Odekon M. Encyclopedia of paleoclimatology and ancient environments

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center stage as one of the main contributors to our understand-

ing of the system that drives Earth’s evolution. ODP focuses its

inquiry into the physics, chemistry, and biology of the Earth’s

processes using two basic data collection strategies: (a) drilling

holes in oceanic crust and extracting core samples and (b)

inserting instruments into the holes to measure conditions such

as pressure, temperature, and chemistry of the pore waters.

Scientists from every major scientific specialty use all the tools

available to them both in ship and shore-based laboratories to

analyze samples and measurements in an effort to compile the

data needed for a thorough, multidisciplinary understanding

of the processes taking place in and on the seafloor.

Deep sampling of the sedimentary layer of oceanic crust

actually began in 1947 with the invention of the Kullenberg

piston corer which was able to penetrate to the Pleistocene and

late Pliocene sedimentary layers. Through the 1950s and early

1960s, scientists were able to gather samples as old as the upper-

Cretaceous (Kennett, 1982;Hay,1988). Long piston cores pro-

vided the first evidence that sedimentary accumulations in deep

sea basins are highly variable and subject to constantly changing

environmental processes.

In the early 1960s, improvements in deep sea drilling technol-

ogy warranted a proposal to launch a national program to drill

through the Mohorovičić Discontinuity, the boundary between

the crust and the mantle. The ambitious plan was to penetrate

6 km of crustal rock below the sea floor in the Pacific Ocean basin.

Known as the Experimental Mohole Project, the plan was

scrapped after a few test holes due to unanticipated technology

problems resulting in substantial cost overruns. Even though the

Project was thought to be a dismal failure, samples obtained

while drilling from a floating vessel demonstrated to scientists

the enormous potential value of geological explorations in oceanic

crust, which covers over 70% of the Earth’s surface, rather than

restricting research to the continental crust.

By 1964, four U.S. oceanographic institutions (Miami Uni-

versity, Columbia University, Woods Hole, and Scripps) had

joined together in a consortium known as the Joint Oceano-

graphic Institutions for Deep Earth Sampling (JOIDES) and

proposed to the National Science Foundation a program of con-

tinuous coring to depths of more than 1 km off the coasts of

Florida and Trinidad. Deep sea sediments recovered from six

drillholes contained long sequences of well-preserved micro-

fossil assemblages useful for global stratigraphic correlations.

The Mohole Project and the JOIDES holes, as they were

called, thus laid the foundation for a campaign of scientific

ocean drilling that, over the past four decades, has yielded a

treasure trove of core samples and downhole measurements

available to scientists all over the world.

On July 20, 1968, the Glomar Challenger, an existing vessel

modified specifically for scientific ocean drilling, set sail as

the flagship of the Deep Sea Drilling Project (DSDP), initially

designed as an 18-month project to drill in the Atlantic and Pacific

oceans. DSDP was so successful that in the early 1970s other

U.S. and foreign institutions joined JOIDES in support of the pro-

gram, thus providing more stable funding for ongoing operations.

DSDP continued operations for more than a decade, gradually

increasing its international participation and its range of deep-

sea explorations. In 1985, a new, larger, and more sophisticated

scientific research vessel began operations, launching the Ocean

Drilling Program, a brand new program of scientific ocean drill-

ing. Operated by Texas A&M University, the drillship JOIDES

Resolution is a “floating university,” fitted with modern drilling

equipment as well as numerous fully-equipped laboratories for

sample measurement and analysis.

ODP has explored all of the world’s oceans, from the Arctic to

the Weddell Sea near Antarctica, collecting sediment and rock

samples, recording downhole geophysical and geochemical data,

and establishing long-term borehole observatories. Since the

JOIDES Resolution first sailed in January 1985, she has put to

sea six times a year, every year with a contingent of scientists,

students, drilling specialists and crew, who leave politics and

national identity behind in favor of getting the job done. As of

mid-2002, ODP has collected almost 700,000 feet (215 km) of

cores from over 1,700 holes. Participants in the Ocean Drilling

Program include 18 U.S. institutions and 22 international partners.

An estimated 2,700 scientists from more than 40 nations have

participated in ODP expeditions and countless more have used

ODP samples and data in their research. All ODP core samples

and related measurements are carefully preserved in depositories

and archives and are made available for study to scientists from

all countries, institutions, and disciplines.

ODP research objectives

The impetus for deep sea drilling has been to collect sediment

and rock samples from oceanic crust in an effort to define and

understand the evolution of the planet. One of the most fortui-

tous aspects of ODP is that evolving technology for scientific

ocean drilling coincided with recently developed theories of

continental drift, sea-floor spreading, and shifts in the planet’s

magnetic field. Theories of slowly drifting landmasses, known

as plate tectonics, were first proposed in the early twentieth

Century by Alfred Wegener, but scientists lacked the tools

and technology to test his intriguing hypotheses. The advent

of scientific ocean drilling has allowed the collection of data

to test and confirm plate tectonics as well as many other

hypotheses concerning the geodynamics of natural events like

earthquakes and volcanoes, and theories of crustal age and its

relationship to the magnetic properties of rocks.

