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latitudes. This can affect atmospheric circulation and hence the

wind-driven circulation of the ocean surface (Haywood et al.,

2000b), thus providing a mechanism for forcing changes in

oceanic heat transport. The key to understanding Pliocene

warmth may therefore be as simple as knowing why the global

coverage of ice was reduced compared to the present. This

of course assumes that ice sheets of greater size than those of

the Pliocene existed before that epoch.

One tectonically-driven process that may have had a signif-

icant impact on oceanic circulation during the Pliocene is the

closing of ocean gateways and changes in the character of

ocean sills. The sill depth of the Central American Seaway

(CAS) decreased from approximately 1,000 m during the

Miocene to near breaching of the sea surface during the early

Pliocene. This may have had the effect of redirecting the flow

of warm and salty water to the North within the Atlantic

(e.g., Haug and Tiedemann, 1998). The sill depth between the

North Atlantic and Norwegian-Greenland seas may have been

lower during the Pliocene. This would have allowed the over-

flow of Arctic waters into the North Atlantic, thereby stimulat-

ing North Atlantic Deep Water (NADW) Formation (Wright

and Miller, 1996). Changes in the depth of the Indonesian

throughflow between 5 and 3 million years before present

may have enhanced oceanic heat transports away from the tro-

pics (Cane and Molnar, 2001). Finally, the first opening of the

Bering Strait was an oceanographic event of global importance.

Stratigraphic information suggests that the minimum age range

for the strait’s first opening was between 4.8 and 7.3–7.4 mil-

lion years before present (Marincovich and Gladenkov, 1999).

The fact that many more Pacific invertebrates invaded the Arc-

tic Ocean than the reverse (Fyles et al., 1991) suggests that the

main current flow was to the North. Heat transfer through the

strait represents only a small part of the Arctic energy budget

but the event may have warmed and moderated the Pliocene

climate of the Arctic (White et al., 1997).

Climatic deterioration

Explanations for the climatic deterioration that occurred during

the late Pliocene have centered on the effects of tectonics,

alterations in atmospheric composition and the effect of chan-

ging orbital forcing on ice-sheet growth. The onset of Northern

Hemisphere glaciation may be related to the progressive uplift

of the Tibetan-Himalayan and Sierran/Coloradan regions

(e.g., Ruddiman and Raymo, 1988). It is possible that the uplift

in these two regions altered the circulation of atmospheric pla-

netary waves so that summer ablation was decreased, allowing

for the progressive build up of snow and ice in the Northern

Hemisphere. The coincidence of Tibetan-Himalayan uplift

and the sudden climatic deterioration during the late Pliocene

remains to be confirmed; Copeland et al. (1987) and Spicer

et al. (2003) have suggested that the majority of Himalayan

uplift was completed between 20 and 15 million years before

present and not during the mid-Pliocene as suggested by paleo-

botanical studies (Mercier et al., 1987). Until the dating of

Tibetan-Himalayan uplift has been established it will be diffi-

cult to assess the true significance of this mechanism in forcing

the climatic deterioration of the late Pliocene.

The Tibetan-Himalayan debate has broadened to include the

impact that such uplift could have on atmospheric concentra-

tion of CO

. Raymo (1991) discussed how Himalayan uplift

might have increased chemical weathering rates during the late

Cenozoic (see Weathering and climate). Weak carbonic acid

(rainwater) weathers rocks via hydrolysis. The chemical weath-

ering of carbonate rocks returns CO

to the atmosphere. How-

ever, the weathering of silicate rocks has the potential to reduce

atmospheric concentrations of CO

. During chemical weather-

ing of silicate rocks, bicarbonate (HCO



) is produced as a

bi-product. The bicarbonate is metabolized by marine plankton

upon reaching the oceans and converted into calcium carbonate

(CaCO

). The calcite skeletons of the marine plankton are ulti-

mately deposited as deep-sea sediments which lock up the

stored CO

in the calcium carbonate for the lifespan of the

oceanic crust on which they were deposited. This process

continually removes CO

from the atmosphere, cools global

climate and hence could have initiated Northern Hemisphere

glaciation. If left unchecked, this mechanism would very

quickly deplete the entire atmospheric CO

reservoir (Berner,

1994). However, subduction-related volcanism would release

back into the atmosphere. Resolution of this problem

awaits our improved understanding of the global carbon cycle

for the Cenozoic.

The closing of the CAS is also considered to be a potentially

important event in forcing the build up of substantial ice sheets

in the Northern Hemisphere. At present, it is not known if the

closing of the CAS and the abrupt shift in climate of the late

Pliocene are coincident. Furthermore, the current scientific

understanding of whether the closing of the CAS would have

helped or hindered the intensification of Northern Hemisphere

glaciation is incomplete. One theory is that the closing of the

CAS would have increased the salinity of the Caribbean, which

would have increased the salinity and strength of the Gulf

Stream and ultimately NADW formation. Mikolajewicz et al.

(1993) argued that an increase in the Gulf Stream and NADW

formation brought about through the closure of the CAS would

have worked against the progress of Northern Hemisphere gla-

ciation since the oceanic heat transport to the high latitudes

would be enhanced. In contrast Haug and Tiedemann (1998)

argued that enhanced Gulf Stream flow would have brought

more moisture as well as heat to the higher latitudes. The

increased flux of moisture may have promoted the growth of

the Greenland Ice Sheet.

Changes in orbital forcing have been suggested as an

important mechanism contributing towards the global cooling

of the late Pliocene. Maslin et al. (1995) suggested that the

observed increase in the amplitude of the orbital obliquity

cycles from 3.2 million years before present increased the sea-

sonality of the Northern Hemisphere, thus initiating the long-

term global cooling trend. Furthermore, the rise in amplitude

of precession and variation of insolation between 2.8 and

2.55 million years before present may have forced the rapid

glaciation of the Northern Hemisphere. Inevitably, like the

warmth that preceded it, the climatic deterioration that charac-

terizes the late Pliocene is likely to have more than one origin.

Tectonic changes might have brought the global climate to a

critical threshold with relatively rapid variations in the Earth’s

orbital parameters triggering the intensification of Northern

Hemisphere glaciation.

Summary

The 3.5 million year long Pliocene epoch appears to have been

generally warmer than at present, particularly at mid to high lati-

tudes. This greater warmth may have been generated and/or sus-

tained by a number of forcing mechanisms working concurrently.

