Odekon M. Encyclopedia of paleoclimatology and ancient environments

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be capable of affecting North Atlantic fresh water balance and

thus THC. Therefore, the disagreement between the model and

paleoclimate data could be a result of differences in the timescale

of investigation (i.e., decadal vs. millennial).

Solar forcing and future climate

It has been difficult to confirm solar influence in decadal cli-

mate variability, possibly because irradiance records are too

short and proxy reconstructions contain much uncertainty.

Another reason may be anthropogenic climate forcing obscur-

ing the solar influence. In the future, greenhouse gases

(GHG) and tropospheric aerosols such as sulfates and dust

could alter Earth’s radiative balance with comparable, if not lar-

ger magnitudes than solar forcing (Figure S45; Hansen, 2000).

The role of solar forcing therefore depends on future anthro-

pogenic emissions. If efforts to reduce CH

and CO

emission

are implemented, solar forcing may re-emerge as the domin-

ant climate influence on centennial to millennial timescales

(Hansen, 2000).

Summary and conclusions

The Sun is a variable star, and from satellite measurements,

total solar irradiance varies by 0.1%. Observational records of

sunspots, geomagnetic activity, and cosmic ray flux are used

as proxies for irradiance variability, though the relationship

with irradiance is largely correlational rather than physical.

Observations of Sun-like stars, if applicable to our Sun during

the Maunder Minimum, suggest an irradiance reduction of

around 0.25% relative to today, though key assumptions with

this method have not been validated.

Cosmogenic isotopes such as

Be and

C are among the

best records of solar magnetic activity, as they are available

for the entire Holocene at high resolution. While the records

are subject to complications of accumulation rate variations

(ice cores) or changes in deep ocean circulation (atmospheric

C), their agreement despite large differences in production

and transport chemistry supports their use as proxies for solar

variability.

Holocene climate change in the North Atlantic region is

influenced by solar variability based on correlations between

cosmogenic isotopes and ice-rafted detritus from sediment

cores. The Little Ice Age appears to have been the most recent

cold phase of this centennial to millennial cycle. Additionally,

several Holocene records of Asian monsoon strength consis-

tently indicate a weaker monsoon during solar minima.

General circulation model simulations suggest a global cool-

ing of around 0.35



C given an irradiance forcing of 0.25%. North

Atlantic paleoclimate records suggest larger regional changes,

implying amplification mechanisms such as ozone photochemis-

try, stochastic resonance, and atmosphere-ocean dynamical

responses. Dynamical responses can change climate by affecting

the advection of air or water from warmer or colder regions in the

absence of large changes in global mean temperature.

Efforts to establish the importance of solar forcing on deca-

dal time scales for recent past climates and the future may be

hampered by the growing contribution of anthropogenic for-

cing. If efforts to curtail emissions are implemented, solar

variability may continue to dominate climate change.

Peter F. Almasi and Gerard C. Bond

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Figure S45 Anthropogenic emissions exert a radiative influence on climate larger than that expected for solar irradiance variability. Solar variability

is dominant among natural forcing on decadal and longer time scales (adapted from Hansen, 2000).

934 SUN-CLIMATE CONNECTIONS

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by cosmic rays. Phys. Rev. Lett., 85, 5004–5007.

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and Matter, A. 2001. Strong coherence between solar variability

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Cross-references

Astronomical Theory of Climate Change

Beryllium-10

Cosmogenic Radionuclides

Dendroclimatology

Ice-Rafted Debris (IRD)

Little Ice Age

Maunder Minimum

Monsoons, Quaternary

North Atlantic Deep Water and Climate Change

North Atlantic Oscillation (NAO) records

Thermohaline Circulation

SUN-CLIMATE CONNECTIONS 935

TEPHROCHRONOLOGY

Introduction

Tephrochronology is a dating method based on the identifica-

tion, correlation and dating of tephra layers (Thórarinsson, 1944;

Thórarinsson, 1981). It is important because it can give unusually

precise and accurate dating control and may be applied over

large areas of both land and sea. As a compound word “tephro-

chronology” follows the spelling conventions for the use of Greek

loanwords in English where the “a” is replaced by an “o.”

Tephra

Tephra is a “catch-all” term to describe essentially unconsoli-

dated pyroclastic material produced by a volcanic eruption.

The term tephra is derived from the Greek word tefra, and

was introduced as a modern scientific term by Sigurdur

Thórarinsson in his 1944 doctoral thesis. Thórarinsson wanted

a general term for pyroclastic ejecta, a definition that could

not be satisfied by existing terms such as “volcanic ash”,

“lapilli” and “bombs” which described very specific types of

volcanic products. He proposed using tephra, noting how it

complemented the use of other Greek loanwords “magma”

and “lava” already in use in geology. There has been debate

as to what is, or is not, strictly defined as tephra. Its scope

may be clarified by considering what it is not: tephra does

not include volcanic gases (yet tephra can be an aerosol); it is

not a consolidated rock (yet tephra can become one); it is not

a liquid (although tephra is formed from molten rock). Tephra

is often composed of glass particles formed by the rapid chil-

ling of droplets of magma. These grains may be vesicular or

composed of solid fragments or plate-like bubble walls. Tephra

can also include varying proportions of crystal and mineral

components derived from either magma or the walls of the

vent. Violent eruptions may result in the incorporation of sig-

nificant fractions of lithic fragments either from the vent or,

in the case of large-scale eruptions, the edifice of the volcano.

