Evolution of desert pavements and the vesicular layer in soils of the Transantarctic Mountains

Geomorphology 118 (2010) 433–443

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Geomorphology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / g e o m o r p h

Evolution of desert pavements and the vesicular layer in soils of the Transantarctic Mountains James G. Bockheim ⁎ Department of Soil Science, University of Wisconsin, 1525 Observatory Drive, Madison, WI 53706-1299, USA

a r t i c l e

i n f o

Article history: Received 1 June 2009 Received in revised form 11 February 2010 Accepted 15 February 2010 Available online 1 March 2010 Keywords: Desert soils Soil chronosequences Pavement development index Ventifaction Desert varnish

a b s t r a c t Compared to mid-latitude deserts, the properties, formation and evolution of desert pavements and the underlying vesicular layer in Antarctica are poorly understood. This study examines the desert pavements and the vesicular layer from seven soil chronosequences in the Transantarctic Mountains that have developed on two contrasting parent materials: sandstone–dolerite and granite–gneiss. The pavement density commonly ranges from 63 to 92% with a median value of 80% and does not vary significantly with time of exposure or parent material composition. The dominant size range of clasts decreases with time of exposure, ranging from 16–64 mm on Holocene and late Quaternary surfaces to 8–16 mm on surfaces of middle Quaternary and older age. The proportion of clasts with ventifaction increases progressively through time from 20% on drifts of Holocene and late Quaternary age to 35% on Miocene-aged drifts. Desert varnish forms rapidly, especially on dolerite clasts, with nearly 100% cover on surfaces of early Quaternary and older age. Macropitting occurs only on clasts that have been exposed since the Miocene. A pavement development index, based on predominant clast-size class, pavement density, and the proportion of clasts with ventifaction, varnish, and pits, readily differentiated pavements according to relative age. From these findings we judge that desert pavements initially form from a surficial concentration of boulders during till deposition followed by a short period of deflation and a longer period of progressive chemical and physical weathering of surface clasts. The vesicular layer that underlies the desert pavement averages 4 cm in thickness and is enriched in silt, which is contributed primarily by weathering rather than eolian deposition. A comparison is made between desert pavement properties in mid-latitude deserts and Antarctic deserts. © 2010 Elsevier B.V. All rights reserved.

1. Introduction Desert pavements play a dynamic role in geomorphic, hydrological, and ecological processes of mid-latitude deserts. They have been widely used for relative dating and correlation of Quaternary-aged deposits (Dan et al., 1982; McFadden et al., 1987, 1989, 1998; Al-Farraj and Harvey, 2000; Pelletier et al., 2007; Al-Farraj, 2008). The predominant features of a desert pavement include a continuous mantle of flat-lying, densely packed or partially overlapping clasts that typically overlie a soft, silt or very fine sand layer filled with gas vesicles. Some of the most comprehensive studies of the origin and evolution of pavements in mid-latitude deserts are those of McFadden et al. (1987, 1989), Wells et al. (1995), and McFadden et al. (1998, 2005). In contrast to mid-latitude deserts, there is minimal published information on the properties and genesis of pavements in high-latitude deserts of Antarctica (Lindsay, 1973; Bockheim, 1982; Campbell and Claridge, 1987; Matsuoka, 1995; Li et al., 2003). Nevertheless, desert

⁎ Tel.: + 1 608 263 5903; fax: + 1 608 265 2595. E-mail address: [email protected]. 0169-555X/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.geomorph.2010.02.012

pavements are ubiquitous in ice-free areas of Antarctica, which total 49,000 km2. Five hypotheses have been advanced regarding the formation of desert pavements in mid-latitude deserts: (i) deflation, (ii) overland flow, (iii) upward migration of clasts, (iv) in situ formation from dust deposition, and (v) physical and/or chemical weathering. In a comprehensive review, Cooke (1970) noted that early investigators favored the deflation theory and proposed that a lag surface gradually develops as finer materials are selectively removed by wind. The overland flow theory or sheet-flood theory explains desert pavement formation from episodic and catastrophic rainfall events in specific regions where the fine materials are removed by raindrop splash and sheet erosion (Williams and Zimbelman, 1994). Where there is sufficient moisture, clasts may be uplifted by wetting and drying or freezing and thawing and gradually accumulate at the surface (Springer, 1958; Cooke and Warren, 1973; Ugolini et al., 2008). McFadden et al. (1987) and Anderson et al. (2002) proposed that pavement clasts are continuously maintained at the land surface in response to deposition and pedogenic modification of windblown dust, which they referred to as “being born and maintained at the surface” (McFadden et al., 1987, p. 504). Finally, Al-Farraj and Harvey (2000) and Al-Farraj (2008) have favored physical and chemical

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weathering as leading to the development of desert pavements in the Middle East and Australia. A vesicular layer is often observed immediately beneath the desert pavement in mid-latitude desert soils (Springer, 1958; McFadden et al., 1998; Anderson et al., 2002; Ugolini et al., 2008). This layer is attributed to gradual accumulation of fine materials from wind deposition to the desert pavement. Vesicular layers have been reported in Antarctica (Campbell and Claridge, 1969), but they have not been assessed with regard to their origin and they have not been linked to the desert pavement. The objective of this study was to provide information on the properties of desert pavements and the underlying vesicular layer in the Transantarctic Mountains, with an emphasis on the McMurdo Dry Valleys, and to illustrate how these features evolve over time. 2. Regional setting The University of Wisconsin Antarctic Soils Database (http://nsidc. org/data/ggd221.html) was used in this study. The database contains information about surface boulder weathering features and soils from some of more than 800 sites in the Transantarctic Mountains from northern Victoria Land through southern Victoria Land, with an emphasis on the McMurdo Dry Valleys (Fig. 1). For this study, seven chronosequences were selected from the Transantarctic Mountains, including sequences in Wright Valley from alpine glaciers and from grounding of ice in the Ross embayment, and sequences from outlet glaciers from the East Antarctic ice sheet, including the Taylor, Hatherton, and Beardmore Glaciers. The chronosequences were selected on the basis of having a long time interval, the availability of numerical ages, and uniformity in parent material composition of member soils

