Odekon M. Encyclopedia of paleoclimatology and ancient environments

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Dendroclimatology

Eolian Sediments and Processes

Eolianite

Geochemical Proxies (non-isotopic)

Glacial Geomorphology

Glaciofluvial Sediments

Glaciomarine Sediments

Glendonite/ ikaite

Ice Cores, Antarctica and Greenland

Lacustrine Sediments

Laterite

Loess Deposits

Marine Biogenic Sediments

Mineral Indicators of Past Climates

Nearest-Living-Relative Method

Nitrogen Isotopes

Ocean Paleoproductivity

Ocean Paleotemperatures

Organic Geochemical Proxies

Ostracodes

Oxygen Isotopes

Paleobotany

Paleo-Ocean pH Indicators

Paleosols, Pre-Quaternary

Paleosols, Quaternary

Paleotemperatures and Proxy Reconstructions

Periglacial Geomorphology

Phosphorites

Pollen Analysis

Radiolaria

Red Beds

Sedimentary Indicators of Climate Change

Stable Isotope Analysis

Strontium Isotopes

Sulfur Isotopes

Transfer Functions

Varved Sediments

PALEO-EL NIN

O-SOUTHERN OSCILLATION (ENSO)

RECORDS

Introduction

The El Niño-Southern Oscillation (ENSO) phenomenon involves

two extreme states, El Niño and La Niña, and their connection

with the atmosphere through the Southern Oscillation. The term

El Niño (or “the boy child”) was originally used by Peruvian

fisherman in the 19th century to refer to a Christmas-time warm-

ing of coastal sea surface temperature (SST), often associated

with a decrease in the productivity of local fisheries. Today, El

Niño refers to the well known, large-scale warming and deepen-

ing of the thermocline across the eastern equatorial Pacific affect-

ing climate over much of the globe. In the mid to late 1980s,

Philander (1990) coined the term La Niña (“the girl child”)for

the opposite oceanic conditions to El Niño – large-scale cool-

ing across the tropical Pacific. The Southern Oscillation (SO)

portion of ENSO describes the global-scale surface pressure

oscillation documented by workers around the turn of the cen-

tury, and first studied by Sir Gilbert Walker. Subsequently, the

normalized sea level pressure anomaly records from Tahiti and

Darwin, Australia, have been widely used to index the atmo-

spheric pressure gradient across the tropical Pacific basin.

The early physical explanations for ocean-atmosphere

interactions at the heart of the ENSO phenomenon deduced

by Jacob Bjerknes, and the subsequent oceanographic

contributions by Klaus Wyrtki, have formed the basis for much

of what is known about ENSO. The term ENSO was first used

by Rasmusson and Carpenter (1982) to describe the interaction

of the oceanic El Niño with the negative phase of the Southern

Oscillation phenomenon. Bjerknes (1969) popularized the

word teleconnections to describe the complex of overturning

cells consisting of the Hadley (north-south) and Walker (east-

west) circulations that direct mass and energy from the equator-

ial Pacific to other regions of the globe. Thorough reviews of

the historical development and our modern understanding

of the ENSO climate system are presented by Philander

(1990) and Allan et al. (1996).

On interannual time scales, ENSO and its teleconnections

generate impacts in both marine and terrestrial environments

over a large part of the globe (Rasmusson and Carpenter,

1982; Ropelewski and Halpert, 1987). During the El Niño phase,

anomalous cloudiness and convection are generated over the

band of unusually warm water that develops across the central

to eastern equatorial Pacific. This coincides with lower than nor-

mal atmospheric pressure occurring in the eastern Pacific and

higher than normal atmospheric pressure in the Australasian

region. These patterns cause widespread drought across many

parts of Australia, southern Africa, northern India, Sahelian

Africa, Indonesia and Southeast Asia (Ropelewski and Halpert,

1987). During the La Niña phase, impacts are generally the

opposite of those described above. ENSO also influences high-

latitude processes, including North American air temperature,

Antarctic sea ice extent and ice chemistry, as well as aspects

of Atlantic hydrography relevant to thermohaline circulation

(Philander, 1990; Allan et al., 1996).

One of the most significant contributions to the longer-term

record of El Niño events was made during the 1970s–1980s by

William Quinn, who compiled historical evidence of meteoro-

logical anomalies in Peru and adjacent countries back to

A.D. 1525 (Quinn et al., 1987). According to this chronology,

moderate to strong El Niño events occur at irregular intervals

and may appear two years in succession and then not appear

for another 3–12 years. An average period of 3.8 years

between El Niños was arrived at by considering all events

(moderate to very strong) over the period 1803 to 1987. How-

ever, these events were not always representative of the wider

ENSO phenomenon across the Indo-Pacific basin. As a conse-

quence, Quinn (1992) looked at evidence for responses to

large-scale ENSO events, such as droughts in India, Australia

and Indonesia, and weak Nile River floods.

Changes in the magnitude and frequency of ENSO events

observed in instrumental records since the 1970s have generated

considerable debate about recent ENSO behavior, and the possi-

bility that it may be responding to global warming. There is also

a growing body of evidence indicating that the near-global influ-

ence of ENSO teleconnections can wax and wane on decadal

time scales. Decadal variations in ENSO frequency, strength

and teleconnections are suggested by 20th century instrumental

records. For example, the well-known link between ENSO and

the Indian monsoon has virtually disappeared since 1976, in par-

allel with a recent shift in 1976 to warmer and wetter (El Niño-

like) conditions in the tropical Pacific. Another style of decadal

ENSO variability consists of modulating the frequency of

extreme events; some decades have stronger interannual variabil-

ity than others do. Fluctuations in ENSO and its teleconnection

patterns are probably caused by changes in the frequency, mag-

nitude and spatial characteristics of ENSO events between differ-

ent climate epochs.