These include enhanced levels of atmospheric CO

, enhanced

PLIOCENE CLIMATES 811

oceanic heat transports, and ice-albedo feedbacks. Climatic

variability occurred during the epoch on Milankovitch periodici-

ties. A climatic deterioration occurred during the late Pliocene

which saw the initiation of significant Northern Hemisphere

glaciation. This downturn, like the warm period that preceded

it, is likely to have a complex origin. Current hypotheses

include the effect of tectonic uplift, drawdown of atmospheric

, and changes in oceanic circulation, heat transports and

orbital forcing.

Alan M. Haywood

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Cross-references

Albedo Feedbacks

Antarctic Glaciation History

Astronomical Theory of Climate Change

Atmospheric Circulation During the Last Glacial Maximum

Carbon Isotope Variations over Geologic Time

Cenozoic Climate Change

Climate Change, Causes

Climate Forcing

Cryosphere

Diatoms

Foraminifera

Glacial Eustasy

Heat Transport, Oceanic and Atmospheric

Marine Biogenic Sediments

Mid-Pliocene Warming

Mountain Uplift and Climate Change

Neogene Climates

North Atlantic Deep Water and Climate Change

Obliquity

Ocean Paleocirculation

Ocean Paleotemperatures

Ostracodes

Oxygen Isotopes

Paleobotany

Paleocean Modeling

Paleoclimate Modeling, Pre-Quaternary

Paleotemperatures and Proxy Reconstructions

Plate Tectonics and Climate Change

Sea Level Change, Quaternary

Thermohaline Circulation

Weathering and Climate

PMIP

Objectives and project design

The Paleoclimate Modeling Intercomparison Project (PMIP),

initiated in 1992, is an international undertaking designed to

evaluate the performance and reliability of global atmospheric

general circulation models by comparing past climates for per-

iods that had significantly different external forcings from the

modern climate. This initiative has been endorsed by the World

Climate Research Programme (WCRP) and the International

Geosphere-Biosphere Programme (IGBP). The first phase of

this project (PMIP 1) initially focused on simulations of the

Last Glacial Maximum (LGM, 21,000 y

BP) and the mid-

Holocene (6,000 y

BP). The LGM was selected to assess the

models’ ability to simulate conditions characterized by exten-

sive ice sheets, cold oceans, and reduced atmospheric CO

levels. Boundary conditions for the LGM experiment included

specification of the ice sheet extent and height with lower ice

elevations (Peltier, 1994) than those used in CLIMAP (1981)

and an atmospheric CO

concentration of 200 ppm (as com-

pared to a modern pre-industrial level of 280 ppm). Two differ-

ent ocean scenarios were specified: the first employing fixed

sea surface temperatures (SSTs) based on CLIMAP (1981),

and the second computing SSTs from coupled atmosphere-

mixed layer ocean models, assuming the same ocean heat trans-

ports as today (Joussaume and Taylor, 2000). Earth’s orbital

parameters were also adjusted to correspond to their values

21,000 years ago (see Eccentricity, Obliquity, and Precession,

climatic, this volume). For the mid-Holocene experiment, only

two factors differed from those of the control run: the atmo-

spheric CO

concentration was reduced to a pre-industrial

level of 280 ppm and orbital parameters were adjusted to their

6,000 y

BP values. The difference in obliquity 6,000 years ago

induces only minor changes in low- to mid-latitude annual

mean insolation from that of the present, but the change in pre-

cession causes an intensified Northern (and lessened Southern)

Hemisphere seasonal cycle (see Astronomical theory of climate

change; Precession, climatic, this volume). SSTs were assumed

to be the same as at present; land-surface characteristics were

also assumed to remain unaltered.

Simulations were compared with observations using inverse

and forward-modeling techniques (Harrison, 2000). The

inverse technique derives climate-related parameters by means

of statistical techniques such as transfer functions, modern ana-

log analyses, or response surfaces (see Transfer functions,

Paleobotany, Nearest living relative method, this volume). Cor-

relations are established between modern distributions of spe-

cies and specific climate parameters, from which past

climates are inferred, based on the paleodata. The forward

modeling approach predicts the response of the paleoclimate

proxy to the simulated climate change by means of process-

based models. The predicted response is then compared with

the original paleoclimate record. In PMIP, this approach has

enabled closer comparisons between terrestrial vegetation and

lacustrine data sets.

Model performance was evaluated using paleoenvironmental

data bases (Harrison, 2000), including: (a) the Global Lake

Status Data Base – a compilation of geomorphological and bios-

tratigraphic data relating to changes in lake level, area, or

volume; (b) BIOME 6000 – global paleovegetation data sets

PMIP 813

for the LGM and mid-Holocene (Prentice and Webb, 1998;Pre-

ntice et al., 2000); and (c) the 21 ka Tropical Terrestrial Data

Synthesis – a compilation of data for sites between 32



Nand



S from diverse land-based paleoclimate proxies (e.g., pollen,

lake level records, speleothems, etc.). These enable derivation

of several climate-related parameters (e.g., mean temperature of

the coldest month (MTCO), mean annual ground temperature

(MAT), plant-available moisture (PAM), and stream runoff

(precipitation minus evapotranspiration, P-E)).

More recent experiments initiated under PMIP 2, and which

started in 2002, were designed to address several shortcomings

identified in the initial simulations, such as a need for improved

representation of ocean and vegetation feedbacks and the role

of freshwater fluxes, using fully coupled ocean-atmosphere

(OAGCM) and ocean-atmosphere-vegetation (OAVGCM) mod-

els. A more realistic recreation of ocean-vegetation interactions

is necessary in order to account for discrepancies between

climate models and land surface proxies identified in earlier

experiments (e.g., in mid-Holocene monsoon intensity,

Joussaume et al., 1999). Several more time periods have been

added (Harrison et al., 2002; Crucifix et al., 2005). Time slices

investigated in PMIP 2 now include the “glacial inception”

(115,000 y

BP), the early Holocene (10,000–9,000 yBP), the

Younger Dryas (12,700–11,700 y

BP) and the abrupt cooling

event (8,200 y

BP), in addition to the LGM and mid-Holocene,

which were examined previously (Crucifix et al., 2005). The gla-

cial inception experiment examines the role of vegetation and

ocean feedbacks on interannual snow accumulation in northeast-

ern North America at the end of the last interglacial period in

order to better understand how an ice age is initiated. The early

Holocene experiment explores the way in which climate reacts

to changes in insolation and ice sheet coverage. The Younger

Dryas and 8,200 year experiments examine the effects of melt-

water additions on thermohaline circulation and their role in

abrupt climate change.