(Table 1). Individual sampling sites were selected on key moraines representing a particular glacial advance. Table 2 provides a provisional correlation of glacial deposits in the Transantarctic Mountains. The composition of the drifts can be grouped into two broad categories. Mixed, light-colored igneous (granites) and metamorphic (gneisses) materials dominate the three sequences in Wright and Taylor Valleys. The sequences in Arena and Beacon Valleys and the Hatherton and Beardmore Glacier regions are comprised of Beacon Sandstone and Ferrar Dolerite (Table 1). Locally, some of the drifts may contain primarily dark-colored volcanic materials or diabase dike rocks. All age categories have samples from both rock types except for Miocene-aged soils derived from granite–gneiss. The chronosequences represent the three major soil climate zones identified in the Transantarctic Mountains, including subxerous, i.e., comparatively moist coastal regions, xerous, i.e., valley floor and sidewalls, and ultraxerous, i.e., high-elevation valleys (Campbell and Claridge, 1969). The approximate mean annual water-equivalent precipitation in these zones is 100–150 mm yr− 1, 50–100 mm yr− 1, and b50 mm yr− 1, respectively. Most of the soil chronosequences span one soil climatic zone. However, the sequences in lower Wright Valley and along the Hatherton and Beardmore Glaciers span two soil climate zones, because their distance from the coast ranges from 35 to 220 km. The chronosequences range in elevation from 200 to 2200 m a.s.l. and span the time period from Holocene or late Quaternary to the Pliocene and/or the Miocene. 3. Methods During the investigation, at least three photographs were taken by the author or his research associate at each site, including the

Fig. 1. Location of place names and study sites in the McMurdo Dry Valleys.

J.G. Bockheim / Geomorphology 118 (2010) 433–443

435

Table 1 Site factors of soil chronosequences in the Transantarctic Mountains. Total no. Glaciation pedons

Driftsa

Wright Valley

58

Alpine

A1, A2, A3, A4

3.7 ka–N3.7 Ma

Arena Valley

54

Taylor

117 ka–N15 Ma

Dolerite–sandstone Ultraxerous

900–1500

Beacon Valley

20

Taylor

T2, T3, T4a, T4b, Ar, Al, Q T2, T3, T4, Al

117 ka–15 Ma

Dolerite–sandstone Ultraxerous

800–1350

Taylor Valley

64

Taylor

Granitic-gneiss

200–1350

Wright Valley

42

Hatherton Glacier

53

Area

Beardmore Glacier 38

Age span

T2, T3, T4a, 117 ka–2.7/3.5 Ma T4b Wilson B, H1, L, H2, T, 3.7 ka–N2 Ma Piedmont O, W, V, Lp Hatherton Ha, Br1, Br2, 8 ka–N600 ka D, I, pre-I Beardmore Pl, Be, M, 14 ka–2 Ma pre-M, D, S

Drift composition

Granitic-gneiss

Soil climatic Elevation zoneb range (m)

References Soils

Parent material

Xerous

Bockheim and McLeod (2006) Bockheim (2007) Bockheim (2007) Bockheim et al. (2008) Bockheim and McLeod (2006) Bockheim et al. (1989) Denton et al. (1989)

Prentice and Krusic (2005), Hall and Denton (2005) Marchant et al. (1993)

Xerous

Granitic-gneiss

Subxerous, xerous Dolerite–sandstone Xerous, ultraxerous Diorite–sandstone Xerous, ultraxerous

250–950

275–350 1000–2200

Bockheim (2007) Wilch et al. (1993) Hall and Denton (2005) Bockheim et al. (1989) Denton et al. (1989), Ackert and Kurz (2004)

a Drift names: A = Alpine; Al = Altar; Ar = Arena; B = Brownworth; Be = Beardmore; Br = Britannia; D = Danum; H = Hummocky; Ha = Hatherton; I = Isca; L = Loke; Lp = Loop; P = Peleus; Pl = Plunket; O = Onyx; Q = Quartermain; S = Sirius; T = Trilogy; V = Valkyrie; W = Wright. b Soil climate zones: subxerous = coastal; xerous = valley floor and sides; ultraxerous = upland valleys (after Campbell and Claridge, 1969).

landform, desert pavement, and soil profile. Additional photographs were taken of special features such as the vesicular layer. The desert pavement photographs used in this analysis were taken with an Olympus OM-1 single-lens-reflex camera equipped with a 50-mm lens. On the advice of Eastman Kodak Company, we used high-speed (ASA 160) Ektachrome film, which gave the truest color rendition of rock and landform surfaces. The photographs were taken from a vertical distance ranging from 1.0 to 1.4 m and included a metric scale. The photographic slides were scanned using a Hewlett-Packard Photosmart S20 slide scanner at a resolution of 185 dots per square centimeter. The images generally were of a high enough quality that further processing of them for exposure, color, and sharpness was not required. The images were opened in Photoshop CS4. A transparent grid containing a minimum of 117 intersections was added as a layer. At each intersection the following information was collected: the presence or absence of a clast larger than 4 mm in diameter (i.e., coarser than granules in the Wentworth classification scheme), the

lithology, and weathering features such as staining (desert varnish), ventifaction, and macropitting. Macropits are defined here as those with a diameter greater than 2 mm; they originate from physical and chemical weathering. Pitting does not include vesicles from scoriaceous rocks. These weathering features have been important in determining the relative age of geomorphic surfaces in Antarctica from surface boulders (Bockheim, 1982, 1990). Not all sites were used in the analysis because of poor quality images taken on overcast days or snow cover obliterating the interstices of the desert pavement. There are several pitfalls to the methods used. It was difficult to detect desert varnish on black rocks such as diabase and basalt. The identification of weathering features from photographs when the dominant clast size was b8 mm was also problematic. The “pavement density,” a measure of the percent coverage by clasts of granule-size (4 mm) and larger (Quade, 2001), was determined from the images. The dominant size range for clasts on

Table 2 Provisional correlation of glacial depositsa in the Transantarctic Mountains. Geologic time scale

Taylor V. Taylor Gl.