PALEO-EL NIN

O-SOUTHERN OSCILLATION (ENSO) RECORDS 721

Paleo-ENSO reconstruction

Concerns about the future behavior of the ENSO, and its poten-

tial far-reaching effects on societies, have promoted research

into its long-term history via proxy records. There is intense

interest in documenting ENSO variability in the past, during

times when Earth’s climate was different from today. Docu-

menting ENSO behavior during intervals when climate bound-

ary conditions (e.g., orbitally determined seasonal insolation,

global ice cover, sea level, aerosols) were markedly different

from those of today is particularly important. Proxy records

can be used to answer specific questions, such as how long

has ENSO been operating in its present form and how robust

is the ENSO phenomenon during the major climate changes

accompanying glacial-interglacial cycles? Furthermore, is

ENSO an unstable oscillation that can switch between being

“El Niño-like,” or permanently “La Niña-like”?

The paleoclimate record can be used to understand both

high- and low-resolution changes in the ENSO phenomenon.

The ideal situation would be to have a suite of paleo-ENSO

records from the tropics that record both the oceanic and atmo-

spheric components of the phenomenon at seasonal time scales.

However, with the exception of records of SST and changes in

surface-ocean salinity extracted from annually banded corals,

no such records exist. High-resolution records, such as tree

rings, fire scars, varved sediments and ice cores, register the

effects of droughts, fires, floods and air temperature fluctua-

tions that can be attributed to single ENSO events. Neverthe-

less, high-resolution records for long time periods are still rare.

On the other hand, low-resolution records can also be used,

but these need to be interpreted with caution. Former impacts

of the ENSO phenomenon can be recorded by riverine depos-

its, pollen assemblages in lake deposits, beach ridge sequences,

marine sediments and geoarcheological remains. Low-resolution

studies of ENSO rely on networks of sites, especially from

regions known for their sensitivity to ENSO. Multi-proxy ana-

lyses dramatically reduce the possibility that a non-ENSO

environmental factor was responsible for ENSO-like changes

in climatic patterns. Utilizing networks of sites and a multi-

proxy approach is particularly useful when studying the distant

past, when climate boundary conditions and ENSO teleconnec-

tion patterns may have been vastly different from those

observed today.

Evidence for Plio-Pleistocene onset

How old is the ENSO? ENSO-like phenomena, both oceanic

and atmospheric, have probably existed since the separation

of the eastern Pacific Ocean from the western Atlantic Ocean.

During the course of the mountain building process that pro-

duced the Andes and Central American chains, and the forma-

tion of the land bridge between North and South America,

exchanges of seawater between the tropical Pacific and Atlantic

Oceans ended. On paleogeographical grounds, the modern

oceanic circulation of the Pacific basin has probably existed

for about the last three million years (since the late Pliocene),

following the final closure of the Isthmus of Panama (Keigwin,

1978). At this time, anticyclonic ocean circulation and high-

pressure cells were established in the Pacific Ocean leading to

the thermal gradients across the Pacific (cold in the east, warm

in the west) that are essential for the development of ENSO

events (Romine, 1982 ). However, the existence of Pliocene

ENSO activity awaits confirmation with high-resolution sedi-

mentary records.

Nicholls (1989) argued that ENSO must have persisted con-

tinuously throughout the Cenozoic because the fauna and flora

of Australia are highly adapted to precipitation variability.

Rainfall variability tends to be 30–50% greater in areas

affected by the ENSO, such as Australia, compared with areas

of similar mean rainfall. Therefore, marked adaptation of the

fauna and flora to high precipitation variability in ENSO-

affected areas may indicate that ENSO has been operating

long enough for these characteristics to have evolved. The

Australian red kangaroo, long-haired rat and several species

of bird, for example, have all developed opportunistic life-

history strategies tuned to erratic rainfall (Nicholls, 1989).

These adaptations cannot be attributed to the arid or semi-arid

Australian environment alone because they are absent in other

arid regions. However, this theory of an ancient ENSO may

underestimate the potential range of climatic conditions to which

plants and animals can adapt. Comparative studies of adapta-

tions to high rainfall variability in other ENSO-sensitive regions

of the world are necessary in order to conclude that ENSO

has helped shape the evolutionary paths of fauna and flora.

Pleistocene glacial-interglacial cycles

Low-resolution records

How did the ENSO respond to changes in background climate

during glacial-interglacial cycles? One of the first discussions

of the past expression of ENSO under an altered glacial climate

state was presented by Quinn (1971). Quinn suggested that the

paleoenvironmental evidence for enhanced upwelling in the east

Pacific during the late Pleistocene, indicating permanently

enhanced trade winds, would imply that ENSO was locked in

the La Niña state. Anderson et al. (1990) came to a similar con-

clusion based on high-resolution analysis of late Quaternary

marine varves indicating high biological productivity in response

to enhanced upwelling along the coast of California. More

recently, studies of the Mg/Ca paleothermometer in the sur-

face-dwelling foraminifer, Globigerinoides ruber, from the equa-

torial Pacific Ocean showed that glacial intervals (marine oxygen

isotope stages 2, 4, 6, 8, 10) were characterized by markedly

stronger upwelling, suggesting enhanced Pacific trade winds

and a La Niña-like state for the mean climate (Lea et al., 2000).

In contrast to marine proxies, terrestrial paleoclimate evi-

dence for the last full glacial cycle from Australasia and South

America does not show patterns that resemble modern La Niña

teleconnection patterns (Markgraf and Diaz, 2000). Paleovege-

tation records from Java, Papua New Guinea, north-eastern

Australia and South America invariably show colder and drier

conditions during the last glacial. A high-altitude tropical

ice core record from Huascaran, Peru, also shows cooler and

drier conditions during the Last Glacial Maximum (LGM;

Thompson et al., 1995). If, as suggested by the marine records,

La Niña conditions had become a permanent mode, northern

Australia and the western Pacific should have experienced high

levels of precipitation, and only north-eastern South America

should have become arid.

