Reconstruction of the Hydrology of the Deep
Ocean
Main Features of Modern Circulation
On a rotating planet like the Earth, surface ocean circulation
is governed by the winds and the position of the continents
that define the shape of the ocean basins. Deep circulation,
on the other hand, is governed by the small variations in the
density of the water masses. Since density depends on
temperature and salinity, the term ‘thermohaline circulation’
is used. Reconstructing past changes in temperature, salinity
and density in the intermediate and deep ocean is thus
needed to understand the temporal variations of the circulation and distribution of water masses; it also provides a
benchmark for simulations provided by general circulation
models.
As we saw in Chap. 1, deep waters are currently formed
in winter in highly localized areas of the high latitudes: the
Norwegian Sea and the Labrador Sea in the northern
hemisphere and the Weddell Sea and the Ross Sea near
Antarctica. During winter, surface waters here become
denser as they cool down but also because the formation of
sea ice is accompanied by a release of salt. When surface
waters become as dense as deep waters, large-scale convection movements are initiated and the waters sink into the
abyssal depths. Once at depth, very small changes in density
of the various deepwater masses govern their circulation
through the different basins. Surface waters sinking in the
Norwegian Sea cross the sills separating it from the Atlantic
Ocean to form North Atlantic Deep Water (often referred to
by the acronym NADW). This water mass then follows the
American coast to join the Southern Ocean and is caught up
in the Antarctic divergence, a large upwelling zone, where it
mixes with the surface waters of southern high latitudes.
Here, these very cold surface waters increase their density
through winter sea ice formation, and sink along the
Antarctic continental shelf forming the densest waters in the
world. This water mass is called Antarctic Bottom Water (or
AABW), and it lines the bottom of all ocean basins. At
present, abyssal waters are not formed from the surface
waters of the Indian and Pacific Oceans. The Antarctic
waters that rush into these basins create the Pacific and
Indian Deep Waters (PDW and IDW, respectively) by
mixing with the waters of the main thermocline, and then
return to the Southern Ocean at around 3 km depth. We can
devise a simplified view of the global ocean circulation,
where the Norwegian Sea is the main source of deep waters,
and the Southern Ocean acts as a recirculation pump
returning to the depths the surface waters surrounding
Antarctica that have received upwelled NADW via the
Antarctic divergence. This circulation pattern is critically
dependent on the climate of the high latitudes of the
Northern Hemisphere (North Atlantic Ocean, Norwegian
and Labrador Seas) and of the Southern Ocean in the
Southern Hemisphere.
Reconstructing the Temperature and Salinity
of Deep Waters
The past evolution of the deep ocean has been the subject of
extensive research. However, the reconstruction of the basic
properties of bottom waters has been hindered by the lack of
transfer functions linking the abundance of benthic species
to the temperature of seawater in the vicinity of the sediment. In many environments, these abundances are essentially governed by the availability of food and by the
proportion of dissolved oxygen. However, paleoceanographers have attempted to apply other approaches to the
reconstruction of bottom water temperature.
Searching for a Reference Zone with Constant
Temperature
As early as 1967, Shackleton (1967) suggested that the d
18
O
of benthic foraminifera must closely resemble the d
18 O of
deep waters, because these waters are formed close to
freezing conditions and their temperature cannot drop much
further during a glacial period. Labeyrie et al. (1987) further
elaborated on this concept by analyzing the d
18 O of benthic
foraminifera from a core in the Norwegian Sea where temperature, well below 0 °C in the deep basins, is constrained
by exchanges with the ice. By comparing the isotopic record
from this core with others from the Pacific and Indian
Oceans, the authors were able to demonstrate, contrary to the
assumptions of Shackleton, that deep water temperature in
the major ocean basins did change significantly at the
beginning of the last glaciation, with a cooling in all deep
oceans to a temperature close to the freezing point (*−1 °
C) during the LGM. This result has been confirmed since by
other tracers such as the Mg/Ca ratio of benthic foraminifera
(see Sect. “Estimating the Temperature Independently of the
Paleotemperature Formula”). Unfortunately, the isotopic
benthic record of the Norwegian Sea is far from continuous
and this method could not be successfully applied to
reconstruct the evolution of the abyssal water temperature
over the whole of the Quaternary.