Holocene Late Quaternary

Wright V. Alpine

Wright V. Wilson Pied. Gl.

Arena V. Taylor Gl.

Beacon V. Taylor Gl.

A1 B T2

A2a

T2

T2

T3

T3

H1 T3

A2b

Middle Quaternary Early Quaternary

Loke, H2 T T4a

T4a

T4a

T4b

T4b

T4b

Pliocene

O, W

Hatherton Glacier

Beardmore Glacier

Numerical dating

Hatherton Br1, Br2

Pl Be

3.7 ky 10 ky 117 ky

D

M

I

Pre-M

200 ky

Pre-I

Do

A3

1.0– 1.1 My 1.1– 2.2 My b3.4 My b3.5 My

V N3.7 My

A4 Lp P Miocene References

S Brook et al. (1993), Hall and Denton Wilch et al. (1993), (2005) Higgins et al. (2000)

Hall and Denton (2005)

Al, Ar Marchant et al. (1993)

Al Bockheim (2007)

Bockheim et al. (1989)

7.7 My N11.3 My

Denton et al. (1989), Ackert and Kurz (2004)

a Drift names: A = Alpine; Al = Altar; Ar = Arena; B = Brownworth; Be = Beardmore; Br = Britannia; D = Danum; Do = Dominion; H = Hummocky; Ha = Hatherton; I = Isca; L = Loke; Lp = Loop; M = Meyer; P = Peleus; Pl = Plunket; O = Onyx; Q = Quartermain; S = Sirius; T = Trilogy; V = Valkyrie; W = Wright.

436

Glaciation

Approximate age

Predom. size range (mm)

Boulders (%)

Cobbles (%)

Pebbles (%)

Pavement density (%)

Ventifacts (% of clasts)

D/La

Varnish (% of clasts)

Macropitting (% of clasts)

DPDIb

Thickness Bv (cm)

D/Bv pebble ratio

% Si D/Bvc

Depth of ghosts (cm)

Arena Valley Taylor 2 Taylor 3 Taylor 4a Taylor 4b Miocene p

Late Quaternary Late Quaternary Early Quaternary Early Quaternary Miocene

16–32a 32–64a 32–64a 32–64a 32–64a 0.210

27ab 38a 41a 14b 9b 0.001

27a 32a 28a 34a 24a 0.658

30ab 18b 21b 43a 51a 0.006

67b 85a 83a 84a 73ab 0.003

9c 5c 19b 20b 37a 0.000

18a 35a 18a 23a 4.6a 0.406

60b 91a 95a 99a 90a 0.000

0a 0.4a 0.7a 1.0a 1.7a 0.182

17c 21b 23ab 25a 25a 0.000

4.8a 3.5a 6.8a 3.9a 6.3a 0.09

2.8a 1.6a 5.1a 2.9a 6.5a 0.142a

1/6 3/5 2/9 1/6 2/8 –

13a 18a 29a 29a 21a 0.29

Beacon Valley Taylor 2 Taylor 3 Taylor 4 Altar p

Late Quaternary Late Quaternary Early Quaternary Miocene

32–64a 32–64a 16–32a 16–32a 0.260

21b 26ab 26ab 28a 0.011

34a 17ab 12b 8b 0.004

31ab 37a 29ab 23b 0.062

87a 86a 83a 83a 0.710

21a 30a 28a 36a 0.110

61a 5.8b 9.4b 24a 0.021

70b 85ab 95a 95a 0.011

0a 2a 2a 2a 0.300

21b 26a 26a 27a 0.006

1.0a 7.2a 9.5a 8.5a 0.16

1.1a 2.3a 6.2a 6.6a 0.14

– – – – –

1.2a 18a 34a 32a 0.19

Taylor Valley Taylor 2 Taylor 3 Taylor 4a Taylor 4b p

Late Quaternary Late Quaternary Early Quaternary Early Quaternary

16–32a 16–32a 16–32a 16–32a 0.119

14a 11a 9a 2a 0.134

20a 23a 15a 17a 0.184

46ab 45b 60ab 68a 0.004

78a 79a 81a 85a 0.120

17b 15ab 25ab 30a 0.019

2.0a 1.6a 1.8a 1.2a 0.370

35a 28a 48a 50a 0.110

0a 0a 1a 0a 0.110

17b 17b 22a 21a 0.004

5.3a 4.5a 6.5a 5.0a 0.34

1.8a 2.1a 2.5a 2.1a 0.72

3/7 (1) 3/16 (2) – 1/10 (6) –

8.4a 9.2a 16a 11a 0.33

Wright Valley Alpine 1 Alpine 2 Alpine 3 Alpine 4 p

Holocene Late Quaternary Pliocene Pliocene

16–32a 16–32a 16–32a 16–32a 0.450

20 16a 3b 4b 0.000

10 19a 14ab 10b 0.008

20 51b 74a 74a 0.000

76a 83a 83a 83a 0.520

11b 20b 36a 38a 0.002

2.7ab 0.8b 6.2a 4.1ab 0.044

19b 32b 69a 72a 0.000

0a 0a 0a 0a 0.530

16b 18b 23a 24a 0.000

2.2ab 2.2b 7.3a 6.8a 0.000

0.75ab 1.5b 4.3a 3.0ab 0.05

– 2/9 (1) 8/10 (1) – –

0b 3.7b 17a 17a 0.000

(5) (4) (3) (2) (2)

J.G. Bockheim / Geomorphology 118 (2010) 433–443

Table 3 Age-related trends in properties of desert pavements in the Transantarctic Mountains. Values followed by the same small-case letter within a column are not significantly different at p = b0.05 based on one-way analysis of variance and Fisher's individual error rates. Values lacking a small-case letter contained too few replications to include in the analysis.

