 12

5-10 m a.s.l.). However, slope processes are localized in this side. The southern exposition 13

favours a longer persistence of snow-patches in this slope, which has promoted the 14

formation of three pronival ramparts in the contact between the moraine and the uppermost 15

marine terrace. By contrast, though the elevation difference between the inner moraine 16

ridge (50-55 m a.s.l.) and the glacier front (15-20 m a.s.l.) is lower, mass wasting processes 17

in this slope are widespread and very frequent phenomena. This is related to the presence 18

of unstable sediments in this recently deglaciated terrain. 19

20

4.2 Paraglacial response to recent deglaciation 21

Slow (solifluction) and rapid mass movements (landslides, debris flows, mudflows) are 22

reworking the unconsolidated glacial debris, especially in the north-facing slope of the 23

moraine. 24

25

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Solifluction is a widespread process in the moraine terrain, but only a few well-shaped 1

solifluction landforms were identified in the southern face (Figure 2). Evidences for the slow 2

down-slope movement of the soil everywhere across the moraine come from the presence 3

of shattered stone fragments spatially separated from the source by solifluction 4

mechanisms. 5

6

Three translational landslides affect the moraine, distributed in the eastern part of the 7

northern and southern slopes (Figure 1). In the upper part of the landslides erosive scars 8

showed evidence of recent activity, with convex accumulations of debris deposited at distal 9

areas. The dimensions of the landslides ranged from 20 to 50 m long and 15 to 25 m wide. 10

Landslides were triggered by the water saturation of the active layer and flowed along the 11

permafrost table that acted as the failure plane. In all cases they were shallow, with 12

average thickness of only 2-5 m. 13

14

Debris flows were only observed in the southern slope of the moraine, in the main central 15

bay of Elephant Point. These fast-moving landslides are composed by coarser and less 16

cohesive sediments than mudflows, forming bouldery levees in the margins of the debris 17

channels. Several generations of active and inactive debris channels have generated an 18

alluvial fan at the foot of the moraine, dismantling the three uppermost levels of raised 19

beaches in this area. 20

21

However, mudflows are the most common type of downhill mass wasting degrading the 22

moraine, especially during the summer season, when a large amount of unstable sediments 23

is being transferred down-slope by these processes (Figure 1). In this sense, up to 27 24

slumps have been identified mostly in the northern hillside of the moraine as well as along 25

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the western coastline. Slumps are concave-upward surfaces of highly variably dimensions, 1

ranging from a few to tens of meters long and between 1 and 5 m deep. Inside these 2

slumps mudflow processes drain sediments towards the streams flowing between the 3

glacier and the moraine. 4

5

Table 2 and figure 3 show data concerning the spatial distribution of the active mass 6

movements in the moraine area. Slumps are the most frequent and abundant type of mass 7

movement. The individual surface of the slumps ranges between 211 m2 and 3,282 m2, with 8

an average of 945 m2. The total area affected by slumping extends over 25,515 m2, 9

representing 9.6% of the surface of the moraine. However, slumps are mainly distributed in 10

the northern and western slopes, where they occupy an area of 24,172 m2, 9.1% of the total 11

moraine surface. The highest percentage of surface affected by these processes 12

corresponds to the western side of the moraine, where it accounts for 38.1% of the slope 13

surface. 14

15

Table 2 16

17

Figure 3 18

19

The three identified landslides have a much smaller extent of only 3,417 m2, which merely 20

represents 1.3% of the total moraine surface. The four debris flows distributed in the 21

southern slope only affect 0.2% of the moraine. Therefore, the total area occupied by all 22

mass movements is 29,469 m2, which accounts for 11.1% of the moraine slopes. However, 23

they are mostly concentrated in the northern and western fringes of the moraine (26,151 24

