treatise on geomorphology || 11.18 aeolian paleoenvironments of desert landscapes

11.18 Aeolian Paleoenvironments of Desert LandscapesDSG Thomas, University of Oxford, Oxford, UK

r 2013 Elsevier Inc. All rights reserved.

11.18.1 Introduction 357
11.18.1.1 The Nature of Aeolian Paleoenvironments in Desert Landscapes 357 11.18.1.2 Sources of Evidence of Aeolian Paleoenvironments 357 11.18.1.3 History of Research and Major Issues 357 11.18.2 Sandy Paleoenvironments 358 11.18.2.1 Distribution 358 11.18.2.2 Interactions between Controlling Variables 361 11.18.2.3 Circulation Changes 363 11.18.2.4 Sediment Supply 363 11.18.3 Chronologies of Paleo-Aeolian Systems 365 11.18.3.1 Multiple Events and Compound Dune Landscapes 365 11.18.3.2 Multiple Events and Complex Accumulation Records 365 11.18.3.3 Sampling Stratigraphy 368 11.18.3.4 Preservation Potential 369 11.18.4 Future Prospects 370 References 371
35

Th

In

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GlossaryErodibility The susceptibility of the ground surface to

erosion. In an aeolian context, this refers to susceptibility to

wind erosion. For bare, dry, surfaces, this is determined by

the particle size of surface sediments. For other surfaces,

erodibility is contributed to by factors such as plant cover,

surface crusting, and soil moisture.

Erosivity The potential of the ground surface to be

eroded. In an aeolian context, this refers to wind energy

and strength. In simple terms, low-velocity winds have

less (or no) erosive potential compared to stronger

winds. In general terms of aeolian transport, when

erosivity exceeds erodibility, sediment movement will

occur.

Geoproxy A landform that provides evidence of past

environmental or climatic conditions. Geoproxies are a

valuable source of data for Quaternary studies, especially in

environments where other sedimentary or biological proxy

records are lacking.

Treatise on Geomor6

omas, D.S.G., 2013. Aeolian paleoenvironments of desert landscapes.

: Shroder, J. (Editor in Chief), Lancaster, N., Sherman, D.J., Baas, A.C.W.

ds.), Treatise on Geomorphology. Academic Press, San Diego, CA, vol. 11,

olian Geomorphology, pp. 356–374.

Mobility index A numerical model that integrates data

representing the climatic parameters that have an impact on

aeolian sediment transport. Various schemes exist, but most

indices integrate data for parameters that have an impact on

both erodibility and erosivity.

OSL dating Optically stimulated luminescence dating has

superseded thermoluminescence (TL) dating as the

principal direct means of determining the age of sand/silt,

and therefore the landforms (e.g., sand dunes) with which

the sediment is associated. OSL dating measures the time

that has elapsed since a sediment was last exposed to direct

sunlight (i.e., was subject to transport processes). The

method quantifies the accumulation of a radiation-related

charge in ’traps’ within the fabric of sediment grains, which

builds up on burial, and divides this by the rate of charge in

the surrounding environment. OSL/TL refers to the method

of laboratory stimulation used to liberate the accumulated

dose.

Abstract

The wind is an important agent of sediment movement and landform development in deserts and some coastal en-

vironments today. In line with major climate changes affecting earth history, the extent and locations of wind-shapedlandscapes have changed over time. For the Quaternary period, these changes have commonly left a record in the land-

scapes of today: evidence of aeolian paleoenvironments in the form of sandy sediments and landscapes that are not

experiencing aeolian activity today. This chapter analyzes the evidence for Quaternary desert aeolian paleoenvironments:what that evidence is; how it can be interpreted in terms of extent and timing of occurrence; what the evidence means for

understanding past climate changes; and examines key issues and debates in reconstructing past aeolian desert conditions.

phology, Volume 11 http://dx.doi.org/10.1016/B978-0-12-374739-6.00311-0

Aeolian Paleoenvironments of Desert Landscapes 357

As new techniques to interrogate Quaternary aeolian sediments have developed, especially optically stimulated lumi-nescence dating, so simple models of climate change-desert expansion have been replaced by recognition of the complexity

and multiple occurrence of aeolian system expansions and contractions. These in turn have led to debates about the climate

signals that Quaternary aeolian sediments may represent, with simple assumptions of greater past aridity needing to be

placed into contexts integrating changes in erosivity conditions as well as those pertaining to erodibility. A further issue thatchallenges successful interpretation relates to record preservation. The chapter concludes by addressing these issues and the

prospects for enhanced interpretations of aeolian paleoenvironmental data in the future.

11.18.1 Introduction

11.18.1.1 The Nature of Aeolian Paleoenvironments inDesert Landscapes

The term ‘aeolian paleoenvironments’ refers to the spatial

distribution, extent, and nature of wind-shaped landscapes in

the past. In reality, there are two main categories of records of

aeolian paleoenvironments. First are records preserved in the

ancient rocks – for example, the Devonian Old Red Sandstone

of NE Scotland and other parts of Europe – that are sedi-

mentary deposits derived from ancient dune sands. Second is

evidence preserved in today’s landscapes and surface sedi-

ments that attest to the extensive operation of the wind as a

geomorphic agent in the Quaternary period (last B2 Ma with

evidence usually focused on the last B250 ka). These are

commonly interpreted as representing past extensions of

warm or cold desert environments, due to climatic changes,

into areas that today are not prone to wind operating as a

major geomorphic process.

11.18.1.2 Sources of Evidence of AeolianPaleoenvironments

Evidence for extensions, or changes, in the spatial distribution

of aeolian environments during the Quaternary period com-

pared to today comes from three primary sources:

1. Aeolian sediments (sand or silt) identified in marine cores

where it has accumulated from material deflated from

nearby, or sometimes distant, land masses. For example,

work includes: the aeolian sands identified in marine cores

in early work by Sarnthein and Diester-Haass (1977) and

Pokras and Mix (1985) off the northwest and west coast of

Africa and inferred to be indicative of more active sand seas

in the western Sahara in the late Quaternary; research using

dust fractions of different grain sizes from marine cores off

the coasts of southern Africa and Chile (Stuut and Lamy,

2004) and eastern Australia (Hesse and McTainsh, 1999) to

infer greater sediment transport out of expanded contin-

ental deserts and greater wind strengths.

2. Aeolian sand landscapes, either with dune forms present or

as relatively flat sand sheets, in low (e.g., Stokes et al.,

1997a, in southern Africa; Fitzsimmons et al., 2007a, in

Australia), mid (e.g., Bateman et al. (1999) in the UK and

Wolfe et al. (2006) in Canada), and high (e.g., Bateman

and Murton (2006) in northern Canada and Juan Federico

et al. (2011) in southern Argentina) latitudes. Dunes are

particularly significant evidence of major environmental

changes, though paleoclimatic interpretations of stable

dunes are not as simple as initially thought.

3. Loess deposits, sometimes extremely thick (tens to hun-

dreds of meters) and blanketing large areas of the high and

mid-latitudes in North and South America, Europe, and

Asia and also occurring in some peridesert locations, e.g. in

North Africa (e.g., Dearing et al., 2001).

This chapter focuses on point 2 above but with reference to

1, whereas loess is fully considered elsewhere in this volume.

