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similarities to many other climate records in its clear pattern of

100,000-year eccentricity cycles. For example, Figure P42

compares the DH isotope record to the deuterium records from

the Vostok, Antarctica, ice core (Petit et al., 1999). Controversy

has arisen, however, because the Devils Hole record indicates

that climate warming at the inception of the last interglacial

period began about 135,000 years ago, about 8,000–10,000

years earlier than predicted by the Milankovitch theory of cli-

mate. This early age for the penultimate interglacial remains

an unresolved challenge to the Milankovitch theory.

Lake records of temperature

Several floral, faunal and geochemical proxies have been used

to reconstruct atmospheric temperatures from lake sediment cores.

Fossil pollen assemblages are commonly used to infer past

temperatures (and precipitation) using a variety of quantitative sta-

tistical techniques. The modern analog technique (MAT) com-

pares fossil assemblages to a large modern pollen database

linked to winter and summer climate parameters. Except during

time of rapid climate change, when forest and pollen assemblages

Figure P42 The Devils Hole, Nevada, core DH-11 vein calcite oxygen-18 and the Vostok, Antarctica, ice core deuterium time series for the past

400,000 years. These sites are separated by 114 degrees of latitude: the Devils Hole record reflects northeast Pacific sea surface temperatures and

the Vostok deuterium record reflects East Antarctic regional temperatures. The Devils Hole data are from Landwehr et al. (1997); detailed

discussions of these time series appears in Landwehr and Winograd (2001), Winograd et al. (1992, 1997) and Petit et al. (1999).

Figure P41 Isotopic record from the GRIP ice core, Summit, Greenland for the past 100,000 years. Oxygen isotopes are 55 cm averages in parts per

mil. Isotopic excursions between 90,000 and 15,000 y

BP represent Dansgaard-Oeschger millennial climate events when rapid atmospheric

warming and cooling occurred over Greenland and many other parts of the world. Isotopic data from Dansgaard et al. (1993), GRIP

Project members (1993), obtained from World Data Center for Paleoclimatology.

PALEOTEMPERATURES AND PROXY RECONSTRUCTIONS 761

sometimes had no modern analogs, the MAT method has provided

excellent temperature history for North America during the past

15,000 years (Prentice et al., 1991). In Europe, using a slightly

different statistical approach to fossil pollen, Guiot et al. (1989)

reconstructed a 140,000-year long temperature record from La

Grande Pile and Les Echets in France. These studies show that

mean annual temperature in France varied from as low as 12

to –13



C during peak glacial conditions, to þ2to4



Cduring

the Eemian interglacial compared to today.

Geochemical methods applied to fossil lacustrine ostracodes

recovered from lake cores also provide quantitative estimates

of aquatic and atmospheric temperatures (Curtis and Hodell,

1993; Holmes and Chivas, 2002). Von Grafenstein et al.

(1999) applied the relationship between mid-European oxygen

isotopic composition of precipitation and atmospheric tempera-

tures to reconstruct a 15,500-year temperature record from the

O of fossil ostracodes from Lake Ammersee, Germany

(Figure P43). This decadal resolution achieved at Ammersee

allowed the researchers to correlate fine-scale European tem-

perature changes during the deglacial period with the record

from Greenland.

Other methods

A wide variety of methods have used qualitative (i.e., relatively

warm, cool) and semi-quantitative (i.e., < 4



C, between 15

and 20



C) interpretations of paleotemperature from the geolo-

gical record (Table P4; see Frakes et al., 1992; Parrish, 1998;

Bradley, 1999). These approaches rely on expert interpretation

of glacial and periglacial sediments and geomorphological fea-

tures, lithologic types (i.e., gypsum, peat) and paleogeographic

distribution, fossil plant associations and morphologies (leaf

shape), ancient soils (paleosols), ancient snowline and treeline

positions, among others. As is the case with more quantitative

methods, some of these indicators are influenced as much by

precipitation and other climatic factors as they are by tempera-

ture. Even when firm estimates of temperature can be obtained,

for example, in the case of fossil insects (Coope, 1994; Elias

and Coope, 1994), many of these records are temporally and

spatially discontinuous, making it difficult to achieve interre-

gional paleotemperature correlations and to infer causal factors.

Nonetheless, for many regions of the world, these methods pro-

vide a rich and valuable literature providing local and regional

information on mean annual or seasonal temperature conditions

for many periods in the geological past.

