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In addition to a sea-level pressure (SLP) signature, the NAO

has a coherent signal in sea-surface temperature (SST) invol-

ving a tripole pattern of almost zonally-oriented anomalies

with subtropical and high-latitude SSTs varying in-phase and

mid-latitude SSTs varying out-of-phase (Seager et al., 2000).

Figure N7 presents the classic SLP and SST pattern asso-

ciated with the NAO. While the NAO reflects changes in

the strength and orientation of mid-Atlantic westerlies that

dictate heat and moisture flux trajectories, it is also expressed

in terms of distinct North Atlantic SST, sub-surface tempera-

ture (Molinari et al., 1997 ; Reverdin et al., 1997) and sea-ice

extent anomalies (Deser and Blackmon, 1993). During the

positive phase of the NAO, SSTs in the eastern Labrador Sea,

eastern Mediterranean, and subtropical Atlantic are anoma-

lously cold by 0.5–1



C, whereas SSTs in the North Sea and

Sargasso Sea are anomalously warm by approximately the

same amount (Cullen et al., 2002).

Given that there is no unequivocal method for defining the

NAO, it follows that there is no universally accepted index

of the temporal variability of the NAO. However, as the signa-

ture of the NAO is strongly regional, a simple NAO index has

been defined as the difference between the normalized mean

winter (December through March; DJFM) SLP anomalies at

locations representative of the relative strengths of the AH

and IL. A positive NAO index implies more meridional storm

tracks while a negative NAO index implies more zonal

storm tracks. It must be remembered, however, that indices of

the NAO merely provide a temporal signature for the behavior

of a phenomenon whose dynamics are not clearly understood.

Historical development

The NAO was first documented by the Danish missionary

Hans Egede Saabye (Egede, 1745) who described it as a see-

saw in temperature between Greenland and Denmark. Sir

Gilbert Walker eventually defined the NAO as a meridional

alternation of atmospheric mass in the 1920s (Walker, 1924).

In addition to the NAO, Walker was also the first to describe

the Southern Oscillation and the North Pacific Oscillation in

his attempt to forecast Indian monsoon rainfall (Brown and

Katz, 1991).

Beginning with the work of Bjerknes (1964), considerable

effort has been made to understand the extent to which pre-

ferred timescales apparent in observations of the North Atlantic

Ocean are associated with interactions between the ocean and

atmosphere. These processes can be either coupled, whereby

the two systems generate important positive and negative feed-

backs, or uncoupled, where communication does not lead to

feedbacks. Coupled, or two-way, interactions between the

ocean and atmosphere suggest the potential for a degree of pre-

dictability. Predictability resides primarily within the long-term

memory of the ocean and provides the opportunity for long-

range forecasting. One example, in fact the only unquestioned

example, of a coupled ocean-atmosphere interaction is the El

Niño-Southern Oscillation (ENSO). Like the ENSO, the NAO

has been shown to have significant climate impacts. However,

there is still considerable doubt as to whether the NAO is a

coupled ocean-atmosphere process. The NAO is a fundamental

mode of the atmosphere, that is, it appears in atmospheric gen-

eral circulation models (AGCMs) with fixed surface boundary

conditions, whereas the Southern Oscillation’s existence is

entirely dependent upon active ocean-atmosphere coupling.

Significance

Given its importance as a modulator of climate in the Atlantic

sector, an increased understanding of how the NAO has

varied in past centuries is of considerable interest to the cli-

mate community. However, unlike the tropics, where abundant

maritime proxy climate data (e.g., coral records) are preserved

for high-resolution reconstructions of ENSO variability over

past centuries, extra-tropical regions such as the North Atlantic

lack these primary archives for reconstruction. The best proxy

archives of NAO variability come from terrestrial regions

and include tree ring records from North America, Eurasia,

Scandinavia, and the Middle East, documentary evidence from

Europe, and ice cores from the Arctic.

The NAO has exhibited a significant trend towards more

positive values over the past 30 years, steadily rising from a

negative phase in the 1960s to a persistent positive phase in

the early 1990s. This trend accounts for a significant portion

of the wintertime (DJFM) temperature increase over Eurasia

Figure N7 Correlation over the period 1865–2001 between the NAO index (Hurrell, 1995) and North Atlantic sector sea-level pressure (SLP; Kaplan,

2000) anomalies and sea-surface temperature (SST; Kaplan et al., 1998) anomalies. SLP correlation is shown as contoured r values (correlation

coefficient) from 1.0 to 1.0 and SST correlation is shown as shaded r values (correlation coefficient) from 1.0 to 1.0.