From the outset, ODP researchers have sought to understand

the dynamics of Earth’s geophysical and geochemical pro-

cesses, focusing on:

1. climate and sea level variability

2. the nature and movements of sediments and fluids in and

on the oceanic crust

3. the deep structure of continental margins and mantle-crust

interactions

4. the nature and functions of life forms within the oceanic

crust

ODP contributions to Earth science

The Ocean Drilling Program’s approach to scientific investigation

derives from the recognition that Earth is a complex, multidimen-

sional, dynamic system that is constantly changing. Some changes

may be considered rapid in geologic time – for example, ice sheets

can fluctuate dramatically over a few thousand years. Other

changes occur so gradually that their effects are not at all apparent,

even over hundreds of millennia. Geologists have learned to ana-

lyze data within theoretical models that can collapse millions

of years into time scales in which changes can be described and

interpreted. Although any division of the components of Earth’s

system is arbitrary, oceanographers must subdivide their theore-

tical approaches and hypotheses in order to tackle manageable

scientific questions. Over the years, ODP has concentrated its

630 OCEAN DRILLING PROGRAM (ODP)

efforts within a number of broad scientific themes: climate and sea

level variability; sediments, fluids, and bacteria as agents of sys-

tem change; heat and mass transfer within the Earth’s crust and

mantle;and deformation mechanisms withinthe lithosphere which

contribute to natural catastrophic events such as earthquakes, vol-

canoes, and massive landslides under the ocean.

Climate change

The Earth’s climate exhibits cycles of warming and cooling

as well as episodes of rapid climate change. Geologists from

a wide variety of disciplines have employed an extraordinary

number of investigative approaches to identify and quantify cli-

mate variability, and the various agents of change within

Earth’s dynamic climate system. This area of research is critical

to understanding natural climate cycles, as well as impacts on

local and global climate of natural and man-made perturbations

in the system.

Climate is affected by atmospheric composition, planetary

orbital oscillations, ocean circulation patterns and carbon flux,

biological productivity and evolution, land surface alterations,

sea-level fluctuations, and the global distribution and volume

of ice. Climate change in turn impacts many of these factors

such that a feedback system exists between physical, chemical,

and other ecosystem elements (Figure O4). Identifying cause

and effect relationships requires data from widely diverse geo-

graphic locations and from as many time periods as possible.

One method for measuring climate change comes from exam-

ining stable isotopes of oxygen, carbon, and nitrogen, and the

presence of particular elements within microfossils in sediment

deposits. The ratio of oxygen isotopes and the concentration

of certain elements in the fossils vary predictably with the tem-

perature at which the animals lived. These measurements allow

scientists to extrapolate their findings to make predictions about

other parts of the climate system, such as the waxing and waning

of ice volume over time (Lear et al., 2000).

Variations in carbon deposition is another indicator of cli-

mate change, since biologic productivity and turnover varies

with climate, and rates of biological activity can be measured

in depositions of biogenic sediments of the deep sea. The degree

of preservation of organic carbon in sediments is controlled both

by rates of productivity and degree of oxygenation of the bottom

waters.

Plate tectonics, popularly known as continental drift, has also

been implicated in major climactic variation due to alterations in

the distribution of freshwater and heat transport in the global

ocean. Through cores obtained from ocean drilling, scientists have

identified shifts in ocean currents, seawater salinity, and rates

and pathways of heat transport due to the reconfiguration of

oceans and continents, particularly the opening and closing

of oceanic gateways. One example is the closure of the Central

American Seaway, near Panama, which initially resulted in cli-

mate warming but ultimately led to major ice sheet development

in the Northern Hemisphere (Haug et al., 2001).

Fluctuations in sea-level are also seen as indicators of shifts

in the global climate. While local sea-level changes can be the

result of sediment shifting and plate movement, global sea-

level fluctuation is usually the result of changes in the global

volume of ice, and is directly associated with variations in

atmospheric and ocean temperatures. ODP has drilled a number

of sites, particularly in the New Jersey continental shelf and

Bahama Islands, in an effort to identify those sea level varia-

tions that are the result of local effects versus those that result

from global climate change.

Large shifts in climate can also occur as a result of cata-

strophic natural events. An event such as a volcanic eruption

or a meteor striking the Earth can produce sufficient change

in the composition of the atmosphere to cause an overall warm-

Figure O4 Earth system components, processes, and phenomena (courtesy of Asahiko Taira, Japan Marine Science and Technology Center).

OCEAN DRILLING PROGRAM (ODP) 631

ing of the global climate and even widespread extinctions of

biological species. Such an event is thought to have occurred

about 65 million years ago. A large extraterrestrial object, prob-

ably a meteor, apparently slammed into Earth at what is now

Mexico’s Yucatan Peninsula. In 1997, ODP drilled into sites

300 miles off northeastern Florida, extracting cores containing

sedimentary layers that reveal in beautiful detail a cataclysmic

story of destruction and biotic upheaval. The cores exhibit sedi-

mentary layers of debris from the impact consisting of a layer

of green, glassy globules and mineral grains, a succeeding

layer showing remains of the meteorite but devoid of almost

all life, and succeeding layers showing the return of micro-

organisms. The cores that document what is now known at

the Cretaceous-Tertiary (K/T) boundary enable scientists to

conduct geochemical and paleontological studies of the post-

apocalyptic repopulation of the ocean (Norris, 1997).

Architecture of planet Earth

By convention, our planet is described as being made up of three

major sections, an inner molten core, the mantle, which is com-

posed mostly of solid igneous rock, and the crust, consisting of

regions of consolidated rock, layers of sediment, and pockets

of gases and fluids inhabited by microorganisms. In fact, these

components of the planet are in motion and interacting with each

other all the time. We see the effects of crustal and upper mantle

motion in the form of volcanoes, earthquakes, faults, geysers,

and other natural phenomena. The Ocean Drilling Program has

focused a great deal of attention on describing the movement of

rock, sediments, gases, and fluids in the oceanic crust and mantle.