Thomas M. Cronin

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(1999), GRIP record of Dansgaard et al. (1993); Courtesy of U. Von Grafenstein, Laboratoire des Sciences du Climat det de l’Environnement,

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Cross-references

Astronomical Theory of Climate Change

Beetles as Quaternary and Late Tertiary Climate Indicators

Climate Change, Causes

Climate Variability and Change, Last 1,000 years

Dansgaard-Oeschger Cycles

Dendroclimatology

Glacial Geomorphology

Ice Cores, Antarctica and Greenland

Ice Cores, Mountain Glaciers

Lacustrine Sediments

Mineral Indicators of Past Climates

Ocean Paleotemperatures

Ostracodes

Paleobotany

Paleolimnology

Paleosols, Pre-Quaternary

Paleosols, Quaternary

Palynology

Sedimentary Indicators of Climate Change

Speleothems

Stable Isotope Analysis

PALEOTEMPESTOLOGY, THE SEDIMENTARY

RECORD OF INTENSE HURRICANES

Introduction

Intense storms can modify landforms, and in appropriate

depositional settings a geologic record can be preserved (e.g.,

Liu and Fearn, 2000; Donnelly et al., 2001a, b). Due to the

relatively short period of reliable instrumental and historic

records, little is known about past intense hurricane landfalls

and tropical cyclone activity in general, and the processes that

govern the formation, intensity, and track of tropical cyclones

are poorly understood (Goldenberg et al., 2001). Reliable

records of Atlantic tropical cyclones maintained by the

National Oceanic and Atmospheric Administration (NOAA)

only extend back to the mid-nineteenth century. An often

incomplete historical record of North Atlantic hurricanes dates

back several hundred years.

Given the potential for changes in the intensity and fre-

quency of storms due to anthropogenic warming of the global

PALEOTEMPESTOLOGY, THE SEDIMENTARY RECORD OF INTENSE HURRICANES 763

climate system, decision makers, scientists and the general pub-

lic have become increasingly concerned about potential risks to

coastal populations. Gaining an understanding of how changes

in hurricane activity may link to changes in climate is impor-

tant in order to forecast future changes and possibly mitigate

socio-economic impacts. Potential connections between the fre-

quency and intensity of hurricanes and changes in the Earth’s

climate system can be examined if long-term records of hurri-

cane activity can be developed.

Hurricanes and climate

Some studies have suggested that the intensity and frequency of

hurricanes may increase in a warmer climate. In a case study from

the western Pacific, model results suggest a 5–12% increase in

hurricane intensity with a 2.2



C increase in sea-surface tempera-

tures (SSTs) (Knutson et al., 1998). Globally SSTs increased by

0.31



C in the last half of the twentieth century, possibly as a result

of anthropogenic greenhouse gas emissions (Levitus et al., 2001).

Goldenberg et al. (2001) attribute the recent increase in North

Atlantic hurricane activity to increasing SSTs and lower amounts

of vertical wind shear. Long-term records of SSTs derived from

oxygen isotope data (e.g., Keigwin, 1996) and faunal data (deMe-

nocal et al., 2000) document centennial- to millennial-scale fluc-

tuations in SST in the North Atlantic. These records indicate that

SSTs were as much as 1



C greater than present during the Medie-

val Warm Period about 1,500–900 years ago and again for a brief

interval, interrupting the Little Ice Age, about 500–600 years ago.

Recently numerous researchers have postulated links bet-

ween changing modes of interannual climate variability, like

El Niño-Southern Oscillation (ENSO) and the North Atlantic

Oscillation (NAO), and patterns of tropical cyclone activity.

Studies relying on recent climatology indicate that North

Atlantic hurricane activity is greater during strong La Niña

years and suppressed during strong El Niño years (Bove et al.,

1998). In strong El Niño years, increased vertical wind shear

associated with a strengthening of the subtropical jet over the

tropical North Atlantic hinders tropical cyclone development.

Several reconstructions of past ENSO variability have been

published in recent years. For example, Moy et al. (2002 )

reconstructed a 12,000-year sedimentary record of ENSO

variability from southern Ecuador. Significant millennial and

sub-millennial scale ENSO variability is evident in this record.

Warm ENSO (El Niño) events became more frequent in the

Holocene. Approximately 1,200 years ago, a trend of decreas-

ing frequency of warm ENSO events began. Several intervals

in the last 2,000 years had notably few warm ENSO events.

Given that warm ENSO phases are thought to diminish North

Atlantic tropical cyclone activity, including intense hurricanes,

more North Atlantic hurricanes may have occurred in intervals

with relatively few warm ENSO events.