620 NORTH ATLANTIC OSCILLATION (NAO) RECORDS

(Hurrell and van Loon, 1997). Consequently, the NAO has

been introduced into the global warming debate with a search

for mechanisms that could resolve whether this trend is due

to anthropogenic perturbation or natural variability (Rodwell

et al., 1999; Shindell et al., 1999). Coherent, large-scale

changes in the NAO since 1981 were linked to recent dry con-

ditions over western Greenland and the Mediterranean, and

wetter and warmer than normal conditions in northern Europe,

the northeast US, and parts of Scandinavia (Hurrell, 1995;

Hurrell and van Loon, 1997). Using multivariate linear regres-

sion to quantify temperature variability associated with the

NAO, it was shown that the NAO accounts for 31% of NH

interannual variance (Hurrell, 1996). Moreover, the NAO

accounts linearly for 0.15



C of the 0.29



C NH extratropical

temperature increase for the period 1981–1994, with respect

to the 1935–1994 mean. It should be noted, however, that the

winters since 1995 have shown negative or neutral values –

thereby interrupting the trend. In addition to being linked to

widespread warming over the past three decades (Hurrell,

1996), it has been suggested that the NAO may have also

played a role in maintaining climate anomalies during the Little

Ice Age (Luterbacher et al., 1999, 2000; Shindell et al., 2001).

Proxy NAO records lengthen the relatively short observa-

tional record and provide the opportunity to assess both the

low-frequency behavior of the NAO and the recent positive

trend. Since the NAO is a natural mode of atmospheric varia-

bility, one could argue that much of the recent warming is nat-

ural and not related to the build-up of greenhouse gases in the

atmosphere since the industrial revolution. This viewpoint,

however, ignores the possibility that anthropogenic climate

change might influence modes of natural variability, perhaps

making it more likely that one phase or another of the NAO

might be preferred.

Observational records

Table N1 details the most commonly used NAO indices. Most

of the more commonly used NAO indices are derived from dif-

ferent representative SLP stations of the IL and AH. The prin-

cipal component (PC) time series of the leading empirical

orthogonal function (EOF) of Atlantic SLP is also commonly

used. Walker and Bliss (1932) constructed the first index of

the NAO using a linear combination of surface pressure and

temperature measurements. Rogers (1984) constructed an

NAO index beginning in 1895 using SLP anomalies from

Ponta Delgados, the Azores, and Akuyreyri, Iceland. 1995)

demonstrated that data from Lisbon, Portugal better captured

the southern node variability of the NAO and used it along

with data from Stykkisholmur, Iceland to develop an index that

extends back to 1864. Figure N7 presents the correlation of the

Hurrell (1996) NAO index with winter (DJFM) SLP (Kaplan

et al., 2000) and SST (Kaplan et al., 1998) data. Jones et al.

(1997) further extended the NAO index back to 1821, using

pressure data from Gibraltar and a composite southwest Iceland

series. Jones et al. (2003) show that all of these indices are

significantly correlated at interannual and decadal timescales

but that choice of the southern node does make a difference.

Cullen et al. (2002) developed an SST anomaly index of the

NAO, extending back to 1857, to extract the NAO-related com-

ponent of Atlantic SST variability – similar in principle to the

NINO3 SST-based index, which tracks ENSO in the tropical

Pacific Ocean. Luterbacher et al. (1999) have developed the

longest observational NAO record to date, going back to

1675, using instrumental station pressure, temperature and pre-

cipitation measurements, and proxy data. Each method has

advantages and disadvantages. Indices based on station data

are fixed in space and are therefore affected by small-scale

transient non NAO-related weather events (Trenberth, 1984).

Given the shifting of the NAO centers-of-action through the

annual cycle, these indices cannot sufficiently capture the

NAO over all months of the year (Hurrell and van Loon,

1997). One advantage of the PC time series approach is that

it includes a larger spatial domain and hence provides a more

optimal representation of the NAO pattern.

In addition to these commonly used NAO indices, several

other methods for capturing temporal variability exist. During

the 1950s, Namias, (1950) and Lorenz, (1951) were among the

first to develop indices known as the “zonal index cycle.”

