Odekon M. Encyclopedia of paleoclimatology and ancient environments

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of d

O events (Oi-events, Miller et al., 1991; Pekar and Miller,

1996) to sea-level estimates (Pekar et al., 2002a) and then

applying them to high-resolution d

O records (Pekar et al.,

2006). At 100- to 400-kyr timescales (corresponding to orbital

eccentricity cycles) glacial-eustatic changes ranged between

20 and 35 m, and at the 40-kyr timescales (obliquity cycle),

20–30 m eustatic changes were inferred.

Recent studies indicate that during the late middle to late

Eocene (42–34 Ma) there were at least episodically significant

ice sheets in Antarctica (Browning et al., 1996; Miller et al.,

1998). Sequences boundaries, interpreted to represent sea-level

changes and dated between 42–34 Ma, were correlated with

O increases identified in deep-sea records, indicating a glo-

bal causal mechanism, such as glacial eustasy (Browning et al.,

1996; Miller et al., 1998).

The early to middle Eocene (55–42 Ma) has been dubbed

the “doubt house” world. Although Browning et al. (1996)

determined that late Middle to late Eocene (<42 Ma) sequence

boundaries in New Jersey were controlled by glacioeustasy,

they concluded that it was unlikely that early to early-middle

Eocene sequence boundaries were the result of ice-volume

changes. Subsequently, Miller et al. (1998) concluded that there

were hints of glacioeustasy operating during the early to early-

middle Eocene. Pekar et al. (2005) identified seven sequence

boundaries of late early to Middle Eocene age (51–42 Ma) in

core material from ODP Leg 189 Site 1171 (South Tasman

Rise). These sequence boundaries correlate well with deep-

sea d

O increases and sequence boundaries identified from

other margins. The synchronous nature of sequence boundary

development from globally distal sites together with d

increases suggests a global control. The only mechanism that

can explain these significant and rapid changes is glacial

eustasy, indicating that large ice sheets existed in this suppo-

sedly ice-free world. Modeling studies and atmospheric pCO

Figure G16 Flow chart showing method for determining paleobathymetric estimates for benthic foraminiferal biofacies, apparent sea-level, and

eustatic changes using two-dimensional paleoslope modeling and two-dimensional flexural backstripping.

GLACIAL EUSTASY 357

estimates show that the first time pCO

levels decreased below

a threshold that could support the development of an Antarctic

Ice Sheet occurred at 51 Ma (Pearson and Palmer, 2000;

DeConto and Pollard, 2003). Sea-level amplitudes estimated

from d

O records ranged from 20 m for the early Eocene

(51–49 Ma) and 25 m to 45 m for the middle Eocene

(48–42 Ma) (Pekar et al., 2005).

The glacial-eustatic record for the Paleocene remains cryp-

tic. Oxygen isotope values during this time were relatively

low ( 1.0%), although these values are similar to those of

the middle Eocene. Additionally, stratigraphic studies have

identified sequence boundaries that may be due to ice volume

changes (Haq et al., 1987; Liu et al., 1997). Thus, it is reason-

able to infer that small (<25 m), ephemeral ice sheets may

have existed during this time.

The Mesozoic era

The Mesozoic was a time of generally warm equable climates

that extended to high latitudes, thus the existence of significant

ice sheets was unlikely. Yet, rapid and large sea-level changes,

recognized in stratigraphic records, present a conundrum in this

supposedly ice-free world (Haq et al., 1987; Hallam, 1992).

These greenhouse sequences can be explained as follows:

(a) they are locally controlled (e.g., delta lobe switching, local

tectonics); (b) they formed from low amplitude sea-level

changes (sea-level changes of up to 10 m on a million-year

timescale can be caused by a number of mechanisms such as

thermal expansion or contraction of ocean water); (c) in as

much as the mechanisms of large rapid sea-level changes are

poorly understood, there could have been significant ice sheets

during so-called greenhouse worlds (Miller et al., 2004).