Elsner et al. (2000a) propose a possible link between a weak

NAO and increased intense hurricanes in the North Atlantic, pos-

sibly related to the strength of the trade winds. Using sandy wash-

overs preserved in a coastal lake from the Florida Panhandle, Liu

and Fearn (2000) inferred two intervals of intense hurricane activ-

ity in the Gulf of Mexico from present day to 1,000 and 3,400–

5,000 years ago. They suggest that this millennial-scale variability

may be associated with changes in the position of the Bermuda

High that may alternately focus hurricanes into the Gulf Coast

and East Coast of the USA. Elsner et al. (2000b) document this

kindof see-saw pattern in Atlantic hurricane tracks associated with

the position of the Bermuda High and NAO intensity over the last

two centuries, where the Gulf Coast experiences more hurricane

activity during a weak NAO and the U.S. East Coast experiences

more activity during a strong NAO.

Storm-induced deposits

Washovers (overwash deposits) occur when wave energy com-

bined with high water levels (storm surge) overtop or breach

barrier beaches and transport nearshore and barrier sediments

into the backbarrier environment. If the barrier is breached, a

lobate fan will form at the terminus of the breach. If multiple

breaches occur close together, these lobes may coalesce. When

the entire barrier is overtopped by storm surge, sheet washovers

that extend the length of the barrier are deposited. In a regime

of rising sea level, low-energy estuarine deposits are typically

deposited over washovers between overwash events. If these

deposits are not reworked during subsequent storms, the over-

wash fans will be preserved as laterally continuous horizons

of sand within backbarrier sediments and provide a record of

past storm surge (Figure P44).

Washovers are composed of locally-derived material

from the foredune, beach and nearshore environments and are

typically well-to poorly-sorted, fine-to-coarse sand. The mean

grain-size within overwash deposits generally decreases as a

function of distance from the barrier (Hennessy and Zarillo,

1987). Washovers often exhibit horizontal stratification, and if

the deposit terminates in a coastal pond or lagoon they can

exhibit medium- to small-scale delta foreset stratification

(Schwartz, 1975). However some washovers can also lack stra-

tification. Cores from backbarrier environments in southern

Rhode Island contain washovers with little or no horizontal

stratification (Donnelly et al., 2001a). In some cases post-

depositional processes like ice scour and bioturbation may era-

dicate laminations.

Another characteristic of washovers preserved in backbarrier

environments is the abrupt nature of the contact with the under-

lying peat or estuarine mud (Donnelly et al., 2001a, b). Evi-

dence of soft-sediment deformation is common where a

washover is deposited over saturated, fine-grained sediments

(Klein, 1986). In addition, rip-up clasts are frequently encoun-

tered at the basal contact of washovers and are indicative of

high-velocity currents associated with overwash deposition.

Washovers preserved in coastal sediments have provided the

basis for constructing prehistoric records of intense-hurricane

strikes in Alabama, Florida, Rhode Island, and New Jersey

(Liu and Fearn, 2000; Donnelly et al., 2001a,b). In Alabama

and Florida, sand deposited in coastal lakes provided evi-

dence of apparent intense-hurricane strikes dating back several

Figure P44 Cross-section of conceptual model of overwash deposition

and the landward translocation of the barrier-marsh system in a regime

of rising sea level. Overtopping of the barrier beach by storm surge

results in washover deposition across backbarrier marshes. Washover

fans are preserved as sea level increases and they are covered with

marsh deposits. Vertical exaggeration = approximately 100 to 500.

764 PALEOTEMPESTOLOGY, THE SEDIMENTARY RECORD OF INTENSE HURRICANES

thousand years (Liu and Fearn, 2000). Higher-resolution

records obtained from backbarrier marshes in Rhode Island

(Donnelly et al., 2001a) and New Jersey (Donnelly et al.,

2001b) contain evidence of intense-hurricane strikes dating

back approximately 700 years.

Succotash Marsh in Rhode Island contains six distinct wash-

overs dating back to

A.D. 1300 (Donnelly et al., 2001a). Four

of these deposits (Figure P45) date to the historic period and the

ages are consistent with known hurricane strikes (

A.D. 1954, 1938,

1815, and 1635) that resulted in significant levels of storm surge

(>3 m). T wo additional washovers at Succotash Marsh date to

A.D. 1411–1446 and A.D. 1295–1446 and were probably deposited

during prehistoric hurricane strikes. At Whale Beach, NJ, a large-

scale washover is attributed to an intense-hurricane strike in

A.D. 1821 (Donnelly et al., 2001b). An earlier land-falling hurricane

probably deposited an older, larger -scale washover at Whale Beach

dated to between

A.D. 1278 and 1434.