Revised boundary conditions for the LGM under PMIP 2

include new estimates of ice sheet extent and height. In the

latest reconstruction (ICE-5G), the thickness of the central

Greenland ice sheet at the LGM has been reduced from that

used in PMIP 1, but is still ~100–300 m thicker than at present

(Peltier, 2004). Antarctica remains approximately the same,

whereas the Laurentide Ice Sheet has increased in thickness

relative to the earlier reconstruction. Paleoclimate databases

used to define and constrain initial boundary conditions have

been updated, including additional pollen and macrofossil data,

revised ice sheet extent, and a river routing scheme that pro-

vides a more naturalistic representation of the hydrological

cycle (Crucifix et al., 2005). A series of experiments probe

the role of major feedback loops, by successively shutting off

the contribution of oceans or vegetation singly, and ocean/

vegetation in combination (Harrison et al., 2002). Another set

of experiments simulate climate responses to 6,000 ky

changes in solar insolation and removal of ice sheets.

Results

The PMIP 1 LGM model runs forced by CLIMAP SSTs pro-

duced a global mean cooling of 4



C, which was stronger over

Northern Hemisphere ice sheets and landmasses, especially

around the North Atlantic (Joussaume and Taylor, 2000). Com-

puted SSTs yielded a wider range of cooling, from 2 to 6



below present, as well as a sharper hemispheric temperature

gradient. Both types of simulations found greater LGM aridity

over most northern continents and tropics. The relatively warm

CLIMAP-specified tropical SSTs let to higher land tempera-

tures than suggested by paleotemperature proxies, except for

equatorial Africa. Slab ocean models, on the other hand, gener-

ated a range of land cooling, related to the intensity of tropical

ocean cooling, which in some cases was much too high.

Regionally, the Afro-Asian monsoon was reduced in strength

and intertropical aridity increased. The presence of ice sheets

modified mid-latitude atmospheric circulation by inducing

anticyclonic circulation over the ice and intensifying planetary

waves. However, splitting of the jet stream (e.g., COHMAP,

1988) was not clearly seen in PMIP 1. Increased meridional

temperature gradients due to the presence of ice sheets

enhanced baroclinicity and shifted storm tracks eastward, parti-

cularly over the North Atlantic. While all models displayed an

increase in the summer monsoon over Africa during the mid-

Holocene, they tended to underestimate its intensity relative

to that suggested by pollen data and lake models (Joussaume

et al., 1999). The models also missed some important features,

such as a colder/wetter Mediterranean growing season than at

present, as implied by lake level data.

Initial PMIP 2 results show that global mean temperatures

at the LGM are 3–6



C below present, with a median value

of 4



C (Masson-Delmotte et al., 2006). Polar amplification

(ratio of polar to global annual mean temperature change) at

the LGM ranges between 2.1 and 2.6 for Greenland, but only

1.2–1.9 for central Antarctica. However, the models underesti-

mate the extent of glacial cooling for Greenland, whereas the

polar amplification is realistically simulated for Antarctica.

PMIP 2 simulations find mid-Holocene global mean tempera-

tures close to present values (Masson-Delmotte et al., 2006).

Temperatures are 0.35



– 0.9



C higher in central Greenland

during the mid-Holocene, and 0.1



– 0.7



C warmer in Antarc-

tica. The OAGCMs reproduce mid-Holocene seasonal cycle

and regional changes, such as higher mid-latitude summer tem-

peratures in many regions, which stem from the altered preces-

sional parameters (Zhao et al., 2005). They produce consistent

changes in summer monsoon behavior, such as increased preci-

pitation over northern India and Sahelian Africa, and strength-

ened monsoon flow from the western Pacific warm pool to the

Indian Ocean, although the areal extent and magnitude of the

monsoons appears model-dependent. The changes in monsoonal

flow appear to be associated with a reduction in Hadley-style cir-

culation and a westward shift in Walker-type circulation from the

Pacific to Indian Oceans (Zhao et al., 2005).

Illuminating insights have emerged from new paleodata

used to validate the PMIP 2 climate models. For example, pol-

len and macrofossil data reveal that the warmest period during

the Holocene (see Hypsithermal , this volume) varied sharply

over time in different regions, beginning as early as 10,000

years ago in Alaska-eastern Siberia, and ending less than

6,000 years ago in northeastern North America. This was in

part related to lingering remnants of the Laurentide Ice Sheet

(Crucifix et al., 2005). New information about ice sheet history

also contributes to the climate modeling efforts. For example,

catastrophic outflow from Glacial Lake Agassiz had long been

considered to be the main source of freshwater outflow that led

to a decrease in rate of North Atlantic Deep Water Formation,

presumably triggering the Younger Dryas cooling. Recent dat-

ing of glacial landforms suggests, instead, that ice retreat

around southern and western Hudson Bay occurred approxi-

mately 1,000 years after the start of the Younger Dryas (Lowell

et al., 2005), casting doubt on the role played by Lake Agassiz.

Moreover, detailed geophysical models of the Laurentide Ice

814 PMIP

Sheet retreat, constrained by

C-dated geomorphological fea-

tures, relative sea level data, and geodetic measurements point

to large meltwater influxes via the Arctic Ocean, Fram Strait,

into the North Atlantic Ocean during the Younger Dryas

(12,700–11,700 y

BP) (Tarasov and Peltier, 2005). Freshwater

input via this route rather than outflow from Lake Agassiz

may have led to the abrupt Younger Dryas cooling.

Conclusions

PMIP, by comparing and contrasting results from a number of

general circulation models under climate regimes that differ sig-

nificantly from the present, enables one to test the ability of these

models to reproduce key elements of climate, such as tempera-

ture, precipitation, atmospheric dynamics, and seasonal and

regional variations. Results to date point to the importance

of using fully coupled ocean-atmosphere models and the role

played by vegetation. The sensitivity of various climate features,

such as changes in the North Atlantic Oscillation or the role of

tropical SSTs, to altered global conditions is also best evaluated

with a “super-ensemble” of models. Concomitant with the PMIP

modeling program is the growth and expansion of supporting

paleoclimate databases that are used to validate these models.