One explanation for this apparent discrepancy between mar-

ine and terrestrial records could be that a 3–4



C reduction in

SST in the warm equatorial western Pacific would result in

weaker atmospheric convection and an overall reduction in pre-

cipitation over both the western Pacific and adjacent land areas.

Furthermore, the most recent SST reconstructions, based on

Mg/Ca paleothermometry using surface-dwelling foraminifera,

suggest that the Pacific was in an El Niño-like state during the

722 PALEO-EL NIN

O-SOUTHERN OSCILLATION (ENSO) RECORDS

LGM (Koutavas et al., 2002, Stott et al., 2002). An El Niño-

like state for Pacific SSTs would help explain the precipitation

patterns indicated by the circum-Pacific terrestrial paleoclimate

records for the LGM. However, until we have high-resolution

paleo-ENSO records from the equatorial Pacific “center of

action,” ENSO reconstructions based on teleconnected precipi-

tation patterns during glacial periods should be interpreted with

caution because it is likely that ENSO teleconnections do not

remain stationary as the background climate changes.

High-resolution records

The best high-resolution paleo-ENSO records for the Last Inter-

glacial epoch (128–118 kyr

B.P.) come from massive corals

from the tropical Pacific region. Spectral analysis of time-series

of skeletal Sr/Ca and

O ratios extracted from annually

banded fossil corals from Indonesia (Hughen et al., 1999) and

Papua New Guinea (Tudhope et al., 2001) reveals power at

periods of 3–6 years during the Last Interglacial. Hughen et al.

(1999) showed that the periodicity of the Last Interglacial

ENSO was similar to that for the period 1856–1960, but different

from that of 1960–1998. Thus it appears that interannual rain-

fall and SST variability in the western Pacific region was

forced by ENSO during the Last Interglacial, when the back-

ground climate was similar to, or slightly warmer than, the

modern climate.

The most complete study of past ENSO variability to date

used a suite of coral records from Papua New Guinea to exam-

ine the behavior of ENSO over the last glacial-interglacial cycle

(Tudhope et al., 2001). Multi-decadal sections of annually

banded coral analyzed at near-monthly resolution showed that

ENSO interannual variability is a common feature of the cli-

mate system over the past 130 kyr (Figure P14). While none

Figure P14 Comparison of paleo-ENSO variability in fossil coral oxygen isotope (d

O) records from Papua New Guinea and climate parameters

over the past 130 kyr (after Tudhope et al., 2001). a Standard deviation of the 2.5–7 year (ENSO) filtered d

O records for modern (black bars)

and fossil (grey bars) corals. Dashed lines show the maximum and minimum standard deviations for sliding 30-year periods in the modern coral

records. b Global sea level (Shackleton, 2000); sea surface temperature (SST) for the western equatorial Pacific (Lea et al., 2000); and changes in

ENSO power forced by changing orbital parameters (Clement et al., 1999). Shading marks the eight time periods covered by the coral records.

PALEO-EL NIN

O-SOUTHERN OSCILLATION (ENSO) RECORDS 723

of the coral specimens dated to the LGM, all the records

showed ENSO variability in the characteristic 2.5–7 year band.

However, the amplitude of ENSO variability was weaker, rela-

tive to modern ENSO variability, during the time intervals

examined (130, 128–118, 112, 85, 40, 6.5, 2.5 kyr

B.P.).

Paleo-ENSO forcing mechanisms

Three possible mechanisms could lead to weakening of ENSO

activity during glacial periods. During glacial sea-level low

stands and periods of cooler tropical SSTs, weaker ocean-

atmosphere interactions may have dampened ENSO variability

(Quinn, 1971). Secondly, shallow continental shelves exposed

by lower sea level in the Indonesian Maritime Continent

may also have stabilized ENSO variability by anchoring the

Indonesian Low convection system. Finally, precessional for-

cing of the seasonal distribution of insolation in the tropics

on 21,000 year cycles may also contribute to suppressing

ENSO variability. A numerical model of the equatorial Pacific

Ocean and atmosphere shows that seasonal insolation changes

associated with precession of the Earth’s equinoxes influence

the seasonal strength of the Pacific t rade winds (Clement et al.,

1999). When perihelion (the time that E arth is closest to the

Sun) falls in the boreal summer/fall, stronger trade winds in

that season inhibit the development of warm El Niño anoma-

lies in the eastern-central equatorial Pacific. This response

may b e sufficient to have generated significant changes in

ENSO frequency and recurrence over the late Quaternary

(Clement et al., 1999).

LGM to early Holocene transition

Paleo-ENSO records

There have not been sufficient paleo-ENSO records to evaluate

ENSO behavior during the postglacial to early Holocene transi-

tion until recently. Although late-glacial paleo-ENSO records

are still scarce, the overall early Holocene trend exhibited in

the records includes a low frequency and amplitude of events.