Conclusions

Faced with expectations of a potentially rapidly changing global

climate system, concern has increased about potential risks to

coastal communities and ecosystems from possible increased tro-

pical cyclone activity. An improved understanding of the rela-

tionship between changes in tropical cyclone activity and

changes in climate will help to anticipate future changes and

possibly mitigate socio-economic impacts. Long-term records

of these events can show how hurricane activity has varied in

the past as the Earth’s climate has changed. The relatively new

field of paleotempestology is focused on extending the limited

instrumental and historic records of intense storms by recovering

and quantifying natural archives of storm occurrence.

Backbarrier wetlands, lagoons, and coastal ponds are often

well situated to receive allocthonous sediments during intense

hurricane landfalls. These normally low-energy environments

are dominated by fine-grained highly organic sediments, with

the exception of episodic deposition of coarser-grained mineral

sediments from the beach and near-shore environment during

extreme storms. Results from the northeast and Gulf coasts of

the United States indicate that intense hurricane strikes produce

a distinctive sedimentary signature that can be used to recon-

struct long-term records of these events.

Jeffrey P. Donnelly

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Figure P45 (a) Tracks of historic hurricanes resulting in over 3 meters of storm surge in the northeastern U.S. and the location of Succotash

Marsh, RI (SM) and Whale Beach, NJ (WB). (b) Photographs of sandy washovers (light layers) interbedded with backbarrier marsh sediments

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Cross-references

Coastal Environments

Little Ice Age

Medieval Warm Period

North Atlantic Oscillation (NAO) Records

Paleo-El Niño-Southern Oscillation (ENSO) Records

PALYNOLOGY

Palynology is the study of microorganisms, and microscopic

fragments of mega-organisms, that occur in sediments. “Palyno-

morph” is a broad term for a variety of acid-resistant microfossils

produced by plants, animals, or Protista that have existed since

the late Proterozoic (approximately one billion years ago). The

oldest known palynomorphs are arcritarchs (Figure P46), which

are likely marine algal bodies that appeared in the Proterozoic

eon. Other palynomorphs that appeared in the Paleozoic and are fre-

quently studied are chitinozoans (marine fossils of uncertain origin);

scolecodonts (annelid worm jaws); microscopic colonial algae;

embryophyte spores; megaspores; pollen (Figures P47–P51); and

dinoflagellates. All of these are still present except the chitinozo-

ans. Chitinous fungal spores are first found in Jurassic rocks,

and microforaminiferal inner tests can be dated to the Lower Cre-

taceous (Traverse, 1988). Fragments of wood and cuticle

are additional acid-resistant plant materials that may be present

in palynological preparations and are frequently studied in

conjunction with palynomorphs. Palynomorphs range in size

from 5 to 500 microns. Identification to the specific, generic, or

higher taxonomic rank requires study with a light microscope.

With few exceptions, palynomorphs are at least partly composed

of sporopollenin, chitin, or pseudochitin; resistant organic mole-

cules that survive the standard laboratory treatments with hydro-

fluoric and hydrochloric acids (Traverse, 1988). Thus neither

siliceous microorganisms (e.g., diatoms and radiolarians), nor

Figure P46 The arcritarchs Inderites spp. This example occurs 5.8

meters above the Carboniferous-Permian boundary stratotype section

at Aidaralash Creek, Ural Mountains, Kazakhstan (Dunn, 2001). Photo

courtesy of M. Dunn.

Figure P47 The pollen grain Vittatina costabilis Wilson, 1962, identified

from 7.1 meters below the Carboniferous-Permian boundary stratotype

section at Aidaralash Creek, Ural Mountains, Kazakhstan (Dunn, 2001).

This grain is striate (deep parallel grooves) and slightly saccate. The

palynomorph was well-established in the late Carboniferous and early

Permian and has been referred to the Gnetales order by some authors,

and conifers by others (Traverse, 1988). Photo courtesy of M. Dunn.

Figure P48 The pollen grain Limitisporites monstruosus Lyuber and

Val’ts, from 7.4 meters above the Carboniferous-Permian boundary

stratotype section at Aidaralash Creek, Ural Mountains, Kazakhstan

(Dunn, 2001). This pollen grain has a bisaccate form. Bisaccate pollen

with residual three-rayed slits (called trilete) may be intermediate forms

between ancestral trilete spores and the modern bisaccate conifer

pollen, which have no slits. Photo courtesy of M. Dunn.

766 PALYNOLOGY

calcareous microorganisms (e.g., foraminifera and coccolitho-

phores) are palynomorphs.