It should be emphasized, however, that very low
deep-water temperatures during glacial periods are to be
expected. Indeed, under current conditions, NADW is
formed from very cold water (close to the freezing point)
that overflows from the Norwegian Sea through the sills
located between Scotland, the Faroe Islands, Iceland and
Greenland. However, the water that crosses these shallow
21 Climate and the Evolution of the Ocean: The Paleoceanographic …
237
Ocean
Main Features of Modern Circulation
On a rotating planet like the Earth, surface ocean circulation
is governed by the winds and the position of the continents
that define the shape of the ocean basins. Deep circulation,
on the other hand, is governed by the small variations in the
density of the water masses. Since density depends on
temperature and salinity, the term ‘thermohaline circulation’
is used. Reconstructing past changes in temperature, salinity
and density in the intermediate and deep ocean is thus
needed to understand the temporal variations of the circulation and distribution of water masses; it also provides a
benchmark for simulations provided by general circulation
models.
As we saw in Chap. 1, deep waters are currently formed
in winter in highly localized areas of the high latitudes: the
Norwegian Sea and the Labrador Sea in the northern
hemisphere and the Weddell Sea and the Ross Sea near
Antarctica. During winter, surface waters here become
denser as they cool down but also because the formation of
sea ice is accompanied by a release of salt. When surface
waters become as dense as deep waters, large-scale convection movements are initiated and the waters sink into the
abyssal depths. Once at depth, very small changes in density
of the various deepwater masses govern their circulation
through the different basins. Surface waters sinking in the
Norwegian Sea cross the sills separating it from the Atlantic
Ocean to form North Atlantic Deep Water (often referred to
by the acronym NADW). This water mass then follows the
American coast to join the Southern Ocean and is caught up
in the Antarctic divergence, a large upwelling zone, where it
mixes with the surface waters of southern high latitudes.
Here, these very cold surface waters increase their density
through winter sea ice formation, and sink along the
Antarctic continental shelf forming the densest waters in the
world. This water mass is called Antarctic Bottom Water (or
AABW), and it lines the bottom of all ocean basins. At
present, abyssal waters are not formed from the surface
waters of the Indian and Pacific Oceans. The Antarctic
waters that rush into these basins create the Pacific and
Indian Deep Waters (PDW and IDW, respectively) by
mixing with the waters of the main thermocline, and then
return to the Southern Ocean at around 3 km depth. We can
devise a simplified view of the global ocean circulation,
where the Norwegian Sea is the main source of deep waters,
and the Southern Ocean acts as a recirculation pump
returning to the depths the surface waters surrounding
Antarctica that have received upwelled NADW via the
Antarctic divergence. This circulation pattern is critically
dependent on the climate of the high latitudes of the
Northern Hemisphere (North Atlantic Ocean, Norwegian
and Labrador Seas) and of the Southern Ocean in the
Southern Hemisphere.
Reconstructing the Temperature and Salinity
of Deep Waters
The past evolution of the deep ocean has been the subject of
extensive research. However, the reconstruction of the basic
properties of bottom waters has been hindered by the lack of
transfer functions linking the abundance of benthic species
to the temperature of seawater in the vicinity of the sediment. In many environments, these abundances are essentially governed by the availability of food and by the
proportion of dissolved oxygen. However, paleoceanographers have attempted to apply other approaches to the
reconstruction of bottom water temperature.
Searching for a Reference Zone with Constant
Temperature
As early as 1967, Shackleton (1967) suggested that the d
18
O
of benthic foraminifera must closely resemble the d
18 O of
deep waters, because these waters are formed close to
freezing conditions and their temperature cannot drop much
further during a glacial period. Labeyrie et al. (1987) further
elaborated on this concept by analyzing the d
18 O of benthic
foraminifera from a core in the Norwegian Sea where temperature, well below 0 °C in the deep basins, is constrained
by exchanges with the ice. By comparing the isotopic record
from this core with others from the Pacific and Indian
Oceans, the authors were able to demonstrate, contrary to the
assumptions of Shackleton, that deep water temperature in
the major ocean basins did change significantly at the
beginning of the last glaciation, with a cooling in all deep
oceans to a temperature close to the freezing point (*−1 °
C) during the LGM. This result has been confirmed since by
other tracers such as the Mg/Ca ratio of benthic foraminifera
(see Sect. “Estimating the Temperature Independently of the
Paleotemperature Formula”). Unfortunately, the isotopic
benthic record of the Norwegian Sea is far from continuous
and this method could not be successfully applied to
reconstruct the evolution of the abyssal water temperature
over the whole of the Quaternary.
It should be emphasized, however, that very low
deep-water temperatures during glacial periods are to be
expected. Indeed, under current conditions, NADW is
formed from very cold water (close to the freezing point)
that overflows from the Norwegian Sea through the sills
located between Scotland, the Faroe Islands, Iceland and
Greenland. However, the water that crosses these shallow
21 Climate and the Evolution of the Ocean: The Paleoceanographic …
237