This simple typology, however, has the potential to ignore two

other important issues. First is that evidence of wind erosion, in

the form of yardangs and other erosive bedforms, can also

demonstrate spatial changes in aeolian activity in the past, in

the same manner as points 2 and 3 above. Erosive aeolian

landforms have not received the same degrees of investigation

as aeolian depositional landforms. Second is that dunes within

current arid and hyperarid sand seas can preserve within their

sedimentary bodies histories of environmental changes and

accumulation records over long periods of time. These are being

revealed by the application of optically stimulated lumi-

nescence (OSL) dating (e.g., Bristow et al., 2007a) in the same

manner that the histories of presently inactive dune fields

(category 3 above) are being revealed. Thus, they will not be

treated separately in this chapter, but are considered alongside

the records from inactive dunes and sand seas.

11.18.1.3 History of Research and Major Issues

In a significant paper based on air photograph analysis and

field investigation, Grove (1958) proposed that vegetated dunes

on the southern side of the Sahara were palimpsests of former

southerly expansions of drier conditions beyond the limits of

present dune activity in the Sahara. By comparing the spatial

distributions of rainfall today, as represented by mean annual

isohyets, Grove (1958) proposed that conditions drier than

about 150 mm pa, seen as the limited of rainfall for dune ac-

tivity in the Sahara, had prevailed in areas that today receive

over 1000 mm pa. Mapping and interpreting vegetated dunes

in, for example, Australia (Jennings, 1975), the Thar Desert,

India (Allchin et al., 1978), The Kalahari (Grove, 1969),

and equatorial South America (Tricart, 1975) provided

similar hypotheses and evidence of extended aeolian paleoen-

vironments in the late Quaternary. In many cases, linear dunes

were the predominant dune form found in a vegetated state.

Vegetation was not the only criteria used to infer palaeo-

status, or the lack of aeolian processes operating today, on

dunes. The development of soils, gullying, degradation of

dune forms, and the presence of stone-age artifacts have all

contributed to designation of palaeo-status (Table 1). How-

ever, two major issues existed with these early studies: when

paleo-aeolian activity had actually occurred, and whether

Table 1 Examples of criteria used to designate palaeo-status to dunes

Criteria Location Example references

Vegetation on dune surfaces Australia Grove (1958)Dune form degraded relative to equivalent active dunes elsewhere Zimbabwe Flint and Bond (1968)Dune flanks gullied Niger Talbot and Williams (1978)Dunes drowned in marine location NW Australia Jennings (1968)Dunes drowned in lacustrine location Botswana Cooke (1984)Lithified dune sediments NW India Sperling and Goudie (1975)Palaeosols within dune body Strezelecki Desert, Australia Fitzsimmons et al. (2007b)Stone age artefacts on dune surface NW India Allchin et al. (1978)Higher silt/clay content than in active dunes Kalahari Thomas (1984)

358 Aeolian Paleoenvironments of Desert Landscapes

precipitation change was the principal or only cause of the

transition from active to stable status.

Prior to the 1980s, the timing of paleo-aridity could only be

achieved through a relative rather than numerical chronology.

For example, Grove (1969) noted for the Kalahari Quaternary

record involving stable dunes, lake shorelines, and calcretes

that: ‘‘The morphological evidence y is difficult to interpret.

No dates can be attached so far to the various features that have

been described and the following sequence of climatic events is

merely the simplest that can be devised to fit the facts’’ (p. 210).

In the Kalahari subsequent studies employed: (1) the relative

degree of post-formation dune degradation (Thomas, 1984) to

suggest differing periods of time over which degradation had

affected different dune systems and (2) different dune system

orientations to suggest changes in atmospheric circulation be-

tween phases of dune construction (Lancaster, 1981). Very

rarely, fortuitous contexts where material for the application of

radiocarbon dating was found in sediments overlying or

underlying dune sands could add a numerical age that might

hint toward the ages of dunes, for example, dated stromatolites

beneath a lunette dune at Urwi Pan, in the southern Kalahari,

were used to suggest that the lunette could not have formed

prior to c. 16 ka (Lancaster, 1986).

More widely around the globe, however, the principal

means by which dune systems were dated was through the

application of a principle espoused in the important hypoth-

esis of Sarnthein (1978): that globally, aridity was at its

greatest extent at times of glacial maxima, at which times dune

systems within deserts and beyond their present margins were

active (Figure 1). This hypothesis was based on the age of

aeolian sediment components occurring in some marine cores

(see above; Sarnthein and Diester-Haass, 1977), and on the

theoretically-based principles that it would have been both

drier and windier than today during glacial maxima.

In 1982 however, the first paper applying thermo-lumi-

nescence (TL) dating to dune sands, in the Thar Desert, India,

was published (Singhvi et al., 1982), generating burial ages

directly derived from the feldspars within dune sands them-

selves. Subsequently, technical developments and the transi-

tion to OSL dating (usually applied to the more abundant

quartz in sands) have, along with advances in field and la-

boratory procedures, revolutionized the ability to produce

chronologies of aeolian sand accumulation (Figure 2).

With dunes covered by dense vegetation, for example, in the

northern savannahs of southern Africa (O’Connor and Thomas,

1999), or dunes that are gullied through active runoff, for ex-

ample, in Nigeria (Talbot, 1984), it is relatively easy to ascribe a

palaeo-status and an increase in rainfall since the time of for-

mation under the operation of aeolian processes. However,

dunes, as modern studies of aeolian processes demonstrate, are

not landforms that can be simply regarded in terms of an ‘on’

or ‘off ’ status. Active dunes may only experience sand transport

on a seasonal basis, or episodically during drought events,

whereas only a small proportion of the total dune sediment

volume may be subject to transport under contemporary en-

vironmental conditions (Wiggs et al., 1995).

It is notable that the vast majority of dunes utilized in

continental desert paleo-aridity studies are linear forms, which

are relatively less mobile than other dune types because of

their extending nature of development (Livingstone and

Thomas, 1993), rendering them more liable to vegetation

colonization, at least on lower slopes, than, say, transverse

dune forms. Conditions can exist, therefore, where dunes are

seasonally or episodically active and experience vegetation

growth, sparsely across a dune body or on lower slopes,

whereas dune crests are active, so that (1) vegetation per se is

not necessarily an indicator of inactivity and (2) factors other

than, or in addition to, precipitation may account for whether

a dune is active to the wind or not.

These questions have, in recent years, led to studies that

challenge the utility of dunes as paleoenvironmental indicators,

especially in the context as proxies of the extent of past aridity

(e.g., Chase and Meadows, 2006). These authors instead invoke

wind energy changes as the dominant control on dune activity

status (Chase and Thomas, 2006). Others have attempted to use

the findings of process studies (Livingstone and Thomas, 1993)

or a broader understanding of dryland environmental dynamics

(e.g., Thomas and Burrough, 2012) to nuance the environ-

mental data that can be achieved from the investigation of

paleo-aeolian features. What is clear, however, is that studies of

paleo-aeolian records are most likely to advance productively

when they embrace in their analysis an understanding of the

controls on, and nature of, aeolian processes, and strong

chronometric control (Thomas and Wiggs, 2008).