Recent stratigraphic studies from a number of sites around

the world (New Jersey: Miller et al., 2004; NW European sec-

tions: Hancock, 1993; and Russian sections: Sahagian et al.,

1996) have lent credence to the idea that significant ice sheets

existed at least intermittently during the Cretaceous, by identi-

fying and dating large and rapid sea-level changes. The good

correlation between the timing of these sea-level changes and

increases observed in oxygen isotope records supports this

hypothesis (Miller et al., 2004). The paradox of high-latitude

warmth during the Cretaceous together with significant ice

volume can be resolved if one assumes ephemeral, areally lim-

ited ice sheet development in Antarctica. The estimated size

of ice sheets would have been restricted to the interior of the

Antarctic Continent. Thus coastal waters around Antarctica

could have remained relatively warm, while at the same time,

ice sheets equivalent to 10–40 m of sea-level change could

have grown and decayed (Miller et al., 2004).

Evidence for extensive glacial-eustatic changes for the

remainder of the Mesozoic era remains largely uncertain. This

period of rising sea level during the fragmentation of the

super-continent Pangaea was associated with generally warm

and equable climates and was largely ice free. A number of stu-

dies suggest numerous rapid sea-level changes during the lower

Cretaceous down through the base of the Triassic (Haq et al.,

1987; Hallam, 1992). Nevertheless, it is uncertain whether

these were due to tectono-eustasy or local changes in sedimen-

tation or tectonics.

The Paleozoic era

Large-scale waxing and waning of ice sheets in the southern

super-continent Gondwanaland occurred episodically throughout

the Paleozoic (Figure G17). Glacial-eustatic changes during the

late Paleozoic have been ascribed to at least three glacial periods

in Gondwanaland (e.g., Isbell et al., 2003). The late Carboniferous

and early Permian contain perhaps the best evidence for large

(>30 m) glacial-eustatic events. This includes glaciogenic sedi-

ments in Gondwanaland and transgressive/regressive cycles

observed mainly in North America and Europe. Stratigraphic

evidence indicates large eustatic amplitudes ranging from 60–

70 m (Moore, 1964; Adlis et al., 1988; Crowley and Baum,

Figure G17 The paleolatitudal distribution of glacial evidence during

geological time (adapted from Crowley and Burke, 1998).

358 GLACIAL EUSTASY

1991)to100 m (Smith and Read, 2000; Heckel, 1994)andas

high as 200 m (Soregham and Giles, 1999; Ross and Ross,

1987). Cyclothem deposits (transgressive/regressive sedimentary

packages) characterize Carboniferous to lower Permian strata in

both North America and Europe and have been ascribed to gla-

cial-eustatic changes. The deposits display a cyclicity between

44 and 412 kyr, which falls within the range of Milankovitch orbi-

tal cycles (Isbell et al., 2003; Imbrie, 1985). In contrast, smaller

eustatic amplitudes have been suggested based on the aerial extent

of Gondwanaland glaciogenic sediments of no more than 50 m

(Isbell et al., 2003). Two other glacial events (late Devonian and

late Mississippian to early Pennsylvanian) were probably smaller

in extent and consequently resulted in smaller glacial-eustatic

changes. Cyclothems and longer timescale sea-level changes have

been correlated between stratigraphic sections in North America

and Europe, indicating a global mechanism. The lower Paleozoic

records contain evidence for glacial events in Gondwanaland

and glacial-eustatic sea-level changes in the stratigraphic record.

One of the largest Paleozoic glaciations occurred during a two mil-

lion year span (437–435 Ma) in the late Ordovician, as northern

Africa drifted over the South Pole (e.g., Berry et al., 1995). Glacio-

genic sediments typical of large continental ice sheets have been

identified in Saharan Africa and Saudi Arabia (e.g., Vaslet,

1990). Geological data suggest that southern ice sheets may have

reached paleolatitudes near 50



S (Brenchley and Newell, 1984).