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m2, 9.8% of the total). The surface of the alluvial fan affected by thermokarst processes in 1

the western part of the proglaciar area has been quantified in 6,598 m2. 2

3

Statistically, the distribution of slumps shows significant differences among slope aspects 4

regarding the average surface of each landform (Kruskal-Wallis H= 7.43, p

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1

Within the slumps there are several tongue-shaped deposits originated by mudflow 2

processes. In the northern slope of the moraine these deposits are constituted by a matrix 3

of clay sediments with few subangular clasts of heterogeneous lithologies. In the steeper 4

western fringe of the moraine the mudflows are clast-supported sediments with less 5

abundance of fine particles. Mudflows are triggered by the thawing both of permafrost and 6

active layer. As observed in the field the permafrost table is the sliding surface through 7

which the water-saturated sediments flow down-slope from the headwall scars (Figure 4a). 8

9

Gradually, as the scars retreat the slump deepens and expands. In fact, different 10

generations of active and inactive slumps were identified, both in the northern slope and 11

along the western face of the moraine (Figure 4b). In the most recent and active slumps 12

ice-rich permafrost is exposed to thaw at the surface, whereas those contemporarily 13

inactive show the concave scar surface covered by debris. 14

15

Table 3 16

17

Figure 5 18

19

Mudflow processes can transfer debris down-slope from a unique slump or they can 20

mobilize saturated sediments also from coalescent slumps (Table 3, Figure 5). Depending 21

on the microtopography as well as on the amount of sediment transfer downwards, 22

mudflows can generate alluvial fans in the flat surface at the foot of the moraine in contact 23

with the present-day glacier front (Figure 1). 24

25

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Another land surface disrupted by thermokarst processes is the upper plateau 50-70 m long 1

distributed in between the two main ridges of the moraine. In this plateau several endorheic 2

lagoons of metric dimensions are fed by the snow-melting and active layer thawing during 3

the summer season. Regarding the proglacial area, thermokarst processes in the western 4

alluvial fan have generated tens of kettle-lakes in this hummocky terrain (Figure 2) 5

6

5- Discussion 7

8

Up to 17.3% of the present-day deglaciated surface in Elephant Point turned ice-free 9

between 1956 and 2010. This glacier shrunk is parallel to the ice loss recorded in the SSI 10

during the second half of the XX century, which has been quantified in 4.5% of the total ice 11

volume stored in this archipelago in 1956 (Molina et al., 2007). Glacier retreat is 12

generalized in the AP, where 87% of the glaciers are thinning and receding (Cook et al., 13

2005; Cook & Vaughan, 2010). Besides, the rates of glacial retreat have increased over the 14

last years (Pritchard & Vaughan, 2007). This pattern is a consequence of the significant 15

warming of ca. 0.5ºC/decade recorded in the AP region over the last decades (Turner at al., 16

2005; Steig et al, 2009). 17

18

5.1 The current paraglacial environment in Elephant Point 19

The advances and retreats of Rotch Dome glacier during the Late Holocene (Bjork et al., 20

1991, 1996) have aggregated sediments in the northern fringe of the moraine, generating a 21

succession of ridges. It is interpreted as a polygenic moraine (Osborn, 1986). 22

23

No absolute datings are available for the formation of the moraine ridges, neither in 24

Elephant Point nor in the adjacent Byers Peninsula. However, if we assume that the highest 25

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marine terrace in Elephant Point corresponds to the same level than in Byers Peninsula 1

where whale bones were dated at 1.8 ky cal BP (Hall & Perry, 2004), a maximum age of 1.8 2

ky cal BP could be inferred for the formation of the moraine overlying the highest terrace in 3

Elephant Point. Therefore, the age of deglaciation of the southern slope is much older than 4

in the northern side, ice-free only since the recent decades. 5

6

The gradual adjustment of the slopes characteristic of the paraglacial phase took place a 7

while ago in the southern slope, immediately after the deglaciation of this environment. This 8

can be observed in the frequency and magnitude of slope processes and rates of sediment 9

yield down-slope: low to moderate in the southern slope of the moraine and extremely high 10

in the northern side, the area that is experiencing today very active paraglacial dynamics. In 11

fact, paraglacial processes in this area of Elephant Point are much more intense than those 12

described in other Antarctic maritime permafrost environments experiencing a paraglacial 13

regime (Zwolinski, 2007; Francelino et al., 2011; Davies et al., 2013). 14

15

Geographically, paraglacial processes in Elephant Point are limited to the two main 16

geomorphological units distributed next to the present-day glacier front: the proglacial area 17

and the moraine system (Figure 1). Before 1956 the glacier was in contact with the 18

moraine, supplying sediments and providing geomorphic stability to this area (Figure 6a). 19