As the density of these different fractions varies, their rela-

tive abundance will vary with distance from source, due to

gravitational sorting. Originally, an “airborne” qualification

was considered in the definition of tephra, but in practice, this

condition has been dropped so that the term may be robustly

applied to essentially all unconsolidated deposits. This applies

whether or not they have ever been truly airborne (as in a volca-

nic plume), or transported at the ground surface in a flow (as in

co-ignimbrite ashes, pyroclastic flows or surges), or generated

and transported in water (as in the products of sub-marine,

sub-glacial or other sub-aqueous volcanic eruptions). This

wide-ranging, inclusive definition of tephra is important when

it comes to the use of tephrochronology because it removes

any need to consider the exact mode of formation and, the

wider the definition of tephra, the wider the applications of

tephrochronology.

Characterization

The fundamental properties of tephra that underpin tephro-

chronology are its potentially rapid and widespread dispersal

and distinctive physical and chemical characteristics. Atmo-

spheric dispersal in a volcanic plume may be considered effec-

tively instantaneous. As a result, tephra deposits may be used

to define “isochrons”, or horizons of the same age, which are

most precisely defined by the basal surface of the tephra

deposit. This horizon may be used to define an area of environ-

ment at a “moment” in time. The destruction of the Roman

cities of Pompeii and Herculaneum by tephra from the

AD 79

eruption of Vesuvius defines an isochron and has been used

to study cityscapes frozen at one point in time. Likewise tephra

horizons may be used to study the characteristics and spatial

patterns of other past environments, be they peat bogs, glacier

forelands or ocean floors. They can be employed for precise

correlation of contemporaneous records from radically different

environments.

Significant temporal patterns caused by changing winds can

occur within the horizon defined by a single tephra unit. For

example, an early stage of an eruption may produce different

types of tephra than a later stage, and may inject material into

different levels of the atmosphere, where it will be affected

by different types of wind. The result can be different dispersal

patterns for contrasting stages during the formation of tephra.

In this case, one should recognize that somewhat different

tephra deposits could record a single event and that associated

deposits could be contemporaneous. Characterization of tephra

deposits is best achieved by combining a variety of data and

utilizing physical characteristics of the layers as a whole, such

as: layer thickness; particle size ranges and particle shapes;

composition in terms of glass shard, crystal mineral or lithic

content; layer color (although this may be significantly affected

by the nature of the enclosing strata); stratigraphic location and

stratigraphic relationships. Bulk chemistry has been used to

characterize tephra layers, but this has been superseded by

discrete grain analyses, generally undertaken by electron probe

microanalysis and capable of accurately determining major

(and minor) element abundances. Where tephras form layers

invisible to the naked eye (“micro” or crypto tephras), the

approach has to be modified and generally focuses on the phy-

sical and chemical properties of separate grains and stratigra-

phic data. Collectively, these data may be used to determine

characteristics best conceived as “signatures” rather than “fin-

gerprints”; while chemical compositions may be distinctive,

they are too frequently similar to be described as a “fingerprint.”

Examples of this similarity may be found for a wide range of

tephra compositions from different eruptions, both silicic and

basic (Larsen et al., 1999, 2001). One unambiguously unique

characteristic is age. This in combination with the consequential

differences, such as dispersal pattern and stratigraphic location,

can be used to achieve unambiguous identifications and so

correlate widely separated deposits of the same tephra.

Tephra dispersal may follow a series of different environ-

mental pathways leading to distinctively time-transgressive

characteristics for the deposit as a whole. For example, fallout

from the volcanic plumes of Late Glacial eruptions in Iceland

has produced extensive isochronous deposits, but tephra fallout

also occurred on sea ice that, through time, has transported the

tephra to new locations where the ice melted, releasing

the tephra to fall to the seabed. Other more lengthy dispersal

routes have resulted from tephra fall onto glaciers, incorpora-

tion into glacier ice, transport within the glacier system and

later deposition at the ice margin. As another twist, ice calving

can result in deposition from melting icebergs long after the

formation of the tephra and far from the source volcano. In this

case, tephra can form horizons within strata that were not

necessarily produced at the same time as the volcanic eruption.

This is not, however, a problem, as tephrochronology can be

used to determine the time taken for movement through

these contrasting environmental pathways, and so it can be

used to constrain assessments of environmental process. This

may be considered to be “tracer” tephrochronology.

An important aspect of the application of tephrochronology is

the use of multiple tephra horizons to constrain periods of time.

This may be crucial for the effective integration of some types of

environmental data. For example, sediment accumulation rates

are best compared over clearly defined time periods rather than

in reference to a single moment or “spot” date. If tephrochronol-

ogy (based on the same tephra layers) is used, then the intervals

of time considered at different sites will be precisely comparable.

Chronology

Tephrochronology is indivisible from volcanic history but

forms a subset of it, insomuch as some key aspects of volcanic

history (such as lava production) may not be associated with

tephra production. Dating tephra layers can harness the full

range of chronological methods available to earth science.

The age of tephras may be determined either directly or indir-

ectly: direct dating of tephra may come from historical records

or dating the tephra itself. Depending on age and composition,

these direct dating methods may include K/Ar, Ar/Ar, fission

track, U-series or luminescence dating.

Frequently, tephras are dated by the analysis of related mate-

rial in which the tephra is found. In this case, depending on the

age and location, these indirect dating methods may include

ice core, varve, radiocarbon dating and dendrochronology. Cru-

cially, the extensive dispersal of tephra layers means that dating

may be optimized by finding the best place within the overall

distribution to apply any particular dating method. This has

the added benefit that dating specific to one environment (such

as ocean core isotope stages) may be applied in another (the

distribution of the Toba tephra across India).