These indices characterize NAO variability by looking at

zonally averaged mid-latitude surface westerly wind fields.

Stephenson et al. (2003) describe several less well-known

NAO indices that have been developed by European scientists.

An investigation of the power spectrum of these NAO

indices suggests little evidence that the NAO varies on any

preferred timescale. Spectral analysis, used to quantify periodi-

cities within time series, shows a slightly “red” spectrum

(power increasing with period). There is somewhat enhanced

variance on interannual (2–3 years) and decadal (8–10 years)

bands as well as a trend over the last 30 years – but there

are no significant peaks. Using a station-based index, Hurrell

and van Loon (1997) demonstrated that the NAO index

showed a greater degree of interannual variability during late

nineteenth and early twentieth centuries, whereas decadal

variability increased during the late twentieth century. Fur-

thermore, Feldstein (2000) concluded that a role for forcings

external to the atmosphere (i.e., the ocean) could not be ruled

out despite the fact that interannual variability of the NAO

arises primarily from noise. Czaja et al. (2003) caution that

spectral analyses of dynamical NAO indices are not optimal

for detecting the impact of the ocean.

Paleorecords

Instrumental records of the NAO, while useful in assessing sea-

sonal and interannual variability, are not long enough to answer

fundamental questions about low-frequency variability. In addi-

tion, recent observational records are not pure reflections of

natural climate variability – they carry an imprint of anthropo-

genic forcing. As a result, a clear need exists to extend climate

records back beyond the industrial era in order to understand:

(a) the range of natural climate variability, (b) how quickly cli-

mate regimes can switch, and (c) how human activity might be

influencing natural climate variability.

At present, paleoreconstructions are unable to accurately fin-

gerprint the climate response to increasing greenhouse gas

Table N1 Instrumental records

Index Season Field Begin

Cullen et al. (2002) DJFM SST 1856

Hurrell (1995) DJFM SLP 1864

Jones et al. (1997, 1999) Monthly SLP 1780

Luterbacher et al. (1999) Monthly SLP, SAT, P 1675

Rogers (1984) DJFM SLP 1874

Thompson and Wallace (2000) Monthly SLP 1899

Thompson and Wallace (1998) JFM SAT 1851

NORTH ATLANTIC OSCILLATION (NAO) RECORDS 621

emissions as they cannot achieve the spatial (regional level)

and temporal (season-specific) resolution required (Jones et al.,

2001). Nevertheless, several attempts have been made to

extend the NAO record back based on the rationale that

large-scale climate variability can be represented successfully

by integrating across a number of high-resolution “proxies” of

past variability. Proxies can either be natural (physical or biolo-

gical) or from a documentary source and generally act as an

indicator of either temperature or precipitation or some combi-

nation of the two. The most widely used proxy for NAO recon-

structions are trees, where ring density, ring width, and wood

isotopic composition are generally measured. Ice cores are the

second most widely used proxy, where isotope ratios, accumu-

lation rates, and melt layers serve as proxies. Corals and stalag-

mites (Proctor et al., 2000) can also be used by measuring

isotopic concentration, cation ratios, and growth thickness to

provide an annual reconstruction; however, these records are

generally restricted to the tropics and have not been widely

employed as a tool for reconstructing the NAO. Whereas

instrumental measurements have unique space and time coordi-

nates, climate proxies often represent the combined influences

of climatic and non-climatic factors, recorded at different times

of the year depending on the type and location of the proxy.

Calibration against instrumental records is critical for all proxies,

and statistical regression is generally used to establish a relation-

ship between observations of the climate in a given region and

the variability of the proxy over the same time period.

Table N2 details the most commonly used NAO paleoproxy

reconstructions. The longest record is that of Cook et al.

(2002), extending back to

AD 1400. The reconstructions of

Luterbacher et al. (1999, 2002a,b) are the only reconstructions

providing monthly resolution. Despite the inconsistencies

between different records, this rich archive of tree rings,

ice cores, and documentary data indicate that prolonged posi-

tive and negative NAO phases have occurred in the past

(Appenzeller, 1998; Cook et al., 1998; Luterbacher et al.,

1999; Schmutz et al., 2000; Cullen et al., 2001). This suggests

that the extreme positive values of the index evident since the

late 1980s are not unprecedented over the past five centuries

(Luterbacher et al., 2002a,b). Proxy records also indicate that

there is a degree of “intermittency” present in the NAO. This

suggests the NAO does not contain a single, persistent fre-

quency – but rather oscillates between low-frequency variability

and periods more akin to white noise (Appenzeller et al., 1998).