Vivien Gornitz

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Loope, H.M., Schaeffer, J.M., Rinterknecht, V., Broecker, W., Denton, G.,

and Teller, J.T., 2005. Testing the Lake Agassiz meltwater trigger for the

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mid-Holocene vegetation patterns from palaeoecological records.

J. Biogeogr., 25, 997–1005.

Prentice, I.C., Jolly, D., and BIOME 6000 participants, 2000. Mid-

Holocene and glacial-maximum vegetation geography of the northern

continents and Africa. J. Biogeogr., 27, 507–519.

Tarasov, L., and Peltier, W.R., 2005. Arctic freshwater forcing of the

Younger Dryas cold reversal. Nature, 435, 662–

665.

Zhao, Y., Braconnot, P., Marti, O., Harrison, S.P., Hewitt, C., Kitoh, A.,

Liu, Z., Mikolajewicz, U., Otto-Bliesner, B., and Weber, S.L., 2005.

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Cross-references

Astronomical Theory of Climate Change

CLIMAP

COHMAP

Eccentricity

Holocene Climates

Hypsithermal

Last Glacial Maximum

Late Quaternary-Holocene Vegetation Modeling

Nearest-Living-Relative Method

Obliquity

Paleobotany

Paleoclimate Proxies, an Introduction

Precession, Climatic

Paleoclimate Modeling, Quaternary

Quaternary Climate Transitions and Cycles

Transfer Functions

The 8200-year BP Event

Younger Dryas

POLLEN ANALYSIS

Introduction

Pollen analysis is a scientific method that can reveal evidence

of past ecological and climate changes: it combines the prin-

ciples of stratigraphy with observations of actual (modern)

pollen-vegetation relationships in order to reconstruct the

terrestrial vegetation of the past.

Early history

Pollen analysis plays a critical role in climate change studies

during the Quaternary (Fægri and Iverson, 1975) and is the sin-

gle most important branch of paleoecology for the late Pleisto-

cene and Holocene (Roberts, 1998). Its scientific scope is broad

since it encompasses knowledge from many disciplines includ-

ing botany, geology, ecology, climatology and archeology. The

extent to which a pollen analysis study may be slanted toward

botanical, ecological or geological questions will depend on the

sedimentary deposits analyzed and the researcher’s back-

ground. Pollen analysis may be utilized for the evaluation of

vegetation changes occurring as recently as the Holocene. It

may also be applied as far back as the late Devonian, when

the first seed plants evolved, although presumably not until

the Pennsylvanian or later did distal germination of the pollen

evolve in the early gymnosperms (Traverse, 1988, p. 163).

A brief summary of key historical developments shows how

pollen analysis evolved as a methodology, and how distinct

disciplinary characteristics have been imprinted upon it. An

early history of pollen analysis is found in the book entitled

“An Introduction to Pollen Analysis,” written by G. Erdtman

(1943). Erdtman recounts how European scientists had laid

the groundwork for pollen analytical studies in the late nine-

teenth and early twentieth centuries. The Swiss geologist,

J. Früh had identified many of the common tree and herb pol-

len types, and spores from ferns and mosses, found in the

regional environment, in an 1885 paper. The Swedish geologist

F. Trybom, in 1888 showed that pine and spruce pollen grains

were found in sediment samples from the bottoms of Swedish

lakes. The microscopist G. Lagerheim introduced a quantitative

aspect: he and N. O. Holst published a paper in the Geological

POLLEN ANALYSIS 815

Survey of Sweden, in 1909, entitled, “Postglaciala tidsbestäm-

ningar.” In this paper, common northern European tree genera

pollen had been identified (i.e., Pinus (pine), Betula (birch),

Alnus (alder), Ulmus (elm), Corylus (hazel), Fraxinus (ash),

Quercus (oak), and Tilia (basswood)) from different levels

of lacustrine sediment layers within Kallsjö bog in southern

Sweden. Lagerheim identified pollen within the layers, and pre-

sented the quantity of each genus as a percentage of all pollen;

furthermore, he examined the shifts in these pollen percentages

from layer to layer. Lagerheim (as translated by Erdtman, 1943,

p. 4) concluded that the varying pollen frequencies are “a reli-

able method with which to follow, step by step, from one layer

to another, the immigration of all plants whose pollen or spores

are preserved as fossils as well as the relative frequency of

these species.” A decrease in the percentage of pine pollen

from the lower to the upper layers caused Holst to speculate

that, “the pine disappeared from southernmost Sweden in his-

torical times, exterminated probably by the activity of man.”

Lennart von Post (a pupil of Lagerheim at the University Col-

lege of Stockholm) transformed pollen analysis into a refined

method in Quaternary geology, he “indicated how pollen analy-

sis should be applied in order to give information on problems

related to Quaternary geology and paleontology (Erdtman,

1943).” Von Post drew the first pollen diagrams and defined dis-

tinctive tree pollen “horizons” and “limits” in 1909. A “spruce

pollen limit” or “spruce horizon” could be used as a datum line

to trace time synchronous horizons in different bogs within a

small area. In 1916, von Post presented a paper with many pollen

diagrams at the Sixteenth Meeting of Scandinavian Naturalists in

Oslo, examining the pollen of forest trees in bogs in southern

Sweden (von Post, 1967 [1916]). His presentation is considered

the realization of pollen analysis as a science.

Thus by the early 1900s, the basic principles underlying

pollen analysis were already in place. Plants produce pollen

grains, which are dispersed and then preserved in sediments

for thousands and exceptionally even millions of years. Then,

samples taken from a column of sediment will show shifts

in pollen occurring over time, assuming that the original

stratigraphy has been maintained. Any apparent shifts may be

interpreted as due to changes in vegetation, perhaps forced by

climatic, geological, or ecological factors.

Subsequently, the number of pollen studies vastly expanded

to include other geographical regions and other geological time

periods. The advent of radiocarbon dating and its extensive

application in the late 1900s allowed relatively firmer chronol-

ogies to be developed from the late Quaternary. Extensive

research of pre-Pleistocene deposits is also carried out by geol-

ogists (Traverse, 1988). Pollen analysis has also evolved with

the increasingly interdisciplinary aspect of many sciences

(including geology and ecology), as results from pollen analy-

sis are now interpreted with respect to site data of a limnologi-

cal, isotopic, or microfaunal nature, to name just a few.