Tephrochronologies

Parts of the world where tephrochronologies have been success-

fully developed and are now an integral part of paleoenviron-

mental research include the British Isles (Dugmore et al., 1995);

Iceland (Thórarinsson, 1981); Japan (Machida, 1999); New

Zealand (Shane, 2000); Mexico (Newton and Metcalfe, 1999);

and Alaska (Carson et al., 2002).

Andrew J. Dugmore and Anthony Newton

Bibliography

Carson, E.C., Fournelle, J.H., Miller, T.P., and Mickelson, D.M., 2002.

Holocene tephrochronology of the Cold Bay area, southwest Alaska

Peninsula. Quaternary Sci. Rev., 21, 2213–2228.

Dugmore, A.J., Larsen, G., and Newton, A.J., 1995. Seven tephra

isochrones in Scotland. The Holocene, 5(3), 257–266.

Larsen, G., Dugmore, A.J., and Newton, A.J., 1999. Geochemistry of

historic silicic tephras in Iceland. The Holocene, 9(4), 463–471.

Larsen, G., Newton, A.J., Dugmore, A.J., and Vilmundardóttir, E., 2001.

Geochemistry, dispersal, volumes and chronology of Holocene silicic

tephra layers from the Katla volcanic system, Iceland. J. Quaternary

Sci., 16(2), 119–132.

Machida, H., 1999. The stratigraphy, chronology and distribution of distal

marker-tephras in and around Japan. Glob. Planet. Change, 21,71–94.

Newton, A.J., and Metcalfe, S.E., 1999. Tephrochronology of the Toluca

Basin, central Mexico. Quaternary Sci. Rev., 18, 1039–1059.

Shane, P., 2000. Tephrochronology: A New Zealand case study. Earth-Sci.

Rev., 49(1–4), 223–259.

Thórarinsson, S., 1944. Tefrokronologiska studier på Island. (Tephro-

chronological studies in Iceland). Geogr. Ann., 26, 395–398.

Thórarinsson, S., 1981. The application of tephrochronology in Iceland. In

Self, S., and Sparks, R.S.J. (eds.), Tephra Studies. Dordrecht: D. Reidal.

pp. 109–134.

Cross-references

Dating, Fission-Tracks

Dating, Luminescence Techniques

Potassium-Argon/Argon-Argon Dating

Uranium-Series Dating

Volcanic Eruptions and Climate Change

THE 8,200-YEAR BP EVENT

Overview

The 8,200 year event, alternatively called the 8.2 kyr event

or more simply the 8k event, refers to the last major abrupt

climate change, a Northern Hemisphere cooling event that

occurred approximately 8,200 years before the present (

BP).

The 8.2 kyr event is the largest anomaly in the Greenland ice

938 THE 8,200-YEAR BP EVENT

cores during the Holocene (see Holocene climates), a period

otherwise marked by a relatively stable climate. Paleoclimate

records from Europe, North America, South America, and

Africa indicate an abrupt cooling event in the Northern Hemi-

sphere. Separate geologic evidence documents the collapse of

the remnants of the Hudson Bay ice dome dam and subsequent

catastrophic final drainage of glacial Lakes Agassiz and Ojib-

way into the Hudson Bay, at approximately the same time.

These lakes once occupied northern portions of the Canadian

provinces of Manitoba and Ontario, respectively.

A potential atmospheric link between the disappearance of

the glacial lakes and the far-field climate events was pointed out

first by Hillaire-Marcel and de Vernal (1995). Further evidence

for cooling and speculation on the connection to drainage of

the glacial lakes was presented by de Vernal et al. (1997)and

von Grafenstein et al. (1998). Barber et al. (1999) provided stron-

ger evidence for the connection between the cooling event in

Greenland and the disappearance of glacial Lakes Agassiz and

Ojibway through improved interpretation of dates derived from

the geologic evidence. Dating suggests the disappearance of

the glacial lakes and drainage occurred at 7.6

C BP,whilethe

anomaly in Greenland ice cores, dated using layer counts,

occurred about 8,200 calendar years ago. Reasons for the differ-

ence between the calendar date and the

C date include changes

in the production of

C through time as well as “reservoir age”

corrections (see Radiocarbon dating).

The pattern of cooling around the North Atlantic basin

and related changes in precipitation patterns from paleoclimate

studies of the 8.2 kyr event is consistent with a decrease in

Atlantic thermohaline circulation (THC, see Thermohaline

circulation). This outpouring of glacial freshwater in a melt-

water pulse (MWP) has been widely hypothesized as a

mechanism for abrupt climate change (Broecker et al., 1985).

Additional freshwater input into the North Atlantic decreases

the density of surface waters and thus the formation of North

Atlantic Deep Water (NADW, see North Atlantic Deepwater

and climate change); this decreased THC reduces the amount

of heat transported by the Atlantic Ocean and causes wide-

spread cooling. However, the 8.2 kyr event is thus far the only

well-established example of coincident cause and effect.

The well-quantified cause and climate changes of the

8.2 kyr event provide an opportunity to evaluate the effective-

ness of global climate models. The ability of climate models

to simulate this past abrupt climate change improves confi-

dence in simulating future climates (Schmidt and LeGrande,

2005). In particular, modelers need to be able to evaluate their

models’ ocean responses to climate change. The spread in

model projections for the North Atlantic THC as a function

of increasing greenhouse gases is extremely large, ranging from

an almost 50% decrease to a small increase by 2,100. In part,

this uncertainty stems from tuning the models for the existence

of a stable North Atlantic circulation, but not being able to tune

for its sensitivity because of the lack of appropriate data. The

8.2 kyr event thus provides an excellent opportunity to test

the ability of the models (“model skill”) to simulate the oceans’

response to freshwater forcing (Schmidt and LeGrande, 2005).