Several studies have investigated the paleo-ENSO in north-

western South America where torrential rain, floods and ero-

sion occur along the normally arid coast of Peru during strong

El Niño events. Based on the analysis of flood deposits from

coastal Peru, Wells (1987) estimated that about 100 El Niño

events had occurred over the last 40,000 years. Sediment pro-

files from archaeological sites along the coast of Peru indicate

a lack of strong flood events (interpreted as El Niños) between

12–5.7 kyr

B.P. at Quebrada Tacahuay (Keefer et al., 1998) and

between 8.9–3.4 kyr

B.P. at nearby Quebrada de los Burros

(Fontugne et al., 1999). Nevertheless, a debris flow in northern

Peru, dated at 7.5 kyr

B.P. (Wells, 1990), has been interpreted

to reflect a major El Niño event, suggesting that ENSO

may have operated at that time. The most continuous, high-

resolution record for the postglacial transition period comes

from laminated clastic deposits in a high-altitude lake, Laguna

Pallcacocha, in Ecuador (Rodbell et al., 1999). Today, such

clastic laminae are associated with anomalously high rainfall

during El Niño events. The sedimentary record shows a

clear suppression of ENSO variability, with periodicities of

15 years, between 12 kyr

B.P. (the beginning of the record)

and 7 kyr

B.P.

Terrestrial paleo-ENSO records from outside the core

ENSO impact area rely on the interpretation of teleconnec-

tion patterns. A late-glacial varve chronology from New

England, USA, spanning 17.5 to 13.5 kyr

B.P., shows a distinct

interannual band (3–5 years) of variability suggestive of ENSO

teleconnections into North America (Rittenour et al., 2000).

The interannual variability largely disappears by 13.5 kyr

B.P.,

with only the highest frequency components persisting. Pollen

records from South America, New Zealand and Australia indi-

cate that early Holocene vegetation did not include types

adapted to the periodic droughts associated with the ENSO

(McGlone et al., 1992; Markgraf and Diaz, 2000). By compar-

ing early Holocene vegetation, lake level and fire history

records from these regions, McGlone et al. (1992) concluded

that the circum-Pacific precipitation patterns, reduced environ-

mental variability and absence of fire all suggest the suppres-

sion of ENSO between 10–8 kyr

B.P.

Marine records of the paleo-ENSO during the postglacial

transition are more sparse than terrestrial records. Nevertheless,

the character and intensity of major El Niños has been

traced back to 17 kyr

B.P., based on evidence from the pollen

of conifer and oak species in marine sediments from the Santa

Barbara basin, California (Heusser and Sirocko, 1997). High-

resolution marine records during postglacial sea-level low

stands are also rare, but coral reconstructions of the paleo-

ENSO exist for the end of the deglaciation, when sea levels

reached near-modern positions. Coral records from Papua

New Guinea indicate that ENSO variability was 60% weaker

at 6.5 kyr

B.P. than during the 20th century (Tudhope et al.,

2001). ENSO-related interannual variability was also weaker

in a coral record from the Great Barrier Reef, Australia, for

6.2 kyr

B.P. (Gagan et al., 1998). However, mid-Holocene

reconstructions of SST variability in the tropical south-western

Pacific suggest there is a significant mismatch between strong

El Niño SST variability and weak precipitation variability, indi-

cating some degree of decoupling of ENSO ocean-atmosphere

interactions during this period (Gagan et al., 2004). These

observations agree with results from a coupled climate system

model (Otto-Bliesner, 1999), which indicate that ENSO tele-

connections may have differed from modern patterns at 6 kyr

B.P. because of the overriding influence of regional climate

changes.

Models of insolation forcing of ENSO

A recent modeling study of orbitally-induced changes in inso-

lation in the tropical Pacific reproduced the observed reduction

in ENSO amplitude and frequency during the early Holocene

(Clement et al., 1999). The authors attributed the reduced early

Holocene ENSO to the peak in boreal summer/fall insolation

brought about by precession of the Earth’s equinoxes. Accord-

ing to the numerical model, the additional heating of equatorial

Pacific surface waters in the early Holocene produced an east-

erly wind anomaly that suppressed the development of El Niño

events. Ocean-atmosphere feedbacks drive the ENSO system

towards a La Niña state by increasing SST and pressure gradi-

ents across the Pacific, in good agreement with the paleocli-

mate records. A similar effect has been observed in a global

coupled ocean-atmosphere model, whereby the intensified

Asian monsoon during the early Holocene further enhances

Pacific trade winds, thus cooling the eastern equatorial Pacific

and reducing ENSO interannual variability (Liu et al., 2000).

Mid-Holocene onset

Several lines of paleoclimate evidence suggest that the onset of

modern ENSO variability occurred between 7 and 4 kyr

B.P.

Evidence for the demise of the period of weakened ENSO

activity during the early Holocene is most clear in the tropical

724 PALEO-EL NIN

O-SOUTHERN OSCILLATION (ENSO) RECORDS

eastern Pacific and northern South America. Spectral analysis

of the 15,000-year high-resolution record of storm-derived

clastic sedimentation in Laguna Pallcacocha, Ecuador (Rodbell

et al., 1999; Moy et al., 2002) shows that the transition to

modern ENSO periodicities (2–8 yr) began 7–5 kyr

B.P.

(Figure P15). A similar conclusion was reached by Sandweiss

et al. (1996, 2001), based on their analysis of fossil mollusk

assemblages and geoarcheological evidence from coastal Peru.

There, the onset of El Niño events at 5.8 kyr

B.P. is indicated by

the demise of thermally anomalous molluskan assemblages.

The beginning of monumental temple construction on the

Peruvian coast at 5.8 kyr

B.P. is thought to reflect the end of

early Holocene hyperaridity brought about by the resurgence

of El Niño activity (Sandweiss et al., 2001). More indirect

evidence of Holocene ENSO variability is provided by titanium

concentrations in sediment from ODP site 1002 in the Cariaco

Basin, off northern Venezuela (Haug et al., 2001), which reflect

variations in runoff associated with shifts in the position of

the Intertropical Convergence Zone (ITCZ). Enhanced runoff

variability beginning at 3.8 kyr

B.P. indicates a mean southward

shift in the position of the ITCZ, thought to be linked to the

strengthening of El Niño events.

On the western side of the Pacific basin, pollen records from

northern Australia yield a slightly later time for ENSO onset.