However, it should also be noted that the NAO index might sim-

ply be one particular realization of a white noise atmosphere

initiating a red noise ocean response (Wunsch, 1999).

Due to the different methods, different time intervals, and

different observational time series used for calibration, a direct

comparison of the different indices shows little agreement

before the 1820s (Schmutz et al., 2000; Cullen et al., 2001).

Schmutz et al. (2000) suggest that proxy-based NAO indices

are unable to capture NAO variability in a consistent manner

in the eighteenth and nineteenth centuries and speculate that

the NAO includes several different circulation patterns with

specific impacts on a given proxy, resulting in degrees of cov-

ariability. Prior to the twentieth century most NAO index

reconstructions show an inconsistent picture and in the period

before the 1820s these inconsistencies are greater.

Of the NAO records listed in Table N2, the reconstructions

by Cook et al. (2002 ) and Luterbacher et al. (2002a,b) appear

to have somewhat more consistent covariability – at least on

interannual timescales. The winter (DJFM) multiproxy recon-

struction by Cook et al. (2002) was constructed using tree ring

and ice core data from the North Atlantic sector whereas the

monthly (1659–1995) and seasonal (1500–1658) reconstruc-

tions by Luterbacher et al. (2002a,b) used both instrumental

and documentary data from Eurasia. When the seasonal indices

are compared, explained variances are higher in Luterbacher et al.

(2002a,b) than in the Cook et al. (2002) reconstruction during

and before the eighteenth century. Furthermore, Luterbacher

et al. (2002a,b) suggests predominantly negative-phase NAO

values before 1800, whereas Cook et al. (2002) indicate more

positive-phase NAO values.

Predictability

In addition to the importance of assessing the role of increasing

greenhouse gases, an improved dynamical understanding of the

NAO may help provide a better understanding of NAO ocean-

atmosphere dynamics as well as the potential predictability of

large-scale climate fluctuations in the Atlantic sector. Current

research efforts suggest a small degree of seasonal predictabil-

ity of North Atlantic climate via two potential pathways. The

ocean, with its significant thermal inertia, may provide one

pathway for limited predictability (Rodwell, 2003). Statistical

analyses have revealed that knowledge of North Atlantic

(Kushnir et al., 2002) and tropical Pacific (Hoerling et al.,

2001) SSTs may provide information about the NAO with a

lead time of several months.

A second pathway that offers hope for improved predictability

of the NAO involves links through which changes in stratospheric

wind patterns might exert some downward control on surface cli-

mate (Thompson et al., 2003). The NAO is most active in the win-

ter, a time when stratospheric circulation favors communication

between the stratosphere and troposphere, which is consistent

with the stratosphere as a potential mechanism. Traditionally, it

was thought that the more massive troposphere was forcing the

stratosphere, however, now it appears things may work the other

way around. Changes in stratospheric circulation can be forced

by several different mechanisms including ozone depletion, volca-

nic dust, and CO

. Rising CO

concentrations cool the strato-

sphere and strengthen the stratospheric winter vortex, which

Table N2 Paleoproxy records

Reconstruction Source Location Period

Appenzeller et al. (1998) Ice core Greenland 1648–1990

Cook et al. (2002) Multiproxy Northern Hemisphere 1400–1979

Cook et al. (1998) Tree rings North Atlantic sector 1701–1980

Cullen et al. (2001) Multiproxy North Atlantic sector 1750–1979

Glueck and Stockton (2001) Multiproxy Northern Hemisphere 1429–1983

Luterbacher et al. (2002a,b) Instruments/documentary Eurasia 1500–1995

Rodrigo et al. (2001) Documentary Spain 1501–1997

622 NORTH ATLANTIC OSCILLATION (NAO) RECORDS

generates stronger surface winds through a complex dynamical

interaction. These enhanced westerly surface winds are consistent

with a positive NAO index. A statistical connection between

the month-to-month variability of the NH stratospheric polar

vortex and the tropospheric NAO was established several years

ago (Perlwitz and Graf, 1995) and more recently it has been

documented that large amplitude anomalies in the wintertime

stratospheric winds precede anomalous behavior of the NAO by

1–2 weeks. (Baldwin and Dunkerton, 2001), perhaps providing

some extended range predictability. Similarly, processes that

affect the stratospheric circulation on longer timescales, such as

reductions in stratospheric ozone, volcanic forcing, and increases

in greenhouse gases, could factor into the trend in Atlantic surface

climate observed over the past several decades (Gillett, 2003).