Core samples from the far reaches of the global ocean have con-

firmed the theory of plate tectonics by comparing sedimentary

deposits, magnetic signatures, and crustal rocks from geographi-

cally disparate sites. Over the past two decades, ODP research has

improved our understanding of Earth’s deep mantle using numeri-

cal simulations, images derived from global seismic tomography,

and experimental studies of materials at simulated mantle condi-

tions. These studies suggest mantle architecture and dynamics that

are considerably more complex than simple layering.

A dramatic example of plate tectonics, or continental drift, is

apparent in the Atlantic Ocean. Until about 140 millions years

ago, a narrow, shallow seaway lay between Europe and North

America. The European and North American plates began to

shift, resulting in a spreading of the seafloor and the formation

of the Atlantic Ocean. A combination of geophysical measure-

ments and studies of drill cores obtained by ODP off the coast

of Portugal documents this gradual phenomenon by measuring

the thickness of the crust and determining that the continental

crust became shallower as it spread. It broke up when it had

thinned to about 7 km, resulting in a crust-free zone of exposed

mantle at the seafloor. Many of the rock cores have now been

dated using radioactive isotopes to demonstrate that the focus

of stretching of the crust and mantle migrated from the conti-

nent towards the ocean (Whitmarsh et al., 2001).

Another major focus of ODP technological and research effort

has been the placement of long-term observatories in the ocean

floor. Dubbed CORKs, these observatories are designed to docu-

ment the flow of heat and chemicals within the interior of the crust

by measuring temperatures, pressures, and compositions of fluids

(Figure O5). CORKS have been placed on the axis and flank of

the Juan de Fuca Ridge in the northern Pacific, and the flank

of the Costa Rica Rift and the Nankai and Mariana subduction

zones. Data from CORKs show that changes in mechanical and

fluid-flow properties of rock formations result from plate strain,

notably at sites where earthquakes occur. Thus, research from

CORKs is helping to explain the relationship between episodic

plate motion, deformation, and earthquake energy release (Davis

and Becker, 1998).

Observatories can also be used to measure seismic activity

in crustal deformations offshore, too far for land-based measur-

ing instruments to reach. ODP emplaced such an observatory

offshore of northeast Japan, more than 1,000 m below the sea

floor in water depths of 2,000 m. This observatory is designed

to sense movement at plate boundaries, called subduction

zones. Interpretations of measurements allow geophysicists to

determine whether plate movement is creating plate boundary

strain that could eventually erupt in a violent earthquake. Earth-

Figure O5 Advanced Cork System (courtesy of Earl Davis, Geological

Survey of Canada and Keir Becker, University of Miami).

632 OCEAN DRILLING PROGRAM (ODP)

quakes are devastating manifestations of plate movement, but

only represent a portion of the total energy storage and release

being created at subduction zones. ODP observatories help us

see the whole picture, allowing better prediction of natural

hazards (Suyehiro, 2002).

Drilling through the entire thickness of Earth’s crust has

been a challenge since the very beginning of scientific ocean

drilling. Scientists define the architecture of oceanic crust by

comparing the seismic signature of the crust in the oceans to

the various pieces of oceanic crust found on continents. ODP

recently drilled one hole in the Pacific to a depth of 2,111 m

below the sea floor – the deepest scientific hole drilled in the

ocean. Another expedition extracted 1.5 km of rock core from

below the seafloor at the Mid Atlantic Ridge. Core samples

from the deep crust provide valuable information on deforma-

tion structures and rock composition from which interpretations

of tectonic activity can be derived (Dick et al., 2000).

Deep biosphere

Living microorganisms have found a comfortable home in the

various ecological niches of Earth’s crust, from shallow sedi-

mentary layers to deep regions of basalt rock. Until recently,

it was assumed that the deep crust was a sterile, lifeless place,

undisturbed by the messiness of organic metabolic processes.

In the past two decades, ODP research has discovered a wide

variety of microbial life forms deep within oceanic crust. Dril-

ling has unearthed bacteria living as deep as 800 m below the

seafloor, much deeper than was previously thought possible

given the extreme temperatures and pressures in the deep crus-

tal environment. Estimating the size and ecological impact of

this so-called deep biosphere is a major scientific goal of

ODP. While the concentration of living organisms in Earth’s

crust is small, the global volume of crust is enormous, indicat-

ing that crustal organisms may represent a significant fraction

of the Earth’s biomass (Parkes et al., 1994).

Recent research indicates that the ability of bacteria to cycle

between active and dormant states in response to nutrient levels

may answer one of the mysteries of how they survive in such

inhospitable environments. Indeed, microbial activity has even

been found in reservoirs of volcanic glass, suggesting that

the microbes are metabolizing minerals as an energy source.

Microbes in volcanic crust may be important catalysts of che-

mical change, and may help to regulate the cycling of elements

between seawater and oceanic crust. The existence of such life

forms that derive their energy from inorganic chemical reac-

tions suggests that life may thrive in previously unsuspected

places, such as on Mars and Europa (Fisk, 1997).

Bacteria isolated from continental hydrothermal environ-

ments have already made a major impact on biotechnology in

areas like bioremediation, waste treatment, microbial enhanced

oil recovery, and biominerals. Some bacterial populations

appear to have survived for millions of years, suggesting they

have some remarkable enzymes and metabolic processes. The

seafloor may turn out to be an especially suitable setting for

sampling the deep biosphere and measuring microbiological

processes and fluid flow in support of biological communities.

Subseafloor environments are often more stable and struc-

turally less complicated than terrestrial environments. Drilling

and core collection strategies are being developed to assure

samples relatively uncontaminated by human activity.

Microbiology has become an integral part of ODP explora-

tions. The drillship has a well-equipped microbiology labora-

tory that allows seagoing scientists to collect samples from

cores for experiments and analysis both immediately onboard

and preserved for shore-based study. The results of these

studies will contribute to our understanding of how the Earth

and its inhabitants have co-evolved (Smith, 2002).