Table 3 (continued) Approximate age

Predom. size range (mm)

Boulders (%)

Cobbles (%)

Pebbles (%)

Pavement density (%)

Ventifacts (% of clasts)

D/La

Varnish (% of clasts)

Macropitting (% of clasts)

DPDIb

Thickness Bv (cm)

D/Bv pebble ratio

% Si D/Bvc

Depth of ghosts (cm)

Hummocky 1 Loke Hummocky 2 Trilogy Onyx Wright Valkyrie Loop Peleus p

Late Quaternary Middle Quaternary Middle Quaternary Middle Quaternary Pliocene Pliocene Pliocene Pliocene Pliocene

16–32a 16–32 16–32a 16–32 16–32a 16–32a 16–32 16–32a 16–32a 0.070

3a – 4a – 14a 10a – 7a 4a 0.576

10a – 8a – 13a 16a – 7a 12a 0.365

64a – 58a – 59a 66a – 78a 73a 0.709

80a 86 75a 85 80a 82a 73 82a 81a 0.700

26b 41 31ab 39 35ab 23b 33 43a 37ab 0.047

2.0a 5.3 2.5a 2.0 2.2a 2.1a 1.7 2.7a 26a 0.760

37a 61 42a 51 50a 32a 36 55a 40a 0.190

0a 0 0a 0 0a 0a 0 0a 0a 0.590

19a 23 20a 22 21a 19a 19 22a 20a 0.360

1.4b – 3.8ab – 4.2ab 3.9ab – 7.0a 6.5a 0.04

2.4a – 1.7a – 1.6a 1.3a – 3.1a 3.2a 0.07

0/1 (4) – – – 1/3 (2) 1/3 (3) – – 2/11 (2) –

1.6b – 11a – 9.4ab 10ab – 7.3ab 9.5ab 0.07

Beardmore Glacier Plunket Holocene Beardmore Late Quaternary Meyer Late Quaternary Pre-Meyer Middle Quaternary Dominion Pliocene Sirius Miocene p

32–64a 16–32a 16–32a 32–64a 16–32a 16–32a 0.740

10a 22a 22a 19a 8a 8a 0.199

20ab 25a 23ab 30a 10ab 9b 0.015

50ab 35b 38ab 43ab 67ab 71a 0.042

74b 91a 94a 92a 91a 94a 0.000

14b 25b 26b 24b 30b 50a 0.000

2.5b 3.3b 8.4b 7.2b 84a 23b 0.000

24c 38bc 48b 60b 5b7 84a 0.000

0ab 0b 0ab 1ab 0ab 2a 0.047

15c 19b 21b 22b 22b 28a 0.000

1.5b 1.6ab 2.8ab 5.6ab 7.7a 5.1ab 0.02

0.8ab 1.2b 1.2b 1.8ab 3.0ab 3.5a 0.01

– – – – – – –

0b 0.6a 6.8ab 11a 13a 12a 0

Hatherton Glacier Hatherton Holocene Britannia 1 Late Quaternary Britannia 2 Late Quaternary Danum Late Quaternary Isca Middle Quaternary Pre–Isca Pliocene p

16–256 16–32a 16–32a 16–32a 16–32a 16–32 0.790

75 32a 26ab 21ab 17ab 5b 0.008

15 24ab 24ab 24ab 26a 14b 0.080

4 38b 42ab 47ab 46b 70a 0.019

84 84ab 91a 85ab 87a 76b 0.022

36 24b 25b 35ab 39a 41a 0.030

2.3 15ab 6.2b 4.0b 6.0b 44a 0.001

55 46b 48b 59ab 64ab 82a 0.003

0 0a 0a 1a 2a 4a 0.107

20 20b 18b 23ab 25ab 29a 0.007

– 2.0a 2.2a 4.0a 3.1a 3.3a 0.28

– 0.84b 0.98ab 1.5ab 1.4ab 2.0a 0.04

– – – – – – –

– 2.1b 11ab 14ab 21a 24a 0.06

a b c

Dark (dolerite, diabase):light (granitic, sandstone) ratio. Desert pavement development index. Number of profiles analyzed given in parentheses.