The first occurrence of drought-adapted pollen taxa in lake

sediment cores from tropical northern Australia indicates

ENSO onset at 4 kyr

B.P. (Shulmeister and Lees, 1995). A

composite charcoal abundance record derived from 10 lake

and wetland records from eastern Indonesia and Papua New

Guinea reflects changes in the pattern of regional burning from

the LGM to the present (Haberle et al., 2001). Higher charcoal

concentrations from the middle to late Holocene (5 kyr

B.P.to

the present) are interpreted to reflect higher precipitation varia-

bility associated with the onset of modern ENSO variability.

Late Holocene ENSO variability

The picture emerging for the most recent 5,000 years of ENSO

history indicates that it began to operate as it does now, but

with variability on millennial timescales and a peak in ENSO

frequency and magnitude at 1.8–1.2 kyr

B.P.(Figure P15).

The record of clastic sedimentation from Laguna Pallcacocha,

Ecuador shows that El Niño events became more frequent

over the Holocene until about 1.2 kyr

B.P., and then declined

towards the present (Moy et al., 2002). Superimposed on this

long-term trend are periods of relatively high and low ENSO

activity, alternating at a time scale of about 2,000 years.

Geoarcheological evidence from the coast of Peru also indicates

an increase in ENSO event frequency after 3.2–2.8 kyr

B.P.

(Sandweiss et al., 2001). Titanium concentrations in sediment

from the Cariaco Basin (Haug et al., 2001) also show enhanced

runoff variability after 3.8–2.8 kyr

B.P., which is thought to be

linked to a southward shift in the ITCZ related to warming of

equatorial eastern Pacific SSTs in response to more frequent

El Niño events.

Coral

O records from massive fossil Porites micro-

atolls at Christmas Island in the central equatorial Pacific

record the migration of warm SSTs and convective rainfall

during El Niño events (Wood roffe et al., 2003). Three multi-

decadal records from this locality indicate that ENSO amplitudes

were 20–40% weaker than modern values during the period

3.6–2.9 kyr

B.P., in good agreement with terrestrial paleo-ENSO

records from tropical South America. The coral records from

Christmas Island, like the Ecuadorian lake record, indicate that

ENSO variability reached, or exceeded, modern values from

2.5–1.7 kyr

B.P.

Coupled ocean-atmosphere model simulations forced by

precessional changes in the seasonal cycle of insolation gener-

ally agree with paleo-ENSO records showing an increase

in large El Niño events during the Holocene, with a peak

3.1–1.0 kyr

B.P. However, according to the paleo-ENSO

records, the abrupt onset of ENSO and millennial-scale variabil-

ity was more complex than that predicted by the models, sug-

gesting that factors other than insolation forcing may be at

work. It has been suggested that the millennial variability in

ENSO observed at Laguna Pallcacocha may be due to internal

ENSO dynamics. Another possibility is that since 3kyr

B.P.,

tighter coupling in the Pacific between the more southerly ITCZ

and the Southern Oscillation (Haug et al., 2001) has served to

amplify ENSO precipitation variability and associated telecon-

nections (Woodroffe et al., 2003). Such a scenario is consistent

with terrestrial paleoclimate records indicating a marked increase

in El Niño activity since 3kyr

B.P.

The last millennium

Evidence for the behavior of ENSO over the last millennium

comes from historical records, tropical ice cores, subtropical

to mid-latitude tree rings, and corals from the Pacific basin.

Historical records of Nile River flood level from as early as

A.D. 622 (Quinn, 1992; Kondrashov et al., 2005) record the

effects of summer monsoon (June–September) rainfall over

the highlands of Ethiopia, the major source region for the Nile

where El Niño events produce below-average flood levels at

Cairo. At the start of the last millennium, during the Medieval

Warm Period, the percentage of weak Nile floods from

A.D.

1000–1290 was low (8% of years), suggesting that a La

Niña-like climate mode operated when the climate of the

Northern Hemisphere was warmer. In contrast, the period

1694–1899, spanning the latter part of the Little Ice Age,

shows more frequent occurrences of weak Nile floods

(35% of years), indicating that El Niño-induced droughts were

common.

Figure P15 Number of El Nin

o events in 100 year intervals since

12,000 cal. yr

B.P., based on the analysis of clastic laminae in Laguna

Pallcacocha, Ecuador (after Moy et al., 2002). Line indicates the

minimum number of events (5) required to produce ENSO-band

variance.

PALEO-EL NIN

O-SOUTHERN OSCILLATION (ENSO) RECORDS 725

Analysis of drill cores from the Quelccaya ice cap in the

high Andes of southern Peru has provided a record of ENSO

variability back to about

A.D. 1450 (Thompson et al., 1992).

In contrast to the Nile River flood record, El Niño events were

rare at Quelccaya during the Little Ice Age (1550–1850),

suggesting that the spatial signature of ENSO may have been

different under the altered background climate at that time.

Annually laminated ice cores from Dasuopu Glacier, on the

southern Tibetan Plateau, allow examination of the teleconnec-

tions between El Niño events and the Asian monsoon

(Figure P16). At Dasuopu, fluctuations in

O, dust and

chloride concentrations are associated with major monsoon

failures (e.g., 1790–1796, 1876–1877) during very strong El

Niño events. The 560-year record also shows rare, but large,

monsoon failures associated with El Niño-induced drought in

the region (Thompson et al., 2000). The full Dasuopu ice core

record (not shown) reveals earlier moderate monsoon failures

in the 1230s, 1280s, 1330s, 1530s, 1590s and 1640s.