Modern palynology addresses the morphology and biochemis-

try of the microorganisms, and their production and distribution

within the present environment. Palynological studies are used in

plant taxonomy, plant pathology, phytogeography, ecology, foren-

sics, medicine (allergies), entomology, melissopalynology (study

of honey), and archeology. An extant palynomorph’s environmen-

tal preferences can help define the ecological preferences of

the same (or possibly similar) palynomorph in the geological

record;this is the“modernanalogue” interpretivetechnique,which

is applied to paleopalynology. Recently, some palynomorphs have

been increasingly utilized for environmental monitoring (e.g.,

algal blooms, including the “red tide” phenomena).

Palynomorphs may be found in consolidated or unconsoli-

dated marine and terrestrial sediments of siliclastic or carbonate

composition. Fossil palynomorphs are typically studied from

small samples (even 10 grams) of fine-grained sediments or sedi-

mentary rock, such as siltstone retrieved from sediment cores, drill

cores, or rock outcrops. The age of the sedimentary matrix will

determine the geological age of the constituent palynomorphs.

Paleopalynology is utilized in biostratigraphy, geochronology,

and in paleoenvironmental studies (Traverse, 1988). Biostratigra-

phy involves dating and correlation of spatially distant rock strata,

based upon assemblages of palynomorphs found within the sedi-

mentary matrix. Geochronology, or relative age dating, is based

Figure P50 An example of a pollen grain, Patinasporites densus,

Leschik, 1955 (Upper Triassic, North America), which is monosaccate in

form. Monosaccate pollen appeared during the Carboniferous and

proliferated during the Lower Permian. Transitional forms between

monosaccate and bisaccate grains suggest that at least some

bisaccate pollen types originate by modification of the monosaccate

condition. P. densus has been recovered from Triassic conifer

cones (Cornet, 1977). Abundant in Upper Triassic rocks, P. densus

disappears at the Triassic-Jurassic boundary. Monosaccate pollen is

still produced by modern conifers such as Tsuga (hemlock). Photo

courtesy of S. Fowell.

Figure P49 Lunatisporites sp. (Upper Permian, UK) pollen. This is a

pollen grain that is bisaccate in form, with a striate sculpture

consisting of parallel grooves and /or thickenings on the main

body (corpus). Striate bisaccates, which appeared during the

Carboniferous and flourished during the late Permian and early

Triassic, were produced by coniferous and glossopterid gymnosperms,

which have since vanished from the paleobotanical record. However,

striate, non-saccate pollen is known from the extant gnetalean

gymnosperms Ephedra and Welwitschia (Traverse, 1988). Photo courtesy

of S. Fowell.

Figure P51 Corollina torosa (Reissinger, 1950) Klaus, 1960 emend.

Cornet and Traverse, 1975 (Lower Jurassic, North America) pollen. The

palynomorph is found as a tetrahedral tetrad, as shown here, or as a

single grain, monad. The pore-like thinning, or tenuitas, and the

encircling colpus of the Corollina genus are features generally associated

with angiosperms. However, pollen grains of Corollina have been

extracted from Mesozoic conifer cones of the family Cheirolepidaceae. C.

torosa appears in the late Triassic and becomes extremely abundant in

early Jurassic assemblages. In North America, the acme zone of Corollina

spp. can be used to distinguish basal Jurassic palynofloras from those of

latest Triassic age. Photos courtesy of S. Fowell.

PALYNOLOGY 767

on correlation of palynomorph taxa from a sedimentary rock of

unknown age with established palynomorph reference sequences.

Paleopalynology is also utilized in paleoecological studies, since

many palynomorphs are indicative of specific environments. Pol-

len and spores in particular are sensitive indicators of terrestrial

environments and thus have been used in terrestrial paleoenviron-

mental studies (Traverse, 1988). Palynomorphs have been used

extensively in exploration geology and hydrocarbon studies,

because the color of constituent pollen and spores reflects the

thermal maturity of the rock. Regional studies can elucidate paleo-

geographic patterns, biodiversity, phylogeny, taxonomy, and ecol-

ogy of the plant or animal group under examination.

The scientific issues addressed by palynological studies vary

considerably, dependent on the organism and time period ana-

lyzed. In Pliocene to Holocene studies, the ecological and climatic

range of the vascular plant taxa that produced the fossil pollen

(or spore) is almost always considered relevant to the scientific

interpretation. Thus, studies of Pliocene to Holocene fossil pollen

will often have a strong ecological component. Similarly, marine

dinoflagellate species are distributed according to distinct envir-

onments (e.g., temperature, salinity, nutrients). Thus a paleo-

environment may be interpreted based upon the present-day

environmental affinity of a taxon. Palynological studies of Pro-

terozoic to middle Miocene strata are generally conducted in

order to resolve biostratigraphic and geochronologic questions,

to provide information regarding the past distribution of organ-

isms, or to place broad constraints on past climatic conditions.