Theory of pollen analysis: the pollen-vegetation

relationship

The basic theory of pollen analysis is that pollen is released by

plants, transported through the air, and then deposited uni-

formly upon the Earth’s surface. In fact, the pollen represents

the actual surface vegetation growing over an area (the area

to be defined by the researcher). The pollen that is transported

in the air is called pollen rain. The pollen rain may be depos-

ited over land or any water body. The pollen that is deposited

in lakes or other water bodies (such as bogs and marshes) sinks

to the bottom of the basin and becomes part of the accumulat-

ing lacustrine or peat sediment. When pollen is deposited in

water, the pollen’s outer layer may remain practically intact

for thousands and perhaps even millions of years. Thus, the

pollen becomes part of the fossil record. This fossilized repre-

sentation of the terrestrial vegetation may then be used to inter-

pret the vegetation, or plant geography, which existed in the

past. Furthermore, since climate is one of the key determinants

of the plant taxa growing at any place on the Earth’s surface,

the fossil pollen may be used to interpret past climates. This

is highly useful when attempting to reconstruct the Earth’s cli-

mate and ecology, as it existed before any instrumental records

or observations made by humans.

A fundamental operating assumption is that the pollen-

vegetation and vegetation-climate relationships that can be

observed now also operated in the past (Roberts, 1998). This

is the principle of uniformitarianism, which is applied in many

disciplines related to Earth history. Paraphrasing from Roberts

(1998, p. 29) and applying to the field of pollen analysis, the

uniformitarian assumptions are: (a) the environmental factors

affecting present-day vegetation are understood, (b) a species’

natural history has not changed over time, (c) the vegetation

was in equilibrium with its environment (this means that the

environment remained unchanged during the plant’s lifespan),

(d) modern analogs exist, (e) the origin (taphonomy) of the fos-

sil pollen assemblage can be established, (f) the pollen assem-

blage has not been biased by contamination (from outside the

sample) or differential preservation, and (g) the pollen can be

identified to a meaningful taxonomic level.

In practice, all these assumptions are never perfectly met. In

fact, pollen analysis as applied to the fields of paleoclimatology

and paleoecology exemplifies the concept succinctly phrased

by the ecologist Edward Deevey (1969), “coaxing history to

conduct experiments.” Essentially, the sciences of paleoecology

and paleoclimatology do not utilize data from controlled

experiments conducted in an artificial laboratory. Instead, the

experiment has been conducted by nature, in the natural envir-

onment, in the geological past. The data, or evidence, from

these experiments remains within the natural environment,

waiting to be retrieved and interpreted by the discerning

researcher via a variety of tools.

Pollen production and dispersal

Pollen and spores are produced by green plants (Viridophytae)

(taxonomy according to Pennisi, 2003). What is commonly

called “pollen” comes from the conifers (sometimes referred

to as gymnosperms) and angiosperms. Green plants that are

neither angiosperms nor conifers produce what are commonly

called “spores.” Mosses, lycophytes, and ferns are green plants

that produce spores. Both pollen and spores are typically iden-

tified by the researcher conducting the pollen analysis.

Pollen is produced in a flower’s anthers (microsporangia or

pollen sacs). It is the haploid microgametophyte, male partner,

in sexual reproduction. Pollen grains germinate on the female

(stigma) surface by forming a pollen tube, which emerges

through one of the apertures in the pollen grain wall (Twell,

2000). In pollen analysis, the scientist studies those pollen

grains that have been dispersed, but rather than germinating,

have accumulated as sediment. This is because the pollen shape

and surface texture, which is called morphology, is highly dis-

tinctive when magnified.

816 POLLEN ANALYSIS

The most striking structural feature of the pollen or spore

grain is the tough, resistant outer coat, called the exine, which

protects the reproductive cells from environmental injury.

Beneath the exine is a second major layer surrounding the

pollen grain protoplasm, called the intine. The exine is com-

posed of a material called sporopollenin, which is a very inert

C–H–O compound (Traverse, 1988). Sporopollenin has been

identified in acritarchs in Precambrian rocks, and is a compo-

nent of many higher green plants. What is unique about sporo-

pollenin is its ability to withstand desiccation and oxidation.

Sporopollenin within a sediment matrix is able to survive for

extended time periods, even as the other components of the

pollen or spore degrade. Oxidizing, highly alkaline, and high

temperatures environments are likely to accelerate sporopolle-

nin’s degradation. In short, the accumulating sediment within

a water body is an ideal place to store the exine of pollen and

spores. This exine may survive for thousands to even millions

of years in a non-oxidizing environment, and thus becomes

the fossil utilized in pollen analysis.

The pollen of different plant taxa manifest considerable var-

iation in size, shape and surface characteristics. To the naked

eye, it is a fine granular or powdery substance. Pollen grains

may vary in length from 5 mminMyosotis to more than 200 mm

in the Curcurbitaceae; the shape varies from spherical to ellip-

tical to more unusually and irregularly shaped objects. The

exine has a three-dimensional patterned surface, several

microns deep, called sculpturing, which helps the grain adhere

to insect pollinators and the stigma. The exine does not develop

over certain regions; these define the positions of the germi-

nation apertures. Apertures also show wide variation in shape,

ranging from elongated furrows to circular pores, and com-

binations of furrows and pores. The pollen’s shape, aperture

number and position, and exine sculpturing are unique taxo-

nomic features that allow the identification of species, genus,

or family (Twell, 2000).

The dispersal of the pollen strongly conditions the pollen-

analytical studies. Fægri and Iverson (1975) describe the

different means of pollen production and dispersal. Those

plants that are pollinated by wind-carried pollen, called anemo-

philous, produce the greatest quantities of pollen. In zoophilous

plants, the pollen is carried from the anther of one flower to the

stigma of another by an animal vector such as insects, birds or

bats; these plants produce relatively less pollen. Hyp-hydroga-

mous (pollination under water) and obligate autogamous plants

produce very little pollen and are rarely analyzed. The earliest

pollen studies involved the anemophilous species and even

now, wind pollinated species are those most commonly

addressed. Because of the larger quantities of pollen dispersed,

the anemophilous species are more likely to leave pollen

deposited within the sediment. Typical quantities of pollen pro-

duced by mid-latitude forest tree species are: 10,000 pollen

grains in a single Betula anther, 28 million pollen grains pro-

duced by 10-year old Betula branch system, up to 350 million

pollen grains produced by a 10-year old Pinus branch system

(Fægri and Iverson, 1975). A common mid-latitude zoophilous

tree, Acer (maple), may have only 1,000 pollen grains per

anther. Generally, those trees reaching the upper canopy will

produce the most pollen, while the shrub and herb plants under

the canopy will produce less. Zoophilous species are usually

more frequent in the lower strata. Pollen production will also

vary seasonally and even annually.