11.18.2 Sandy Paleoenvironments

11.18.2.1 Distribution

Sands attributed to an aeolian depositional origin cover 5% of

the Earth’s land surface, plus narrow coastal dune strips

(Figure 3). Active continental dunes today occur primarily in

dryland areas, and in areas where drylands and other

Holocene

LGMActive sandseasIce caps

0°0° 60°60° 120°120° 180°180°60°60°120°120°

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Figure 1 Hypothesized extent of active desert sand seas at the Last Glacial Maximum (LGM) and in the Holocene. This frequently reproducedfigure was based not primarily on the analysis of terrestrial records, but on the presence of wind-transported sediments in some marine coresections dated loosely to the LGM. This generated a working hypothesis regarding the timing of dune field accumulation around the world, whichforms the underpinning of much research on Quaternary aeolian paleoenvironments to the present day. Redrawn from Sarnthein, M., 1978. Sanddeserts during glacial maximum and climatic optimum. Nature 272, 43–46, with permission from Nature.


conditions conducive to aeolian activity have been more ex-

tensive in the past. This includes:

1. Peridesert locations in the low latitudes, adjacent to today’s

hot desert environments. In the past, activity in these areas

has represented either a growth in the area of aeolian activity

relative to today or a displacement of aeolian activity from

areas presently active to adjacent locations. Examples in-

clude dune systems in the Sahel belt, south of today’s ex-

tremely arid Sahara (e.g. Grove, 1958), the Kalahari of

Fusa


central southern Africa (Thomas, 1984), interior Australia

(Twidale, 2008), and the Thar Desert, India (Singhvi and

Kar, 2004). In the last three examples, aeolian activity is

extremely limited in its operation today, relating either to

sand movement on otherwise stable dunes during drought

events or to sand movement on dunes where human ac-

tivities have removed the stabilizing effect of natural vege-

tation (e.g., Thomas and Leason, 2005). These stable dune

systems are often dominated by linear dune forms.

Sand seas Active Probably limited activity Fixed

Loess Major deposits Thin or uncertain deposits

Dune coasts

Equator

Tropic of capricorn

Tropic of cancer

0

Figure 3 Global distribution of aeolian sediments. Dune sands are classifieactive, partially active, or inactive under modern climatic conditions. IncorpoReproduced from Thomas, D.S.G., Wiggs, G.F.S., 2008. Aeolian system resintegration. Earth Surface Processes and Landforms 33(9), 1396–1418, with

1982–870

20

40

60

80

100

120

140

160

180

1988–91 1992–97 1998–2002 2002–2007

Num

ber

of r

esea

rch

pape

rs

igure 2 Five-yearly record of primary research papers publishedsing TL/OSL dating to produce ages of aeolian accumulation. Thetart point of the time line is the publication of the first paperpplying TL dating to dune sands, in 1982 (see text).

2. Dune systems in the mid-latitudes that are not close to

modern active dune systems and which represent a sig-

nificant change in the distribution of both drier and

windier environmental conditions, though they may be

located in presently semi-arid environments (e.g., Wolfe

et al., 2004, 2006). Such systems are commonly relatively

small compared to the fixed dune systems in 1 above, and

may occupy discrete basins. Sediment may have been

supplied to dunes from local lake basins or fluvial systems.

Examples include dune fields in the mid-west of the USA,

for example, in Nebraska, USA (Hanson et al., 2010),

Saskatchewan, Canada (Wolfe et al., 2006), and in parts of

Argentina (Tripaldi et al., 2010). A mix of dune forms may

be present, but parabolic forms, indicative of sand trans-

port on partially vegetated surfaces, may be significant

features. In some locations, for example, on the high plains

of the Free State, South Africa (Holmes et al., 2008), and

Texas, USA, lunette dunes may particularly be associated

with deflation basins (pans).

3. High-latitude dune systems, for example, in northern China

and northern Canada (Bateman and Murton, 2006), where

dune sands have potentially been derived from glacially

generated sediments. The dune forms present may be similar

to 1 or 2 above, depending on the scale of dune fields and

the nature of surfaces on which the dunes formed.

4. Cover sands, where morphologically-distinct dune forms

are lacking, though not totally absent. These tend to

occur in mid- and high-latitude locations, and are com-

monly associated with former glacial-margin periglacial

environments, for example, in many parts of central and

3000 km

d into those in coastal locations and continental sands that arerating information in various sources including Thomas (2010).

ponses to global change: challenges of scale, process and temporalpermission from Wiley.

Table 2 Examples of studies where mean annual precipitation values of active and stable dunes have been compared

Place Active dunes (mm) Stable dunes (mm) References

Linear dunesSouthern Africa (Kalahari and Namib) 150 200–1000 Grove (1969)

Lancaster (1981)Thomas (1984)

Southern Sahara/Sahel 150 750–1000 Grove (1958)Western Australia 200 1000 Glassford and Killigrew (1976)

Parabolic and linear dunesArizona 238–254 305–380 Hack (1941)NW India 200–275 800 Allchin et al. (1978)

Source: Thomas and Tsoar, 1990.


eastern (e.g., Zeeberg, 1998) and western Europe (e.g.,

Koster, 1988, 2005), and the UK (e.g., Bateman, 1998;

Clarke et al., 2001).

Figure 2 distinguishes, rather crudely, active, fixed, and

‘limited activity’ dunes. This simple classification attempts to

capture the two extremes of dune environmental contexts:

locations where surface conditions today are dry, where ae-

olian transport can operate unhindered by significant vege-

tation, and those where surface conditions, usually in the form

of an extensive vegetation cover, would inhibit all aeolian

sediment transport, however much wind energy were avail-

able. This is analogous to the ‘on’ and ‘off ’ context mentioned

previously. In terms of the distinction between the two, pre-

cipitation thresholds have been quoted in the literature, cer-

tainly ever since Hack (1941) and Grove (1958), with

identified threshold values differing from location to location

(Thomas and Tsoar, 1990; Table 2). As Livingstone and

Thomas (1993) noted however, sand transport potential oc-

curs along a spatial environmental continuum, so that in

many places the potential for activity requires more complex

consideration, especially in locations where wind energy and

precipitation display marked seasonal and inter-annual vari-

ability. In Figure 2, such areas may mainly fall in the ‘limited

activity’ categorization. In these locations, episodic sand

transport is a likely occurrence (Bullard et al., 1995; Thomas

and Leason, 2005), and the distinction between episodic and

localized surface sand movement today, and conditions that

in the past were more favorable for dune construction, is an

important consideration in assessments of aeolian

paleoenvironments.

11.18.2.2 Interactions between Controlling Variables

That suggested precipitation threshold values for aeolian activity

should differ from place to place in Table 2 clearly demon-

strates not only the limits of such an approach to dune in-

activity, but importantly that precipitation alone is not the sole

controlling variable impacting on dune activity. As Livingstone

and Thomas (1993) noted, ‘‘often the argument has been, first,

that vegetated dunes are fixed relicts, and second, that fixed

dunes indicate past aridityyneither of these maxims is neces-

sarily true’’ (p. 91). As process studies indicate, sand transport

and dune activity result from the interaction between erosivity,

or wind energy and the potential for aeolian entrainment, and

erodibility, the resistance offered by a surface to that energy,

through factors such as plant cover and surface moisture.

This interaction was captured effectively in Lancaster’s

(1988) mobility index (M), where M¼W/(P/PE), with W being

the amount of time wind blows above a threshold for sand

entrainment, and P/PE capturing rainfall effectiveness through

the relationship of precipitation to potential evapotranspiration.

Values of M were calibrated as 4200¼ fully active dunes,

100–200¼plinths and interdunes vegetated, 50–100¼only

activity on dune crests, and o50¼dunes fully inactive. More

sophisticated variants of this index have since been produced,

for example, incorporating rainfall lag effects on soil moisture

and rainfall seasonality (Thomas et al., 2005), but importantly,

Lancaster’s version was used to assess the degree of integrated

(erodibility and erosivity) environmental change that had oc-

curred since dunes in the southwest Kalahari were formed

(Figure 4).