The application of palynomorphs to biostratigraphy is exempli-

fied by a study of the Carboniferous-Permian boundary strato-

type section at Aidaralash Creek, in the southern Ural Mountains

of Kazakhstan (Dunn, 2001). The Lower Permian boundary

has already been established based upon conodont biostrati-

graphy. Palynomorphs found in this stratotype section include

abundant and diverse species of the pollen genus Vittatina,

Limitisporites monstruosus pollen, and the acritarch Inderites

(Figures P46–P48). Documentation of the palynomorph assem-

blages may help determine which microfossils can be correlated

from the stratotype section to other palynomorph-bearing Permian

sections and permits correlation with stratigraphic sections

where the conodonts (other marine invertebrates) are rare or absent

(Dunn, 2001). In North America, identification of Corollina spp.

pollen grains (Figure P51) has biostratigraphic and evolutionary

plant taxonomy implications. The Corollina pollen grains were

apparently produced by conifers, yet show morphology charac-

teristics usually unique to angiosperms (Cornet, 1977). The acme

zone of Corollina spp. can be used to distinguish basal Jurassic

palynofloras from those of latest Triassic age.

Margaret Kneller and Sarah Fowell

Bibliography

Cornet, B., 1977. The Palynology and Age of the Newark Supergroup.

Unpublished PhD thesis, Pennsylvania State University.

Dunn, M.T., 2001. Palynology of the Carboniferous–Permian boundary

stratotype, Aidaralash Creek, Kazakhstan. Rev. Palaeobot. Palynol.,

116, 175–194.

Traverse, A., 1988. Paleopalynology. Winchester, MA: Allen & Unwin

Inc., 600pp.

Cross-references

Dating, Biostratigraphic Methods

Dinoflagellates

Pollen Analysis

PATTERNED GROUND

Patterned ground occurs in a variety of different geographical

environments, but is perhaps best developed within soils subject

to regular freeze-thaw cycles. It provides a good example of self-

organization within a geomorphological system (Hallet, 1990a).

The term-patterned ground is used to describe a plethora of

different phenomena with a variety of characteristics and sizes.

On horizontal surfaces, patterned ground usually consists of iso-

lated circles, polygonal nets, or closely spaced mounds, while on

slopes it commonly occurs as either transverse steps or down-

slope stripes. The most spectacular forms of pattern ground

involve size sorting (Figure P52a, b). The formation of patterned

ground has been the subject of considerable debate ever since

earlier explorers of Arctic regions marveled at its regularity. It

is now clear that there is no single mechanism to explain all

examples of patterned ground, most types are polygenetic, and

several different processes may result in very similar forms

(Washburn, 1979). Three suites of processes are required to give

the range of forms observed (Figure P53). Firstly, a sorting pro-

cess (or processes) is required that can separate and concentrate

particles of different sizes. Secondly, a process (or processes) is

needed that can give rise to the regular patterns and spacing

observed. Finally, it is clear, as one would expect, that gravity

must play an important role in modifying the patterning and sort-

ing processes on slopes. Of these three process suites, it is sorting

and patterning that provides the greatest challenge and remains

controversial despite considerable research (French, 1996).

It is possible to provide a size sorting mechanism by exam-

ining the behavior of a soil of mixed grain size subject to

repeated freeze-thaw cycles (Mackay, 1984; French, 1996).

As a soil freezes, it expands as the water within it changes

phase. As the freezing front (0



C isotherm) descends through

the soil, the upper part of a large stone is likely to freeze firmly

to the adjacent soil before the lower part. Subsequent ice

growth and expansion will tend to raise each stone relative to

the surrounding unfrozen soil beneath it. Most of this relative

motion will be lost during a thaw, as the stone settles. However,

the reversal is unlikely to be complete as finer particles will

tend settle and work beneath the larger ones. The net result of

this process after successive freeze-thaw events is the upward

movement of larger stones with respect to the finer matrix.

The movement of these stones will parallel the direction of

maximum heat flow (Hallet, 1990b). If the freezing front des-

cends parallel to the surface, movement will occur perpendicu-

lar to it. However, given slight heterogeneities in natural terrain

due to local relief, cracks, varying soil texture, or vegetation

cover, the freezing front may not be a planar surface. The con-

sequence of this is that stones will be drawn towards certain

areas on the ground surface. For example, freezing is likely

to be slowest in finer grained soils, in part because they retain

more moisture and hence release more latent heat upon freezing

than coarser soils (Hallet, 1990b). The freezing front will there-

fore propagate faster in areas of coarser texture and will be

lowered in these regions with respect to areas of finer texture.