J.G. Bockheim / Geomorphology 118 (2010) 433–443

Glaciation

437

438

J.G. Bockheim / Geomorphology 118 (2010) 433–443

each pavement was determined using the Wentworth system. Size ranges consisted of N256 mm, 64–256 mm, 32–64 mm, 16–32 mm, 8– 16 mm, 4–8 mm, and 2–4 mm. Based on random, repeated counts, the error in determining pavement density and proportion of clasts with varnish or ventifaction is estimated to be 10%. A “desert pavement development index” was developed by taking the sum of the coded particle-size range, the pavement density (times 0.1), the percent of clasts with ventifaction (times 0.1), the percent of clasts with desert varnish (times 0.1), and the percent of clasts with macropits. The coded particle-size range included N256 mm = 1, 64–256 mm = 2, 32–64 mm = 3, 16–32 mm = 4, 8–16 mm = 5, and 4–8 mm = 6. A total of 329 desert pavements were analyzed. Samples of the desert pavement and the underlying horizons were collected and the proportion of cobbles (64–256 mm), granules + pebbles (2–64 mm), and fine-earth material (b2 mm) were determined by sieving and weighing and in less than 20% of the pavements by visual estimation. The fine-earth samples were taken to the laboratory and analyzed for pH, electrical conductivity, and watersoluble cations and anions in 1:5 soil:water extracts (APHA et al., 1975). Particle-size fractionation of the fine-earth materials was determined on selected samples using the hydrometer technique (Day, 1965). Chemical data are given in previous publications by the author and are not reported here. The thicknesses of the vesicular layer and of the layer containing “ghosts” (pseudomorphs of weathered clasts) were measured directly in the field. Changes in properties of the desert pavement and vesicular layer in relation to time of exposure were analyzed using one-way analysis of variance (Minitab, 2000). Fisher's individual error rate was used to compare individual means. The coefficient of determination (R2) was employed to compare desert pavement properties and to illustrate the quality of curvilinear equations. 4. Results 4.1. Properties of desert pavements Desert pavements in Antarctica normally are thin, commonly range from 0.5 to 2 cm, and average 1 cm in thickness. The pavements generally are one clast in thickness with clasts imbedded in a clastdepleted layer which may have a vesicular porosity. Pavements are comprised dominantly of granules and pebbles, which range from 27 to 73% (one standard deviation) and average 50% of the total coarse fraction (N2 mm) (average of all data used to construct Table 3). The boulder fraction (N256 mm) ranges from 0 to 30% and averages 15%; the cobble fraction (64–256 mm) ranges from 9 to 33% and averages 21% of the total coarse fraction. Mean values of the remaining fineearth fraction (b2 mm) are 96% sand, 2.1% silt, and 1.8% clay (data not shown).

Fig. 2. Examples of soils with vesicular porosity. (A) Vesicular porosity of Bwv horizon (pedon 84-69) exposed by removal of desert pavement. (B) Close-up of vesicular porosity in pedon 84-87. (C) Massive, structureless conditions in pedon 77-36.

Table 4 Occurrence of a vesicular layer in relation to composition and age of parent material and soil climate (percentage of total in parentheses). Site factor

4.2. Properties of the vesicular layer Forty percent of the pedons in the dataset contain a horizon immediately below the desert pavement with vesicular porosity (Fig. 2A and B). These horizons generally are massive and have a slightly hard consistency (Fig. 2C). The vesicular layer ranges from 0 to 18 cm and averages 4 cm in thickness (average of all data used to construct Table 3). Nearly 80% of the vesicular horizons did not react to 10% hydrochloric acid, indicating that there is little carbonate present in most of the samples. The concentration of silt averages 5% greater in the vesicular layer than in the desert pavement (Table 3). The occurrence of soils with vesicular porosity was examined relative to soil climate zone, parent material composition, and age of the parent materials. More than half (54%) of the soils in the ultraxerous zone have vesicular porosity; in contrast, 29% and 10% of the soils in the xerous and subxerous climatic zones, respectively, have vesicular porosity (Table 4). Of the soils derived from sandstone–

Parent material Granitic–gneissic Sandstone–dolerite Diabase Volcanic Total Soil climate Subxerous Xerous Ultraxerous Total Relative age Holocene, late Quaternary Middle Quaternary Early Quaternary Pliocene Miocene Unknown Total

Pedons with vesicular horizon

Pedons without vesicular horizon

Total

83 (24) 154 (53) 2 (100) 2 (22) 241 (59)

260 135 0 7 168

(76) (47) (0) (78) (41)

343 289 2 9 643

4 (10) 103 (29) 134 (54) 241 (37)

35 251 116 402

(90) (71) (46) (63)

39 354 250 643

73 (26) 1 (10) 51 (43) 51 (44) 43 (77) 22 (37) 241 (37)

210 9 67 65 13 38 402

(74) (90) (57) (56) (23) (63) (63)

283 10 118 116 56 60 643

J.G. Bockheim / Geomorphology 118 (2010) 433–443

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are derived from sandstone and dolerite of Miocene age in ultraxerous regions. More than half of the soils examined lack a vesicular layer, but the horizon immediately underlying the desert pavement has fewer clasts, especially pebbles + granules, than the desert pavement (Table 3). 4.3. Age-related trends in the desert pavement

Fig. 3. Distribution of boulders, cobbles and granules + pebbles in desert pavements in relation to time of exposure and composition (SS–D = sandstone–dolerite; G–G = granite–gneiss).

dolerite materials, 53% have vesicular porosity, whereas only 24% of the soils derived from granitic-gneiss materials have vesicular porosity. The proportion of soils with vesicular porosity increased with age from 26% in Holocene and late Quaternary soils to 44% in early Quaternary and Pliocene soils, to 77% in Miocene-aged soils. Therefore, soils most likely to have a horizon with vesicular porosity

Although the differences were not statistically significant, the dominant size range of clasts on desert pavements averaged 32–64 mm on Holocene surfaces and 16–32 mm on late Quaternary and older surfaces (Table 3). In general the proportions of boulders and cobbles decrease with time of exposure, and the proportion of granules +pebbles increases with time (Fig. 3). Statistically significant differences were recorded in 4 of 7 comparisons for boulders, 3 of 7 comparisons for cobbles, and 5 of 7 comparisons for granules +pebbles (Table 3). The pavement density does not vary significantly with the time of exposure and averages narrowly between 78 and 85% for surfaces ranging between Holocene and Miocene in age (Table 3). An example of a chronosequence of desert pavements is given for Arena Valley (Fig. 4). Ventifacts are found on geomorphic surfaces of all ages. However, the proportion of ventifacted clasts averages 20% on Holocene and late Quaternary surfaces and 35% on middle Quaternary and older surfaces (Table 3). There were statistically significant age-related differences in the proportion of ventifacted clasts for 6 of the 7 chronosequences. Desert varnish is ubiquitous and averages 33% on Holocene surfaces, 52–78% on late Quaternary to Pliocene-aged surfaces, and more than

Fig. 4. A chronosequence of desert pavements derived from sandstone and dolerite drifts from the Taylor Glacier in Arena Valley: (A) Taylor 2 drift (pedon 76-38); (B) Taylor 3 drift (pedon 86-23); (C) Taylor 4a drift (pedon 82-14); (D) Taylor 4b drift (pedon 76-29); (E) Altar drift (pedon 82-17); and (F) Arena drift (pedon 86-20).