Gradually, the accelerated thinning and retreat of the glacier promoted paraglacial stress 20

release in the northern moraine slope due to differential deglacial unloading (Ballantyne, 21

2008) (Figure 6b). 22

23

Figure 6 24

25

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The rapid glacier retreat can also provide insights about the age of formation of the 1

permafrost in this peninsula. The broad spatial distribution of the ice-rich permafrost across 2

the moraine suggests that it already existed before the deglaciation of this environment. It is 3

unlikely to consider that these permafrost conditions (up to several meters thick as 4

observed in some slumps) may have developed solely during the last half century. The 5

presence of permafrost strongly conditions the distribution, magnitude and intensity of the 6

geomorphological processes taking place in the recently deglaciated area. The relief in the 7

northern fringe of the ice-free area in Elephant Point is controlled by the moraine 8

topography extending between the W and E edges of the peninsula. However, the 9

geomorphological processes occurring in the northern and southern slopes are significantly 10

different. 11

12

The snow melting period usually starts in late spring and early summer in the SSI, though 13

the timing is highly variable due to the large internannual and intraanual climate variability in 14

this archipelago (Bañón et al., 2013). The snow remains longer in the southern slope, 15

where several late-lying snow patches were observed at late January 2014. These snow-16

patches protect the underlying sediments of being mobilized down-slope, but snow melting 17

leads to the saturation of the active layer in the lower parts of the moraine favouring the 18

activity of debris flow in these areas. These debris have created several alluvial fans of 19

different size in contact with the highest raised beaches. The sliding of the sediments on 20

these snow-patches has also generated three pronival ramparts at the foot of the larger 21

late-lying snow patches. These arches composed by angular to subangular boulders are 22

located in the distal part of the moraine. The existence of pronival ramparts in moraine 23

environments has been already identified other ice-free areas in the SSI (López-Martínez et 24

al., 2013). The presence of coarser sediments may explain the inexistence of frost mounds 25

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in the southern slope, since these landforms require fine-grained sediments for their 1

formation. In this slope the long-term mass wasting of the moraine has cleared this area of 2

fine-grained sediments leaving an armouring lag of coarser sediments (Cossart, 2008). This 3

is typical of a later stage of the paraglacial period, since it is a question of time until these 4

easily transportable unconsolidated sediments are mostly removed (Ballantyne, 2002b; 5

Iturrizaga, 2008). 6

7

By contrast, the higher abundance of fine particles in the northern slope favours the 8

presence of frost mounds, effectiveness of freeze-thaw cycles and mass movements. 9

Freeze-thaw processes are particularly active in areas with cold oceanic climates of low 10

annual temperature range (French, 2007), such as in the Maritime Antarctic, and enhance 11

cryoturbation processes. In gentle slopes of the recently deglaciated northern margin of the 12

moraine as well as in the plateau existing between the two main moraine ridges, small-13

scale patterned ground phenomena are distributed on these frost-susceptible fine-grained 14

soils. 15

16

5.2 The impacts of thermokarst processes on ice-rich permafrost terrain 17

The lowering and disruption of land surface due to melting of ground ice in recently 18

deglaciated slopes of moraine environments is intense by means of different thermokarst 19

processes. Tens of retrogressive-thaw slumps were mapped in the western and northern 20

margins of the moraine. Retrogressive-thaw slumping activity has been particularly studied 21

in the Arctic, where permafrost thawing is increasingly affecting coastal and river erosion 22