A successful model simulation of this last major abrupt climate

change will improve confidence in the model’s accuracy,

including its ability to simulate future climate changes.

Long term changes versus the abrupt 8.2 kyr event

Naming the “8.2 kyr event” for its occurrence in time lends

itself to confusion – an anomaly that occurred around 8.2 kyr

ago need not be an expression of the “8.2 kyr event.” Orbital,

orographic, and potentially solar-forced climate changes during

the early to mid Holocene may be distinct from the 8.2 kyr

event that was likely caused by the final drainage of glacial

Lakes Agassiz and Ojibway.

The 8.2 kyr event appears to overlay a strong quasi-

millennial-scale oscillation that is potentially linked to small

changes in solar forcing (see Sun-climate connections;Bondetal.,

2001). The production of cosmogenic isotopes (

Be and

from ice cores and

C from tree-ring records) is related to solar

activity. An increase in Greenland

Be flux in the GRIP ice core

around 8.2 kyr could be interpreted as a decrease in solar activity

(Yiou et al., 1997). However, this increased flux is not unique –

several such events occur throughout the Holocene, suggesting

that solar activity changes are not responsible for the 8.2 kyr

event (Muscheler et al., 2004; Field et al., 2006), though they

might be responsible for century to millennial scale oscillations

during the Holocene.

Another potential confusion results from orographic changes,

or changes that occurred as a result of the retreat of the Lauren-

tide Ice Sheet. In North America at roughly 8,000 y

BP,theshift

in atmospheric circulation towards more westerly flow became

more pronounced (e.g., Dean et al., 2002) – in archeological

records, the subsequent mid-Holocene period is referred to as

the “Altithermal,” or “Hypsithermal” period (see Hypsithermal).

Ancillary to the orographic changes could be long term shifts in

meltwater and precipitation run-off from the Laurentian Ice

Sheet. Although these climate shifts are also related to the col-

lapse of the Hudson Bay ice dome dam, they are more generally

related to the retreat of the Laurentide ice sheet and thus may be

distinct from the abrupt 8.2 kyr event.

Orbital forcing of the Northern Hemisphere was stronger

during the early Holocene, and many records document a

“Holocene Thermal Optimum” in the early to mid-Holocene

(see Hypsithermal) that is distinct from the 8.2 kyr event. The

timing of the retreat of the ice sheet is tied to these long term

changes, but the 8.2 kyr event is ostensibly linked to the

climate changes that resulted from the outburst flooding.

The 8.2 kyr event thus refers to Greenland ice core and other

records that document an abrupt (less than a few centuries)

Northern Hemisphere cooling event that likely resulted from

the collapse of the Hudson Bay ice dome dam and subsequent

drainage of the glacial freshwater lakes and perturbation

of THC in the North Atlantic. Simple “anomaly hunting”

for events around 8,200 years ago is insufficient. Since the

8.2 kyr event is extremely abrupt, only high resolution records

with excellent chronologies can be confidently correlated with

the ice core event – extreme caution should be applied before

linking any single anomaly in a record to the 8.2 kyr event

(Morrill and Jacobsen, 2005).

Geology of the flood

During the final stage of Lake Agassiz, it was combined on its

eastern edge with Lake Ojibway (thus called the “Ojibway

Phase”), and the total surface area of these lakes is estimated

at 841,000 km

, corresponding to a volume of 163,000 km

(Leverington et al., 2002). Subglacial outflow (run-off beneath

the ice sheet), similar to modern Icelandic jökulhlaups, likely

initiated the collapse of the ice sheet dam and allowed rapid

drainage of these combined lakes into the proto-Hudson Bay

(called the Tyrrell Sea) down to the Kinojévis level (also

referred to as the Ponton Level for Lake Agassiz) (Teller et al.,

2002; Clarke et al., 2004). Complicating this simple drainage

THE 8,200-YEAR BP EVENT 939

scenario is the Fidler beach level (Klassen, 1983) which lies

below the Kinojévis level, possibly suggesting a two-stage

drainage of the glacial lakes – first Lake Ojibway and some

of Lake Agassiz, followed shortly by the remnants of Lake

Agassiz (Teller et al., 2002). Either way, this final drainage of

the two lakes exceeded any previous drainage of Lake Agassiz

by an order of magnitude (Dyke, 2004).

Scours along the Hudson Bay sea floor, as well as other

geomorphic evidence of the quick movement of icebergs, point

to a rapid, large flood event along the western edge of the

Hudson Bay (Josenhans and Zevenhuizen, 1990). Kerwin (1996)

identified a red, hematite-rich, clay layer through the Hudson

Strait, deposited at approximately 8,000

CyBP that was likely

deposited as material fell out of the Agassiz-Ojibway MWP.

From the Hudson Strait, the waters traveled through the Labrador

Sea and into the North Atlantic Ocean.

Paleo-hydraulic modeling studies (Clarke et al., 2004), or stu-

dies that take into account the geometry of the lake basins as

well as the estimated volumes and rates of discharge, have con-

strained the magnitude and length of the event to approximately

5Sv(1Sv=10

1

)for0.5–1 year; this is equivalent to

0.79  10

–1.58  10

or 22–44 cm of sea level given

the current surface area of the ocean. Alternatively, the best guess

for the volumes and rates of a two-phase event are an initial

drainage of about 3.6 Sv for 1 year, followed shortly (decades)

by 1.6 Sv for 1 year (Teller et al., 2002). The volumes in these esti-

mates are smaller than the upper boundary of 1.2 m of sea level rise

indicated by Mississippi delta sediments (Törnqvist et al., 2004).