ODP technology

The ODP collects samples from the Earth’s crust in locations

that are not only great distances from shore-based laboratories

but incredibly difficult to reach. ODP drilling sites may be

thousands of miles from shore and more than 8 km below the

ocean surface. ODP adapts technology developed by oil explo-

ration companies, as well as engineering specialized drilling

tools as required by scientists’ research goals. The centerpiece

of the ODP is the 143 m long drillship JOIDES Resolution

(Figure O6). For more than two decades, this ship has made at least

six expeditions a year to drill cores and take measurements inside

the boreholes in all but very shallow, very deep, or ice-covered

oceans. The ship is equipped with over a thousand square feet of

state-of-the-art geophysical, geochemical, and microbiological

laboratories, and accommodates 50 scientists and technicians

and an equivalent complement of crew members.

ODP cores and downhole measurements are the priceless

data that are used by both seagoing and shore-based scientists

to conduct analyses, test hypotheses, and develop theories of

how all the elements of the Earth system function. Cores are

obtained by lowering a string of drill pipe through an opening

in the ship’s hull, called the moon pool, down to the seafloor.

Core barrels, containers for the core, are lowered within the

pipe. They collect core samples as the drill bit chews through

the seafloor. Once the core arrives on deck, all scientists and

technicians are busy taking measurements of its geophysical

and geochemical properties, such as density, porosity, and mag-

netism. The core is then halved, with the working half being

prepared for sampling and the archive half kept in pristine

condition and saved for future research. Scientists spend much

of their time on board measuring and analyzing core samples

for composition, grain size, age, and color. At the end of the

cruise, all cores are sent to one of four onshore repositories

where they are stored in a temperature-controlled environment

for future research.

The borehole left behind is also a source of valuable data

that is collected and stored with care equal to the cores them-

Figure O6 ODP drillship JOIDES Resolution.

OCEAN DRILLING PROGRAM (ODP) 633

selves. Downhole measuring tools are placed in the hole on

wirelines both during and after extracting the core. Instruments

attached to the wireline continuously measure geophysical con-

ditions such as temperature, pressure, fluid flow, and magnet-

ism, all of which are critical to understanding the dynamic

processes in the environment from which the cores were

extracted. These logging tools also include imaging tools that

create a detailed “picture” of the inside of the borehole. These

data help scientists to develop models and theories of sediment

and rock structure, conditions and movements at plate bound-

aries, and shifts in crustal composition such as magma flows

and intrusions (Goldberg, 1997).

ODP organization and funding

The ODP is an international partnership of scientists and research

institutions organized to explore the evolution and structure of the

Earth. Its composition and membership has grown throughout

its thirty-year history to include the United States, Germany,

Japan, the United Kingdom, the Australia/Canada/Chinese

Taipei/Korea Consortium for Ocean Drilling, the European

Science Foundation Consortium for Ocean Drilling (Belgium,

Denmark, Finland, Iceland, Ireland, Italy, The Netherlands,

Norway, Portugal, Spain, Sweden, and Switzerland), the People’s

Republic of China, and France.

Member countries pool financial resources to fund ODP

research. Program resources are managed by a non-profit cor-

poration that subcontracts with various institutions to pro-

vide ship and logging operations, shore-based laboratories

and core curation, and dissemination of samples, data, and

research results.

ODPs research agenda has always been proposed and eval-

uated by participating ODP scientists. Individual scientists or

groups of scientists propose research projects to ODP. Propo-

sals are reviewed and evaluated by multidisciplinary panels

representing scientists from all ODP member countries and

institutions. The consortium of Joint Oceanographic Institu-

tions for Deep Earth Sampling (JOIDES) constitutes a series

of science advice panels and committees, which are charged

with evaluating not only the scientific merit of proposals, but

also considerations such as safety, likelihood of success, pollu-

tion prevention, and technological requirements.

This unique format brings together researchers and students

from universities, industry, and government laboratories in

member nations to work in modern, fully equipped shipboard

laboratories on focused scientific projects. The results of

research based on ODP cores, samples, and data are published

openly in leading scientific journals and in ODP publications.

Joanne C. Reuss

Bibliography

Davis, E., and Becker, K., 1998. Borehole observations record driving

forces for hydrothermal circulation in young oceanic crust. Eos, Trans.

Am. Geophys. Union, 79(31), 377–378.

Dick, H.J.B., et al., 2000. A long in-situ section of the lower ocean crust:

results of ODP Leg 176 drilling at the Southwest Indian Ridge. Earth

Planet. Sci. Lett., 179,31–51.

Fisk, M.R., 1997. Evidence for Microbes inOceanic Balasts: Glass-Eating

Bacteria? ODP’s Greatest Hits; Contributions from the U.S. Scientific

Community. Washington, DC: Joint Oceanographic Institutions, 15.

Goldberg, D., 1997. The role of downhole measurements in marine

geology and geophysics. Rev. Geophys., 35–3, 315–342.

Haug, G.H., Tiedemann, R., Zahn, R., and Ravelo, A.C., 2001. Role

of Panama uplift on oceanic freshwater balance. Geology, 29, 207–210.

Hay, W.W., 1988. Paleoceanography: A Review for the GSA Centennial.

Geol. Soc. Am. Bull., 100, 1,934–1,956.

Kennett, J., 1982. Marine Geology. Englewood Cliffs, NJ: Prentice-Hall,

811pp.

Lear, C.H., Elderfield, H., and Wilson, P.A., 2000. Cenozoic deep-sea

temperatures and global ice volumes from Mg/Ca in benthic forami-

niferal calcite. Science, 287, 269–272.