(Lantuit & Pollard, 2008; Wang et al., 2009; Lantuit et al, 2012). Slumping has been also 23

monitored in moraine environments in high northern latitudes, such as in Svalbard (Ronnert 24

& Landvik, 1993; Lukas et al., 2005), Iceland (Kjær & Krüger, 2001) and in the Canadian 25

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Arctic, where very high mean headwall retreat rates between 8.6-11.4 m/year were 1

recorded (Lewkowicz, 1987). Former studies in Maritime Antarctica briefly described the 2

existence of active slumps (Horne, 1968, Zinsmeister, 1979). Only during the last decade 3

the degradation of ground ice has been studied in more detail, both in the Maritime 4

Antarctica (e.g. Vieira, et al, 2008; Davies et al., 2013), as well as in continental Antarctica 5

(Campbell & Claridge, 2003; Levy et al., 2013). In the Dry Valleys region, Levy et al. (2013) 6

have inferred a present rate of erosion 10 times higher than the mean Holocene retreat due 7

to increasing insolation and sediment/albedo feedbacks. 8

9

In Elephant Point the impact of thermokarst processes in the moraine and proglacial 10

environment is rapidly degrading this ice-rich permafrost terrain. During the summer season 11

the erosion produced by mudflow activity exposes permafrost to sunlight and surface air 12

temperatures, which favours its thawing. Slumping has a direct impact on the increase of 13

ground temperatures, especially during the melting season and in areas where permafrost 14

is close to 0ºC. In fact, the thermal disturbance due to slumping was quantified in about 4°C 15

in mean annual ground temperatures in a retrogressive thaw slump in the Yukon Territory, 16

Canada (Burn, 2000). Therefore, the exposure of permafrost constitutes a feed-back 17

mechanism contributing to further erosion and degradation of the moraine. This process is 18

particularly effective during the snow melting season and with rainy events or sunny days, 19

when radiation heats the dark surface of the moraine terrain and ablates the permafrost. 20

Then, the water percolating through the active layer and the same water generated by the 21

melting of the exposed permafrost table, lubricates the fine-grained sediments. Under these 22

circumstances, these saturated unstable sediments are subject to mass movement down-23

slope by mudflow activity. 24

25

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Slumps are the most widespread mass movement in Elephant Point. Several generations of 1

slumps of highly variable size were identified, probably related to different time-scale 2

processes. Up to 9.6% of the surface of the moraine is affected by retrogressive thaw 3

slump processes. In some cases slumps are individual landforms of small extension (200-4

300 m2), whilst in others active coalescent slumps occupy larger surfaces (> 3,000 m2). In 5

those features where higher sediment yields are produced, alluvial fans are distributed at 6

the contact between the moraine and the glacier front (Figure 7). This occurs in the 7

northern slope of the moraine, and the glacier and snow-fed streams draining to the sea 8

redistribute the sediments down-valleys. By contrast, in the western-facing side of the 9

moraine the sediments mobilized down-slope reach the shoreline, being washed by storms 10

and wave action. 11

12

Figure 7 13

14

The presence of tens of kettle-lakes in the westernmost fringe of the recently deglaciated 15

environment is also related to the degradation of the ice-rich permafrost in this area. It is 16

noteworthy to mention that this area occupies a significant 3.3% of the deglaciated area 17

since 1956 in Elephant Point. In the SSI these thermokarst features have been observed at 18

elevations below ca. 20 m a.s.l. and are assumed to be indicative of the degradation of the 19

local permafrost conditions (Serrano et al., 2008; Bockheim et al., 2013). 20

21

5.3 Future climate and environmental scenarios 22

During the last decade a deceleration trend in the rate of mass loss has been recorded in 23

Livingston island (Navarro et al., 2013). The authors relate this pattern to increased 24

precipitations as well as lower summer surface temperatures in the SSI. Despite this recent 25

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slow down on the long-term warming climate trend recorded over the previous decades in 1

this archipelago (Turner et al., 2005), it is expected that the degradation of this recently 2

deglaciated terrain will continue in the near future. This is because the area is experiencing 3

nowadays the natural readjustment typical of the recent paraglacial period in an ice-rich 4

permafrost environment (Ballantyne, 2002a). 5

6

Higher temperature conditions and increased precipitations are forecasted in the northern 7