The link between the geologic evidence of a drainage

event and paleoclimate records of an abrupt climate change

was initially stymied because the initial dating of the geologic

evidence indicated that the large MWP that followed the

drainage of the glacial lakes pre-dated the ice core and other

paleoclimate records by several centuries. Barber et al. (1999)

determined that an additional marine reservoir age correction (see

Radiocarbon dating) to the

C dates was required, and they estab-

lished that the two events were close enough in time – both around

8,400 100 y

BP – for the MWP to be the catalyst that caused

the abrupt climate cooling events in the paleoclimate records.

Ice cores and important records of the 8.2 kyr event

The largest Holocene abrupt climate change in the GISP2,

GRIP, and NGRIP Greenland ice core records occurs from

8,200 until 8,000 years ago (Dansgaard, 1987; Alley et al.,

1997). An approximate negative 2% excursion in the oxygen

isotopic composition of Greenland ice cores indicates that tem-

peratures may have been as much as 6



C cooler for approxi-

mately 200 years at about 8.2 kyr (Cuffey et al., 1995; Alley

et al., 1997); although some estimates for the cooling suggest

cooling of up to 11



C (Leuenberger et al., 1999). Paired with

this abrupt cooling is decreased snow accumulation (Alley

et al., 1993; Spinelli, 1996) and increased deposition of aero-

sols – e.g., salt, dust, sulfates (and associated cosmogenic iso-

tope

Be) – characteristic of a cooler, drier regional climate

(O’Brien et al., 1995; Muscheler et al., 2004). The approximate

75 ppbv abrupt decrease in methane concentration in bubbles

in the Greenland ice cores at 8.2 kyr (Chappellaz et al., 1993;

Brook et al., 2000) indicates a hemisphere-wide cooling and

drying, since this concentration change would most likely have

resulted from decreased wetland methane emissions that

were the dominant pre-Industrial source for methane emissions

(Houweling et al., 1999). Many records corroborate this indi-

cation of cooling and drying in both boreal (Northern

Hemisphere) as well as tropical regions, and an extensive review

is available in Alley and Ágústsdóttir (2005). Here, the focus is

on records with high resolution and a clear 8.2 kyr event or on

records that directly correlate to records with high resolution

and a clear 8.2 kyr event.

Glaciers in Scandinavia advanced and quickly retreated

roughly synchronously with the 8.2 kyr event; this advance in

southern Norway is called the “Finse Event” (Dahl and Nesje,

1994). Tree-ring thickness changes and alterations in the

oxygen isotopic composition of a lake from Germany reflect

a cooling of 1–2



C (Klitgaard-Kristensen et al., 1998; von

Grafenstein et al., 1998).

Ocean sediment core records off the south-western coast of

Norway document an increase in abundance of the cold water

planktonic foraminifera species, Neogloboquadrina pachyderma

(left) (Klitgaard-Kristensen et al., 1998), but no change in its

oxygen isotopic composition of planktonic foraminifera (Klit-

gaard-Kristensen et al., 2001). However, further north in the East

Norwegian Sea, the oxygen isotopic composition of N. pachy-

derma (left) does become enriched by 0.7%, indicating a cool-

ing of 3–4



C (Risebrobakken et al., 2003).

Interestingly, many Labrador Sea sediment core records

do not show a significant change in the oxygen isotopic com-

position of foraminifera at this time (Hillaire-Marcel et al.,

1994). Keigwin et al. (2005) suggest that the MWP may have

traveled southward along the coast of North America as a

boundary current, thus preventing a significant oxygen isotope

signal within the Labrador Sea.

Deep Lake, Minnesota (USA) records a spike in varve thick-

ness, interpreted as an increase in aeolian deposition, at around

8.2 kyr (Hu et al., 1999). A spike in montane conifers in

Virginia indicates a short-lived cooling period approximately

synchronous with the 8.2 kyr event (Kneller and Peteet, 1999).

Model studies consistently indicate that when the Northern

Hemisphere cools, the inter-tropical convergence zone (ITCZ)

shifts south (Stouffer et al., in press). The ITCZ is essentially

a tropical rain “belt” that generally occurs along the zone of

highest sea surface temperature. Hughen et al. (1996) and Haug

et al. (2001) used a gray-scale record from an ocean sediment

core in the Cariaco Basin, off the Venezuelan coast to infer a

southward shift of the Atlantic ITCZ roughly synchronous with

the 8.2 kyr event. Brazilian speleothems (stalagmites) suggest a

southward shift in the ITCZ as well (Wang et al., 2005). Tropi-

cal ice cores from Kilimanjaro, Africa show a dramatic increase

in dust deposition, suggesting a drop in African lake levels

(Thompson et al., 2002) that could be associated with this

southward migration of the ITCZ. Lake records from Africa

indicate a dry spell around the same time (e.g., Gasse and

Van Campo, 1994), but they lack the resolution to be defini-

tively tied to the 8.2 kyr event.

At about the same time, the ice core record of dD in Vostok,

Antarctica shows a negative 22% excursion, indicating a

warming of approximately 2.5



C (Petit et al., 1999); however,

the dating of the ice core is sufficiently uncertain (500 years)

to question the link with the 8.2 kyr event of the Northern

Hemisphere. Furthermore, not all Antarctic ice cores indicate

such an oscillation (Masson et al., 2000). This implied warm-

ing Southern Hemisphere coupled with the colder Northern

Hemisphere might be an expression of the bi-polar seesaw

(Broecker, 1998).