Studies of tree rings have the potential to provide annual and

even seasonal data from which the long-term behavior of

ENSO may be derived (Cook et al., 2000). However, tree-ring

analysis of tropical trees has been problematic due to a paucity

of long-lived species and a lack of annual growth rings. Alter-

natively, Lough and Fritts (1985) developed a network of tree-

ring width series back to

A.D. 1660 based on the connection

between precipitation anomalies and ENSO in western North

America, particularly moisture limitation and its effect on tree

growth during El Niño events. The result was a series of annual

maps of seasonal climate features such as temperature, precipi-

tation and sea-level pressure indicative of ENSO variability in

North America. Stahle et al. (1998) developed a 272-year

(1706–1997) reconstruction of the Southern Oscillation Index

based on drought-sensitive tree-ring chronologies (Figure P17).

The proxy data set comprises 20 sub-tropical tree-ring chron-

ologies from the southern United States and Mexico, and a

single tropical teak chronology from Java. This index targets

the Northern Hemisphere cool season (October–March),

when El Niño and La Niña events typically mature in the

equatorial Pacific. Their reconstruction indicates stronger

interannual ENSO variance in the late 19th and 20th centu-

ries as compared to earlier periods. However, a network of

drought reconstructions over the continental USA shows that,

although the ENSO-drought link in the southwest is relatively

robust, drought in other regions (the Atlantic region) shows a

more variable connection to tropical Pacific climate variabil-

ity (Cole and Cook, 1998).

Coral records from the tropical Pacific clearly show that the

characteristic timescale of ENSO variability has changed over

the past four centuries (Dunbar et al., 1994; Urban et al.,

2000). A 400-year long coral record from the Galapagos

Islands, in the eastern equatorial Pacific, reveals that decadal

SST variability and ENSO change in strength simultaneously

(Dunbar et al., 1994). The Galapagos record indicates that

the dominant oscillatory ENSO mode shifted from 6 years to

4.6 years by

A.D. 1700, and to 3.4 years after A.D. 1850. Another

coral record from Maiana Atoll, in the central equatorial

Pacific, registers a long-term trend from cooler/drier to war-

mer/wetter conditions that is associated with changes in ENSO

variance since

A.D. 1840 (Urban et al., 2000). When the back-

ground climate was cooler/drier during the 19th century,

ENSO variability occurred on a decadal time scale, in contrast

to the shorter, dominantly interannual variance of the 20th

century (Figure P18).

Figure P16 Annual averages of oxygen isotopes (d

O), insoluble dust

and chloride in ice from the high-altitude (7,200 m a.s.l.) Dasuopu

Glacier in southern Tibet since

A.D. 1440 (after Thompson et al., 2000).

Dust concentrations are per milliliter of melted ice. El Nin

o events (low

monsoon precipitation) are indicated by higher d

O values, dust and

chloride concentrations in the ice. Diamonds mark very strong El Nin

events (monsoon failures) in 1876–1877 and 1790–1796.

Figure P17 Boreal winter (October–March) Southern Oscillation Index

(SOI) reconstructed using tree-ring data from Mexico, the southwestern

USA and Indonesia (after Stahle et al., 1998). The tree-ring

reconstruction indicates a significant increase in the interannual

variability of the winter SOI, and more La Nin

a events, beginning in the

mid-19th century.

726 PALEO-EL NIN

O-SOUTHERN OSCILLATION (ENSO) RECORDS

A composite record of ENSO-related rainfall and river run-

off variability since

A.D. 1615, compiled from eight multi-

century coral cores from the Great Barrier Reef, Australia

(Hendy et al., 2003), shows strong anti-correlations with SSTs

in the equatorial eastern Pacific during the mid-17th to late

18th centuries. This suggests similar, strong ENSO-related

teleconnections occurred then as in recent decades. However,

disappearance of the anti-correlation between the eastern and

western Pacific for most of the period from the 1800s to

1870s suggests that ENSO-related teleconnections with north-

east Australian rainfall were not operating during much of the

19th century. During this period both interdecadal and interann-

ual variability were suppressed, in good agreement with other

coral and tree-ring records. The late 20th century increase in

ENSO variability and the strength of ENSO teleconnections

are, therefore, not solely recent phenomena.

Conclusions

In summary, significant progress has been made in our under-

standing of past ENSO behavior. However, well-calibrated

records from the tropical Pacific region of strongest ENSO

influence are still sparse. A full understanding of the paleo-

ENSO will require the comparison of El Niño temperature

forcing in the Pacific with rainfall perturbations that may

be modulated under the influence of what is potentially an

evolving ocean-atmosphere system.

Michael K. Gagan

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Cross-references

Ancient Cultures and Climate Change

Atmospheric Circulation during the Last Glacial Maximum

Climate Variability and Change, Last 1,000 Years

Coral and Coral Reefs

Dendroclimatology

Holocene Climates

Ice Cores, Mountain Glaciers

Last Glacial Maximum

Little Ice Age

Medieval Warm Period

Monsoons, Quaternary

Ocean Paleotemperatures

Paleoclimate Modeling, Quaternary

Paleoclimate Proxies, an Introduction

Paleoceanography

Paleotemperatures and Proxy Reconstructions

Paleotempestology, the Sedimentary Record of Intense Hurricanes

Pleistocene Climates

Quaternary Climate Transitions and Cycles

Teleconnections (see Encyclopedia of World Climatology)

Time Series Analysis

Varved Sediments

PALEOGENE CLIMATES

Introduction

The Paleogene spans the first half of the Cenozoic era, from

65 to 23 million years ago. It includes the Paleocene epoch

(65–55 million year ago), the Eocene epoch (55–34 million

years ago), and the Oligocene epoch (34–23 million years ago).