AP for the following decades (IPCC, 2014). These conditions would reinforce the 8

degradation of permafrost in the near future and, thus, of the moraine terrain, with more 9

abundant retrogressive thaw slumping transferring more sediments down-slope. 10

Subsequently, the natural evolution would evolve towards a phase of decreasing 11

paraglacial activity in Elephant Point. However, the persistence of these warm and wet 12

climate conditions may result in renewed paraglacial sediment release (Ballantyne, 2002b). 13

In parallel, these conditions would also enhance glacier retreat in many coastal 14

environments of the SSI where mean temperatures are close to the melting point. 15

Therefore, as glaciers retreat – even at a faster rate than previously expected in the AP and 16

Western Antarctica (McMillan, 2014; Rignot et al., 2014) –, the terrestrial surface affected 17

by paraglacial dynamics in Antarctica is expected to increase in the following decades. 18

Considering this accelerated increase of new ice-free surfaces, it is expected that the 19

greater surface affected by paraglacial processes will not be compensated by the 20

geomorphic stabilization of anciently-deglaciated areas. 21

22

6- Conclusions 23

24

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Future climate scenarios anticipate a strong warming in the AP region with very significant 1

impacts on maritime and terrestrial ecosystems. One of these environmental implications 2

will be the acceleration of the glacier retreat that has been already recorded in this area 3

over the last decades. This increased glacier loss will expose new ice-free environments, 4

especially in those areas where mean annual temperatures are closer to 0ºC, as it is the 5

case of the SSI. It is crucial to examine the impacts that the deglaciation process operates 6

on the land system since this may provide insights about the environmental response in the 7

future deglaciated landscapes. Therefore, the analysis of the geomorphic response of the 8

terrestrial ecosystem in a very recently deglaciated environment of the Maritime Antarctic, 9

Elephant Point, may be used as an analogue for future widespread paraglacial conditions in 10

an increasing number of enclaves in the AP. 11

12

In the natural laboratory of Elephant Point, the accelerated retreat of the Rotch Dome 13

glacier has exposed 17.3% of the total ice-free surface in this peninsula between 1956 and 14

2010. The northern slope of the moraine is now ice-free, as well as new flat surfaces 15

between the moraine and the glacier. A significant difference is detected in the typology and 16

intensity of geomorphological processes prevailing in the two slopes of the moraine. 17

Although the southern slope of the moraine is steeper than the northern one, slope 18

processes are more frequent and extend over larger surfaces in the latter one. This is 19

related to the different degree of consolidation of the sediments due to the different time-20

exposure in both slopes to ice-free conditions. Therefore, both slopes are in different stages 21

of paraglacial adjustment. The northern environment of the moraine has been deglaciated 22

over the last decades, which promotes very intense paraglacial processes reworking the 23

fine-grained unconsolidated debris, whereas the southern slope has been ice-free for 24

longer and sediments show an armouring pattern, being much more consolidated. The 25

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grain-size distribution of the sediments shows evidence of a strong negative feedback in 1

sediment transfer characteristic of the paraglacial period (Cossart, 2008). 2

3

Both the moraine area and the present-day proglacial environment are experiencing a rapid 4

degradation process and are losing volume through slow and rapid mass wasting 5

processes. Retrograde thaw-slumping is extremely effective degrading the moraine in its 6

northern and western fringes, where mudflow processes transfer a large amount of 7

sediments down-slope. Slumps are polycyclic, showing diverse degrees of development 8

and variable geometries. A significant percentage of 9.6% of the slopes of the moraine is 9

affected by this type of mass movement. In the most recent slumps ice-rich permafrost is 10

exposed at the surface, which may indicate that the permafrost in these areas may have 11

formed before the rapid deglaciation of these enclaves. The exposure of permafrost to solar 12