Pollen analysis is based upon the assumption of uniform

dispersal of this great quantity of pollen grains (Fægri and

Iverson, 1975). The pollen of wind-pollinated species should

be dispersed by the wind, then settle out and be deposited over

a region, including any lakes and bogs, in which strata they

will remain, until finally being retrieved by the researcher.

All aspects of pollen production and dispersal continue to be

studied by botanists and ecologists, more detailed information

may be found in the specialist literature. Since the earliest pol-

len studies were based in the temperate northern European for-

ests, the earlier methodologies were constructed around the life

history of the local, predominantly anemophilous species.

Newer studies also address the tropical and even high-latitude

habitats. Understanding key species in pollen production in tro-

pical habitats is especially challenging as the number of species

composing the local vegetation formations is high, and many

are zoophilous.

Pleistocene palyno logy

Living organisms evolve over time. The assumption that the

ecological and climatological affinities of extant vegetation

have the same paleoecological and paleoclimatological affi-

nities as paleovegetation (interpreted via pollen), is accepted

as generally holding true for the Pleistocene, that is the most

recent 1.8 million years. This conclusion is based on the studies

of Neogene fossil flora remains, both megafossils and micro-

fossils. Fossil pollen are usually identified by genus, although

sometimes a species identification is possible. Some pollen

can only be identified to its taxonomic family, a good example

being the grasses (Poaceae) and sedges (Cyperaceae). By the

beginning of the Pleistocene, nearly 100% of flora species

recognized are still extant. Nearly 100% of extant plant genera

are present by 10 million years ago; by 20 million years ago,

practically all angiosperm remains are referable to extant

families (Traverse, 1988). Thus, as one moves from the distant

geological past to the present, the precision with which a

researcher can link the pollen fossil to an extant plant, and

hence that plant’s natural history, improves. Thus returning to

the uniformitarian assumptions given by Roberts (1998), we

see that at least the first two assumptions (the environmental

factors are understood, a species natural history has not changed

over time) can be applied with greater certainty to the most recent

geological time – the Pleistocene and the Holocene. Hence, pol-

len analysis applied to Pleistocene and Holocene sediments may

aim for relatively precise determinations of climate and environ-

ment. Pollen analysis applied to fossils of the last 10 million

years may still yield information relevant to climate and ecol-

ogy, since most of the plant genera were extant, but the ques-

tions will have more of a geological flavor (Traverse, 1988).

The sample site and laboratory technique

Field sites

Determining which site was a suitable pollen collection basin

will have a decisive impact upon the relevance of the final

results. Equipment and the natural landscape are major limiting

factors. Ideally the basin should have accumulated sediment

continuously over time, thus its geological origins should be

clear. From an ecological perspective, one wishes to know the

distance of the pollen from the source vegetation.

The aim of the field work is to collect uncontaminated

samples, to define as precisely as possible the conditions under

which the samples were taken, and to characterize the sedi-

ments and stratigraphy (Fægri and Iverson, 1975). The investi-

gator must evaluate and balance two (sometimes competing)

POLLEN ANALYSIS 817

factors: the field sites actually available, and the spatial scale

which the pollen analysis will address. Most frequently, the

investigator is seeking to reconstruct changes in the natural

undisturbed vegetation – present day vegetation is frequently

altered by human activity and undisturbed sites are usually

kilometers to hundred of kilometers from roads. In addition,

pollen is best preserved under anaerobic conditions – those at

the bottom of a peat bog or lake.

Given the historical roots of pollen analyses in the northern

temperate latitudes, the ideal sedimentary basin is a lake or bog

of roughly 5,000 m

which collects windblown pollen (Fægri

and Iverson, 1975 ). The pollen sources are defined as local,

regional, and extra-regional (Fægri et al., 1989) – these divi-

sions are usually only qualitative (Figure P82). The researcher

usually wishes to describe the “regional” vegetation; very

approximately, the vegetation within one kilometer of the

basin’s center. The relatively small basins allow this interpreta-

tion of a “regional pollen signal.” Since the pollen deposited

should be transported by wind, input to the basin from streams

must be avoided – this would bring in pollen from outside the

“region,” and also pollen eroded from older sediments.

In the first pollen studies conducted in northern Europe and

Scandinavia, a single person might have cut samples from

exposed peat deposits. Peat sediments were easy to examine,

pollen was frequently well preserved within the peat and mud

matrix, and continuous sedimentation was possible for approxi-

mately 10,000 years; that is, since the last deglaciation.

Pollen analyses have expanded in their scientific and geo-

graphic scope. The ideal 5,000 m

size catchment basin is sim-

ply unavailable in the lower latitudes and tropics, thus other

catchments may be utilized. Small basins also effectively limit

the geological time span of the sediment, since they are almost

always the by-product of a geomorphological regime occurring

after the last glaciation. Hence, large basins are studied: wind-

blown pollen is collected from a much larger “region,” yet sedi-

ment cores may be longer and hence encompass more time.

Pollen accumulated in marine sediments may be very useful

in paleoclimatological studies. Rat middens and cave deposits

have been frequently utilized in the arid southwest of

North America. Archeological sites also contain pollen. These

sedimentary accumulations have the disadvantage of being

relatively stratigraphically discontinuous; however, in more

arid regions they still provide valuable information.