Norris, R.D., 1997. Records of the Apocalypse: ODP Drills the K /T

Boundary, ODP’s Greatest Hits; Contributions from the U.S. Scientific

Community. Washington, DC: Joint Oceanographic Institutions, 9.

Parkes, R.J., Cragg, B.A., Bale, S.J., Getcliff, K., Goodman, P.A., Rochelle,

J.J.F., Weightman, A.J., and Harvey, S.M., 1994. Deep bacterial

biosphere in Pacific Ocean sediments. Nature, 371, 410–413.

Smith, D.C., 2002. Microbes: Life Deep Beneath the Seafloor,

ODP Highlights; International Scientific Contributions from the

Ocean Drilling Program. Washington, DC: Joint Oceanographic

Institutions, 6.

Suyehiro, K., 2002. The Earth’s Next Move, ODP Highlights; Interna-

tional Scientific Contributions from the Ocean Drilling Program.

Washington, DC: Joint Oceanographic Institutions, 25.

Whitmarsh, R.B., Manatschal, G., and Minshull, T.A., 2001. Evolution

of magma-poor continental margins from final rifting to seafloor

spreading. Nature, 413, 150–154.

Cross-references

Carbon Isotopes, Stable

Climate Change, Causes

Cretaceous/ Tertiary (K-T) Boundary Impact, Climate Effects

Deep Sea Drilling Project (DSDP)

Oxygen Isotopes

Plate Tectonics and Climate Change

Sea Level Change, Last 250 Million Years

Stable Isotope Analysis

OCEAN PALEOCIRCULATION

Introduction

Because of its impact on global climate, one of the most

important aspects of ocean circulation is the rate of meridional

overturning (also called “Conveyor Belt” or thermohaline cir-

culation) (Figure O7; see Thermohaline circulation). In the

Atlantic Ocean, the meridional overturning circulation (MOC)

brings heat from the tropics to high northern latitudes, which

ameliorates the climate of the northern continents (Broecker,

1997). Changes in the rate of the Atlantic MOC and in the

associated production of the North Atlantic Deep Water

(NADW) may thus have been directly implicated in the large

climatic cycles and abrupt climate changes that characterize

the Quaternary period and may have contributed to the waxing

and waning of the large continental ice sheets that covered

North America and Northern Europe during glacial times

(see Quaternary climate transitions and cycles). Changes in

the global ocean circulation could have also played an indirect

role in altering the Earth’s climate by influencing atmosphe-

ric CO

and greenhouse warming (see Ice cores, Antarctica

and Greenland). Lower rates of deep-water ventilation would

engender greater sequestration of carbon in the deep sea,

resulting in lower atmospheric CO

and cooler temperatures

(Toggweiler, 1999). Reconstructing past changes in ocean cir-

culation (i.e., ocean paleocirculation) is thus an integral part

of understanding the evolution of climate and has been subject

to intensive scientific scrutiny for several decades.

634 OCEAN PALEOCIRCULATION

Reconstructing paleocirculation

The chemical and isotopic composition of marine sediments

and skeletal remains of organisms that grow on the seafloor

is one of our main sources of information for evaluating

past changes in ocean circulation in relation to climatic varia-

bility. Changes in ocean circulation impart distinct chemical

and isotopic signatures to seawater that are recorded in the car-

bonate shells of benthic foraminifera (i.e., protozoans living on

the seafloor), in the carbonate skeletons of deep-sea corals and

in the sediment accumulating on the seafloor. Results obtained

from these geochemical tracers (or proxies) clearly reveal that

significant changes both in the distribution of water masses

and in the rate of deep-water circulation took place in the past,

in concert with important climatic events. Reconciling the

detailed interpretation of these different and sometimes con-

flicting proxies is yielding an increasingly clear depiction of

how deepwater circulation contributed to or responded to past

global climate changes.

Geochemical tracers of deep-water circulation can be

broadly divided into two categories: (a) water mass tracers

and (b) kinematic tracers of ocean circulation. Both types of

tracers are complementary and necessary to fully document

past changes in deep water circulation. The water mass tracers

are better suited for reconstructing past expansions or contrac-

tions of the different water masses that fill the deep ocean

basins, while kinematic tracers are necessary to constrain past

changes in the rate at which volumes of deep water were pro-

duced (expressed in Sverdrups ¼ 10

1

Underlying principles

Proxies for water mass identification

In the deep Atlantic Ocean, two antagonistic water masses, the

North Atlantic Deep Water (NADW) and the Antarctic Bottom

Water (AABW), vie for spatial dominance. The NADW, which

initiates the global conveyor belt circulation (Figure O7), origi-

nates from the North Atlantic and flows southward towards

the Southern Ocean, while the AABW originates from the

Southern Ocean and flows northward. The latter water mass

is denser, and wedges underneath the NADW.

In the modern ocean, these two deepwater masses can be

clearly distinguished by their chemical and isotopic composi-

tion. Deep waters of southern origin have significantly higher

dissolved nutrient concentrations (Figure O8). Nutrients consist

of dissolved salts (nitrate, phosphate, silicate) that are taken up

in surface waters to sustain the growth of phytoplankton and

regenerated at depth from the decay of sinking organic matter.

This biological cycling results in lower nutrient concentrations

in surface waters and higher concentrations in deeper waters.

Nutrient concentrations in deep waters have two components.

When surface waters sink to produce deep waters, they already

contain nutrients due to incomplete biological utilization and

entrainment of nutrient-enriched water from intermediate

depths. As newly formed deep waters age in deep ocean basins,

they accumulate an increasing load of dissolved nutrients

from the decay of organic matter exported from the surface.