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90% on Miocene-aged surfaces. In 5 of 7 chronosequences, there were significant age-related differences in the proportion of clasts with desert varnish. Although measurements were not taken in this study, the desert varnish on dolerite clasts appears to become thicker with time of exposure. On Miocene-aged surfaces, even sandstone contains a patina (Fig. 4E and F). Pitting is found primarily on clasts of Mioceneaged desert pavements (Fig. 4E; Table 3). The desert pavement development index discriminated among desert pavements on surfaces of different ages. The pavement development index averages 17, 20, 22, 23, 22, and 26 on surfaces of Holocene, late Quaternary, middle Quaternary, early Quaternary, Pliocene, and Miocene age, respectively (Table 3). In 6 of 7 chronosequences, there were significant age-related differences in the desert pavement development index. 4.4. Influence of parent material on desert pavement development Lithology plays an important role in desert pavement development in the study area. The two dominant lithologies examined in this study were dolerite–sandstone and granite–gneiss. Light-colored rocks include sandstone, granites and gneisses. Dark-colored rocks include dolerite, the dike rock diabase, and volcanic rocks such as scoria and basalt. On granite–gneiss materials, the dark:light ratio averages 2.7, 1.6, 3.3, 1.6, and 3.0 for Holocene, late Quaternary, middle Quaternary, early Quaternary, and Pliocene surfaces, respectively (Table 3). On sandstone–dolerite materials, the dark:light ratio averages 2.4, 17, 6.6, 17, and 44 for the same geologic time periods. In general, the dominant clast range is smaller for granitic desert pavements than on dolerite–sandstone desert pavements on surfaces of comparable age (Fig. 3). The proportion of boulders is two- to three-fold greater for pavements of sandstone–dolerite than for pavements of granite–gneiss on surfaces of equivalent age. In contrast, the proportion of granules + pebbles is greater in granite–gneiss pavements than in sandstone–dolerite pavements of the same approximate age. Ventification is favored by the presence of diabase and basalt, but other rock types such as sandstone may also show ventifaction. Granite–gneiss materials are more subject to grusification than to ventifaction and varnishing. 5. Discussion 5.1. Desert pavement chronofunctions The relation between desert pavement property and time of exposure was examined for Arena Valley, one of the better dated chronosequences in the Transantarctic Mountains. Based on logarithmic functions, the proportion of varnished clasts approaches 100% by 1.7 Ma on the doleritic-rich materials (Fig. 5A). Although the proportion of varnished clasts does not increase on Pliocene- and Miocene-aged surfaces, the intensity of the varnish color and thickness of the varnish appears to increase (c.f., Fig. 4). The youngest surface investigated in Arena Valley was the Taylor II surface at 117 ka. Holocene-aged surfaces (ca. 3.7 ka) on comparable materials in the upper Beardmore Glacier region (Plunket drift) had 24% of the clasts varnished (Table 4). This suggests that desert varnish forms very rapidly in Antarctica. The proportion of clasts with ventifaction increases logarithmically on desert pavements in Arena Valley and does not appear to have equilibrated even on Miocene-aged surfaces (Fig. 5B). The pavement development index, an integrated measure of pavement development, suggests that the desert pavement develops rapidly within the first 200 ka and approaches a dynamic steady state within 1.7 Ma (Fig. 5C). However, as mentioned previously, certain properties of the desert pavement continue to change, such as the thickness and intensity of the desert varnish and ventifaction. These findings illustrate the value of desert pavements, including the desert pavement development index, as a relative dating tool.

Fig. 5. Desert pavement properties in Arena Valley in relation to time of exposure: (A) proportion of varnished clasts; (B) proportion of ventifacted clasts; (C) pavement development index.

5.2. Relation of vesicular layer to desert pavement Although it occurs in only 40% of the profiles examined, the vesicular layer, or a similar layer, appears to be linked with the desert pavement. Where the vesicular layer is absent, there is a 0.5-to-18cm-thick layer beneath the desert pavement that has many of the properties of a vesicular layer, including a greater silt content and fewer granules + pebbles than the underlying layer. In view that clast size of the desert pavement diminishes with time of exposure, the vesicular layer is probably the recipient of fine materials released by physical and chemical weathering. These materials fall between the clasts and enable the clasts to “float” upon the vesicular layer. However, unlike mid-latitude deserts, the desert pavement is lifted only a few centimeters, rather than as much as 20 cm in warm deserts (McFadden et al., 1987, 1998). These interpretations suggest that as in mid-latitude deserts, the desert pavement is in “motion.” This interpretation is confirmed by the fact that weathering features such