radiation and summer air temperatures self-reinforces its accelerated thawing, triggering 13

mudflows and inducing the retreat of the headwalls in the slumps. Therefore, permafrost 14

degradation may reduce the effects of the paraglacial negative feedback regarding 15

sediment transfer rates in these recently deglaciated environments. The large amount of 16

mass-wasted material is being redistributed by glaciofluvial processes in the northern slope 17

of the moraine as well as by coastal erosion in the westernmost fringe of the moraine. The 18

flat proglacial environment as well as the plateau existing between the two ridges of the 19

moraine are being also affected by thermokarst processes. In these areas the thawing of 20

ground ice generates depressions, most of which are filled by water (kettle-lakes) coming 21

from the snow melting and from the thawing of the active layer. 22

23

Future research in the SSI should conduct a long-term monitoring of these mass wasting 24

processes and of other degrading permafrost features in order to better understand how the 25

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terrestrial ecosystems is responding to present climate trends. The efficient coupling 1

between the degradation the ice-rich permafrost and paraglacial activity in the recently 2

deglaciated area in Elephant Point is expected to continue during the following years, more 3

or less pronounced depending on the rate and magnitude of climate change. Warmer 4

climate conditions may prolong the time of paraglacial activity in the recently deglaciated 5

areas in Elephant Point, as well as extend these conditions to other environments today still 6

glaciated in Maritime Antarctica. 7

8

Acknowledgements 9

This research has been funded by the Portuguese Science Foundation through the projects 10

HOLOANTAR (Holocene environmental change in the Maritime Antarctic. Interactions 11

Between permafrost and the lacustrine environment, reference PTDC/CTE-12

GIX/119582/2010) and PROPOLAR (Portuguese Polar Program). Special thanks also to 13

the invaluable support in the field of the Brazilian and Chilean Antarctic Programs. Marc 14

Oliva acknowledges the AXA Research Fund for funding a postdoctoral research contract 15

during which this paper was written. We also thank Prof Dr Gonçalo Viera (University of 16

Lisbon) for supporting our field work logistics as well as Prof Dr Paulo Pereira (Mykolas 17

Romeris University) and Prof Dr James Bockheim (University of Wisconsin-Madison) for his 18

constructive comments on an earlier draft of this paper. 19

20

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Table 1. Processes and resulting landforms generated in the moraine area.

Processes Landforms Area Characteristics

Frost shattering

Rock fragments

Everywhere across the moraine This process affects mainly basalt lithologies, granodiorite boulders are less affected by frost physical weathering. The shattered fragments are reworked by periglacial slope

processes.

Ice segregation

Frost mounds Northern slope of the moraine Triangular-shaped landforms (1-1.5 m height, 1 m long) composed by a body of ice in the

subsurface. Their growth mechanism is related to the injection of ice.

Cryoturbation

Sorted circles Relatively flat areas (4-6º) with

fine-grained sediments Landforms of centimeter size in the proglacial area as well as in the upper flat surfaces of

the moraine.

Stone stripes Gentle slopes (8-12º) with fine-

grained sediments

Landforms of centimeter size in the proglacial area as well as in the upper gentle surfaces of the moraine. These pattern ground surfaces are more developed in areas with higher

water availability.

Thermokarst

Hummocky terrain

Western alluvial fan and moraine plateau

Recently deglaciated area with undulating surfaces composed of mounds (5 to 20 m of diameter) and hollows.

Kettle-lakes Western alluvial fan and

moraine plateau Some of the depressions in this hummocky terrain are occupied by snow and glacier

melting water. Their diameter range from 2 to 20 m.

Mass movements

Slumps Very abundant in the northern

slope of the moraine Individual and/or coalescent scars of variable morphology and dimensions. See Table 3.

Landslides Both slopes of the moraine Erosive scars triggered by water saturation of the active layer of permafrost. At the foot of these shallow landslides (2-5 m depth) convex accumulations of debris (20-50 m long, 15-

25 m wide) are distributed.