The heat flow vectors will therefore be directed towards areas

of coarser soil, and consequently coarser particles within the

soil will tend to move towards these areas during repeated

freeze-thaw cycles, thereby reinforcing the textural inhomo-

geneities present. This provides not only a vertical but also a

lateral sorting mechanism.

768 PATTERNED GROUND

The next issue to consider is the range of possible patterning

processes. The simplest of these processes is contraction cracking

of frozen ground and the formation of polygonal nets (Washburn,

1979). These result from horizontal tension within the soil asso-

ciated with a decrease in volume caused by thermal contraction,

and resemble patterned ground produced in arid regions by the

desiccation of surface soils. The size and configuration of the poly-

gons depends on the rheological behavior of the soil and the nature

of the volume change, which is in part determined by the climatic

regime. In permafrost areas, the cracks become ice-filled and

demarcated by ice wedges (Figure P52c; French, 1996). Thermal

contraction does not explain all patterned ground and additional

patterning processes are required. Research in recent years has

concentrated on the presence of convective systems within soils

subject to regular freeze-thaw cycles (Hallet, 1990b). There are

two main types of convective systems. The first of these involves

the movement of water through porous soils (Gleason et al., 1986;

Krantz, 1990).This process operates during summer months when

the temperature near the ground surface is a few degrees above



C, while at depth it remains at 0



C along the thaw surface. In

the case of permafrozen soils, this corresponds to the top of the per-

mafrost table. Water reaches its maximum density at 4



C and con-

sequently the warmer surface waters are denser than the water at

the thaw front, leading to a density-driven water circulation

through the soil pore network. This circulation will result in the

movement of heat, and therefore variations in the rate of thawing

at different depths within the soil. Irregularities in the downward

propagationof the thaw front may develop, producingpatterns that

are dictated by the size and geometry of the circulation cells.

Although water movements have been observed and the theoreti-

cal geometry of the cells corresponds well with the scale of

observed patterning, problems remain in understanding how this

process controls the long-term formation and maintenance of

patterned ground (Hallet, 1990b;French,1996). The alternative

view involves the convective movement of soil. Again, theoretical

considerations suggest that potential cell dimensions correspond

well with the scale of patterned ground, and soil movement has

been observed at rates of 1 cm per year, but the mechanism that

drives this circulation remains elusive (Hallet et al., 1988; Wash-

burn, 1989). Research is underway to observe and model potential

convective movement of soil and water and to relate this move-

ment to the formation and maintance of patterned ground.

Figure P52 (a, b) Sorted circles and nets, Bro

ggerhalvoya (Photo: M. R. Bennett); (c) ice wedge below polygonal patterned ground, Tuktoyaktut,

MWT Canada (Photo: R. I. Waller).

Figure P53 Processes involved in the formation of patterned ground

(after Washburn, 1979, fig. 5.41, p. 160).

PATTERNED GROUND 769

Irrespective of the lack of consensus about the formation of

patterned ground, it remains one of the most spectacular exam-

ples of self-organization within a geomorphological system.

Matthew R. Bennett

Bibliography

French, H.M., 1996. The Periglacial Environment. Harlow, UK: Longman,

341pp.

Gleason, K.J., Krantz, W.B., Caine, N., George, J.H., and Gunn, R.D.,

1986. Geometrical aspects of sorted patterned ground in recurrently

frozen soil. Sciences, 232, 216–220.

Hallet, B., 1990a. Spatial self-organization in geomorphology: From peri-

odic bedforms and patterned ground to scale-invariant topography.

Earth Sci. Rev., 29,57–75.

Hallet, B., 1990b. Self-organization in freezing soils: From microscopic ice

lenses to patterned ground. Can. J. Phys., 28, 842–852.

Hallet, B., Anderson, S.P., Stubbs, C.W., and Gregory, E.C., 1988. Surface

soil displacements in sorted circles, Western Spitsbergen. In Proceedings,

Fifth International Conference on Permafrost, vol. 1, Trondheim, Nor-

way, Tapir Publishers, pp. 770–775.

Krantz, W.B., 1990. Self-organization manifest as patterned ground in

recurrently frozen soils. Earth Sci. Rev., 29,117–130.

Mackay, J.R., 1984. The frost heave of stones in the active layer above per-

mafrost within downward and upward freezing. Arct. Alp. Res., 16,

439–446.

Washburn, A.L., 1979. Geocryology: A Survey of Periglacial Processes

and Environments. London, UK: Edward Arnold, 406pp.