Unpacking the relative contributions of erodibility and

erosivity changes that contributed to once-active dunes be-

coming less active or stable is more complex. In many studies

reporting paleo-dune environments, precipitation change is still

regarded as the principal parameter to have experienced change

since dune accumulation (e.g., Fitzsimmons et al. (2007a) for

the Strezlecki Desert, Australia). Some studies report wind en-

ergy changes as significant (e.g., Nanson et al. (1992a) for the

Simpson Desert, Australia) or dominant (e.g., Chase and

Thomas (2007) for western South Africa). In other studies, the

contributions of precipitation and wind energy change are not

differentiated. Reality is that without recourse to other proxy

records of the same age as the dune deposits, such distinctions

are difficult to achieve and somewhat arbitrary. For example,

Chase and Thomas (2006) inferred wind energy changes as the

dominant control on dune activity in SW South Africa at 16–24,

30–33, 43–49, and 63–73 ka because this coincides with

identified increases in aeolian dust in equivalent-age marine

core sections from the adjacent Atlantic (Stuut et al., 2002).

For parts of the southern hemisphere, Chase (2009) recently

extended the case for the greater recognition of, or even a

dominant role for, increased wind energy as an important dri-

ver of late Quaternary dune construction. Without wind energy,

there would be, of course, no dunes. It is, however, a mis-

conception to assume that high wind speeds alone will have led

to dune activity in the past, and it is necessary to consider and

establish the relative roles of both erodibility and erosivity

elements of the potential for dune development, as well as

SouthwesternKalahari dunefield

Namibsandsea

50100

200

50100200

50100200

50

100

200

Today

At time of SW Kalahari dunefield contruction

20°

25° S

20°

25° S

200 km0

(a)

(b)

MI values>200 = Fully activedunefield

100–200 = Dune basesand interdunesvegetated

50–100 = Crests onlyactive

<50 = Dunes inactive

Figure 4 Dune mobility index values for the southwest Kalahari dune field, (a) today and (b) inferred for 18 000 BP. See text for an explanationof dune mobility indices. Redrawn from Lancaster, N., 1988. Development of linear dunes in the southwestern Kalahari, Southern Africa. Journalof Arid Environments 14, 233–244.


issues of the chronometric basis on which comparisons be-

tween different paleoenvironmental proxy records are com-

pared. In the context of paleo-dunes, Thomas and Burrough

(2012) have recently made two observations:

1. The problems of the temporal precision of dated records,

and issues of comparing dated dune records with other

dated proxy records, are significant. The size of the errors

attached to ages (expressed as 71 sigma on OSL ages, for

example), commonly c. 10% of the mean age, makes the

treatment of errors critical to how dated records are com-

pared. This can have a substantial impact on whether or

not temporally coincident records (between dated dunes

and records interpreted as proxies for wind strength or

precipitation, or between dune formation and key en-

vironmental marker periods) are found. For example,

Lancaster et al. (2002) noted that the late glacial dune

building period of Mauritania closely parallels the dust

record (interpreted as a windiness record) from the ad-

jacent Atlantic. They also recorded that the dune building

phase is markedly longer than the dust accumulation epi-

sode. This could be due to imprecision in ages (a technical

issue), or because dune construction has responded to

different drivers or combinations of drivers at different

times within an overall period of dune building (an en-

vironmental issue). Even though wind energy is a key


variable in terms of sand transport and dune construction,

sand can only be mobilized, regardless of wind energy

levels, if surfaces are suitably exposed. Translated into

issues surrounding paleo-dunes, this means that vegetation

cover had to be sufficiently limited in the past for sand

exposure to the wind to have occurred. In areas of paleo-

dunes that are heavily vegetated/forested today due to high

modern rainfall amounts, precipitation would have to have

been substantially less in the past before significant sand

transport could have occurred.

2. The significance of climate variability, notably drought, in

impacting on the potential for sand transport and dune

construction, has not been sufficiently considered, yet is

very important today in areas close to notional thresholds

of dune activity (see, e.g., Thomas and Leason, 2005). It

cannot be discounted as being important in the past, par-

ticularly in areas typical of the ‘limited activity’ regions in

Figure 2. Drought is increasingly being recognized as a

potential contributor to reactivation events affecting small

dune fields in the American Midwest (e.g., Forman et al.,

2008). The principles invoked are equally applicable to the

behavior of currently largely fixed continental dune fields

in the low latitudes.

A key message here is that in considering the interpretation

of paleo-dunes in terms of past environmental changes, it is

vital not to treat all paleo-dunes as necessarily having had the

same controls on sediment supply and activation. Instead, it is

important to consider dunes specifically within the environ-

mental and climatic contexts in which they occur. It is sur-

prising how commonly this simple issue is ignored, but

encouraging that recent research is taking detailed account of

the local contexts and potential controls on dune dynamics in

the past. Such work includes recognition of the importance of

relatively short drought events (as opposed to major climate

changes) for dune activation, as demonstrated from the US

Great Plains by Hanson et al. (2010), and the potential

interactions between fluvial sediment supply and aeolian

dune construction, for example, in the Cooper Creek complex

in Australia (Cohen et al., 2010). The geomorphic role of small

mammals in the destabilization and activation of aeolian

systems has even been invoked in a study from Nebraska, USA

(Schmeisser et al., 2009).

11.18.2.3 Circulation Changes

Changes in atmospheric circulation since the time of paleo-

dune formation have been inferred from mean dune orien-

tation by relating inferred paleo-sand flow directions to

present-day dominant wind flow (e.g., Jennings (1968) for

Australia) or resultant sand flow vector (cf. Fryberger, 1979)

(e.g., Thomas (1984) for the Kalahari). Where patterns of

successive dune systems overlap and have orientation differ-

ences, changes in mean sand transport direction are more

readily suggested than in situations where each dune system is

spatially independent, as in the Kalahari. A good example of

overlapping systems is provided by the analysis of the linear

dune systems of Mauritania (Lancaster et al., 2002; see later

discussion).

Nonetheless, establishing the circulation conditions

responsible for paleo-dune system formation is not a

precise science, for a number of reasons. First is that we now

know, through application of detailed dating programs (see

below), that many dunes are not single-generation features,

and may therefore not have formed under a single circulation

regime. Second is that for active dune systems today, signifi-

cant sand transport is not necessarily a year-round phenom-

enon, and may be comprised of seasonally different, and

directionally distinct, components. For example, in the Namib

Sand Sea, mean annual sand flow comprises 80–90% from the

SSE-SW sector, decreasing to 55–65% in the interior and

35–40% at the easterly margin, whereas E-NE ‘berg’ winds,

which generally occur intensively for short periods in

July–August, account for less than 10% of sand flow at the

coast, rising to 30–55% in the interior and 60–65% in the east

of the system (Lancaster, 1985, 1989). Third is that large

dunes can affect local airflow to a significant degree, which in

turn can impact upon the orientation of secondary patterns of

dunes that develop on the flanks of the initial dune forms.

Thus, dune orientations can reflect modified airflow and

sand transport regimes, not primary atmospheric circulation

patterns.

Taking these factors into account, inferring circulation

trends for paleo-dunes, which requires the application of

analogs from the association between modern dune forms and

wind regimes, will likely be a gross simplification or averaging

of the actual sand flow regimes that occurred in the past. This

is particularly the case for linear dunes, the most common

paleo-dune forms, where modern dune-wind flow associ-

ations for active dunes can be difficult to establish. In the

relatively rare cases where paleo-dunes are transverse or bar-

chans forms (Bristow et al. (2009), for example, reported

barchn dune relicts in the sediments on the floor of Lake

Chad), such relationships may be relatively simpler to deduce

because of the more unidirectional wind flow patterns re-

sponsible for their formation. However, the very nature of

sand transport associated with transverse dunes, leading to

migratory dune forms, means that their potential for preser-

vation is far more limited compared to extending linear dune

forms. Such transverse forms should not to be confused with

the lunette or parabolic dunes that develop on the margins of

pans/playas as a consequence of sediment deflated from bare

pan surfaces being trapped on the downwind margin by

vegetation (first systematically analyzed by Bowler (1973) in

Australia, with more recent studies including the work of

Holmes et al. (2008) in the Free State, South Africa). That

linear dunes are the world’s most common dune type

(Lancaster, 1982), including paleo-dunes, is probably due to a

combination of the fact that unidirectional wind regimes are

relatively unusual, and that the extending and accumulating

nature of linear dune development favors their preservation

relative to more transient and mobile transverse forms

(Livingstone and Thomas, 1993).