The evidence outlined thus far makes a clear case for cool-

ing in the Northern Hemisphere at around 8.2 kyr ago, but it

does not indicate the cause of the abrupt cooling. McManus

940 THE 8,200-YEAR BP EVENT

et al. (2004) use

230

Pa/

231

Th (protactinium-thorium) in an ocean

sediment core from the Bermuda rise as an indicator of changes

in NADW; higher resolution examination of this same record

indicates a decades to century long slow-down of Atlantic

THC at around 8.2 kyr. The dissolution of aragonite index in

an ocean sediment core off the north-eastern coast of Brazil

also implies a reduction of the NADW at approximately

8.2 kyr (Arz et al., 2001).

Modeling of the 8.2 kyr event

The 8.2 kyr event particularly lends itself to paleoclimate

model simulation for proxy data comparisons (Schmidt and

LeGrande, 2005). The outburst flood from the drainage of

Lakes Agassiz and Ojibway into the Hudson Bay provides a

plausible and interesting hypothesis for the forcing. Geologic

evidence paired with modeling efforts has provided specific

constraints for the volume and duration of the event (Clarke

et al., 2004). Abundant, widespread, and clear proxy data of

the climate response provides multiple opportunities for com-

paring model results to the paleoclimate record (Alley and

Ágústsdóttir, 2005). The magnitude of the climate response

implies an intermediate THC response (e.g., not “on” or

“off,” rather “in between”). It has several more features that

make it an especially desirable event to model using fully

coupled (atmosphere-ocean-sea ice) general circulation models

(GCMs) – the boundary conditions for the 8.2 kyr event are

relatively similar to today, and its length is relatively short, an

important characteristic since coupled GCMs can take many

months to simulate 1,000 years of climate (Schmidt and

LeGrande, 2005).

LeGrande et al. (2006) simulated the 8.2 kyr event using

the Goddard Institute of Space Studies ModelE-R by adding

freshwater to the Hudson Bay at the rate and duration sug-

gested by Clarke et al. (2004). This coupled GCM also tracks

water isotopes throughout the hydrologic cycle, and the atmo-

sphere component tracks changes in wetland methane emis-

sions and aerosol deposition. Temperatures over much of the

North Atlantic cooled by up to 2



C, while Europe, Asia, and

parts of North America cooled by 0.5–1



C. Temperatures in

the South Atlantic and Southern Ocean warmed by  0.5



which is an expression of the bi-polar seesaw (Broecker,

1998). Storm tracks in the Atlantic became more zonal, and

the ITCZ shifted southward. This pattern of climate change

is typical for GCM experiments of reduced THC in the North

Atlantic, though the magnitude of the climate change is smaller

than “hosing” experiments that apply a larger volume of fresh-

water over a longer period of time and shut down NADW (e.g.,

Stouffer et al., 2006).

LeGrande et al. (2006) directly compared model tracers to

the paleoclimate proxy record and found that the freshwater

forcing is a viable cause for the climate anomalies recorded

in the Greenland ice cores and elsewhere. Generally, water

isotopes in precipitation became more depleted with colder or

wetter conditions. Their simulations showed that the water iso-

topes in precipitation over much of the North Atlantic, Green-

land, and Europe became depleted. In addition, bands of

enriched and depleted water isotopes in precipitation occurred

north and south of the equator, reflecting the southward migra-

tion of the ITCZ.

The drier atmosphere over Greenland caused an increase in

dust and

Be deposition (LeGrande et al., 2006). Both wet

and dry

Be deposition over Summit, Greenland increased,

implying that

Be flux (important for inferring changes in solar

activity) cannot be estimated using the

Be concentration

and snow accumulation alone (Field et al., 2006). The cooling

and drying of boreal (Northern Hemisphere) areas caused a

decrease in wetland methane emissions that resulted in an

approximate 60 ppbv shift in atmospheric methane concentration.

The LeGrande et al. (2006) results indicate that the lack of

evidence for the 8.2 kyr event in the oxygen isotopic composi-

tion of foraminifera from many ocean sediment core records

may be due to “canceling” effects in the proxy record – in

this case, depleted seawater oxygen isotopes and colder ocean

temperatures.

Two Earth system intermediate complexity models (EMICs)

have studied the 8.2 kyr event by applying freshwater forcing

to “8k appropriate ” simulations that include orbital and green-

house gas forcing. Renssen et al. (2001, 2002) simulated the

addition of 4.67  10

of freshwater to the Labrador Sea

at a range of rates (1.5 Sv for 10 years, 0.75 Sv for 20 years,

and 0.3 Sv for 50 years) using the ECBilt-CLIO model. They

found that the model THC response varied greatly and that

the length of the simulated climate event ranged from a few

hundred to a few thousand years, suggesting that the climate

response to freshwater forcing is strongly dependent on non-

linear dynamics within the climate system. Using another

EMIC model (CLIMBER-2), Bauer et al. (2004) simulated

the addition of 2.6 Sv freshwater for 2 years (1.6  10

)

to a band of the Atlantic Ocean between 50 and 70



and also introduced “white noise” to the climate system in the

form of small (0.01–0.1 Sv), random inputs of freshwater in

this same band. They also found a large range of model

responses to the same freshwater forcing, and suggested that

“chance” played a role in the temporal evolution of the 8.2

kyr event.