This leaves the interesting possibility that, while interannual

to decadal variations in the NAO/AO exist simply because

they represent a fundamental mode of climate variability,

the trend may be anthropogenically forced. Using an AGCM,

Shindell et al. (1999) tested the possibility that these observed

trends could be simulated by realistic increases in greenhouse

gas concentrations. Ultimately, Shindell et al. (1999) showed

that rising greenhouse gases in an AGCM could create a trend

in the NAO/AO. Greenhouse gases, therefore, may induce cli-

mate change through dynamical effects in addition to direct

radiative forcing.

Conclusions

Records, both instrumental and paleoproxy reconstructions of

the NAO, provide a temporal snapshot of the behavior of a

large-scale climate phenomenon whose dynamics are not yet

fully understood. Despite the fact that the NAO is indeed a

mode of variability internal to the atmosphere, these records

of the NAO exhibit low-frequency variability – both decadal

variability and trends – that would not be expected from a fast

acting atmospheric process. That not all of the variability asso-

ciated with the NAO can be attributed to intraseasonal stochas-

tic atmospheric processes suggests a possible role for external

forcings. Furthermore, it suggests a small but perhaps useful

amount of predictability.

Heidi M. Cullen

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Cross-references

Climate Forcing

Climate Variability and Change, Last 1000 Years

Coral and Coral Reefs

Dating, Dendrochronology

Dendroclimatology

Ice Cores, Antarctica and Greenland

Little Ice Age

Medieval Warm Period

North Atlantic Deep Water and Climate Change

Paleo-El Niño-Southern Oscillation (ENSO) Records

Paleotemperatures and Proxy Reconstructions

Speleothems

624 NORTH ATLANTIC OSCILLATION (NAO) RECORDS

OBLIQUITY

The Earth’s axis of rotation is not perpendicular to the orbital

plane (ecliptic). At present, there is an angle of 23.446



between

the axis of rotation and the perpendicular to the ecliptic. This

angle, called obliquity, governs the march of the seasons. The

North Pole receives sun light during the entire boreal summer

while the South Pole remains in the dark. It is the reverse for

the boreal winter.

Obliquity results from the torque that the Sun and the Moon

exert on the equatorial bulge of the Earth. Its value has varied

from 22 to 25



over the last three million years, with a quasi-

period of 41,000 years (Figure O1; Berger, 1978; Berger

and Loutre, 1991). Obliquity plays a similar role in both

hemispheres. Indeed, larger obliquity increases the energy

received in each hemisphere during the local warm season.

The highest latitudes are more sensitive to obliquity changes

because the energy flux at the surface depends on the inclina-

tion of the solar beam (Berger and Loutre, 1994). Around

the poles, a 1



obliquity change induces a change in the

solar radiation at the top of the atmosphere by 4% during

summer time. Moreover, a larger obliquity reinforces the seaso-

nal contrast in insolation because it increases the irradiance in

summer and reduces it in winter. It also reduces the latitudinal

contrast in annual irradiance because it increases the irradiance

at mid-and high-latitudes and decreases it at low latitudes

(Figure O2).

Marie-France Loutre

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Berger, A., and Loutre, M.F., 1991. Insolation values for the climate of the

last 10 million years. Quaternary Sci. Rev., 10, 297–317.

Berger, A., and Loutre, M.F., 1994. Precession, eccentricity, obliquity,

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(eds.), Long Term Climatic Variations, Data and Modelling. Nato ASI

series, Series I: Global Environmental Change, Vol. 22. Berlin:

Springer, pp. 107–151.

Cross-references

Astronomical Theory of Climate Change

Climate Forcing

Eccentricity

Millennial Climate Variability

Precession, Climatic

Pre-Quaternary Milankovitch Cycles and Climate Variability

Quaternary Climate Transitions and Cycles

SPECMAP

OCEAN ANOXIC EVENTS

Although black shales and carbon isotope anomalies are known

from the Paleozoic, ocean crust older than approximately

150 Myr has been subducted and therefore oceanic anoxic

events prior to the Mesozoic cannot be confirmed. Therefore,

this discussion will focus mainly on Mesozoic events.