11.18.2.4 Sediment Supply

Regardless of the prevalence of suitable climatic regimes in the

past, paleo-aeolian systems will never form in the absence of

Tim

e

0 −> + 0 −> + 0 −> +

ClimateSedimentproduction

Aeoliansedimentavailability

Aeoliantransportcapacity

Arid

HumidHumid

Weath-Weath-eringering

Humid

Con

tem

pora

neou

sav

aila

bilit

y-lim

ited

influ

x

Cumulativesystem

response

Aeolian system

response

Destructional

Sand-starvedconditions

Laggedtransport-limited influx

Laggedavailability-limited influx

Storedsediment

Weath-ering Stabilization

Con

stru

ctio

nal

(dun

e gr

owth

and

accu

mul

atio

n)

Figure 5 Theoretical model of dune system responses to changes from humid to arid climatic conditions. Modified from Thomas, D.S.G.,Wiggs, G.F.S., 2008. Aeolian system responses to global change: challenges of scale, process and temporal integration. Earth Surface Processesand Landforms 33(9), 1396–1418, with permission from Wiley.


suitable available sediment for sand transport. Sediment

availability therefore has to be factored into models that at-

tempt to interpret the potential for aeolian systems to accu-

mulate as a function of climatic change (Figure 5).

The cover sands of Western Europe (Koster, 2005), and

the loess of Europe, Asia, North, and South America were

principally sourced from glacial outwash materials. The sands

in desert and peridesert dune systems of lower latitudes pri-

marily have other origins, including coastal sources, lake

basins, fluvial systems, and, to a limited degree, the products

of wind erosion. The reworking of older dune sands is

also important in the creation of new dune forms (Kocurek,

1998). Together, the range of potential sources, and the vol-

ume of available sediment, can have a profound effect on both

the ability of suitable dune building climates to actually lead

to dune construction and, significantly, the potential for a

dune building period to be preserved in the sedimentary

record.

The Kocurek (1998) model of aeolian system responses to

climatic change, derived from first-hand experience of the

dune systems of the USA and Arabia (Figure 5), provides a

framework through which the likelihood for a record of

paleo-aeolian activity to be preserved, or destroyed, can be

assessed. Destruction phases occur when the potential for

sediment movement in a system, determined by climatic

factors, exceeds the supply of sediment to that system. In

Figure 5 this is presented as occurring largely toward the end

of an arid phase, when sediment supply in source areas is

exhausted. These conditions may also be hypothesized to

occur within a preexisting dunefield at the onset of a climatic

phase favoring dune building, especially at locations deep in

a dunefield or at distance from sediment source areas. Here,

existing dune sands are likely to be reworked prior to further

sediment arriving for new accumulation to occur. In a

Saharan context, Mainguet and Chemin (1990) categorized

dunes into ‘dunes d’erosion’ and ‘dunes d’accumulation’, based

on the occurrence of these negative or positive sediment

budgets.

Thus, dune construction and preservation should be con-

sidered not in terms of sediment transport potential per se,

which are what purely climatically-based interpretations do,

but should also include the sedimentary regime that is avail-

able for wind transport too. In reality, this can be exceedingly

difficult in studies that attempt to reconstruct environmental

conditions many millennia ago. The impact of sediment

supply is, however, included in a simple modeling framework

that attempts to theorize the potential for past dune building

records to be preserved in a sediment sequence (Telfer et al.,

2010). This also indicates how preserved records might be

expected to vary in terms of their completeness through a

dunefield, with the most complete preservation of records of

dune-building events likely in locations where net sediment

inputs are greatest.

The same factors also impact the climatic sensitivity of a

paleo-dune record. Close to reliable sediment supply areas in

deserts and peridesert locations, dunes may be constructed

with little sensitivity to precipitation: all that is required is a

regular supply of sediment and that sediment to be available

to the wind. Alluvially sourced dune sands from ephemeral

river systems are important in this respect. In Australia, for


example, Nanson et al. (1995) noted that close to the Finke

River in the Simpson Desert, dunes were supply controlled,

with dunes only extending downwind from source during arid

phases when surfaces were vegetation free and thus able to

facilitate longer-distance sand transport.

11.18.3 Chronologies of Paleo-Aeolian Systems

The sections above serve to identify the factors that impact the

environmental interpretation of paleo-dune records. When the

first direct-dating was achieved through the application of TL

dating in 1982 to dunes in Rajasthan, India (Singhvi et al.,

1982), it appeared that a significant geoproxy archive could be

unlocked to provide a major record of late Quaternary en-

vironmental changes, notably in environments where other

terrestrial proxies are commonly sorely lacking. As the number

of studies utilizing TL and/or OSL dating to provide dune

accumulation chronologies has grown, so the hypothesis of

Sarnthein (1978) has been repeatedly tested. As Singhvi and

Porat (2008) noted, ‘‘Applications of luminescence dating have

yquestioned the conventional textbook statements on syn-

chronous expansion and contraction of deserts, and of a direct

association of sediment with the forcing function of climate.

Global forcing is translated into dune accretion under optimal

combination of precipitation, evaporation and wind regimes

and their variations through time’’ (p. 552).

It is beyond the purpose or need of this piece to recount

region-by-region case studies of dated dune accumulation

chronologies: useful summaries are provided elsewhere (e.g.,

Goudie, 2002; Tchakerian and Lancaster, 2002; Thomas and

Shaw, 2002; Munyikwa, 2005a; Singhvi and Porat, 2008;

Gasse et al., 2008). Rather, a number of key themes and issues

are considered through regional examples that serve to illus-

trate issues that should be borne in mind in all studies of

paleo-dune systems.

?

?

?

?

?

?

24−15 ka 13−10 ka

Figure 6 Overlapping linear dune generations in Mauritania, with ages proModified from Thomas, D.S.G., Wiggs, G.F.S., 2008. Aeolian system responintegration. Earth Surface Processes and Landforms 33(9), 1396–1418, with

11.18.3.1 Multiple Events and Compound DuneLandscapes

Situations where distinct overlapping patterns of dunes exist as

testimony to distinct phases of dune building are rarely rep-

resented in the paleo-aeolian record. The dunes of the Azefal,

Agneitir, and Akchar sand seas in Mauritania (Lancaster et al.,

2002; Figure 6), dated through the application of OSL to 14

dune sediment samples from the system, are an exception

to this.

The dated record provides evidence of three phases of dune

building, resulting in the development of overlapping and

distinctly oriented (N-S, NNE-SSW and NE-SW) linear dune

patterns. It is interpreted that the wind regimes that occurred

during each of three late Quaternary dune-forming periods, at

25–15, 13–10 ka, and after 5 ka, were significantly different,

leading to the formation of dunes with distinct, super-

imposed, trends. There are some sedimentary differences in

the dunes of each generation, and some distinct separating

pedogenic horizons. That dunes of each generation are pro-

gressively smaller is explained in terms of the duration of each

distinct wind regime and transport energy levels in each phase.