Snowball earth episodes in the proterozoic

The largest glacial events in Earth’s history occurred during the

early Proterozoic (2.3 Ga) and Neo-Proterozoic (0.7 Ga)

when cold glacial conditions extended into the tropics. Ice-

volume extent and subsequent glacial-eustatic amplitudes are

uncertain during the Neo-Proterozoic, with estimates ranging

from 160 m, inferred from stratigraphic records of an incised

valley, to perhaps several times that of the Pleistocene glaciation

(Hyde et al., 2000). A debate continues over the extent of the sea

ice and whether the Earth did indeed become a true “snowball”

or whether it remained a “slush ball”, with the tropics remaining

mainly ice-free (Hoffman et al., 1998; Christie-Blick et al.,

1999). In both glacial periods, decreases in greenhouse gases

are thought to have been a primary mechanism for the initiation

of these colder periods. It has also been speculated that the

Earth’s tilt may have been higher (>54



) during these periods,

which could have resulted in low latitude glaciation without

the entire Earth experiencing glacial conditions (Jenkins, 2003).

Stephen F. Pekar

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Cross-references

Antarctic Glaciation History

Cenozoic Climate Change

Cyclic Sedimentation (cyclothems)

Glacial Isostasy

Greenhouse (warm) Climates

Icehouse (cold) Climates

Neogene Climates

Oxygen Isotopes

Paleogene Climates

Sea-Level Change, Last 250 Million Years

Sea-Level Change, Post-Glacial

Sea-Level Change, Quaternary

Sea-Level Indicators

Snowball Earth Hypothesis

GLACIAL GEOMORPHOLOGY

Glacial geomorphology is the scientific study of the processes,

landscapes, and landforms produced by ice sheets, valley gla-

ciers, and other ice masses on the surface of the Earth. These pro-

cesses include understanding how ice masses move, and how

glacial ice erodes, transports, and deposits sediment. Landscapes

and landforms that developed as a result of glaciation are the

dominant focus of this field since they carry distinctive features

and forms related specifically to glacial processes. Likewise, gla-

cial geomorphology encompasses the impacts of glaciation upon

floral and faunal evolution, modification, and distribution; and

includes study of those areas peripheral to glaciated terrains,

where drainage pattern alteration, climate, vegetation, and soil

conditions are all severely affected by glaciation. This field

includes studies of the causes of glaciation, the chronology of gla-

ciation through geologic time (the retreat and advance of ice

masses), glacial sediment stratigraphy, and the global impact of

glaciations on oceans, climate, flora, fauna, and human society

Mickelson & A ttig, 1999; Bennett & G la sser, 1996; Benn & Evans,

1998; M enzies, 2002. Closely allied t o glacial geomorphology

are studies into the physics of ice masses (glaciology), global

climatology (paleoclimatology), and paleoenvironmental recon-

structions (palaeoecology). It is generally accepted that glacial geo-

morphology has become synonymous with glacial geology.

In every geological period, other than the Jurassic, there

appears to have been at least one, if not several, global glacia-

tions. Earth is a “glacial planet” where ice masses have covered

at least 30–60% of the Earth’s surface (continents and ocean

basins) approximately every 100,000 –150,000 years over the

past 800,000 years. Glacial sediments, both lithified and

unlithified, are found on every continent, and the present ocea-

nic basins of the Earth are covered by great thicknesses of gla-

ciomarine sediments. At the maximum of the last Pleistocene

glaciation, sea levels were at least 120 m below the present

sea level, permitting temporary land bridges in the immediate

postglacial period to appear between Siberia and Alaska, South

Asia and the Indonesian Archipelago, and between continental

Europe and the British Isles. Such land bridges acted as

corridors through which flora, fauna, and humans colonized

North America, northwestern Europe, and parts of Australasia.

The impact of glacial geomorphology upon human society is

immense especially in the mid-latitudes and poleward in

both North and South Hemispheres. Soils and construction engi-

neering are, for example, affected by glacial sediments and gla-

ciated terrains. Cities such as Boston, Massachusetts and

Glasgow, Scotland have been built on drumlin fields such that

urban planning and transportation systems must adapt to this

“peculiar” hummocky topography. The siting of dams, the intri-

cacies of groundwater pathways, and the location and discovery

of minerals are tied to glaciations and problems in glaciated ter-

rains. The impact of glaciation is enormous in the continental

USA where at least 60% of the population live and work in areas

once covered by the last glaciation (the Wisconsinan). Likewise,

in Canada, a comparative figure would be closer to 99% of the

Canadian population, and in Western Europe at least 50–60%

of the population inhabit formerly glaciated terrain.