The former contribution to the total nutrient load of deep

waters is the “preformed nutrient” concentration and the latter

contribution is the “regenerated nutrient” concentration. Deep

waters of southern origin have higher preformed nutrient

concentrations because nutrient utilization in the surface waters

around Antarctica is limited by the harsh environmental condi-

tions that prevail in this region. This is the main reason why

AABW has noticeably higher nutrient concentrations than

NADW, which clearly identify its presence and its northward

propagation above the seafloor (Figure O8). Assuming no

changes in the preformed nutrient concentration of the two

deep-water masses, further northward penetration of AABW

would result in higher nutrient concentrations in the deep water

of the Atlantic. Therefore, if we were able to estimate past

Figure O7 The thermohaline circulation of the ocean. Warm, shallow water is transported from the tropics to the high North Atlantic, where it

comes in contact with cold air, cools down and eventually becomes dense enough to sink to the bottom. The newly-formed deep water

flows to the south, goes around Antarctica and fills up the two other major ocean basins where it gradually mixes upward to return as a warmer

shallower water mass to the North Atlantic (Intergovernmental Panel on Climate Change, 2001).

OCEAN PALEOCIRCULATION 635

changes in deep water nutrient concentrations, we could deduce

the relative importance of these two water masses in the past.

However, past changes in deepwater nutrient concentrations

are not directly recorded in marine sediments. Instead, one

has to find chemical and isotopic signatures (i.e., nutrient con-

centration proxies) that correlate closely with dissolved nutrient

concentrations and become incorporated in marine archives

(i.e., benthic foraminifera, deep-sea corals, or sediments). The

most often used water mass proxies are the carbon isotopic

composition (d

C) of dissolved inorganic carbon and the dis-

solved concentration of several biologically recycled trace

metals (Cd, Zn, Ba), which are recorded in the isotopic and

chemical composition of benthic foraminifera and deep-sea

corals that grow in contact with bottom water. Neodymium iso-

tope ratio ( e

) has also been used. Unlike the previous tracers,

is recorded in the oxide coatings of settling particles that

accumulate on the seafloor and is not directly affected by

biological cycling.

C in benthic foraminifera

Carbon has two stable isotopes:

C(1.1%) and

(98.9%). Isotopic compositions are reported in the “delta”

notation, i.e.,

C ratio expressed in permil deviation from

a standard taken as reference:

Sample



Sample



Standard

 1

1;000

During photosynthesis, dissolved inorganic carbon, nitrate and

phosphate are taken up by phytoplankton to produce organic

matter in surface waters. Because

C is taken up preferentially

C, the organic matter produced has a d

C value lower than

the d

C of inorganic carbon dissolved in surface waters. Upon

degradation and oxidation of organic matter in deep waters, the

dissolved inorganic carbon produced has a similarly “light” iso-

topic composition (i.e., low d

C). This isotopically light inor-

ganic carbon is regenerated concurrently with nitrate and

phosphate, resulting in a negative linear correlation between

the nitrate or phosphate concentration and the d

C of dissolved

inorganic carbon in deep waters (Kroopnick, 1985;

Figure O9a). The d

C of inorganic carbon dissolved in bottom

waters is recorded by the carbon isotopic composition of the

calcium carbonate produced by benthic foraminifera living

on the seafloor, which thus indirectly record nutrient concen-

trations in bottom waters. By measuring the d

C of benthic

foraminifera that lived in the past at different water depths,

it is thus possible to reconstruct past changes in the d

of seawater and deduce past changes in the concentration

and vertical distribution of nutrients in deep waters (Sarnthein

et al., 2001).

Cd/ Ca, Ba/Ca and Zn /Ca ratios in benthic foraminifera

or deep-sea corals

A similar principle can be applied to certain trace metals. Cad-

mium (Cd) is biologically recycled with phosphorus, resulting

in a tight positive correlation between phosphate and dissolved

Cd concentration in seawater (Figure O9b). As Cd becomes

incorporated in biogenic carbonate minerals in proportion to

its concentration in ambient seawater, the Cd/Ca ratio in fossil

benthic foraminifera (Boyle, 1988) and deep-sea corals (Adkins

et al., 1998) provides an independent means of reconstructing

past changes in bottom water nutrient concentration. As for

C, this information can be used to contrast the relative con-

tribution of NADW and AABW in the deep Atlantic Ocean, if

we assume (i) no changes in the preformed nutrient concentra-

tions in the water masses involved, and (ii) no changes in the

rate of organic matter remineralization in deep waters.

Barium (Ba) and zinc (Zn) are also biologically recycled,

but their regeneration from settling particles occurs at greater

depth than for Cd. Instead of tightly correlating with phosphate,

their seawater profiles mimic those of alkalinity or dissolved

silicate ( Figures O9c and O9d), which are controlled by the dis-

solution of carbonate and biogenic silica occurring in deeper

water. As for Cd, the concentration of dissolved Ba and Zn

in bottom water is recorded in the Ba/Ca and Zn/Ca ratios in

the shells of benthic foraminifera (Lea and Boyle, 1989;

Marchitto et al., 2000). Because of its deeper regeneration,

the contrast in Zn concentration between NADW and AABW

is more pronounced than for Cd. As a result, paired Zn

and Cd measurement in the same foraminifera shells provides

Figure O8 Latitudinal cross section of phosphate concentration in the Atlantic Ocean. The high phosphate concentrations in the two water

masses of southern origin, the Antarctic Intermediate Water (AAIW) and the Antarctic Bottom Water (AABW), stand out in sharp contrast to the

lower phosphate concentrations in the North Atlantic Deep Water (NADW).