Debris flows Southern slope of the moraine The sediments supplied by debris flows have formed an alluvial fan at the foot of the

moraine. The uppermost marine terraces have been eroded by the activity of debris flows.

Solifluctions landfoms

Both slopes of the moraine

In both slopes the unconsolidated moraine sediments are being transferred downslope by slow mass periglacial movements. In some cases the fragments produced by frost

shattering from the same boulder are being displaced and mobilized down-valleys by solifluction processes.

Nival Pronival ramparts

Southern slope of the moraine Up to three debris ridges (1.5-2 m height, 50-100 m long) were identified.

They are distributed at the foot of long-lying snow patches.

Alluvial Alluvial fans Southern slope of the moraine and proglacial environment

The reworked moraine sediments transported down-valleys by mass wasting processes form fan-shaped deposits of decametric dimensions.

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Table 2. Surface occupied by mass movements in the moraine area. Different letters represent significant differences at a p

For Peer Review

Table 3. Typology of slumps according to the scar geometry, activity and main environmental characteristics.

Geometry Activity Characteristics

Individual

Active

Well-defined landforms with traces of fresh activity

through different mudflows and steep retreating scars. Permafrost is frequently exposed at surface. The size

ranges from 5 to 25 m long and 1-3 m high.

Inactive

Vague landforms showing evidence of former mass wasting activity in the area. The size ranges from 20 to

80 m long and 1-5 m high.

Coalescent

Active

Series of slumps of different size (usually decametric) and geometry which are related to different stages of evolution. The retreating scars may generate a unique larger slump triggering intense mudflow processes that

can form alluvial fans at the foot of the moraine.

Inactive

Succession of adjacent and vague hollows in the moraine suggesting past slope movements in the area.

No steep headwalls are observed.

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Figure 1. (a) Location map of Elephant Point in the Antarctic Peninsula region, (b) geomorphological sketch with the distribution of the main landforms in this peninsula, and (c) geomorphological map of the moraine

and adjacent areas where paraglacial dynamics has been studied in detail. Legend: 1) Basalts, 2) Schists, 3) Fractures, 4) Rock scarp, 5) Sea, 6) Present-day beach, 7) Marine terraces, 8) Marine terrace edge, 9) Streams, 10) Lakes, 11) Lagoons, 12) Alluvial fan, 13) Glacier, 14) Glacial limit in 1956, 15) Till, 16)

Moraine, 17) Glacial diffluence, 18) Striae, 19) Kettle-lakes, 20) Proglacial fan, 21) Snow patch, 22) Pronival rampart, 23) Talus cones, 24) Mudflows, 25) Frost mounds, 26) Stone sorted-circles, 27) Stone stripes, 28)

Earth hummocks, 29) Solifluction lobes, 30) Landslides.

144x115mm (300 x 300 DPI)

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Figure 2. Examples of geomorphological processes and landforms distributed in the recently deglaciated area in Elephant Point.

146x126mm (300 x 300 DPI)

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Figure 3. Individual surface affected by slumps, landslides and debris flows in the moraine terrain according to the slope.

141x117mm (300 x 300 DPI)

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Figure 4. (a) Geomorphological features identified in a very active slump in the northern slope of the moraine, (b) Overlapping generations of slumps with active scars where permafrost is exposed in the

western side of the moraine. 68x27mm (300 x 300 DPI)

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Figure 5. Typologies of slumps in the moraine area: (a) individual slump with active mudflow processes, (b) individual slump with inactive present-day slope processes, (c) coalescent slumps with active mudflows, and

(d) inactive coalescent slumps in the foreground with some active features in the background.

131x101mm (300 x 300 DPI)

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Figure 6. Sketch of the geomorphological dynamics in Elephant Point: (a) before 1956, (b) present-day. 112x74mm (300 x 300 DPI)

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Figure 7. Slump processes in the moraine corresponding to the mass movements identified in the geomorphological map in Figure 1.

135x108mm (300 x 300 DPI)

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for peer review - holoantar · 2018. 10. 4. · for peer review 1 post-glacial rebound also have a...

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