Ágústsdóttir (1998) simulated the 8.2 kyr event using the

GENESIS climate model. This model is an atmosphere GCM

coupled to a slab (e.g., 50 m thick) ocean; thus, the event

was simulated through (a) specifying 8k appropriate green-

house gas, orbital, and ice sheet forcing; and (b) changing the

heat convergence in the Norwegian Sea, where the convection

associated with THC occurs, not through freshwater forcing.

They found the “typical” pattern of THC perturbation: cooling

over the North Atlantic, zonal shift in precipitation bands,

and southward migration of the ITCZ.

Summary

Around 8,200 years before the present, Greenland ice core

records document the last major, abrupt climate change. Multi-

ple paleoclimate records elsewhere in the Northern Hemis-

phere provide other evidence for a short-lived (about 200

year) cooling event (Alley and Ágústsdóttir, 2005). Geologic

evidence indicates the final drainage of glacial Lakes Agassiz

and Ojibway into the Hudson Bay occurred at about the same

time (Teller et al., 2002). The coincidence of these two effects

suggests that the additional freshwater from the drainage of

these lakes may have been the catalyst for this abrupt climate

change (Barber et al., 1999).

The theory for the cause of the abrupt climate change is that

the drainage of the glacial lakes caused changes in thermoha-

line circulation, also called meridional overturning circulation

(e.g., Broecker et al., 1985). The drainage of the glacial lakes

decreased the surface density of the North Atlantic Ocean,

causing a decrease in the formation of North Atlantic Deep

Water. This “slowdown” of the so-called global conveyor belt

caused cooling of around 1–2



C around the North Atlantic

THE 8,200-YEAR BP EVENT 941

Ocean and surrounding land masses, and perhaps up to 6



Cof

cooling in Greenland.

The 8.2 kyr event is well documented in the paleoclimate

record, and it has an interesting viable cause, or trigger, for

the abrupt cooling. These two properties in addition to its short

length make it an excellent candidate for simulation using

GCMs (Schmidt and LeGrande, 2005). Modeling studies of

the 8.2 kyr event support this theory of abrupt climate change

(LeGrande et al., 2006). Conversely, the match between model

simulations and paleoclimate record provides a means to eval-

uate the skill of climate models.

Allegra N. LeGrande

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Cross-references

Cosmogenic Radionuclides

Foraminifera

Glacial Megalakes

Holocene Climates

Hypsithermal

Ice cores, Antarctica and Greenland

North Atlantic Deep Water and Climate Change

Oxygen Isotopes

Paleoclimate Modeling, Quaternary

Radiocarbon Dating

Sea Level Change, Post-Glacial

Sun-Climate Connections

Thermohaline Circulation

THERMOHALINE CIRCULATION

Definition

Formally defining thermohaline circulation is difficult and numer-

ous definitions exist within the literature. Historically, thermo-

haline circulation has been defined as the component of oceanic

circulation driven by fluxes of heat and freshwater (sometimes

combined into a buoyancy flux) through the ocean’s surface. This

particular definition of thermohaline circulation is prevalent

among ocean modelers, wherein ocean models driven exclusively

by boundary conditions on heat and freshwater, with wind forcing

set to zero, lead to a global-scale meridional overturning.

However, in many cases, anomalies of the surface buoyancy

flux, and particularly its thermal component, strongly depend

on the thermohaline circulation itself. For example, an intensi-

fied meridional overturning cell would lead to stronger pole-

ward heat transport in the ocean. At steady state, this heat

transport anomaly must be balanced by more heat loss to the

atmosphere over the regions towards which the heat is being

transported, and by more heat gain by the ocean over regions

from which the heat is being transported (Figure T1).

A further problem with the above definition arises when one

tries to separate the thermohaline ocean circulation from the

wind-driven circulation. For example, turbulent fluxes (latent

and sensible heat fluxes) represent a sizable part of the net

heat flux and are highly dependent on the near surface wind

speed. Furthermore, evaporation, which represents a negative

part of the total freshwater flux into the ocean is essentially

given by the latent heat loss, and hence is also proportional

to the strength of surface winds. In addition, ocean models

and the ocean component of coupled models typically spe-

cify a fixed value of diapycnal mixing (mixing of heat and

Figure T1 A sketch of the two main cells of the meridional overturning

circulation (MOC). One cell is associated with the formation of the

North Atlantic Deep Water (NADW) in the North Atlantic, its upwelling

in the Southern Ocean, around Drake Passage (DP) and its return flow

as a lighter Antarctic Intermediate Water (AAIW); this cell essentially

represents the Atlantic Meridional Overturning (AMO). The second cell

is associated with the formation of Antarctic Bottom Water (AABW)in

high southern latitudes, its spread northward, mixing with NADW, and

its return to the Southern Ocean. Arrows illustrate the notion that an

intensified (weakened) AMO cell would lead to a stronger (weaker)

northward oceanic heat transport in the Atlantic. This would have to be

balanced by stronger (weaker) heat loss in the north and by stronger

(weaker) heat gain in the south.

THERMOHALINE CIRCULATION 943

salt across surfaces of constant density in the ocean). This

representation of diapycnal mixing is a parametrization of inter-

nal wave breaking which in turn originates from wind (and

tidal) forcing.

Thermohaline circulation is also defined as the density-

driven global-scale oceanic circulation. This too has serious

shortcomings, as density gradients in the ocean are largely set

up by the winds. In particular, the density structure in subtropi-

cal and subpolar gyres is determined by Ekman dynamics. The

wind stress anomalies themselves are functions of sea surface

temperature. Finally, in the ocean, which is typically in geos-

trophic balance over the spatial and temporal scales of interest

to climate and paleoclimate researchers (100s of km and years,

respectively) one cannot separate the existence of the density

gradients from the existence of the currents.