11.18.3.2 Multiple Events and Complex AccumulationRecords

Early chronologies for many dune systems were based on

relatively few dune ages. Consequently, simple dunefield

histories resulted, for example, for the the Simpson Desert,

Australia (Nanson et al., 1995), Kalahari, southern Africa

(Stokes et al., 1997a), and the Algodones, California (Stokes

et al., 1997b). Where individual dunes or dunefields have

been subject to more detailed sampling and dating, resultant

chronologies have tended to become more complex. At

Witpan in the southwestern Kalahari for example, initial OSL

age determinations were obtained from samples collected

Post 5 ka

posed from the application of OSL dating to dune sediment samples.ses to global change: challenges of scale, process and temporal

permission from Wiley.

5%

4%

3%

2%

P

1%

0%0 10 20 30 40 50 60

Age (ka)70 80 90 100 110 120

10%

8%

6%

4%

P2%

0%0 10 20 30 40 50 60

Age (ka)70 80 90 100 110 120

30%

25%

15%

20%

10%

P

5%

0%0 10 20 30 40 50 60

Age (ka)70 80 90 100 110 120

(c)

(b)

(a)

Figure 7 Evolving linear dune accumulation chronologies for thesouthwest Kalahari dune field: (a) based on OSL ages published in1997, (b) with additional OSL and TL ages published by 2003, and(c) with further OSL ages published in 2007. Individual ages show byhorizontal bars: center square¼mean age, bars¼ 1 sigma errors.Curves are probability density plots with peaks showing most likelycentral points of accumulation phases in the records. Modified fromTelfer, M.W., Thomas, D.S.G., 2007. Late Quaternary linear duneaccumulation and chronostratigraphy of the southwestern Kalahari:implications for aeolian palaeoclimatic reconstructions andpredictions of future dynamics. Quaternary Science Reviews 26,2617–2630.


from dune sediments exposed in a gully at the southern end of

the dune. These produced a simple chronology of lunette

dune formation centered on 6 and 2–1 ka (Thomas et al.,

1997). Reanalysis and more detailed sampling via drilling

cores along the whole dune profile by Telfer and Thomas

(2006) led to a more complex history of dune development

being proposed. Nearly 33 OSL ages derived from nine sample

sites on the dune again showed that accumulation largely

occurred over the past 2 ka, but marked sectoral variability in

accumulation age was identified and attributed to spatial

variability in sediment supply from the pan surface to differ-

ent parts of the dune body. Similarly, in the dunefields of

interior Australia, the simple aridity model of early work such

as Nanson et al. (1992b) has been greatly embellished, with

regional differentiation of dune histories resulting from recent

studies such as Fitzsimmons et al. (2007b), who also identi-

fied paleosols within dune bodies in the Strzelecki Desert as

markers of dune stabilization, and Hollands et al. (2006),

who presented a complex chronology of dune building in the

northwestern Simpson Desert.

Chronologies of past aeolian accumulation can become

even more complex to interpret when the histories of whole

dunefields are based on successively more dated samples. This

issue is again best illustrated with reference to studies from the

southwest Kalahari, focusing on the linear dunes that dom-

inate the dunefield. The southwest Kalahari dunefield was

considered to be the youngest of three dunefields in the

Kalahari in studies that lacked direct dating of dune sediments

(Lancaster, 1981; Thomas, 1984). In 1997, seven OSL ages

were published for linear dunes in this as part of a Kalahari-

wide investigation of dune accumulation (Stokes et al., 1997a,

1997c). Ages from the southwest suggested that these dunes

had been formed in the last 30 ka in one, possibly two, phases

of dune development (taking mean ages and ages with errors

into account, Figure 7(a)). By 2003, further studies, from

Blumel et al. (1998) and Bateman et al. (2003) had added

11 new OSL and TL ages. These extended the period over

which accumulation was recorded to include the Holocene,

and also suggested more continuous deposition (marked

overlap in individual ages including errors, Figure 7(b)).

By 2007, a total of 88 ages had been published (Telfer and

Thomas, 2007). Many fell within the period covered by the

earlier studies but, because deep drilling of dune profiles had

contributed to sampling, these also extended the period dur-

ing which dune accumulation was recorded to over 110 ka

(Figure 7(c)).

All the samples dated in Stokes et al. (1997c), Blumel et al.

(1998), Bateman et al. (2003), and represented in Figure 7

were obtained from locations in the southern areas of the

dunefield (Figure 8(a)). A further 48 OSL ages, this time de-

termined from samples collected from four full-profile dune

cores from a more northerly part of the dune system, were

produced by Stone and Thomas (2008; Figure 8(b)). Many of

these ages fell within the previously recorded 30 ka accumu-

lation period, as well as back to c. 110 ka as recorded by Thomas

and Telfer. Two further ages also represent deposition in the

170–200 ka period (Figure 8(b)). This account raises two

issues. First is that as more samples are collected and analyzed

for a dunefield, the history of accumulation may change.

A simple picture of punctuated accumulation has, for the

southwestern Kalahari, been replaced by a chronology repre-

senting accumulation over a longer period of time, but with

ages unable to distinguish individual episodes of accumulation.

Second is that sampling may have a profound impact on the

(b)

(c)

200 180 160 140 120 100

Age (ka)

80 60 40 20 0

Mariental−Stampriet dunes (this paper)

? ?

Age (ka)200 180 160 140 120 100 80 60 40 20 0

Mariental−Stampriet dunes: 1m sampling intervals, starting at 0.5 m depth

?

Mariental−Stampriet dunes: 1m sampling intervals, starting at 1 m depth

?

MIS 6 MIS 5 MIS 4 MIS 3 MIS2 MIS1

AuobAuob

AuobAuob

Olifant

Olifant

Auob

Auob

Olifant

Nossob

Orange R

.

MarientalMariental

AranosAranos

UpingtonUpington

StamprietStamprietNossob

Nossob

Rehoboth

Kalkrand

Mariental

Windhoek

Koes

Aranos

NAMIBIA

BOTSWANA

SOUTH AFRICA

Upington

Tshabong

Gobabis

Stampriet

CT 96/10

CT 96/10944944

945945

946946

948948

943943

HDS 139

HDS 139

HDS 140, 141

HDS 140, 141

NAM06/1

NAM06/1

NAM06/4

NAM06/4

NAM06/6

NAM06/6

Witpan

Witpan

Mamatwan

Mamatwan

942942

KAL 98/2

KAL 98/2

KAL 98/2

CT 96/10944

945

946948

943

HDS 133

HDS 133

HDS 133

HDS 139

HDS 140, 141

NAM06/1

NAM06/4

NAM06/2

NAM06/2

NAM06/2

NAM06/6

Witpan

Mamatwan

KAL 98/3

942

18°

25°

23° S

16° E

TROPIC OF CAPRICORN

1000 km

(a)

20°20°20°

Sample site this study

TL sample site (Blümel et al, 1996)

Sample site (OSL) (Bateman et al, 2003)

Witpan site (OSL)(Telfer and Thomas, 2007)

KEY

20°20°20°

30° S

20°

SOUTHAFRICA

NAMIBIABOTSWANA

StamprietStamprietAquiferAquifer

StamprietAquifer

Witpan

Southern KalahariDunefield

0 500

km

Figure 8 (a) Location of sampling sites contributing ages to the records presented in Figure 7. (b) Ages from more northern cored samplesites in the southwest Kalahari dune field. (c) Differing overall chronologies of accumulation derived from samples collected at 1 m samplingintervals from the sample dune drill core, but with sampling depths commencing at 0.5 and 1.0 m below the dune crest (Stone and Thomas,2008).