Extent of glac iation at present and in the past

Today approximately 10% of the Earth’s surface is covered

by ice masses. The two largest ice masses are the Greenland,

and the West and East Antarctic Ice Sheets. The remaining

ice masses are various mountain valley, cirque, hanging, and

piedmont glaciers, small ice caps and fields, and ice shelves

attached to the fringes of the Antarctic and the Canadian Arctic

Archipelago (Figure G18a). Present ice sheet thickness varies

from around 2,500 m in central Greenland to over 4,600 m in

Antarctica. Valley glaciers and smaller ice masses are considerably

thinner. During the maximum extent of the last glaciation in

North America, the thickness of the Laurentide Ice Sheet over

Hudson Bay would have been similar to that of Antarctic today

Mickelson & Attig, 1999.

The extent of glaciation during the Pleistocene may have cov-

ered at least 30% of the continents. If the oceanic basins in the

Arctic, north Pacific, north Atlantic, and Southern Oceans are

included, then over 60% of the Earth’s surface would have been

ice-bound (Figure G18b). Similar percentages of ice cover may

have occurred in earlier geological times.

GLACIAL GEOMORPHOLOGY 361

The Earth as a “Glacial Planet”

The Earth has alternated between interglacial and glacial condi-

tions on an approximate 100,000–150,000 year cycle during

most of the Quaternary. Recognition of the recurrence of

glaciation in the geologic past is a relatively new concept. In

the nineteenth Century, glacial lithified sediment (the Talchir

Boulder Beds) had been recognized in India, and the Permian

glacial sediments of Shropshire in England had been identified,

but a full recognition of repeated global glaciations only be-

came established in the mid-twentieth Century.

Figure G18 (a) Present worldwide distribution of glacial ice (modified from McKight and Hess, 2000). (b) Extent of worldwide glaciation

approximately 18,000 years ago, at the maximum of the Late Wisconsinan (modified from McKight and Hess, 2000).

362 GLACIAL GEOMORPHOLOGY

In Europe and North America, during the late nineteenth and

early twentieth Centuries, only one single glacial period, the

Pleistocene (“The Ice Age,”) was initially recognized. The

“Ice Age” was perceived as an unusual aberration of nature,

not a repetitive cyclic event. During the Pleistocene, which

began around 1.8 million years ago (Ma), much of today’s flora

and fauna developed and evolved, and likewise the emergence

of our human ancestors stemmed from this period. The Pleisto-

cene was also a time of considerable upheaval in terms of plant

speciation and the demise of several larger animals, such as the

wooly Rhinoceros, the giant Elk and the Mammoth, towards

the end of this period.

As ice ages began to be recognized in Europe, a subdivision

of 4 major glaciations was established (Table G1). This four-fold

division was further used in North America. Only with oceanic

deep drilling programs (ODP) in the North Atlantic and Pacific

Oceans that began in 1968 did it become apparent that instead

of 4, at least 17, if not 20, major glaciations had occurred during

what was a much longer and colder Pleistocene Period. In the

past, it was thought that glaciations worldwide were synchro-

nous in their advances and retreats, but it has become apparent

that fluctuations in ice mass margins vary considerably between

continents and along different margins of the same ice sheet. For

example, along the margins of the Laurentide Ice Sheet in North

America, retreat and advance of margins differed considerably

between the east coast of New England and the Canadian

Maritimes as compared with the margin in the Dakotas and the

Canadian Prairies. This lack of synchroneity, certainly over per-

iods of time less that 2,000 years, suggested a much more com-

plex glacial history than previously considered. With modern

dating methods and the use of oxygen isotope ratios (establish-

ing the basis for marine Oxygen Isotope Stages), warm (intergla-

cial) and cold (glacial) periods can be determined where the

sediment record permits (Figure G19). We now appear to live

an interglacial period that may be further prolonged in time or

heightened in temperature due to global “greenhouse” warming.