The Paleogene marked an important transformation in the climatic

history of the Earth, from the “greenhouse” world of the Mesozoic

(which persisted into the early Eocene), through the climatic dete-

rioration to form the “doubthouse” world of the middle and late

Eocene, to the Antarctic glaciation of the early Oligocene, produ-

cing the “icehouse” world that we are still experiencing today

(Miller et al., 1987, 1991;Miller,1992;Zachosetal.,1993, 1994;

ProtheroandBerggren,1992;Prothero,1994;Protheroetal.,2003).

The overall history of Paleogene climates is shown in

Figure P19. Our best proxy of global paleotemperature is the

record of oxygen isotopes in the shells of microfossils from the

deep oceans around the world (see Oxygen isotopes). Other indica-

tors are used as well, such as the known paleoclimatic preferences

of certain fossils, and the presence of certain sediments that are

climatically informative. For example, in deep-sea cores, the pre-

sence of coarse sediments rafted by icebergs a long way from the

continent and dropped into deep ocean muds is considered an

excellent indicator of the growth of ice sheets and the calving of

icebergs (see Ice-rafted debris (IRD)).

In addition to these important marine signals, there are many

climatic indicators on land. Perhaps the most useful is the

record of land plants, which are highly sensitive to climate

and other changes in their growing conditions. The most

728 PALEOGENE CLIMATES

straightforward such index is the shape of leaves (Wolfe, 1978,

1994 see Paleobotany). Other terrestrial indicators, such as the

presence of dune sands and evaporites indicating deserts, or

thick coal deposits, indicating swampy conditions, are also

used. Finally, certain types of terrestrial organisms, such as tur-

tles and crocodilians (Markwick, 1994), and some kinds of land

snails, are highly sensitive to climatic changes, and can also be

used as constraints on the paleoclimate of a region (see Animal

proxies, invertebrates; Animal proxies, vertebrates).

The Paleocene to early Eeocene “greenhouse” world

The Paleocene epoch was the 11-million-year period of recov-

ery from the great extinction at the end of the Cretaceous that

wiped out the dinosaurs, ammonites, marine reptiles, and many

other typically Mesozoic groups. Some groups that had been

decimated by the extinction event, such as the foraminifera

and mollusks, recovered and radiated in the Paleocene. Other

groups, such as the mammals, had lived as tiny animals over-

shadowed by the dinosaurs during the entire Mesozoic. Once

the dinosaurs were gone, mammals underwent an explosive

adaptive radiation in the Paleocene, and evolved from a hand-

ful of rat-sized animals at the beginning of the Paleocene to

over two dozen different orders by the early Eocene, including

all the living orders of mammals. They soon occupied most of

the terrestrial niches, from the tiny insectivore or seed eater, to

the large and small carnivore, and even the huge herbivorous

land animal niche as well. By the middle Eocene, there were

also mammals in the seas (early whales) and the air (bats).

Although there were some perturbations of the oceans and

atmospheres as a result of the end-Cretaceous extinctions, climate

soon returned to the “greenhouse” state that had occurred through

most of the later Mesozoic. Paleocene and early Eocene rocks

above the Arctic Circle and in Antarctica yield temperate-zone

plants, and tropical plants, such as palms and cycads, occurred as

far as 60



N latitude. Alligators and tortoises also lived above



N latitude in a region that today experiences 6 month of dark-

ness and has only freezing climates and tundra vegetation. Calcu-

lations from oxygen isotopes suggest that tropical temperatures

were much like those of today (Pearson et al., 2001), but polar

waters (which are now at or below freezing) were as warm as

10–12



C in the late Paleocene, and 15



C in the early Eocene.

To explain this puzzle, some scientists have suggested that the

Arctic Circle would be much smaller if the Earth’stiltonits

rotational axis was much less than its present angle of 23.5



.How-

ever, such a major change in the Earth’s tilt creates all sorts of

geophysical problems, and some climatic models of an Earth with

a shallower tilt create colder, not warmer , polar regions. The con-

sensus of scientists is that these polar animals and plants were cap-

able of living for 6 months in a near-dark world as long as

temperatures remained warm and mild even in the months of

darkness.

Such conditions require that the entire Earth be much war-

mer than today on average, and that there was a lot of heat

transport from the equators to the poles, so that the difference

between their temperatures was much less than today. Most

scientists agree that a higher level of greenhouse gases (such

as carbon dioxide and methane) was largely responsible for

the overall level of warmth in the early Cenozoic. Although

some scientists point primarily to carbon dioxide (De Conto

and Pollard, 2003), others have shown that the plant record

does not support unusually high levels of this gas (Royer et al.,

2001). Estimates from the geochemistry of carbonate (Pearson

and Palmer, 1999) and climatic modeling (Sloan and Rea,

1995) also indicate that atmospheric carbon dioxide levels were

Figure P19 Climatic history, global deep-sea oxygen, and carbon isotope record correlated with major oceanographic and climatic events from

70 to 23 million years ago (modified from Zachos et al., 2001).

PALEOGENE CLIMATES 729

not much higher than present. Instead, Sloan et al. (1992) argue

that methane might have been a more effective greenhouse gas,

consistent with all the geochemical evidence.

However, plate tectonics and oceanic circulation must have

been important factors as well. To explain the shallow gradient

in temperature between poles and equator, there must be oceanic

circulation that allows warm waters from the equator to mix with

temperate and polar waters, transferring the heat to the poles.

Oceanic circulation models with early Cenozoic continental posi-

tions show that a key factor is the connection of Antarctica to both

Australia and South America. Today, Antarctica is isolated from

these continents, producing the Circum-Antarctic Current, which

circulates in a clockwise direction around the Antarctic. This

shallow-water current is one of the most voluminous in the ocean,

with a velocity of 25 cm s

–1

, and a volume of 233 million cubic

–1

, more than 1,000 times the flow of the Amazon River. The

Circum-Antarctic Current acts as a “refrigerator door” that locks

in the cold around the Antarctic continent, and promotes the

growth of polar ice caps. It also promotes upwelling of deep waters

from the Antarctic margin, which rise, chill, and then sink to gen-

erate the cold Antarctic bottom waters. These cold, oxygenated

currents make up 59% of the world’s marine waters, and flow from

the surface of the Southern Ocean to the bottoms of the deep

oceans all the way to the Northern Hemisphere as far as 50



north

latitude in the Atlantic.