Washburn, A.L., 1989. Near-surface soil displacement in sorted circles,

Resolute Area, Cornwallis Island, Canadian High Arctic. Can. J. Earth

Sci., 25, 941–955.

Cross-references

Cryosphere

Periglacial Geomorphology

PERIGLACIAL GEOMORPHOLOGY

Definition

The periglacial system is characteristic of cold morphogenetic

regions at both high latitudes and high altitudes. It is specific

to cold regions, both in the vicinity of glaciers and not. In peri-

glacial areas, ice can modify the landscape by two ways. The

dominant process is through landforms associated with freezing

and thawing on an annual or shorter-term basis, the seasonal

frost. The second is through landforms associated with perma-

frost or permanently frozen ground that melts only superficially

in summer. As cold climates imply cold temperatures, at least

in winter, water is present during part of the year in the form

of snow or of ice. As ice is needed for most of the specific

mechanisms, water is the sine qua non condition of frost activity

(Wasburn, 1979). Frost also implies desiccation, as the vapor ten-

sion is low at temperatures below freezing. Other processes

such as alluvial and eolian also act specifically in this environ-

ment. For these reasons, the present-day periglacial landscape

preserves morphologies surviving from Tertiary climates. The

periglacial transformation of the landscape is limited at depth.

History

The term “peri-glacial ” was introduced in 1909 by W. Lozinski

and has been used since the beginning of the twentieth century

to describe cold Pleistocene landscapes of non-glacial origin.

Permafrost

Permafrost is a part of the subsoil that remains frozen (T < 0



during at least two successive years. The upper part of the soil

melts each summer; it is the active layer. The depth of the active

layer is controlled by climate and moisture content, being dee-

per in drained terrains and shallower in poorly drained soils.

The upper part of the permanently frozen ground is called the

permafrost table. It is often rich in ice accumulation for the first

10 m of the soil in interglacial conditions, with much poorer ice

accumulation during glacial events. Permafrost records the

thermal deficit induced by climate, especially in summer on a

long time span (10

years). We can distinguish:

 Continuous permafrost. It covers more than 80% of the

topography. Mean Annual Air Temperature (MAAT) is

< 7



C, Mean Annual Soil Temperature (MAST)

is < 3



C, varying with the dynamic of the snow cover.

 Discontinuous permafrost. It covers less than 80% of the

landscape. MAAT mostly ranges from 3to7



C, MAST

from 0 to 3



C, controlled by the snow cover. The depth of

the lower boundary of the permafrost ranges from 20 m to

about 60 m.

 Sporadic permafrost. Patchy permafrost located only in peat

bogs or on north-facing slopes. It covers less than 30% of

the landscape. MAAT is > 3



C, MAST is close to 0



The permafrost reaches a thickness of about 20–30 m.

Thermal and hydrological impacts of permafrost on

the substratum

The permafrost table creates a summer barrier for surface waters.

Its lower boundary is in equilibrium with the long-term climate

(10

years) and the local geothermal gradient. The occurrence

of permafrost has an impact on the hydraulic functioning of the

subsoil. Fluids migrate from the subsoil to the coldest part of this

horizon, at a depth of about 10–20 m, which corresponds to that

of penetration of the annual thermal fluctuations (Williams &

Smith, 1989).

Present extent

At low altitudes, permafrost extends today between 57



Non

the eastern sides of continents and 70



N on the western sides.

Its thickness varies from 20 m in the south, reaching about 300 m

in the deglaciated regions, and more than 600 m in the continental

never-glaciated regions such as Siberia. The deepest record is

more than 1,000 m in the Verkoyansk Mountains, in eastern

Siberia.

Permafrost in polar and middle latitude regions

Antarctic permafrost is very old; it probably occurred prior

to the onset of the Cenozoic glaciations (38 Ma). Arctic

permafrost is a little younger, but still as old as the late Mio-

cene (7–5 Ma). It has probably spread since 20 Ma. Its thermal

inertia allowed its preservation at depth, as well as during the

lower Pliocene warming, and during the warmest interglacials

of the Quaternary. Its extent over geological time has been con-

trolled at polar latitudes by the same parameters as glaciations:

the massiveness of continents, orbital parameters (obliquity and

precession), and eustatic lows. At middle latitudes, permafrost

developed during periods with a depleted summer input in solar

energy without being necessarily linked to large ice sheets. Its

occurrence is a recent feature in Western Europe and temperate

North America (<1 Ma) (Van Vliet and Hallégouët, 2001).

770 PERIGLACIAL GEOMORPHOLOGY