1 2 3 4 5 6 7 8 9

1 2 3 4 5 6 7 8 9

70

60

50

40

30

20

10

0

Age

(ky

r)

INDIA

GanganagarDelhi

AmarpuraBikaner

Didwana JaipurJaisalmer

Pushkar

Bagor

Langhnaj

Ahmedabad

ARID

SEM

I AR

ID

3

8

7

7

7

77 7

7

6

5

4

2

19

Figure 9 Different published dune accumulation records for the Thar Desert, India, as summarized by Singhvi, A.K., Porat, N., 2008. Impact ofluminescence dating on geomorphological and palaeoclimate research in drylands. Boreas 37(4), 536–558, with permission from Wiley. Sourcesof data from Chawla et al., 1992; Singhvi et al., 1994; Singhvi et al., 1982; Thomas et al., 1999; Juyal et al., 2006; Andrews et al., 1998; Bristowet al., 2005, and Thomas et al., 1993.


record that is produced. While the dunefield considered here

has been analyzed in considerable detail, the same cannot be

said for many dune records presented in the literature

and, where individual studies are not integrated, complex

and apparently conflicting records may result, as suggested by

the summary diagrams produced for records from different

desert regions by Singhvi and Porat (2008; Figure 9). For

these dune systems, published records should be considered

as ‘work in progress’ rather than definitive accounts of

dunefield paleoenvironmental conditions. However, conversely,

more ages do not necessarily result in a clear record being

generated.

11.18.3.3 Sampling Stratigraphy

The frequency of sampling for dating dune sediments can also

impact the resultant dune accumulation record. This has al-

ready been presented in the context of sampling sites along a

dune, in the case of the records from the Witpan lunette dune

(Telfer and Thomas, 2007). How samples are collected ver-

tically through a dune body is also important to consider.

Stone and Thomas (2008) collected samples for dating at

0.5 m vertical intervals from sediment cores in the northern

part of the southwestern Kalahari dunefield. Figure 8(c) shows

these data presented as two records at 1 m sampling intervals,


one starting at 0.5 m below the dune surface, the other at 1 m

below the dune surface. The two strategies produce dune ac-

cumulation records with significantly different periodicities of

recorded accumulation. Clusters of ages representing dune

accumulation can therefore be sampling-dependent and may

not fully capture the actual record present with a dune body.

11.18.3.4 Preservation Potential

The dated records of accumulation preserved in paleo-aeolian

systems are, as issues presented above indicate, more difficult

to interpret in climatic terms than was once assumed. This has

become particularly apparent in recent years as investigations

have moved away from focusing on dune systems in two di-

mensions (e.g., studies that have mapped and interpreted

dune patterns: Grove (1958, 1969), Lancaster (1981), and

Thomas (1984), for example), or with limited vertical sam-

pling of dune bodies (e.g., Stokes et al., 1997a), toward three-

dimensional analyses that allow internal sediments to be more

fully examined in the context of the dunes or dunefields from

which they are derived.

This situation has arisen through the growing use of ground

penetrating radar (e.g., Bristow et al. (2000, 2007a) in the

Namib and Bristow et al. (2007b) in central Australia) and deep

drilling (e.g., Preusser et al. (2002) in Oman) to access sedi-

ments deep below dune surfaces. A result of these studies has

been either a greater targeting of sites to sample (e.g., Bristow

et al., 2007a), which might overcome some of the issues dis-

cussed in the previous section, or a marked increase in the

number of dated samples contributing to individual investi-

gations. Somewhat paradoxically, however, the increase in tar-

geted and detailed analyses has led, as previously noted, to

questions being raised about the representativeness of sampled

records of dune accumulation, and how these records may or

may not capture dune system paleoenvironmental histories.

Where the internal structure of a dune is observable, in a

natural or artificial section or in well-resolved GPR imagery, it

might be relatively simple to address these issues on strati-

graphical criteria alone. In the numerous studies that rely on

drilling for sampling, internal dune structures are not ob-

servable. The stratigraphical framework that is developed is

then based not on sedimentary and structural criteria, but on

the luminescence ages that are subsequently produced in the

laboratory. A question that then arises is how much of the

discrepancies between different studies from the same dune-

field result from issues of sampling, and how much from

issues relation to record preservation.

Flint and Bond (1968) noted how conditions post-dune

formation can lead to dune body degradation and erosion by

runoff during rainfall events. Sediment is removed from dune

crest and flanks and deposited in interdune areas, a point re-

visited by MacFarlane et al. (2005). Kocurek (1998) theorized

how temporal variations in sediment availability in a dune

system through arid–humid cycles could lead to phases when,

rather than accumulating sediment, dune bodies are eroded by

the wind itself (Figure 5). Even for linear dunes, which have a

greater likelihood of preserving a record of past dune activity

than for more mobile transverse dune forms (Thomas and

Shaw, 1991; Munyikwa, 2005b), it is necessary to at least

consider issues of dune accumulation and preservation

potential.

Telfer et al. (2010) have done this through the development

of a simple probabilistic model to simulate sand removal and

accumulation at the crest of a linear dune (Figure 10). The

model has been run to investigate how different sand flux

regimes may influence the net accumulation record over mil-

lennial timescales. Even when a linear dune is in an area of net

sand flow gain, the sand arriving at any particular location

on the dune crest is likely to have been sourced elsewhere on

the dune, so that there is a significant possibility that the

accumulated record at any location is discontinuous. It is

therefore important to attempt to factor in the impacts of both

externally forced (i.e., climate change-driven) changes in sand

movement on a dune and those due to the probabilistic

nature of sand transport on the dune surface. The degree of

discontinuity in an accumulated record may therefore differ

according to the interaction between external and internal

factors affecting sand transport: that is, the nature and fre-

quency of sand transport during a climatic regime favorable to

sand movement.

From their experiments, Telfer et al. (2010) concluded that:

1. Periods of markedly enhanced aeolian activity are readily

preserved in dune sedimentary records even if a phase of

reduced sediment transport is absent from the end stages of

activity.

2. When transport is more episodic, the potential for a full

record of activity being preserved is reduced because in-

ternal factors governing erosion or deposition at an indi-

vidual location on a dune become significant.

3. As proposed by Kocurek (1998), transitional phases be-

tween aeolian activity and inactivity are particularly well

represented in the preserved dune sediment record.

A key conclusion from this investigation is that the re-

lationships between climate forcing of dune activity and

resultant sand accumulation are likely to be highly com-

plex and nonlinear, meaning that rigorous field sampling,

and the production of many luminescence ages (‘some

hundreds, if not thousands’) may be necessary to fully

capture the depositional history of paleo-dune systems,

particularly in studies that use age frequency histograms to

represent dune age data. More prudently, it might be sug-

gested that age frequency histograms should be avoided

(Thomas and Burrough, 2012), and gaps in dated records

should not be interpreted as representing the absence of

dune activity (cf. the same issue referred to by Burrough

and Thomas (2009) with reference to lake shoreline

accumulation).