Sampling methods have also advanced. Piston corers of

varying degrees of sophistication are now often utilized: they

consist of drill bit that can bore into the ground, a sampling

tube into which the sediment slides, a piston which maintains

a vacuum inside the tube, and extension rods connecting the

sampling tube to the piston, allowing the entire apparatus to

be taken in and out of the ground. A typical sediment core is

5–10 cm in diameter and from 1 to more than 10 meters in

length, the core is retrieved in sections of usually a meter ’s

length or less. The coring operation may be conducted on dry

land or on water. In the latter case, the machinery and people

operating the coring equipment must utilize a dry and level

platform: the platform may range from portable wooden

planks, to the drilling rigs aboard modern research sea vessels,

even to the ice covering a lake. The goal is always to reach the

deepest depths, while retrieving the most continuous sections

of sediment to the surface. The depths achieved, and the conti-

nuity of the core, depend mostly on the types of sediment cored

and the equipment being used. Marine sediments, usually com-

posed of clays and silts, are easier to penetrate and may yield

cores in the range of tens to hundreds of meters. Lacustrine

cores may exceptionally exceed ten meters in depth, peat and

lake sediments are relatively soft and easy to penetrate. How-

ever, changes in the climatic or geomorphological regime must

be expected as one proceeds deeper into any sediment, essen-

tially going back in time, and thus coarser sediments such as

sands, gravels, and even bedrock may be encountered at the

bottoms of the collecting basins. Indurated or coarse sediments,

or bedrock, effectively stop the drilling equipment.

Laboratory techniques

The laboratory work comprises the separation of pollen grains

from their sediment matrix, and then identification of the pollen

grains. Work in the laboratory is the most time consuming aspect

of pollen analyses, and the identification of the pollen is the most

beautiful aspect of the study. The sediment (in cores, blocks, or

even sub-samples taken from other studies) retrieved during field

work must be described, evaluated for any possible lack of stra-

tigraphic continuity, and then sub-sampled. Sub-samples are

taken from the cores or sediment blocks, the researcher must

decide at what distance interval to sample. A closer sampling

interval usually increases the time resolution of the study.

The description of the sediment combined with any geologi-

cal study of the sampling site should allow a geological recon-

struction of the site’s genesis over time. Furthermore, a correct

interpretation of the changing vegetation or climate based upon

the pollen data requires knowledge of the depositional environ-

ment. The description of sediment may follow geological or

soil classification schemes. Two commonly utilized classifica-

tion schemes are the “Fægri and Gams” and the “Troels-Smith”

descriptions. “Fægri and Gams” is a synthetic-interpretive

approach in which the sediment’s description implies its gen-

esis. The “ Troels-Smith” scheme is an analytic-descriptive

approach. A referenced system of notation and graphic symbols

(called signatures) exists for each system, the signatures are

usually presented in the final pollen diagram. Fægri et al.

(1989) emphasized that classification should include the

identification of deposit-forming vegetation (e.g., sedges,

mosses, wood), degree of humification, mineral content and

grain size. Basins formed by glacial action in the temperate

Figure P82 Schematic representation of wind deposition of pollen

from local, regional, and extra-regional sources.

818 POLLEN ANALYSIS

mid-latitudes, used in early pollen studies, typically show a

change (over several thousand years) from minerogenic sedi-

ment to organic muds or gyttja – this change is a response to

the gradual development of soil and vegetative cover. In the

tropical basins, the more dramatic changes in the sedimentary

matrix are usually linked to fluctuations in water level.

Fossil pollen is separated from the sediment matrix in a

laboratory. Essentially, the extraneous, non-pollen material

must be removed from the sediment samples so that the final

preparation contains only the pollen, which will be easily

visible and unobscured when viewed under magnification.

The laboratory and all air, water and supplies that enter the

room must be pollen-free, only the samples themselves should

contain pollen. A typical pollen laboratory is furnished with a

fume hood, centrifuge and water baths.

The quantity of sediment needed for a single sample is

usually small, approximately 1 cubic centimeter for lacustrine

sediments, and 10 cubic centimeters or more for marine

sediments – hundreds to thousands of pollen grains may be

hidden within. To allow the absolute pollen frequency count

or the density of the individual pollen taxa in the sediment to

be determined, a known quantity of a marker is commonly

added to the sample. Common markers are polystyrene spheres

or a pollen taxon that is clearly foreign to the native vegetation.

Filters and chemicals are used to remove the extraneous

material. The large majority of pollen grains have a minimum

diameter between 7 and 125 microns, while the supporting

sediment matrix ranges from finer-grained clays to coarser-

grained sands. Filters or sieves can be used to separate these

fine and coarse materials from the pollen containing sediment;

the researcher decides which filter sizes to utilize. The basic

chemical processing steps applied to a measured quantity of

sediment are the addition of alkalis (KOH or NaOH solution)

to remove and deflocculate humic acids (associated with peat

and organic matter), and acid baths to chemically remove the

silt and fine sand. Hydrochloric acid removes the calcium car-

bonate components. Siliceous sediments are dissolved by

hydrofluoric acid. Gravity separation, utilizing what are known

as “heavy liquids” (e.g., bromoform, stannic chloride, or zinc

chloride) may be used as alternatives to the hydrofluoric acid

treatments. Both the acids and heavy liquids are noxious and

must be carefully handled. Excessive use of acids may degrade

the pollen exine. Different samples will react quite differently

to the same treatments; the researcher must adjust methods to

fit the nature of the sediment. It is important that no pollen

should be lost during this processing, and the pollen types must

not respond differentially to the physical and chemical treat-

ments (Traverse, 1988; Fægri et al., 1989).

The prepared samples containing only pollen may be stained

in order to increase contrast in the exine’s features. The sample

is mounted on a microscope slide; common mounts are gly-

cerol and silicone oil. It is crucial that the grains can be turned

within a liquid medium so that all sides are visible.

The next step is to make a taxonomic identification of each

pollen grain, and an absolute count of frequency. The resear-

cher is aiming to define two quantities: percentage of total

pollen for each pollen type found; and pollen concentration,

which is expressed as the number of pollen grains per unit

volume of the sediment. These two quantities, combined with

the eventual determination of the rate of sedimentation, allow

the calculation of two additional variables: the sediment matrix

accumulation rate, expressed as thickness of sediment accumu-

lated per unit of time; and pollen deposition rate (also called

pollen accumulation rate,orpollen influx), which is expressed

as number of grains accumulated per unit area of sediment per

unit time (Davis, 1969). It is the pollen percentages of the taxa

of interest, usually graphically presented on a single page, that

are considered the main result of the field work and laboratory

analysis.