For over 30 years now, geologists have known that this cur-

rent and its effects could not have existed in the Paleocene and

Eocene, as long as the connection between Antarctica and both

South America and Australia blocked its path. Instead, models

of Paleocene and Eocene oceanic circulation show that the tro-

pical currents mixed with temperate and polar currents, warm-

ing the poles. In some models (Kennett and Stott, 1990a), the

modern circulation pattern dominated by cold Antarctic bottom

waters is replaced by a different kind of ocean, where warm

saline waters are generated in the tropics by evaporation, and

then sink (due to increased salinity) to form warm saline deep

waters, which traveled along the ocean bottom all the way to

the poles. These ideas are still controversial, but clearly warm

tropical currents were reaching the poles and transferring heat

to this region to keep it so balmy.

The late Pale ocene thermal maximum

On top of the overall warm “greenhouse” climates of the Paleo-

cene and Eocene, there are short-term events that cannot be

explained by slow-moving causes such as changes in atmospheric

gases or oceanic circulation. The most remarkable of these is

the Late Paleocene Thermal Maximum (LPTM) at 55.6 million

years ago, when deep-sea temperatures warmed 5



C, and sea sur-

face temperatures warmed by 4–8



C in less than 200,000 years

(Röhl et al., 2000). Carbon isotope values show a dramatic nega-

tive excursion over the same short interval of time (Kennett and

Stott, 1990b; Zachos et al., 1993), and this carbon isotope signal

is even apparent in the teeth of land mammals and in ancient soil

carbonates (Koch et al., 1992). In addition, there was a major mass

extinction in the benthic foraminifera that live on the ocean bot-

tom, and rapid diversification in the planktonic foraminifera and

blooms of dinoflagellates. An evolutionary radiation in land mam-

mals occurred and they freely migrated over the polar routes from

Eurasia to North America in the early Eocene, and even lived

above the Arctic Circle (papers in Aubry et al., 1998).

The extraordinary rapidity and the peculiarity of the warm-

ing of deep ocean waters led to another explanation of the

LPTM (Dickens et al., 1995, 1997; Thomas et al., 2002).

The most striking evidence comes from the ratio of

(the common isotope of carbon) to

C (a rare isotope of car-

bon). In two short pulses, each less than a thousand years in

duration, the oceans became extremely rich in

C, suggesting

that some source had released enormous amounts of biogenic

carbon in a geologic instant. This rules out the slow oceano-

graphic or plate tectonic changes discussed above. Instead,

many scientists now think that methane (CH

, or natural gas)

was trapped in a chemical form known as methane hydrate

(methane combined with water in complex cage-like, icy com-

pounds). These compounds can form huge volumes of trapped

carbon, frozen on the ocean floor. According to the late Paleo-

cene scenario, as much as 1.12  10

gofCH

was locked up

on the sea bottom. When the methane hydrates broke down and

their methane was released, that flooded the deep ocean with

excess methane and carbon dioxide (hypercapnia). This car-

bon-rich water nearly wiped out many of the bottom-dwelling

organisms (especially the benthic foraminifera). Eventually this

methane escaped to the atmosphere to produce a warm “super-

greenhouse.” Although some of the carbon was eventually

returned to the crust in the form of carbonates and coals, much

remained throughout the early Eocene, and was responsible for

the extraordinary global warming.

The Eocene climatic deteriorati on and the

“doubthouse”

The warm, balmy world of the early Eocene began to deteriorate

by the early part of the middle Eocene. Over the course of the 12

million years of the middle Eocene (49–37 million years ago),

and the 3 million years of the late Eocene (37–34 million years

ago), this climatic deterioration continued so that by the early

Oligocene, glacial ice returned to Antarctica. From the beginning

of the middle Eocene, mean global temperature declined by more

than 10



C, more than during any ice age cycle ( Figure P19). In

North America, the land plants suggest 13–15



C of cooling over

the same interval. This climatic transformation was gradual dur-

ing the entire middle Eocene, although there was an abrupt cool-

ing episode (causing temperatures to drop 5



C globally) at the

end of the middle Eocene (37 million years ago).

During the three million years of the late Eocene (37–33

million years ago), there was a slight warming and recovery from

the long-term cooling trend. At least three major impacts struckthe

Earth in the middle of the late Eocene (35.5–36.0 million years

ago), but they caused no significant changes in climate, nor any

extinction of importance.

In addition to the isotopic and leaf-margin climatic signals,

there are other indicators of cooling climate. Antarctic glacial

deposits overlain by a lava flow dated at 49.4 million years

old have been reported (Birkenmajer, 1987); although most

scientists do not think that there was significant Antarctic gla-

cial ice this early. Gradual changes in the oceanic circulation

patterns are also seen during the middle Eocene (Boersma

et al., 1987), although nothing as dramatic as the inception

of Antarctic bottom waters in the early Oligocene. To many

scientists, the middle-late Eocene is the “doubthouse” world

(Miller et al., 1991): a time of transition between better known

“greenhouse” and “icehouse” conditions, where there are few

modern analogues and it is difficult to describe how the oceans

and atmospheres worked during this time.

The causes of this middle-late Eocene climatic change are

still unclear. As discussed above, some scientists argue that

the climate was changed by addition or withdrawal of green-

house gases, but it is not clear that carbon dioxide was the main

730 PALEOGENE CLIMATES