636 OCEAN PALEOCIRCULATION

Figure O9 Correlations between seawater properties (nutrient concentration and alkalinity) and proxies used for their paleoceanographic

reconstruction. (a) d

C of dissolved inorganic carbon vs phosphate concentration in deep water (adapted from Broecker and Maier-Reimer, 1992).

(b) Dissolved cadmium vs. phosphate concentration in subsurface waters (adapted from Boyle, 1986). (c) Dissolved barium vs. alkalinity in surface

and deep waters (adapted from Lea, 1993). (d) Dissolved zinc vs. dissolved silicate concentration in seawater deeper than 1,000 m (adapted from

Marchitto et al., 2000). (e) e

vs. dissolved silicate concentration in the deep Atlantic Ocean (adapted from Rutberg et al., 2000).

OCEAN PALEOCIRCULATION 637

a means of distinguishing between changes in AABW pen-

etration and changes in preformed nutrient concentration

(Marchitto et al., 2002).

Neodymium isotopic ratio (e

) in sedimentary oxides

Neodymium isotopic composition is reported as:

ðÞ

Sample

143

Nd=

144



Sample

143

Nd=

144



Standard

 1

Seawater e

reflects Nd input from nearby continental masses

(von Blanckenburg, 1999). Deep waters formed in the North

Atlantic acquire a Nd isotopic composition (e

¼13.5)

clearly distinct from that of the deep waters originating from

the Southern Ocean (e

¼7). There is a positive linear cor-

relation between e

and silicate concentration (Figure O9e)

in the deep Atlantic, reflecting a two end-member mixing

between NADW (low silicate concentration and low e

) and

AABW (high silicate, high e

). Dissolved Nd is removed from

seawater by adsorption on iron and manganese oxides that

precipitate in seawater and coat settling particles. The Nd isoto-

pic composition associated with oxides, which can be chemi-

cally leached from sediment, thus provides a complementary

means of estimating the northward penetration of deep waters

of southern origin, which is independent of the biological recy-

cling of nutrients (Rutberg et al., 2000). This is important,

because changes in the concentration of the nutrient-related

tracers described above could be affected by biological recy-

cling, independently of circulation.

Kinematic proxies for estimating rates of circulation

Several proxies that provide information on changes in circula-

tion rates have also been developed. They include (a) recon-

struction of the

C ratio in the atmosphere, (b) paired

planktonic-benthic

C age differences, (c)

C ages of

230

Th-

dated deep-sea corals, (d) geostrophic flow reconstructions

from benthic foraminifera d

O, and (e)

231

Pa/

230

Th in Atlantic

sediments.

C ratio in the atmosphere

C is a radioactive isotope of carbon that is continuously pro-

duced in the atmosphere by cosmic rays (cosmic rays are pro-

tons that shatter nuclei of atoms in the atmosphere, producing

neutrons that become incorporated into the nuclei of nitrogen

atoms while ejecting a proton; as a result

N becomes

C).

After formation,

C enters the global carbon cycle and decays

with a half-life of 5,730 years, thereby providing a clock to

date carbon-containing samples during the last 50,000 years.

However, accurate dating must take into account past variations

in the

C ratio of the atmosphere (expressed as D

atm

;

see Radiocarbon dating). To that end, D

atm

has been

reconstructed for the past 50,000 years (Hughen et al., 2004)

by measuring the

C ratio of samples that could be

independently dated (tree rings,

230

Th-dated corals, annually-

layered sediments, marine sediments correlated to the precise

timescale of Greenland ice cores). An important fraction of

atm

variability results from variations in the intensity

of the Earth’s magnetic field; these affect the influx of cosmic

rays and hence the production rate of cosmogenic nuclides like

C. This variability can be estimated from paleomagnetic data

(e.g., Laj et al., 2002). Additional D

atm

variability can also

result from changes in the rate of deepwater formation, which

affect the residence time of

C in the atmosphere. Thus, the

residual between the reconstructed history of D

atm

and the

atm

predicted from the changes in the intensity of the geo-

magnetic field can be attributed to changes in the rate at which

C is transferred from the atmosphere to the deep sea as a

result of deep water formation (Hughen et al., 2004). D

atm

higher than predicted from paleomagnetic data indicates slower

rates of transfer of

C from the atmosphere to the deep sea,

reflecting lower rates of deep-water formation. However, changes

in the air-sea exchange rate of

C, in the solar component of

C production modulation, and in the terrestrial carbon reservoir,

together with the effect of the biological export of

Ctothe

deep sea, must also be considered.

Paired planktonic-benthic

C age differences

C age differences between the fossil shells of planktonic (i.e.,

living in surface waters) and benthic (i.e., living on the sea-

floor) foraminifera recovered from the same sediment horizon

and presumed to be contemporaneous can potentially provide

quantitative information on past changes in deepwater renewal

rates. Larger differences in age have been taken to indicate

older deep waters and slower renewal rates (Keigwin and

Schlegel, 2002).

There are two potential complications, however, with that

approach. One stems from the variability in D

atm

with time.

Although the planktonic and benthic foraminifera are contem-

poraneous, the benthic species are growing in water that was

in contact with the atmosphere at an earlier time when D

atm

may have been different. The

C ratio of the benthic and

planktonic foraminifera cannot thus be compared directly but

one must take into account this difference in initial conditions

(Adkins and Boyle, 1997). The second complication may be

even more problematic to resolve. It arises from the fact that

foraminifera do not acquire their carbon directly from the atmo-

sphere. Instead, their carbon source is inorganic carbon dis-

solved in ambient seawater. The

C ratio of surface

waters, where planktonic foraminifera grow, is controlled by

exchange of CO

with the atmosphere and mixing with older

dissolved inorganic carbon from below. As a result, the

C ratio of oceanic surface waters and planktonic forami-

nifera is lower than that of the contemporaneous atmosphere.