Mesozoic ocean anoxic events are defined based on the wide-

spread occurrence of fine-grained sedimentary rocks that are

highly enriched in organic carbon (e.g., black shales). Deposits

of organic-carbon enriched strata (generally known as “black

Figure O1 Long term variations of the obliquity over the last 800,000 years.

shales”) were recognized as special features of the rock record

early in the development of the geologic sciences because they

require specific environmental conditions for formation that are

not common or widespread in modern oceans. Nonetheless, these

deposits store quantities of carbon sufficient to source all of the

petroleum and natural gas that humans have thus far consumed,

or will ever produce. In fact, it has been estimated that the highest

proportion of petroleum source rocks were formed during the

times in Earth history that were characterized by ocean anoxic

events (Schlanger and Cita, 1982).

Based on observations of organic carbon burial rates in mod-

ern marine environments that are oxygen deficient, early black

shale researchers concluded that the key factor leading to

enhanced organic carbon preservation was decreased oxygen

content in bottom waters (e.g., less than 1.0 ml O

per L H

O).

Oxygen deficiency was known to occur as a consequence of

either (i) water column stratification, a process that prevents

downward mixing of surface waters enriched in oxygen by

photosynthesizing algae and/or gas exchange with the atmo-

sphere, (ii) increases in primary production, which result in accu-

mulation of excess decomposing organic matter and exhaustion

of oxygen supply by aerobic bacteria. As the petroleum industry

expanded in the twentieth century, the impetus to better under-

stand the formation of organic carbon-enriched petroleum source

rocks led to even more detailed studies of modern and ancient

black shales. Following World War II, the development of

research-related exploratory drilling in the ocean basins (the

Deep Sea Drilling Project) resulted in a major discovery: many

of the black shales known from shallower epicontinental

basins could be traced to time equivalent strata in the deep sea

record, suggesting that the conditions responsible for organic

carbon burial and preservation had been extremely widespread,

if not global, during certain intervals of Earth’s history. This dis-

covery led Schlanger and Jenkyns (1976) to propose the concept

of Mesozoic oceanic anoxic events or OAEs.

The key features that first distinguished ocean anoxic events

were the rapid onset, short duration (0.5 Myr), and widespread

distribution of highly organic-carbon enriched deposits, which

suggested a profound and sudden change in the Earth’s ocean-

atmosphere system (Schlanger and Jenkyns, 1976; Schlanger

et al., 1987). It was later shown that many of the OAEs were also

characterized by significant positive shifts in the stable isotopic

composition of both organic and inorganic (carbonate) carbon

contained within them (Arthur et al., 1985, 1987, 1988). This

evidence indicated that the Earth’s entire surface carbon reservoir

had been depleted in the lighter stable isotope of carbon (

C),

likely due to a global increase in the burial of organic matter

(which preferentially concentrates the lighter stable isotope).

Had the entire global ocean turned rapidly anoxic (as suggested

by the OAE term), leading to a worldwide increase in preservation

of organic carbon? Alternatively, had rates of phytoplankton pro-

duction dramatically risen so that the supply of organic matter

sinking through the water column was so large that the respiratory

demands of decomposers overwhelmed oxygen supplies, secon-

darily causing widespread anoxia? Was carbon burial in ocean

sediments so massive that it turned the oceans into a net CO

sink,

causing drawdown of atmospheric CO

and cooling of the warm

greenhouse climate of the Mesozoic? Were these putative changes

in marine and terrestrial environments responsible for the major

changes in biodiversity revealed in fossil records spanning some

of the OAEs? Lastly, what process(es) had initiated each OAE

and what caused them to terminate? Such questions have been

vigorously debated during almost thirty years of intensive

research on these events, and although much has been learned,

the largest questions remain unanswered. For this reason, Meso-

zoic OAEs remain one of the most actively studied examples of

deep time perturbations in the Earth’s ocean-climate system.