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as ventifaction and varnish occur on all surfaces of clasts and not just on the exposed surfaces. 5.3. A theory for desert pavement formation in Antarctica Five theories for desert pavement formation were identified earlier, including (i) runoff, (ii) deflation, (iii) vertical sorting, (iv) eolian, and (v) weathering in situ. A comparison of the evidence for these theories with regard to desert pavement formation in Antarctica is given in Table 5. Early investigators (Cooke, 1970; Cooke and Warren, 1973; Dan et al., 1982) favored multiple origins of desert pavements, or different origins depending on location. Since the mid-1980s the eolian theory has been espoused by geomorphologists in the American Southwest (McFadden et al., 1987). Our data suggest that the weathering in situ theory is the primary mechanism leading to the development of deserts pavements on the hyper-arid, hyper-cold landscapes of Antarctica. Evidence includes: (i) a reduction in mean fragment size with longevity of weathering; (ii) the existence of a layer below the pavement that has fewer granules + pebbles that does not appear to be eolian but does bear some properties of the underlying horizons; (iii) the existence of abundant freeze–thaw cycles that drive physical weathering; (iv) increases in the proportion of clasts with ventifaction and desert varnish and in the desert pavement development index (DVDI) with time of exposure; and (v) an overall increase in soil development on the seven chronosequences. There were highly significant (p b 0.05) decreases in the proportion of boulders and cobbles in the desert pavement with time and an accordant increase in pebbles + granules (Table 6), implying a gradual

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decline in clast size with time of exposure. These findings are comparable to those of Cooke (1970) in the Huasco Valley, Chile; Dan et al. (1982) in Israel; Al-Farraj and Harvey (2000) in the United Arab Emirates and Oman; and Al-Farraj (2008) in Australia. The granule + pebble-free layer occurring beneath the desert pavement and above the zone of salt enrichment contains an abundance of silt and sand-sized particles and may result from weathering of clasts in the desert pavement rather than from dust deposition. Surface clasts in desert pavements of Antarctica are subject to two kinds of temperature oscillations, both occurring across the freezing point (McKay and Friedmann, 1985). A daily freeze–thaw cycle results from low-frequency (diurnal) and large-amplitude (up to about 20 °C) oscillations on the sunlit surface of rocks. The diurnal changes result from changes in the sun altitude and angle with respect to the rock surface. High-frequency (few minutes) oscillations occur under certain weather conditions, such as sunny days with light winds, and are superimposed on the low-frequency oscillations. They are caused by the cooling effect of wind gusts on rock surfaces that are much warmer than ambient air temperatures. High-frequency oscillations result in a rapid freeze–thaw cycles on the clast surface. Both oscillations seem to have a marked effect on rock weathering. Further evidence of weathering pertains to the abundant significant correlations between time of exposure and the proportion of clasts with ventifaction and varnishing and DPDI (Table 3). Moreover, detailed soil investigations show significant correlations between many soil properties, such as depth of staining, depth of ghosts, depth of coherence, maximum electrical conductivity, and profile quantities of salts (Bockheim, 1982; Bockheim et al., 1989; Bockheim, 1990; Bockheim and McLeod, 2006; Bockheim, 2007; Bockheim et al., 2008).

Table 5 Evidence for different hypotheses of desert pavement formation. Citation

Transantarctic mountains

Deflation Lag pavement at surface Strong winds Low critical-surface-roughness constant

Cooke (1970), Dan et al. (1982) Cooke (1970) Cooke (1970)

X X

Vertical sorting Abundant freeze–thaw cycles Optimum water content Abundant water between the ice-water interface and the stone Ice lenses on the cold (freezing) side of stones Well-graded materials: sand and gravel; sand and silt Slow velocity of freezing, e.g. b0.6 mm/h Increased porosity from displacement of stones Fine grain-size distribution

Corte (1966), Cooke et al. (1993), Viklander (1998) Corte (1966), Viklander and Eigenbrod (2000) Corte (1966, 1994) Mackay (1984) Corte (1994), Viklander and Eigenbrod (2000) Corte (1966, 1994) Corte (1966) Corte (1966)

X

“Born at the surface” Eolian (accretionary or inflationary) layer below pavement Constant pavement density over time Abundant freeze–thaw, wet–dry cycles that interlock clasts Columnar and platy structure to trap eolian materials High dust deposition rates Initial bar-and-swale microrelief Well-sorted materials

Pelletier et al. (2007) Wells et al. (1995) McFadden et al. (1987) Anderson et al. (2002) Reheis and Kihl (1995) Pelletier et al. (2007) Valentine and Harrington, (2006)

Weathering in situ Reduction in mean fragment size with longevity of weathering Gravel-free layer below pavement from weathered sand and silt particles Abundant freeze–thaw, wet–dry cycles that interlock clasts Increase in desert pavement density with longevity of weathering Increase in angularity of clasts from progressive mechanical weathering

Cooke (1970), Dan et al. (1982), Al-Farraj and Harvey (2000) Al-Farraj and Harvey (2000) Cooke (1970), Dan et al. (1982), Al-Farraj and Harvey (2000) Al-Farraj and Harvey (2000) Cooke (1970), Al-Farraj and Harvey (2000)

Runoff Raindrop impaction Sheet wash All Vesicular porosity D/Av ratio = 4.2–90

X

X X

X X X

Williams and Zimbelman (1994)

Springer (1958), McFadden et al. (1987, 1998), Anderson et al. (2002) X Springer (1958) X

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Table 6 A comparison of desert pavement properties and site factors for mid-latitude and Antarctic deserts. Property

Mid-latitude desert

Antarctic desert

References

Pavement density (%) Clast shape Clast cracking Clast weathering Age (yr) Vesicular horizon thickness (cm) MAAT (°C) MAP (mm) Vegetative cover (%) Predominant clast size range (φ) Predominant salts

60–95 Angular to subrounded Yes Varnished, ventifacted 10 to ~ 180 ka Av, 0–80 11–25 b135 0–35 − 2.2 to − 7.3 CaCO3, CaSO4, NaNO3

63–92 Subangular, subrounded No Varnished, ventifacted 3.7 ka to 15 Ma Bv, 0–18 − 20 to − 30 b150 0 − 3 to − 7 NaCl, NaNO3, Na2SO4, CaCO3

8, 1, 1, 5, 1, 2, 1, 1, 1, 1 1,

9 7 7 9, 3, 4, 9 9 3,

10 6, 7, 8, 11, 12 7, 9, 10

8

4, 9, 10, 13, 14

References: 1 = Al-Farraj and Harvey (2000); 2 = Anderson et al. (2002); 3 = Dan et al. (1982); 4 = Graham et al. (2008); 5 = Liu and Broecker (2008); 6 = Marchetti and Cerling (2005); 7 = McFadden et al. (2005); 8 = Quade (2001); 9 = Springer (1958); 10 = Ugolini et al. (2008); 11 = Valentine and Harrington (2006); 12 = Wells et al. (1995); 13 = Bockheim (1982); 14 = Bockheim (1990).