The brief description here cannot do justice to the time

intensive aspect of the pollen analyses. Most identification is

done with an optical microscope; the magnification necessary

ranges from 250 to 1,000. This is the resolution needed to

evaluate the three-dimensional shape of the pollen grain, the

number and shape of the apertures, and exine structure and

sculpturing (examples in Figure P83). Identification is based

upon a combination of modern samples in a pollen herbarium,

and pollen and spore identification keys. A “pollen herbarium”

is essential for learning to identify a plant taxon by its pollen.

Herbariums are usually created by individual researchers,

or members of a research institution: at their smallest, they

may be composed of only the key vegetation relevant to the

studied region, while at their largest they may contain multiple

exemplars of pollen collected from different individuals,

and include both common wind-pollinated and rare insect-

pollinated species. Identification keys are usually composed

for the native flora of a specified geographic region: the “

Iden-

tification Key for Northwest European Pollen Flora” compiled

primarily by Fægri and Iversen (1975), and the “Key to the

Quaternary Pollen and Spore of the Great Lakes Region,” by

McAndrews et al. (1973) are examples of pollen keys that are

utilized extensively for research in the appropriate geographic

regions. In these keys, most grains are identified to the taxo-

nomic level of genus, others to species, and a few only to

family. There have been many recent efforts to compile pollen

keys for other geographic regions including tropical, Southern

Hemisphere, African and Asian flora.

The shape of the pollen or grain, and the arrangement of

the apertures, can be classified into “morphological types

(Traverse, 1988)” or “pollen classes (Fægri and Iverson, 1975).”

The exine morphology also has its own vocabulary. However,

morphology terminology has been, and still to a great extent

is, a controversial and even chaotic subject (Fægri et al., 1989).

The morphological reference the researcher utilizes will depend

on the area and geologic age of the samples being studied. It

can be difficult to discern the features of a fossil pollen grain

even at high magnification. The identification and counting of

pollen grains has still not been successfully automated. A sam-

ple that contains only pollen grains, all of which are in perfect

form, may lend itself to computerized image recognition. In

reality, most samples still contain extraneous material (ranging

from charcoal to mineral dust) and pollen grains that have been

folded and broken – the trained human eye still making the

more definitive identification.

The pollen record

Typically, the researcher takes a few hours to examine the

pollen from a single sediment sample. A minimum number of

pollen grains, the pollen sum, must be counted in order to

ensure the statistical robustness of the pollen percentages; this

number varies from approximately 300 to 1,000 grains. The

number counted will depend on the pollination characteristics

of the vegetation; for example, pine trees produce greater quan-

tities of pollen than other typical North American deciduous

genera (such as beech and elm) and therefore pine pollen is

said to “swamp” the pollen counts. A large pollen sum may

POLLEN ANALYSIS 819

be used in order to discern abundance patterns of less-prolific

pollen producers. It is also common to exclude some pollen

taxa from the pollen sum: grains commonly excluded are those

from aquatic plants, ferns, or all non-arboreal plants. The end

result of the pollen analysis is a pollen record (Fægri et al.,

1989), or a numerical matrix of pollen taxa identified per depth

sample.

Commonly, after the pollen record has been compiled, the

various types of pollen and spores encountered are calculated

as percentages of the pollen sum. Admittedly, “percentage data

have a built in bias, because the percentages must total 100%,

and the percentage of pollen ‘A’ therefore influences the per-

centage of pollen ‘B’” (Traverse, 1988). Nevertheless, a pollen

diagram, which presents the percentages in a simple graphic

form, continues to provide easily readable information, relevant

to further ecological or climatological interpretation. If the

researcher has sufficient chronological control to calculate sedi-

mentation rates, then pollen deposition rates may also be calcu-

lated and graphed. The graphical format of pollen deposition

rates is similar to the pollen percentage diagrams graphs,

although usually only a few key taxa are presented.

Ideally, the pollen percentage diagram should contain chron-

ostratigraphic, lithostratigraphic, and biostratigraphic data

(Fægri et al., 1989). The chronostratigraphic element, on the

ordinate axis, is composed of the depth scale, radiometric or

relative dates, and any geological age assignments. The lithos-

tratigraphy is usually a single column utilizing symbols, with

accompanying legend, which conveys sediment information.

The biostratigraphy is the heart of the pollen diagram: the pollen

spectrum, which are the relative frequencies (percentages) of

the various types of pollen per single sample, on the abscissa.

Graphically the spectrum are shown as histograms, or as points

which connect from depth to depth forming a “sawblade.” A

variety of diagram types is found (see Figures P84 and P85).

The most common diagram type presently constructed is termed

the resolved diagram (Fægri et al., 1989); the varying percen-

tages of many taxa are displayed, side by side.

The pollen percentage diagrams may contain any number of

depth points, and their respective pollen spectra. The more

spectra a diagram contains, the more it becomes practical to

sub-divide the biostratigraphic portion of the diagram into pol-

len zones. A “zone is a common, usually informal, term for a

minor stratigraphic interval in any category of stratigraphic

classification (Hedberg, 1972 in Fægri et al., 1989).” Zones

may be defined by presence/absence relations or by percentage

variations of key pollen taxa. Pollen assemblage zones have

Figure P83 (a) Light micrograph of Pinus palustris P. Miller. Pinus is a vesiculate pollen grain, the two hemispheric bladders show reticulate

sculpturing; the cap of the body (proximal surface) shows verrucate sculpturing (terminology follows Faegri and Iversen, 1975). Courtesy of

Gretchen D. Jones, USDA-ARS, APMRU. (b) Scanning electron micrograph of Quercus alba L. Quercus pollen often has three furrowed apertures

called colpi, this grain is tricolpate, the sculpturing is verrucate (terminology follows Faegri and Iversen, 1975). Courtesy of Gretchen D. Jones,

USDA-ARS, APMRU. (c) Light micrograph of Acer saccharum H. Marshall var. saccharum. Acer pollen is tricolpate, the exine is relatively thick with

spines, echinate sculpturing (terminology follows Faegri and Iversen, 1975). Courtesy of Gretchen D. Jones, USDA-ARS, APMRU. (d) Scanning

electron micrograph of Aster sericeus. Aster pollen, and many genera in the Asteraceae family, have three colpi, each with a pore; this grain is

tricolporate, the sculpturing is reticulate (terminology follows Faegri and Iversen, 1975). Courtesy of Gretchen D. Jones, USDA-ARS, APMRU.

820 POLLEN ANALYSIS