Organic carbon burial processes

The fundamental process underlying the formation of OAEs is

the burial of organic carbon. Organic carbon is produced in suffi-

cient quantities to form geologically significant accumulations

mainly by primaryphotosynthesizers, including algae and bacteria

in oceans and lakes and vascular plants on land. In most deposi-

tional environments, where oxygen is abundant, accumulating

organic matter is efficiently decomposed and the CO

that was

fixed by plants (“photosynthate”) is returned to the surface reser-

voir with no net change in the global carbon cycle. Under certain

conditions, however, where oxygen becomes limited due to a

decrease in supply or an increase in demand (relative to supply),

accumulating organic matter may be protected from aerobic

decomposition long enough to be buried, thus preventing its remi-

neralization to CO

(examples include bogs in which so much

plant matter accumulates that available oxygen is consumed by

respiration, or well-stratified tropical lakes and shallow seas in

which a lack of seasonal overturn leads to oxygen-depleted bottom

waters and improved conditions for preservation of algal organic

matter). In these settings, carbon is removed from the surface

short-term carbon cycle and it enters the long-term (lithospheric)

carbon cycle, where it remains for timescales of millions to many

millions of years. Ocean anoxic events representextremely narrow

time intervals in which large areas of the world’soceansshifted

from carbon respiring to carbon preserving systems.

The exposure of organic matter to dissolved oxygen cer-

tainly exerts an important primary control on organic carbon

burial rate (Hartnett et al., 1998), but it is not the only control.

The main factors that regulate the rate of burial and long term

preservation of organic matter in marine sediments include

the rate of export production (fraction of primary photosynthate

reaching the sediment), the rate of bulk sedimentation (which

controls how fast organic matter is transferred into the anoxic

zone within the sediment column), and the rate of organic

decomposition (which is accomplished by aerobic and anaero-

bic microbial communities) (e.g., Arthur and Sageman, 1994;

Figure O2 Variation of the mean irradiance (W m

–2

) following an

increase of the obliquity from 22 to 24



(at present-day eccentricity,

e = 0.0167 and longitude of perihelion, = 102.04



626 OCEAN ANOXIC EVENTS

Sageman and Lyons, 2003; Sageman et al., 2003). Although

there has been much debate about the importance of production

vs. preservation over the years, the current consensus views

each of the aforementioned factors to be important, if not inter-

dependent elements of organic matter burial. Their relative

importance may vary from one depositional system to another.

Types of OAEs

Mesozoic OAEs occur in two major time intervals that are gen-

erally characterized by widespread development of black shales

(Figure O3). Based on areal extent of organic carbon deposition

and perturbation of the global carbon cycle, as reflected in

carbon isotope excursions, the Toarcian event in the Jurassic

(Jenkyns, 1988) and OAE Ia through III in the Cretaceous

(Leckie et al., 2002) are the major designated OAEs. However,

other stratigraphic intervals such as the Late Jurassic Kimmer-

idge Clay and Oxford Clay, or the Middle to Late Cenomanian

Graneros and Greenhorn Formations (and time equivalent stra-

tigraphic units), are also characterized by significant organic

carbon enrichments, as well as some minor perturbations in

the isotope record.

Although the difference between OAE and non-OAE

black shales appears to be a matter of degree, the key question

is degree of what? Both OAE and non-OAE black shales may

exhibit spatial and temporal differences in the principal types of

organic matter buried; the dominance of production, preservation,

or sedimentation mechanisms; or the nature of the associated

carbon isotope excursion. For example, the Toarcian event,

OAE 1a, OAE 1b, OAE II and OAE III [all] contain mostly

marine organic matter, but OAE 1c and OAE 1d appear to be

dominated by terrestrial organic material (Erbacher et al.,

1996). Increase in primary production is currently viewed as

the dominant mechanism driving carbon burial during OAEs

in most settings. However, evidence for dominance of hydro-

gen-sulfide rich conditions in the deep ocean basins (e.g., Kuy-

pers et al., 2002) combined with the recent demonstration of

enhanced organic matter preservation due to sulfide-mediated

hydrogenation reactions (Hebting et al., 2006) suggests that

preservation may indeed also play an important role. Lastly,

although the archetypal isotope excursion during an OAE is

the positive shift believed to reflect greater burial of

C-

enriched organic matter (e.g., OAE Ia, OAE II), some events

are characterized by distinct negative shifts in d

C followed

by a swing to positive values. This pattern is particularly well

developed in the Toarcian event (Hesselbo et al., 2000, 2003).

Causal mechanisms for OAEs

If enhanced primary production initiates OAEs, then these

events likely reflect major changes in nutrient availability. Several

processes have been implicated as causal factors affecting nutrient

budgets in Cretaceous seas, but none of them individually pro-

vides a satisfactory explanation for the observed features of OAEs.