Wunsch (2002) defined the thermohaline circulation as the

circulation of temperature and salt. This definition, however,

is not one that is readily accessible to paleoclimatologists.

A more practical approach is to move away from the use of

the term thermohaline circulation and to focus on the three

dimensional (3D) global overturning circulation (GOC). For

many paleoclimatological applications, a sufficient representa-

tion of the GOC is in terms of the meridional overturning circu-

lation (MOC) and its manifestation in the Atlantic as the Atlantic

meridional overturning (AMO), which represent mass transport

in the 2D meridional/vertical plane (Figure T1). Inherent in

this approach is the acknowledgement that wind and buoyancy

forcing are inseparable, and that wind and tidal forcing play a

fundamental role in diapycnal mixing within the ocean.

Here, we focus our discussion on the MOC as a fundamental

diagnostic in understanding the role of the ocean in past, pre-

sent and future climate change. Furthermore, sudden changes

in the strength of the MOC, and in particular the AMO, are

associated with the abrupt climate change prevalent in high

resolution proxy records over the last glacial cycle. The impor-

tance of the AMO to climate lies in its association with much

of the total oceanic poleward heat transport in the present-day

Atlantic, peaking at about 1.2  0.3 PW (1 PW equals 10

Watts) at 24



N. Since the MOC represents the ocean circula-

tion in the meridional-vertical plane, its existence and structure

is fundamentally connected with the locations of deep water

formation in the ocean.

Locations of deep water formation

Two distinct forms of deep water formation occur: near-

continent and open ocean. The former involves one of two

simple processes: evaporation, or more typically brine rejec-

tion, above a continental shelf produces dense heavy water

which sinks down and along the slope under the combined

forces of gravity, friction and the Coriolis force (Killworth,

1983); alternatively supercooled water may be formed at the

base of a thick ice shelf during freezing or melting and this

dense water may in turn flow down-slope. By contrast, open

ocean convection is observed in areas remote from land and

is characterized by a large-scale cyclonic mean circulation

which causes a doming of the isopycnals and weakens the

static stability over an area tens of kilometers wide. The con-

vection itself is short-lived, is restricted to a narrow circumfer-

ence and is typically driven through intense surface cooling.

These narrow convective chimneys are almost purely vertical

and usually do not entrain large volumes of surrounding water

as in near-continent convection.

The two main constituent water masses of the deep North

Atlantic Ocean – North Atlantic Deep Water at the bottom

and Labrador Sea Water at an intermediate level – are currently

formed in the Greenland-Iceland-Norwegian Seas and the

Labrador Sea, respectively. Deep convection also occurs at a

number of locations around Antarctica (Adéle Coast, Amery

Ice Shelf, Ross Sea, and Weddell Sea), but the dense bottom

water is susceptible to being trapped by topographic sills (as

in the Bransfield Strait), or by local circulation patterns (not

excluding the Antarctic Circumpolar Current – ACC). In the

Southern Ocean, around the southern tip of South America,

an enhanced formation of low salinity Antarctic Intermediate

Water (AAIW) also occurs.

In today’s climate, there are no sources of deep water in the

North Pacific Ocean, although North Pacific Intermediate Water

(NPIW), characterized by a salinity minimum in the subtropical

gyre at depths of 300–800 m within a narrow density range, is

locally produced. This water mass is mainly constrained to the

subtropical gyre, unlike the Labrador Sea intermediate water

mass which is largely confined to the subpolar gyre of the

North Atlantic. The original source water for NPIW is thought

to be sinking in the Sea of Okhotsk.

Deep convection also occurs in the Mediterranean Sea, but

the dense salty outflow that exits into the Atlantic gains buoy-

ancy by mixing with lighter environmental water and spreads

out at mid-depth rather than sinking to the bottom. Similar,

but weaker, saline outflows have been detected issuing at

mid-depth from the Red Sea and Persian Gulf.

In summary, two main bottom water masses exist whose tra-

jectories can be traced by their temperature and salinity charac-

teristics throughout the rest of the world ocean. These are (see

Figure T1): (a) the component of Antarctic Bottom Water

(AABW) which is largely produced in the Weddell Sea before

mixing with Circumpolar Deep Water in the ACC and then

flowing into the major ocean basins; (b) North Atlantic Deep

Water (NADW), which lies above the AABW at all latitudes

except north of 40



N in the North Atlantic. The North Pacific

Ocean, although a source of North Pacific Intermediate Water

is not a source of bottom water.

In addition, as discussed below, AAIW plays a critical role

in linking the Pacific and Atlantic Oceans and in particular

on the stability of the AMO. In the present climate, about

13 Sv (1Sv = 10

–1

) of cold, fresh Antarctic Intermediate

Water (AAIW) flows into the South Atlantic, about 8 Sv of

which is converted into thermocline water through mixing by



S (Rintoul, 1991). Further mixing converts the remaining

5 Sv of AAIW into thermocline water as it approaches the

equator (Schmitz and McCartney, 1993; Table T1). At 24



there is about 13 Sv of thermocline water which flows north-

ward into the high northern North Atlantic where it is con-

verted into NADW. AABW flows into the South and North

Atlantic where it is modified into and exported as NADW into

the Southern Ocean.

Table T1 Transport (in Sv) summary from Schmitz and McCartney

(1993)

Water type 32



S24



Thermocline water 8 13

Intermediate water (AAIW) 5 –

Deep water (NADW) 17 18

Bottom water (AABW) 4 5

944 THERMOHALINE CIRCULATION