In the modern ocean, at low to mid latitudes, surface waters

have a

C ratio that corresponds to an age of  400 years.

This so-called “reservoir age,” however, can be significantly

larger at high latitudes (up to 1,300 years), particularly in the

Southern Ocean, where upwelling rates of older deeper waters

are higher and CO

exchange with the atmosphere is limited

by extensive sea ice cover (Bard, 1988). Because deep water

is formed at high latitudes, benthic foraminifera growing on

the seafloor at low to mid latitude record a

C ratio

that has been affected by a significantly older “reservoir age”

than their contemporaneous planktonic counterparts living in

surface water. This difference must also be considered when

calculating the ventilation age of deep water.

C ages of

230

Th-dated deep-sea corals

Some species of corals live at great depths, sometimes as deep

as 4,000 m. They can be dated by measuring the growth of

230

Th from the decay of

234

U included in the coral’s carbonate

lattice. Decay-correcting

C measured in the same coral

samples and comparing it to the corresponding

C ratio

of the atmosphere thus provides a means of estimating the

638 OCEAN PALEOCIRCULATION

age of deep water (Adkins et al., 1998). These estimates must

also take into account possible changes in “reservoir age” and

the variability in D

atm

. However, unlike the sedimentary

archives, which provide continuous but relatively low resolu-

tion records (millennial to centennial timescales, depending

on the sedimentation rates), deep-sea corals provide narrow

(150 years dictated by the life span of coral specimens) but

high-resolution (decadal-centennial timescales) windows on

past changes in deep water ages.

Geostrophic flow reconstructions from benthic

foraminifera d

Most of the warm water flowing to the far North Atlantic for

NADW formation is supplied by the Gulf Stream (Figure O10a).

The Gulf Stream enters the North Atlantic through the Florida

Strait at a flow rate of about 30 Sverdrups. As it moves to the

north, it is continually accrued by entrainment of water coming

from the North Atlantic Subtropical gyre, and its flow rate

increases gradually, reaching 150 Sverdrups as it turns eastward.

In the Florida Straits, the Gulf Stream is nearly in geostrophic

balance, i.e., the Coriolis acceleration associated with the flow

of water (C) is balanced by the lateral pressure gradient across

the Strait (P; Figure O10b). This pressure gradient is associated

with a lateral seawater density gradient and results in sloping

surfaces of constant seawater density, called “ isopycnals.” If

the flow rate of the Gulf Stream were to accelerate, the Coriolis

force would intensify. To maintain geostrophic balance, the

lateral pressure gradient would have to increase, which would

be reflected by steeper isopycnals. The slope of the isopycnals

across the Florida Straits can be estimated from the oxygen

isotopic composition (d

O) of benthic foraminifera living on

the seafloor on each side of the Straits, thereby providing a

means of estimating the flow rate of water through the Strait

in the geological past. To the extent that the contribution from

the subtropical recirculation gyre (Figure O10a) remained con-

stant, changes in flow rate in the Florida Strait could be used to

infer changes in the rate of NADW formation (Lynch-Stieglitz

et al., 1999). The main difficulties with this approach are

(a) ascertaining the contribution from the subtropical gyre, and

(b) the requirement that current velocity must be known at one

level taken as reference, which is very difficult to constrain for

past circulation (Wunsch, 2003).

231

Pa/

230

Th in Atlantic sediments

231

Pa and

230

Th are two natural radionuclides produced at con-

stant rates in seawater from the decay of dissolved uranium. Both

are rapidly adsorbed on settling particles and removed to the

underlying sediment (Figure O11). As

231

Pa has a lower affinity

for most marine particles than

230

Th, it has a residence time in

deep water approximately five times longer and equivalent to

the mean residence time of deep water in the modern Atlantic

(ca. 200 years). As a result, nearly 50% of the

231

Pa produced

in the NADW as it transits through the Atlantic is exported to

the Southern Ocean, while most of the

230

Th is removed in the

Atlantic sediment (Yu et al., 1996). Reflecting this differential

export, the mean

231

Pa/

230

Th of Atlantic sediments (activity ratio

0.05) is only slightly above half the ratio of the rate of produc-

tion of the two nuclides in the water column (0.093).

231

Pa/

230

in Atlantic sediments can thus record the rate at which NADW is

produced and exported to the Southern Ocean. As the rate

of NADW formation and export decreases, the extent of

231

Pa export to the Southern Ocean also diminishes, resulting in

higher

231

Pa/

230

Th in Atlantic sediments (Marchal et al., 2000).

The interpretation of this proxy is complicated, however, by

the fact that the removal rate of

231

Pa from seawater depends on

particle flux and composition. Lower

231

Pa/

230

Th in Atlantic

sediments could also be due to higher particle flux or higher

concentration of opal, a biogenic mineral produced by some

plankton species. Past changes in particle flux and composition

in the Atlantic Ocean must therefore be taken into account when

Figure O10 (a) Gulf Stream and recirculation in the North Atlantic subtropical gyre with total transport streamlines in Sverdrups, adapted from

Worthington (1976). Red circle highlights the Florida Straits. (b) Cross-section in the Florida Straits showing sloping horizons of constant seawater

density obtained from the d

O of modern benthic foraminifera (sampling depths are marked by open circles). The geostrophic flow that prevails in

this region results from the balance between the Coriolis force due to the Earth’s rotation (C) and the lateral pressure gradient created by the

lateral density gradient (P). The cross indicates that the water flows away from the reader (adapted from Lynch-Stieglitz et al, 1999).

OCEAN PALEOCIRCULATION 639