Glacial landscapes – today

In the Pleistocene, vast ice sheets covered the Earth’s surface

beginning with a relatively slow build up to repeated maximum

extensions into the mid-latitudes (Figure G18b). These ice

sheets had an enormous impact in moving across terrain and

eroding, transporting, and depositing vast quantities of sedi-

ments both on land and in the ocean basins. The surface topo-

graphy of the continents today, for example in the northern

states of USA and throughout Canada and northern Europe,

was totally modified, with distinctive landscapes and landforms

repeatedly overprinted until the terrain was finally exposed

from beneath the ice some 10,000 years ago. Although ice

Table G1 Major glaciations of North America and western Europe

European Alps Northwest Europe Britain North America

Würm Weichsel Devensian Wisconsinan

R/ W Eem Ipswichian Sangamon

Warthe

Riss Saale Wolstonian Illinoian

Drenthe

M/ R Holstein Hoxnian Yarmouth

Mindel Elster Anglian Kansan

G/ M Cromerian Cromerian Aftonian

Günz Nebraskan

Interglacials in italics; glacials in bold.

Figure G19 Oxygen isotope record of glaciations for the past 130,000 years (modified after Bowen, 1994).

GLACIAL GEOMORPHOLOGY 363

sheet build-up was relatively slow, the downwasting and melt-

ing of the ice sheets was remarkably fast. With the rapid demise

of the ice sheets, sea level very quickly rose and, since the ice

sheets had isostatically depressed the continental land masses,

in many instances sea level rose above present levels, drowning

large coastal areas. Much later, continental land masses read-

justed to the loss of the ice sheet load and slowly rose. Raised

shorelines and cliffs began to re-appear above sea levels. These

raised beaches and strandlines, for example in Europe, became

the colonization pathways for early humans as they moved

northward.

Glacial erosion can be seen on bedrock surfaces in the form

of glacial striae and chattermarks, and the effects of high

pressure meltwater scour (Figure G20). Evidence of glacial

transport can be observed in the form of boulder trains and

isolated erratic boulders left strewn across glaciated land-

scapes (Figure G21). Finally, the effects of glacial deposition

can be seen as thick glacial sediments, and landforms such

as moraines, drumlins and eskers, glacial lake sediments,

and immense thicknesses of glacial sediments within marine

environments (Menzies, 2002).

Glacial erosion – processes, landforms, and landscape

systems

The processes of glacial erosion produce sediments for trans-

port and later glacial deposition. Processes of erosion occur in

all glacial environments. Although rather limited, supraglacial

erosion in the form of abrasion and meltwater action occurs

on the surfaces of ice masses, especially during the summer

months when melted ice and snow move across glacier sur-

faces. In mountainous areas, considerable windblown detritus

and debris deposited by avalanches and mass movement often

ends up in the supraglacial environment where it may undergo

abrasion and meltwater action. Even more limited is the

amount of erosion that occurs within ice masses in englacial

tunnels where glacial sediment is transported spasmodically

depending on meltwater activity within an ice mass. Proglacial

erosion is considerable since significant volumes of debris are

released from ice masses either along lateral and frontal mar-

gins in valley glaciers, along the front margins of large ice

sheets, or into oceanic basins where ice margins are floating

or have attached ice shelves and thus debris is removed via a

grounding-line. In all these locations, sediment entering the

proglacial environment is subject to substantial erosion largely

due to meltwater transport and mass movement. Most glacial

erosion, however, occurs in the subglacial environment where

high stress levels, meltwater under hydrostatic pressures, and

a vast source of erodable material are available (Figure G22).

Figure G20 Striated boulder within a clast-rich till, Steingletscher,

Switzerland (boulder is approximately 11 cm across).

Figure G21 Erratic boulder sitting on outwash plain in front of the Mont Mine

Glacier, Valais, Switzerland (boulder approximately 2 m high).

364 GLACIAL GEOMORPHOLOGY

Processes

Glacial erosional processes can be subdivided under five main

headings: abrasion, plucking/quarrying, meltwater action, che-

mical action, and freeze-thaw processes. In general, all of these

processes may operate on the same rock surfaces such that

examples of glacial erosion typically possess all the character-

istic “marks” of all these processes Benn & Evans, 1998;

Menzies, 2002.