For example, changes in vertical circulation associated with the

opening of ocean gateways has been suggested as a mechanism

that might have upwelled phosphate-rich deep waters (Kuypers

et al., 2002; Poulsen et al., 2003). Some investigators have

suggested that micronutrients, such as reactive Fe, derived from

hydrothermal activity associated with rapidly spreading ridges or

emplacement of large igneous provinces may have played a

role in driving OAEs (Sinton and Duncan, 1997;Kerr,1998;Kerr

et al., 2004; Leckie et al., 2002;Snowetal.,2005). This idea is

quite intriguing, especially since the strontium isotope record

(Figure O3) shows shifts toward less radiogenic values consistent

with global increases in oceanic volcanism and sea floor spreading

during several OAEs (McArthur et al., 2001), and the ages of some

large igneous provinces broadly overlap with OAEs (Wignall,

2001; Leckie et al., 2002). A terrestrial source of increased phos-

phorus is also a possibility given that many studies have shown

changes in paleotemperature proxies consistent with significant

warming during the time intervals containing OAEs (e.g., Huber

et al., 2002,Norrisetal.,2002;Wilsonetal.,2002). Such tempera-

ture increases, initially caused by enhanced volcanic outgassing of

and global warming, could have driven up chemical weather-

ing rates leading to greater terrigenous phosphorous input to the

oceans (Föllmi, 1995). Lastly, it is clear from a comparison of

the global relative sea level record and OAEs (Figure O3) that they

occurred during rising sea level or highstand episodes when the

area of shallow shelf seas was greatly expanded. As originally

recognized by Schlanger and Jenkyns (1976), shelf sea area may

have been a critical prerequisite for OAE development.

Although none of these proposed mechanisms alone explains

the short-term character of all OAEs, some combination likely pro-

vides a plausible means to initiate and terminate these events. In

addition, the role of phosphorus regeneration during anoxic

organic matter decomposition has received increasing attention

in recent years as a potential feedback mechanism for eutrophica-

tion and black shale deposition (Ingall et al., 1993; Van Cappellen

and Ingall, 1994; Ingall and Jahnke, 1997; Murphy et al., 2000;

Nederbragt et al., 2004). This process may have played a role in

maintaining elevated organic carbon burial rates during OAEs.

Summary

Interest in OAEs was originally motivated, in part, by a desire

to better understand the processes leading to formation of

petroleum source rocks. As we look forward to the end of

the Petroleum Era, due to occur in the next century, renewed

interest in OAEs has been driven by a very different motivator:

the need to better understand the processes and feedbacks

involved in rapid perturbations of the Earth’s biogeochemical

cycles. OAEs include some of the most geologically rapid

and pervasive changes of the oceans and atmosphere driven

by changes in the carbon cycle, and they are associated with

warm climates, rising sea levels, and possibly increased nutri-

ent fluxes to the oceans. The development of sound environ-

mental policy for the twenty first Century and beyond will

depend on our ability to predict the response of the Earth Sys-

tem to anthropogenic CO

output, the climate warming pre-

dicted to result from this output, the increases in sea levels as

continental ice sheets continue to melt, and the increase in agri-

cultural runoff of nutrients associated with population-driven

expansion of food production. Studies of Mesozoic OAEs

provide a critical source of information to our ongoing effort

to build accurate and predictive Earth System models.

Brad Sageman

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628 OCEAN ANOXIC EVENTS

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Cross-references

Atmospheric Evolution, Earth

Carbon Cycle

Carbon Isotopes, Stable

Mesozoic Climates

Sapropels

OCEAN DRILLING PROGRAM (ODP)

History

For more than 30 years, the Ocean Drilling Program (ODP) has

sent hundreds of scientists out from their universities and labora-

tories to the far corners of the world’s oceans in search of sedi-

ments and rocks that tell the story of Earth’s restless past. The

ancient ocean depths, long thought to be a barren and unchanging

primordial environment, have yielded their secrets of how our

planet’s powerful yet delicate ecosystem functions.

The Ocean Drilling Program constitutes one part of a syste-

matic geologic investigation of the Earth that has been ongoing

since the eighteenth century. It is only in the past thirty

years that scientific ocean drilling conducted by ODP and its

predecessor, the Deep Sea Drilling Project (DSDP) has taken

OCEAN DRILLING PROGRAM (ODP) 629