Some of the lag coarse fragments that form the desert pavement in Antarctica may originate from ablation during the earliest stages of till deposition (Campbell and Claridge, 1987). In addition, deflation may play an important role in concentrating clasts during the first several millennia following till deposition. The deflation theory is attractive for explaining the origin of desert pavements in Antarctica as very high winds persist in many regions, particularly adjacent to the polar plateau. However, the strong age-related trends in soil properties do not favor sustained periods of deflation. Vertical sorting is an important mechanism for desert pavement formation in the Arctic where ice-cemented permafrost is pervasive (Mackay, 1984). However, there is insufficient soil moisture in most locations of Antarctica for vertical sorting to occur, particularly for the abundant coarse-grained materials, which require near saturation to initiate frost heaving (Corte, 1966). There was no evidence of ice lenses beneath clasts, needle ice, or depressions beneath clasts suggestive of vertical sorting. Based on studies by Corte (1966), freezing may be too rapid for vertical sorting to occur because of the unusually strong thermal gradients in Antarctic soils (Campbell et al., 1998). Runoff does not play a role in desert pavement formation in interior Antarctica as rain has never been recorded, and sheet wash only occurs during the austral summer in floodplains of rivers. The eolian theory accounts only partially for desert pavement formation in Antarctica (Table 5). The pavement density equilibrates rather quickly, within ca. 10 ka, and changes only minimally with increased time. These findings are comparable to those reported in southwestern USA (Wells et al., 1995; Quade, 2001). As mentioned previously, although a layer comprised dominantly of pebbles and granules forms beneath the desert pavement in Antarctica, this layer appears to contain residual materials rather than windborne materials. In addition, the rates of dust deposition are much lower in Antarctic than in mid-latitude deserts, due to a limited supply of sediment (Lancaster, 2002). Moreover, Antarctic soils lack the columnar and platy structure of temperate desert soils which enable entrapment of dust. Although the “bar-and-swale” microrelief that favors eolian entrapment occurs in Antarctica, it is relatively uncommon. 5.4. A comparison of pavements in mid-latitude and Antarctic deserts Mid-latitude deserts and high-latitude Antarctic deserts are comparable in terms of their aridity and generally low vegetative cover (Table 6). The desert pavements are comparable in terms of pavement density, and clast shape, size range, and weathering features. Both systems contain a vesicular layer that plays an important role in development of the desert pavement by promoting surface clast motion and pavement development. Both systems contain a variety of salts from atmospheric deposition and soil weathering. However, unlike midlatitude deserts, CaCO3 is restricted to encrustations beneath clasts

in comparatively moist (subxerous) coastal areas of Antarctica, and petrocalcic horizons have not been identified in Antarctica. Pavements may reach a greater age in Antarctica than in midlatitude deserts, possibly because of the lack of moist pluvial periods when vegetation was more lush and interrupted desert pavement formation (Quade, 2001; Valentine and Harrington, 2006). Another key difference between pavements in mid-latitude and high-latitude deserts is that a vesicular layer does not always occur in Antarctica. Whereas the vesicles in mid-latitude desert soils are attributed to degassing of carbon dioxide (Springer, 1958; McFadden et al., 1998), the vesicles in Antarctic soils more likely originate from sublimation of ice crystals. During the austral summer occasional snowfalls melt on the dark-colored desert pavement, and water infiltrates and percolates into the upper few centimeters of soil, where it may freeze. During subsequent sunny days, the pores are evacuated by sublimation of the pore ice. When the vesicular layer does occur in cold desert soils, it rarely exceeds 18 cm in thickness. Vesicular layers in mid-latitude deserts range from a few centimeters to 20 cm in thickness (McFadden et al., 1998). Whereas the dust in mid-latitude deserts of southern Nevada and California originates primarily from modern playas (Reheis and Kihl, 1995), the limited dust in the McMurdo Dry Valleys of Antarctica originates from floodplains of abrading streams (Lancaster, 2002). 6. Conclusions The key findings of this study are as follows: • Pavements in cold deserts are formed from weathering in situ rather than by wind deflation of fines, overland flow, uplifting of clasts, or eolian addition; • Desert pavements in continental Antarctica are commonly underlain by a silt-enriched and clast-depleted layer that often has a vesicular porosity; • Age-related trends in cold desert pavements include a decrease in clast size and an increase in the proportion of clasts showing ventifaction and varnishing and the pavement development index; • Clast sizes generally were smaller in pavements derived from granite–gneiss than in those from sandstone–dolerite of similar age; • Highly significant age-related trends in properties of desert pavements such as varnish and ventifaction suggest that the desert pavement may be a valuable relative dating tool; • Pavements in Antarctic deserts bear many similarities to those in mid-latitude deserts, including pavement density and clast shape, size range and weathering features; • Desert pavements in continental Antarctica may reach a greater age than in mid-latitude deserts, possibly because they haven't been affected by moist pluvial events.

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