Abrasion is the wearing down of bedrock and rock fragment

surfaces by the scouring of debris-laden ice and meltwater. In

both instances, as the debris at the base of the ice or moving

within high pressure meltwater passes over the surface of bed-

rock or other rock surfaces, scratching and wearing down of

the surface occurs. The evidence takes the form of minute

scratches or striae (Figure G20) on rock surfaces, or extremely

polished rock faces with smoothly sculpted geometries that

illustrate rapid wear. These latter forms of erosion are termed

P-forms (Figure G23). To be effective, such abrasion processes

demand high basal ice debris concentrations and/or elevated

debris content in high pressure meltwater streams – debris that

is sharp, angular, and harder than the rock surfaces being cut;

sufficiently strong basal ice pressures; and finally an effective

means of evacuation of the abraded debris. This latter require-

ment is essential if the abrasion process is to continue and not

become “clogged.” The production of immense volumes of

abraded debris is apparent in the down-stream “milky” nature

of glacial streams that exhibit a blue or greenish-blue color

due to the high fine debris content referred to as “glacial milk”

or “rock flour.”

Plucking or quarrying is a set of processes that removes

fractured, jointed, or disaggregated rock fragments from bed-

rock or the rock surface. Typically, rocks fracture under tensile

or compressive stresses produced by the overall stress levels

created by the overlying moving ice mass, or by percussive

processes where rock fragments crash against rock surfaces

and produce flakes and shards of rock materials. Rock plucking

can produce enormous boulders and minuscule rock flakes.

Where large bedrock knobs have been overridden, roche mou-

tonnée may be produced as distinctive erosional landforms

with smooth up-ice (stoss) sides and steep craggy down-ice

(lee) sides (Figure G24). Where percussive plucking occurs

on rock surfaces, tiny chattermarks and lunate fractures develop

(Figure G23).

Meltwater, in association with high debris content and high

pressure discharge, creates other forms of abrasive wear on rock

surfaces. A little-understood process is the effects of chemical

weathering beneath ice masses where hydrostatically pressured

meltwater, high stress levels, and fluctuating temperatures

lead to carbonate, silicate, and iron solution and re-precipitation

down-ice. Characteristically, such chemical processes can

usually be observed as distinctive stains or “trails” on rock

Figure G22 Subglacial deposits directly beneath a glacier in Switzerland.

Figure G23 P-forms from Espanola, Ontario (scale card 8.5 cm long).

GLACIAL GEOMORPHOLOGY 365

surfaces, often on the lee-side of bedrock protrusions. Finally,

in glacial environments, freeze-thaw processes are active, sea-

sonally and diurnally, producing large volumes of frost shat-

tered rock materials.

Landforms

Glacial erosional landforms can be subdivided into areal and

linear forms. The range of scale of these landforms can be

immense, from centimeter sized “rats-tails” and flutes on rock

surfaces (Figure G25) to roche moutonnée, tens of meters in

height (Figure G24). Vast areas of the Canadian and Fennos-

candian Shields best portray widespread areal glacial erosion

(Figure G26). Linear forms such as fjords, troughs (U-shaped

valleys) (Figure G27), finger lakes, tunnel valleys, and, at a

much smaller scale, fluted bedrock knobs (P-forms) and roche

moutonnée, occur in all glaciated regions. All of these land-

forms reflect the erosive power of glaciers and ice sheets, and

their associated meltwater action.

Landscape syste ms

Perhaps the best known example of a glaciated landscape sys-

tem is that of high mountain glaciation where pyramidal peaks

(horns), arêtes, cirques, tarn lakes, rock steps (riegels), and

hanging valleys are found in close association (Figure G28).

No similar landscape system exists for ice sheet erosive

landscapes except for the areal scour, bedrock trough, fluted

Figure G24 Roche moutonne

e (approx. 25 m in length) in front of Nigardsbreen, Norway; ice moving from right to left.

Figure G25 Various fluted and streamlined bedrock forms – small-scale

flutes, Wilton, Ontario.

366 GLACIAL GEOMORPHOLOGY