Dune age records therefore require extremely careful in-

terpretation in the context of other available paleoenviron-

mental records from adjacent areas and, particularly in

situations where the dunes being investigated are close to a

threshold for dune activity, careful assessment in terms of

the environmental factors that may lead to their activity,

including consideration of the role of climatic variability as

opposed to simply a role for major climatic changes

(Thomas and Burrough, 2012). Overlap between dune and

humid chronologies may not represent flaws in data sets or

limitations in the paleoenvironmental potential of dune

t = 151t = 150

t = 95

t = 69

t = 27

t = 25

t = 1S

edim

ent c

olum

n re

spon

dsS

edim

ent c

olum

n re

spon

dsto

net

sed

imen

t flu

x by

to n

et s

edim

ent f

lux

byge

tting

dee

per

or e

rodi

ngge

tting

dee

per

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rodi

ng

Sed

imen

t col

umn

resp

onds

to n

et s

edim

ent f

lux

byge

tting

dee

per

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rodi

ng

Cored profileat dune crest

Net sediment fluxalong dune crest

At the end of eachdeposition cycle,

reworking may occur

Revisedreworking

probability, Pr,reduces

Substrate

Dune surface

Sedimentinput

at depositionprobability, Pd

Enhanceddeposition

probability, Pd

If deposition continues

If deposition occurs

Sedimentreworking

at reworkingprobability, Pr

Enhancedreworking

probability, Pr

If reworking occurs

If reworking continues

Reviseddeposition

probability, Pd,reduces

Figure 10 Schematic diagram of the probabilistic model of Telfer et al. (2010). Each variable-thickness unit of t in the sediment columnrepresents preserved sediment from 2000 runs of t in the model. Each t is a time step of B10 years during a total period of 20 000 simulatedyears of dune surface variable activity. Modified from Telfer, M.W., Bailey, R.M., Burrough, S.L., Stone, A.E.S., Thomas, D.S.G., Wiggs, G.S.F.,2010. Understanding linear dune chronologies: insights from a simple accumulation model. Geomorphology 120, 195–210.


records, but may capture the reality of environmental

dynamism within the time intervals used to represent data

(Figure 11).

11.18.4 Future Prospects

Evidence of Quaternary aeolian paleoenvironments is wide-

spread across the globe (Figure 2), with dune landscapes

being a significant part of the paleo-aeolian record. Mapping

the distribution of paleo-dunes and cover sands from the late

1950s onward opened the door for their significance at the

global scale to be recognized. Advances in geochronology,

especially in luminescence dating from the early 1990s,

facilitated direct and detailed study of the paleo-archives

contained within aeolian sediments and landforms. Practical

and technical advances in the past 20 years have

gradually increased the detail of the records that have been

produced.

With that increased detail available to Quaternary scien-

tists, we are now faced with a new set of challenges. This

primarily concerns what dated dune records actually mean in

terms of environmental and climatic changes. A number of

studies have challenged the value of these archives because of

the uncertainty surrounding their meaning (e.g., Chase and

Meadows, 2007). The way forward in addressing the issues

raised rests on three main factors. First is better use

of aeolian process studies to inform Quaternary scientists of

the controls on dune development (Thomas and Wiggs,

2008). Addressing that challenge requires dune process studies

that focus on key issues at dune and dunefield scales, such as

the role of vegetation in dune dynamics, how different dune

forms relate to atmospheric circulation conditions (there is,

for example, still considerable debate surrounding how linear

MIS3 MIS2 HOLOCENE

H5 H4 H3 H2 H13

2

1

0−1

−2

−3

−390−400−410−420−430−440−450

475

450

425

Pre

serv

edge

omor

phic

act

ivity

by g

eogr

aphi

cal

unit

(bin

ary

data

)

TsodiloEtoshaMababe/NgamiMakgadikgadiSouthwest dunefieldNorthern dunefieldEastern dunefield

6

0

−32−34−36−38−40−42−44−46−48

Pre

serv

edge

omor

phic

activ

ity in

dex

Epi

ca d

ome

CdD

(pp

m)

50 45 40 35 30 25 20 15 10 5 0

Age (ka)

13

12 11 109

7 6 5 4 32

8

S

N

Nor

Grip

d18

O (

ppm

)

Inso

latio

n(W

m−2

)

LGM (as defined by Gasse et al., 2008) LGM (as defined by Chase and Meadows, 2007)

(b)

(a)

(c)

(d)

(e)

Figure 11 It is common, when assessing regional paleoenvironmental records, to attempt to ‘fit’ them to major global climate proxies such asmarine and ice core records. However, records derived from geomorphological sources may not correlate well, because geomorphic processesrespond to many controls that include climatic change and variability, and their interactions, as well as interactions between geomorphicsystems, sediment supply, and preservation potential. In this example, the dated geomorphological records from lakes and dunes in centralsouthern Africa over 50 ka are plotted against global-scale records of climatic change and selected forcing factors. Geomorphic activity indexdata (Thomas and Burrough, 2011) are acquired by summing binary data representation of dated active dune fields or lake basins preservedwithin any 1000-year time slice: (a) Preserved geomorphic activity by geographical unit, presented as binary data; (b) preserved geomorphicactivity index: total number of geographical units preserving a record in each 1 ka timeslice; (c) EPICA Dome C Ice Cores Deuterium Data (Jouzeland EPICA-community-members, 2004); (d) NGRIP d18O record (NorGRIP, North Greenland Ice Core Project members, 2004) and Insolation at151 North and South (Berger, 1992). (e) Insolation curves for 301 N and 151 S. It is to be noted that: (1) clear relationships between the regionaland global records are poor and (2) for much of the regional record, both dune and lake record accumulation exist (i.e., geomorphologicalactivity is recorded in both dune and lake systems within 1 ka time slices. This does not represent a limitation in the data sets, but rathercaptures the likely occurrence, based on modern analogs, of climate variability at sub-millennial timescales. Reproduced from Thomas, D.S.G.,Burrough, S.L., 2012. Interpreting geoproxies of late Quaternary climate change in African drylands: implications for understanding environmentalchange and early human behaviour. Quaternary International 253, 5–17.


dunes develop), and how dune bodies are preserved in the

landscape.

The second issue relates to the production of chronologies

of dunefield development. There are two main facets to this

challenge: how best to interpret and utilize the growing body

of luminescence ages derived from aeolian sands and how best

to capture dune system histories in field sampling programs.

Stone and Thomas (2008) and Telfer et al. (2010) represented

just a start toward addressing these issues. The third issue

concerns how to integrate dated dune records into wider

portfolios of paleoenvironmental records. The problems that

exist in doing this are substantial, as Thomas and Burrough

(2012) highlighted. Together, these three issues represent a

significant challenge to aeolian geomorphologists and Qua-

ternary scientists alike, but a challenge that has to be addressed

if aeolian paleoenvironments are to be better understood in

the future.

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Biographical Sketch

David Thomas is a geomorphologist whose interests include research into aeolian processes, aeolian systems, and

the Quaternary dynamics of aeolian systems, especially dune fields. He has conducted research in many dryland

areas, especially in southern Africa and also including North Africa, Arabia, and Iran. His interests also include the

application of geochronometric methods to aeolian systems, especially OSL dating.

His career began at the University of Oxford where he studied for three degrees (BA, Cert Ed., D.Phil.). He was

employed at the University of Sheffield for 20 years where he founded the Sheffield Centre for International

Drylands Research and where he became Full Professor and Head of the Department of Geography. In 2004 he

returned to Oxford, as Professor of Geography and Fellow of Hertford College. From 2008–2012 he was Head of

the School of Geography and Environment. In 2006 he was appointed as a visiting professor at the University of

Cape Town, South Africa.

David has authored over 120 refereed scientific papers and authored or edited several books, including Arid

Zone Geomorphology (Third edn., 2010). He has been principal investigator or co-investigator on research grants

funded to over d7million, and is currently participating in projects that include identifying dust source areas in


dryland to improve climate modeling, and the late Quaternary climate and environmental dynamics of the

northern Kalahari.

treatise on geomorphology || 11.18 aeolian paleoenvironments of desert landscapes

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